Someone is selling a MAC MA6500 Integrated claiming its superiority over the Ma6600 due to the fact that "it does not have the degrading autoformer design found in the MA6600". That is the first time I've heard a claim that the autoformer was a hindrance to better performance; I thought quite the opposite. What do you MAC Maves think?
Ramtubes, Le is borrowed from T/S (Thiele Small) for inductance of a coil. Everything is clear with the design. They used a low voltage relatively high current design. Actually, I have stumbled upon opinions about superiority of current amplifiers many times. There we have it. Low frequency is not a problem when there is no DC offset and the permeability is big. It is interesting if the MC2K is even more low voltage. 2kW into 1 Ohm is 44.7V and 44.7A, in 0,5 Ohm it is 31,6V and 63,3A. Power wise, 2 kW requires 20 transistors, current wise 5 to 7.
Hi Ramtubes, The voltage of the amp is confirmed to be +/-80V by the service schematic of the amp. 80/1.414=56.6V RMS 56,6^2=3200 3200/1.6=2000W
I have two excerpts for you, the autoformer photo and schematic with voltages, but don't see how to upload images.
1.6 Ohms is the Z of the so called primary with load on the so called secondary. Z of an transformer is calculated through Le, and Le is dependant on core crossection, lenght of magnetic lines through the core and core material permeability. Double C has less losses, and toroidal has evem less than double C. Also they have different path lenghts and are typically made of different materials. Anyway. 56.6 to the required 126.5V RMS is a ratio of 1:2.23, even within the generally recommended ratio for an autoformer of 1:3 in order to be feasible. For 2kW into 16 ohms 179V RMS are required, making of 1:3.16 turm ratio. Quite manageable. Regarding the output resistance that should be 0.2 Ohms for a DF of 40 into 8 Ohm load, this seems to be the image of the output resistance of the voltage source with some losses or with very shallow NFB.
Regarding the NFB, it is a separate tap from the autoformer and is summed wirh the normal NFB in some unclear way, at least unclear from the schematic of the service manual. And regarding what it is there for... I just saw your comment and am questioning why I participate in this conversation. All amplyfying elements are non linear, be it tubes or transistors... NFB is anty non linearity in the first place.
Anyway, my investigation is journalistic and performed by a non engineering person. Although hard to find, online there is enough knowledge to permit designing and manufacturing of such device by almost completely ignorant designer.
My quest was to unveal the design considerations of the MC2K. Undoubtedly they are using low voltage in the amp and the corresponding high audio grade components. The typical transistor is voltage limited to 70-100V, up to 100W and up to 10A. It is obvious that the hardest limitation is the voltage. With the capacitors again the voltage is the main limiting factor. 126V RMS is upwards of 180V on the supply rails. I went through a electronic parts supply catalogue, big electrolytes diversity ends around the 80-100V rating.
And now I reconsider about the photos. Anyone in this discussion should have seen both the schematic and the manual with the photos. I tell you I've seen that the rails are +/- 80V and the wire seems in the 1mm-ish region. Prove me wrong.
P.S. Regarding the distortion, it is in octaves and in multiples of the
base frequency, thus the 7th harmonic of 100 Hz is 14,800 Hz and the 7th
harmonic of 1000 Hz is 148,000 Hz... That is why the H-Fi standard
measures only to 3rd harmonic only up to 7 kHz.
Hello, I found the thread and read it very carefully. Presently I am reverse engineering this type of block scheme of power amp
.
Thanks for reading the thread carefully, however there is not enough information to calculate the transformer design.
Here is what I have found: Damping factor for MC1200 and 2000 in 8 Ohms is about 40 which translates to 0.2 Ohm output resistance. In my experience with transformers, this translates to about 100 windings of 1.2 mm wire over a large cross section of E+I laminated steel. A little more for double C-core. 2 kW into 8 ohms is 126.5V and 15.8A
What is a large core? The double C results should not be any different than the EI. One needs to know the core area and lowest frequency to calculate the turns. The output impedance is due to many things, not just transformer winding resistance. Feedback makes it low.
MAC output devices are actually step up transformers. Why are they called autoformers is not clear to me.
An auto transformer has just one tapped winding where currents are shared. I wrote an explanation of how it is wound and how it differs from a conventional output transformer. Everything above the 2 ohm tap is step up in voltage. This allowed them to use low voltage transistors.
In the power supply 100V filtering capacitors are used, thus the rail voltage must be less than that voltage, lets say 90V or even less than +/- 80V for longevity. (typical commercial capacitors begin at 1200 hours at rated voltage and temperature and there are some rated at 13000 hours, but some people don’t turn off their gear which makes for a mere 77 weeks in the latter case... lowering the voltage and temperature multiplies the life multiple times) 126.5/80=1.575 is the transmission ratio of the transformer. 100/1.575 2000W and 80V translate to 3.2 Ohms load and 25 Amps current.
Note: output transformers are designed based on allot more complicated calculations.
This is very simple and vague calculation.
It is not unthinkable for the primary winding to be sub 2 ohms and the operating voltage of the supply rails to be under 50V with large currents.
The complication in the reverse engineering comes from the separate winding of the NFB loop, which loads both primary and secondary windings additionally and thus reducing their impedances.
The NFB winding does not load anything. It is there for stability under reactive loads.
From the photo of the autoformer in the McIntosh manual, the wires seem to be in the 0.8-1.2mm range. 1 mm wire can withstand 30 Amperes, which leads to primary resistance of about 2.2 Ohms.
Can I see this photo please?
In addition, aluminum or nickel wires are not unthinkable in regards of 22-30 meters of wire having 2-3 Ohm DCR.
I presume it is wound with copper, why anything else?
Why? Everyone who have tried to lay down the design requirements of a powerful amplifier have come across the design limitations of high voltages. Reducing the voltage allows for high-end audio grade components such as capacitors and transistors. At 140V there are no capacitors and a limited choice of transistors. So the autoformer enables McIntosh to design a no compromise SS amp capable of large current due to the high transistor count and powerful power supply at the cost of some manageable complication.
P.S. Regarding the distortion, it is in octaves and in multiples of the base frequency, thus the 7th harmonic of 100 Hz is 14,800 Hz and the 7th harmonic of 1000 Hz is 148,000 Hz... That is why the H-Fi standard measures only to 3rd harmonic only up to 7 kHz.
Hello, I found the thread and read it very carefully. Presently I am reverse engineering this type of block scheme of power amp. Here is what I have found: Damping factor for MC1200 and 2000 in 8 Ohms is about 40 which translates to 0.2 Ohm output resistance. In my experience with transformers, this translates to about 100 windings of 1.2 mm wire over a large cross section of E+I laminated steel. A little more for double C-core. 2 kW into 8 ohms is 126.5V and 15.8A MAC output devices are actually step up transformers. Why are they called autoformers is not clear to me. In the power supply 100V filtering capacitors are used, thus the rail voltage must be less than that voltage, lets say 90V or even less than +/- 80V for longevity. (typical commercial capacitors begin at 1200 hours at rated voltage and temperature and there are some rated at 13000 hours, but some people don’t turn off their gear which makes for a mere 77 weeks in the latter case... lowering the voltage and temperature multiplies the life multiple times) 126.5/80=1.575 is the transmission ratio of the transformer. 100/1.575 2000W and 80V translate to 3.2 Ohms load and 25 Amps current.
Note: output transformers are designed based on allot more complicated calculations.
This is very simple and vague calculation.
It is not unthinkable for the primary winding to be sub 2 ohms and the operating voltage of the supply rails to be under 50V with large currents.
The complication in the reverse engineering comes from the separate winding of the NFB loop, which loads both primary and secondary windings additionally and thus reducing their impedances.
From the photo of the autoformer in the McIntosh manual, the wires seem to be in the 0.8-1.2mm range. 1 mm wire can withstand 30 Amperes, which leads to primary resistance of about 2.2 Ohms.
In addition, aluminum or nickel wires are not unthinkable in regards of 22-30 meters of wire having 2-3 Ohm DCR.
Why? Everyone who have tried to lay down the design requirements of a powerful amplifier have come across the design limitations of high voltages. Reducing the voltage allows for high-end audio grade components such as capacitors and transistors. At 140V there are no capacitors and a limited choice of transistors. So the autoformer enables McIntosh to design a no compromise SS amp capable of large current due to the high transistor count and powerful power supply at the cost of some manageable complication.
P.S. Regarding the distortion, it is in octaves and in multiples of the base frequency, thus the 7th harmonic of 100 Hz is 14,800 Hz and the 7th harmonic of 1000 Hz is 148,000 Hz... That is why the H-Fi standard measures only to 3rd harmonic only up to 7 kHz.
Put very simply, bifurcation is the process where feedback can cause a 2nd to become a 4th and so on. Norman Crowhurst wrote about this and I think later also Baxandall. If you look into Chaos Theory you will see quite a lot of papers on the topic.
The KT-150 spec sheet you showed us says that "Tube impedance is [f]rom 10.0KOhm to 12.5KOhm.
And is even higher at low currents where we are often listening. It really helps for someone get a scope or some kind of monitoring device to see the voltage the speaker is using. Some listeners are at very low levels and some much higher. Anyone who is going through tubes is up at the high endor has a terrible mismatch.This may not be obvious, but once one goes over the knee at 0 grid volts the extra current comes at the expense of large voltage drops across the tube and the amplifier is surely clipping at this point. .
There is no optimal load for a power tube. The load belongs to the circuit in which it is used and the load that is expected. That is why an RM-9 can do equally well with the KT 88 family of tubes (beam tubes) or EL34 (true pentodes). Since these tubes have very different curves they will certainly sound different. One must realize that installing different tubes, either the same type with slightly different specs, or another type entirely will mostly change the feedback of the amp, thus the damping and distortion.
In most cases the largest difference will be in the bass and overall frequency response due to the higher damping (more feedback) with the beam tubes.
Interestingly, the 6550 "load resistance" is 3000 ohms. But the plate resistance is 15K ohms.
The KT-150 spec sheet you showed us says that "Tube impedance is [f]rom 10.0KOhm to 12.5KOhm.
I think the importance point here is that whatever the relevant resistance spec is, I think your point is that if one has a speaker with a flat impedance function of 4 ohms, using the 4 ohm tap will present an optimal load to the output tubes off the primary winding of the output tranny.
By contrast, if one's speaker has a rock and rock impedance function, ranging from 4 ohms to 20+ ohms, using the 4 ohm tap is still the way to go even if the back impedance presented to the output tubes is higher than the tube's impedance spec, whatever it is. In other word, opt for light loading. It will result in less distortion, longer tube life, and a higher DF.
I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life. Ergo, use the 4 ohm tap. You called it light loading.
What I have been saying is that the primary impedance chosen has nothing to do with the plate impedance of the Pentode. Ideal plate resistance of a pentode is very high as demonstrated by the flatness of the plate curves above the knee. If the plate impedance of a KT150 were 3000 ohms it would produce something close to a 45 degree line on the curves. I just think the 3000 ohms is a mistake thought is it also shown on the KT-120 data sheet, which is curious it would be the same for both tubes.. Modern spec sheets are not done with the care of the good old days and there are few out there to check them.
RCA, GE, Tungsol and Sylvania all had identical plate curves or at least very close and such a mistake would have been found right away. The printed specs for most tubes are IDENTICAL from one maker to the other. There is no other maker for the KT 120 or 150 to monitor the situation.
Thus is the current state of tubes.
Here is a much better data sheet, made not by the manufacturer but someone else..
On the first page are the pentode curves and the plate resistance is the slope of the line above the knee. The top curve has a slope of about 20 ma/200 volts = 10K ohms, the lower curves, where the amplifier typically operates is flatter making the impedance even higher.
Also note on the last page the the the load impedance is given as 3,000 ohms. So that is likely the source of the mistake in the Tungsol data sheet, which was made by the Russians I would think. Perhaps something got lost in the translation.
The simplest way to explain light loading is to look at what heavy loading does. If the load is two heavy (4 ohms on the 8 ohm tap) the tube will have excessive current and voltage across it (ie it wont get to the knee but be to the right of it). That product of voltage and current does two things. It reduces the output of the amplifier and it heats the tube excessively.
I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life. Ergo, use the 4 ohm tap. You called it light loading.
So if the speaker impedance is higher than the nominal tab impedance, resulting in a primary impedance presented to the output tubes that is higher than the plate impedance, it will result is less distortion and longer tube life. Ergo, you said to use the 4 ohm tap.
Of course, the 4 ohm tap has lower output impedance and therefore a higher DF, which is a good thing.
Also, Anthony echoed that even if the 4 ohm taps presents an impedance to the output tubes off the primary coil of the output tranny that is higher than the rated 3000 ohms output impedance of the tubes, no harm will occur.
Lets get clear about matching plate impedance to tranformer impedance. We don't do that in amplifier design. The primary impedance reqired is determined by the voltage and current of the output tubes. For instance a EL84 amp with a 385V B+ uses a 8,000 ohm transformer while my EL84 RM-10 has a 13,500 ohm primary because it runs at higher voltage and lower current.
While the plate impedance will provide some damping this is not a large effect on the damping when the loop feedback is applied.
I have doubts about the plate impedance being 3,000 ohms in pentode. It should be much higher and I will measure it at my first opportunity. In triode it looks like 1 to 2 Kohms.
I agree that output impedance is important, but if you wind up using too little feedback to get there, it can be detrimental to the sound thru the process of bifurcation.
I read the Nelson's article, Figure 11 doesnt make sense to me as there is no reduction in the lower harmonics with added feedback. I wrote Nelson to ask about it.
I would not make any generalization about the proper amount of feedback. I built a Futterman to see what was going on. Sometimes that's the best way to understand something. In his circuit 40-60 dB of feedback is easily applied. Unlike most amplifiers the open loop bandwidth is so wide that the feedback is not a problem. I don't think we can dispute that the Futterman amplifiers are well loved for their sound when used in their power range. The OTL I did for Counterpoint is a Futterman style also.
The pot is not dual but single with a switch attached, thus the click
into the high damping condition which cuts out all the Zmatic circuitry
resistor losses. It is indeed a combination of voltage and current
feedback as seen in this schematic
Yes- any way that the current and voltage feedback is balanced is how you arrive at constant power. Other amps I've seen use dual controls.
Whatever feedback they use is not of importance, its the resulting output impedance that matters.
I agree that output impedance is important, but if you wind up using too little feedback to get there, it can be detrimental to the sound thru the process of bifurcation. There's a range of sorts- 8-15 db or so is where this sort of thing can occur. Here's a nice article by Nelson Pass: https://www.passlabs.com/press/audio-distortion-and-feedback
To clarify a bit I made reference to Class A3 operation as implemented by Jack Elliano and used in the design of his A3-500 300B/6A3 amplifier. As Jack does not design amplifiers with multiple taps, this Class A3 operation achieves similar results as light loading. Jack achieves this with a combination of the transformer winding, as well as changes in the plate dissipation, load requirement, and grid drive of the circuit.
@ramtubes, I was in contact with Anthony today. I will ship off 2 quads of KT-150s next week that I bought from a 3rd party tube vendor. Hopefully, Roger can get some good matched pairs out of these tubes. Damn expensive!! Otherwise, it's back to ARC.
Anthony advised that I should "light load" the amp off the 4 ohm tap, which Atkinson measured as having an output impedance of .55 ohms. Presumably, FR should be flatter and DF higher. The impedance in the bass saddle is a smidge south of 4 ohms. I estimate the DF to be approximately 7 in the saddle. Hopefully, that will be enough to get the bass kick I like.
Also, Anthony echoed that even if the 4 ohm taps presents an impedance to the output tubes off the primary coil of the output tranny that is higher than the rated 3000 ohms output impedance of the tubes, no harm will occur. In fact, it will make for less strain on the tubes and lower distortion. The cost, … about 3db of power.
I can live with that. My speakers have a sensitivity rating of 92 db. They play plenty loud.
@bifwynne Roger has one of the best tube matching services in the country.
I suspect that one reason you have issues with the taps is that the ARC amp really needs a few more db of feedback; 15 db is slightly on the low side to get right.
Thanks for the compliment. I did start the whole computer matching program and feel the only correct way to do it is at fixed plate current which I can enter in the test program.
Whatever feedback they use is not of importance, its the resulting output impedance that matters. For instance, a triode output tube has inherently low output impedance and provides a damping factor typically of 4-5 without feedback. This is childs play to figure out. Its just the output transformer primary impedance divided by the plate resistance. For a 300B tube that is about 3500/700 ohms= 5.
For a pentode output tube its much higher for a 6550 about 3000/27,000 = 0.11 ouch, less than one. Now we need a lot of feedback to get that down. Even triode connected it is only 0.22. Thus not a good triode comparred to the 300B.
This is why SET ampifiers can be made without feedback. These tubes were created to do this job. The 45, 2A3, PX4, PX25 all have similar characteristics as does the 6EM7 which I use because it is very close to a 2A3.
There are three classes of output tubes. Low impedance triodes as mentioned above, Pentodes (EL34, EL84) and Beam Power tubes (6550, 6L6, and all KT series). The pentodes and beam tubes all have very high plate resistance, so high that we don’t even try to match it.
When I was a young lad the idea of measuring feedback was a total mystery until I found out how easy it is to measure. Heres how you do it.
Simply connect the proper load to the amplifier, set the output to a very low voltage, remove the feedback and see how many dB the output rises. ARC looks at another way, which I appreciate and use in my thinking. If you think of an amplifier as a power supply then you can easily measure its regulation, as power supply specs do. It can be noted either in percent or dB. This is measured by noting the output voltage with the load connected and then with no load. We want to see the smallest change, as does the speaker.
This is a great way to demonstrate what is going on with your speaker. If you pull the load on an amplifier with a damping factor of 1 the output will go up exactly 6dB. It will do about the same on your speaker when it goes above 20 ohms!
One of the first hits is an image from YouTube, showing the damping control on the amplifier, which is a dual-gang pot. If you look closely, it is labeled "constant voltage" with the control all the way down, "constant power" with the control at noon, and "constant current" with the control all the way up. In this case, voltage and current feedback are balanced against each other in the noon position.
I dont think you are making it up Ralph. Do look more closely at the video. The pot is not dual but single with a switch attached, thus the click into the high damping condition which cuts out all the Zmatic circuitry resistor losses. It is indeed a combination of voltage and current feedback as seen in this schematic
One can clearly see the the switch that grounds out the current sense resistors and returns the amp to conventional voltage feedback. The damping spec on this amp is 26, quite good for an amplifier of this vintage. It is interesing to me that although the Z matic control was of interest they made the click to entirely remove it and its losses.
I can match your tubes, may have to sub or sell you a few. Will need to know the plate, screen voltages and cathode current which we do know. Write tubeaudiostore@gmail.com to set it up.
I would still like to see a schematic of the amp. Can you get one?
What ARC did to balance things in design is what I do also. Use the 4 ohm tap, it should sound better.
Speaker manufacturers are not so honest about their impedance range and a curve is the only way to know. If they didn’t do so many tricks in the crossover we would not have this problem. The drivers are not the problem, the crossover is.
In my experience most speaker designers do not know much about electronics or care about what the amplifier may have to do.
+1 on this, I’ve felt this for a long time.
I’ve heard this from ESL owners where the impedance rises in the bass to very high values. They believe that OTL amplifiers are better for their speakers because they can supply the extra voltage to drive the high impedance in the bass. They indeed get more bass if the damping factor is low, but not the bass the speaker was designed to give. They get more and perhaps like more but it is one note bass.
[snip]
This is where we get in trouble with the power paradigm which had led people to believe that the speaker wants constant power. It does not. I cannot think of or find a modern speaker that wants constant power, the varying impedance and flat response insure that the designer uses a constant voltage amplifier, ie one with high damping.
@ramtubes
The first paragraph is false. You don’t get a one note bass at all, even if the OTL has no feedback (and most do use feedback). Please keep in mind that one of my employees (who now works at Bel Canto) owned ESL57s, and don’t forget Bill Toberman (RIP) who had ESL63s. The one note bass phenomena only occurs when the panel speaker (it doesn’t have to be an ESL) is too close to the wall behind it, allowing the back wave to reinforce around one frequency. This happens a lot if the ESL user has a solid state amp because getting bass out of the speaker becomes a challenge- most solid state amps can’t make power into 30 ohms or more!
The last paragraph is a bit misleading. The Power Paradigm does not lead people to think speakers want constant power. If anything, most people think the idea is ridiculous. And as you state, most speakers are Voltage Paradigm. Its not until they hear what a properly set up Power Paradigm system can do that they might begin to think that its not bunk (ask clio09). And there are manufacturers that make speakers that are Power Paradigm and I’ve mentioned a few already- Audiokinesis, Classic Audio Loudspeakers, DeVore Fidelity, Lowther, PHY, Feasterex, Onken, Volti Audio, PureAudioProject, Evolution, Sonic Flare.... I can go on and on but I hope you get the point. Most horns and panel speakers are Power Paradigm devices unless something unusual was done to change that (there’s a guy named Wayne Piquet who does just that with Quad ESL63s, by adding an extra panel to the speaker). For example, Magnaplanars are Power Paradigm but because of their resistive nature work fine with Voltage Paradigm (constant voltage) amplifiers.
Sometimes its the intent of the speaker designer that makes the difference, so the speaker might be acoustic suspension, like the AR-1, or it might be bass reflex and so on. Your comment at the top of this post has a lot to do with that- many speaker designers went to the school of ’by gosh and by golly’ and are simply trying to make the speaker work with a particular type of amplifier, like an SET (which is likely the most common application). If they get the latter right, they will have a Power Paradigm device.
One of the first hits is an image from YouTube, showing the damping control on the amplifier, which is a dual-gang pot. If you look closely, it is labeled "constant voltage" with the control all the way down, "constant power" with the control at noon, and "constant current" with the control all the way up. In this case, voltage and current feedback are balanced against each other in the noon position.
Now current drive never really developed into a thing (meaning there are almost no speakers based on current drive), but power drive and voltage drive did. This amplifier was built about the time (mid-late 1950s; Fisher, ElectroVoice and a few others had such ’damping controls’ on their amps during this period) when the voltage rules were first being proposed and so amplifiers had to be adaptable to whatever was out there. Instead of balancing current and voltage feedback, you get similar results with no feedback at all, which is understandable because as you know, current feedback **raises** output impedance while voltage feedback decreases it.
@bifwynne Roger has one of the best tube matching services in the country.
I suspect that one reason you have issues with the taps is that the ARC amp really needs a few more db of feedback; 15 db is slightly on the low side to get right.
I've seen that stated before, but I'm wondering how an amp can be designed with 60 db of feedback. Wouldn't its open loop gain have to be enormous to support a closed loop gain that is reasonable?
The open loop gain is enormous and obtained entirely in the first tube. That tube drives a split load phase inverter which provides EQUAL drives to the output tubes. Many think the top output tube is a follower and not equally driven but it indeed is equally driven.
What is so clever about the circuit is that it is able to employ that high level of feedback without oscillation. This because of the very wide bandwidth of the circuit up to many hundered KHz. Conventional tube amps are limited in feedback by the output transformer.
While the original circuit is quite simple, published in AES Journal, it takes some time to fully appreciate what Futterman created.
So Roger, …. bottom line, do you concur that using the 4 ohms taps on my Ref 150SE should yield a flatter FR, less distortion and longer tube life?? I gather you are saying yes.
P.S. the ARC engineers obviously were trying to balance a lot of design ideas and techniques when putting the Ref 150SE together. Just enough negative feedback to get the output impedance low enough to handle a host of speakers designed to be driven my low impedance SS amps and a robust power supply (1040 joules) to handle the tough spots that require a lot of current (i.e., low impedance and highly reactive phase angles in the bass region).
P.S.S. - I have about 1800 hours on my KT-150s. Can you help me source some matched tubes that will bias well my amp? I have 8 fresh KT-150s sourced from another vendor sitting in a box. They don't bias well. Could you match them for me?
+1. Great thread. And thanks to Ralph and Roger for providing their informative if somewhat different perspectives.
Roger, regarding...
The Futterman amplifiers have over 60 dB of feedback and a good reputation.
I've seen that stated before, but I'm wondering how an amp can be designed with 60 db of feedback. Wouldn't its open loop gain have to be enormous to support a closed loop gain that is reasonable?
Best regards, -- Al
P.S: Bruce (Bifwynne), great to see you posting again recently.
Thanks for the info. Let me clairfy this part which many people make the same mistake.
Max power may be compromised at higher impedance, but if I am not pushing the amp, that shouldn't be a problem.
I've heard this from ESL owners where the impedance rises in the bass to very high values. They believe that OTL amplifiers are better for their speakers because they can supply the extra voltage to drive the high impedance in the bass. They indeed get more bass if the damping factor is low, but not the bass the speaker was designed to give. They get more and perhaps like more but it is one note bass.
Speaker makers are doing a better and better job making speakers have flat response, which is a good thing. To to this they generally let the impedance do what it will. They could add a lot of parts to the crossover to try to flatten the impedance curve but they assume you are using an amplifier with reasonable damping of 10 and above. Ten is not a hard limit but 5 is better than 1.
This is where we get in trouble with the power paradigm which had led people to believe that the speaker wants constant power. It does not. I cannot think of or find a modern speaker that wants constant power, the varying impedance and flat response insure that the designer uses a constant voltage amplifier, ie one with high damping.
The reason that John Atkinson puts the response with the simulated speaker first is that he and I feel it is the first thing you are going to hear when you audition that amplifier.
What the power paradigm is saying is that flat response is not as important as ills of feedback. Sorry, but I am not in that camp. Proper feedback has no ills. The Futterman amplifiers have over 60 dB of feedback and a good reputation. Thats a lot of feedback, more than most transistor amplifiers. I plan to start a thread on the Futterman design when I get done with this topic.
Since everyone hears what they hear and like what they like I would rather not promote one paradigm over the other, I just want to point out what is going on based on my experience and my rules of amplifier and speaker design.
I would rather see all speakers have flat impedance curves but this is not how the world is currently set up. Even the DeVore (which has been recommended) has wide impedance variations.
While I agree with JA and Ralph that it is easy to drive (stays above 8 ohms) its peaks go off the chart above 20 ohms. A damping factor of 1 or 2 or 3 will modify the nicely flat frequency response several dB.
Thanks Roger. Thought you might find the attached websites of interest. Atkinson tested an earlier version of my amp, the Ref 150. The test results should be comparable to my amp, the Ref 150SE, because the Ref 150and Ref 150SE share almost identical electrical characteristics.
Atkinson uses a simulated speaker load, which is actually kind. See below. Impedance doesn't drop much below 8 ohms. Atkinson measured Ref 150 tap impedance as follows: "The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm." Running the Ref 150 into the simulated speaker load, he measured FR as follows: "From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB."
So in summary, I gather that even if speaker impedance swings between 3.9 and 20++ ohms at its peak, it is still better to use the 4 ohm taps because it will draw less current off the output tubes and thereby cause less tube wear and less distortion. Also, the 4 ohm tap is close tothe bass dip impedance, which is good. Max power may be compromised at higher impedance, but if I am not pushing the amp, that shouldn't be a problem.
Btw, based on Atkinson's measurements, I gather that if my speakers do dip to approximately 4 ohms in the bass, the damping factor should be roughly 7.27 (4/.55), which ain't too bad.
Am I getting it now??
Btw, ARC uses 14db of negative feedback in ultralinear mode to achieve these electrical characteristics. The amp is also runs in modified AB mode.
These are examples of speakers that will not play well on low damping amplifiers due to wide impedance swings or very low impedance demanding excessive current. The density of music where dips occurr is important to how much the speaker will overheat the tubes. These are just in the order that I found them in Stereophile under floor standing speakers. I didnt have to look for long. these are the first 4 measured in order.
Connected to an amplifier with a damping factor of one at eight ohms the following will happen. Where the impedance approaches 20 ohms there will be a 4-5 dB peak at those frequencies. Where the impedance dips below 3 ohms the respoinse will dip 4-5 dB making a total error in frequency response of 8-10 dB.
In addition if the low impedance occurrs where there is a lot of music the amplifier will current clip and the tubes will get hot.
Click on the link and open in a new tab for a good look.
The Wilson Alexia 2 is still a current-hungry design. Its impedance drops to 2.6 ohms at 84Hz (fig.1), and there is a demanding combination of 5.1 ohms and –44° electrical phase angle at 57Hz, both frequencies in regions where music can have high energy levels.
Eggleston Viginti June 2018 needs to be connected to the 4 ohm tap and will work it hard in the bass where there is lots of energy. Also note how high the impedance is from 1Khz to 8 Khz where a low damping amplifier will cause a 4-6 dB rise in level making the speaker very bright on trumpet music and anything in that region.
Monitor Audio Silver 300, JA notes "Although the minimum magnitude is 3.6 ohms between 150 and 170Hz and there is a combination of 5.4 ohms and –39° at 97Hz, this speaker won't tax the amplifiers with which it is used".
While this speaker won't tax most SS amps, it will bother a tube amp unless connected to the 4 ohm tap. (RAM)
The tubes will be happiest if you use the tap that is equal to or lower than the lowest impedance of the speaker. So if your lowest is 4 ohms use the 4 ohm tap.
Although David Manley (rest his soul) believed that we match the primary of the tranformer to the tube’s impedance this is not true. Once again its all about the voltage and current that the tube is comfortable with. With that in mind, a reduced impedance will require more current from the tube. If the tube is already at max current minimum voltage (the correct place to be at full power) the reduced load impedance will demand too much current and cause a great increase in voltage drop across the tube and overheat it.
A 10% overcurrent may double or triple the voltage drop aross the tube increasing the dissipation by many times. Its a horrible situation. In fact the RM-9 Special addressed this problem by allowing the output tubes to go into AB2 mode thus saving the tubes. The speaker that prompted this was a Theil whose impedance dropped below 4 ohms in the treble region where loud trumpet music would just bake the tubes in a regular RM-9. The AB2 mode is also used in the RM-200 as i liked what it did.
For normal tube amplifiers it would be best to connect the amplifier to the tap that matches the lowest impedance of the speaker in the region where there is a lot of music.
Speaker manufacturers are not so honest about their impedance range and a curve is the only way to know. If they didn’t do so many tricks in the crossover we would not have this problem. The drivers are not the problem, the crossover is.
In my experience most speaker designers do not know much about electronics or care about what the amplifier may have to do.
Roger, you wrote that "a typical 100 watt amp puts out 100 watts into any matched tap and somewhere around 75 watts into a tap mismatched by one step. In this case the tube are loafing along, distortion is reduced and damping increased. But this requires that the load does not go significantly below the tap impedance.
However going in the other direction where the load is lower than the tap impedance bad things happen. In that case the amplifier puts out less power, works harder and the tubes get overly hot to the point or radically shortening their life."
Let me understand better what you are saying. If my speakers have a roller coaster impedance curve (4 ohm to 20+ ohms), is my tube amp "happier" if I use the 4 ohm taps. I think you are saying that the amp may produce less power, but it will produced better sound with less distortion and less tube wear and tear.
I assume this is so because the back impedance presented to the output tubes off the primary winding of the output tranny will be equal to or be higher than the output tubes' output impedance.
If you have the tubes tested in a rigorous manner as Roger does, it makes sense that they would hold up better :) It *also* makes a difference that they are mil spec- that might even be the bigger difference!
That does not seem like the sort of tube of which ample supplies exist. We stay away from the collector market as a general rule but if you mess with NOS tubes of course you can get better results.
We've had such poor luck with the garden variety of GA-style tubes that we don't mess with them. Usually what happens is someone buys a mess of them and installs them in the amps without any sort of testing. Then we get a call about a malfunctioning amplifier, but then it turns out to just be tubes. With the Russian tubes (which is what we've mostly been shipping recently) we have few enough problems that doing a warranty for a year is easy.
After the MkIIIs went into production about 14 years ago, we found we were shipping about 10% of replacement tubes that we were doing before that. Its good that you have a set of American tubes working for you- IMO and from all the feedback we have, they sound the best (and the amp makes more power too, an extra 20 watts).
I neglected to mention that the 6AS7GA tubes I am using are General Electric military spec. In addition, as Roger had performed measurements and had data on the tubes he was able to match them so they were drawing an equal amount of current. Matching the tubes also allowed the position of the DC offset adjustment screw to be set at the middle (near 12 o'clock) of its range to achieve the appropriate reading on the meter. My previous experience with other tubes was such that while you could still set the offset, you may not have had as much flexibility for future adjustments. In some cases I also had to swap tubes around a number of times to achieve the proper DC offset. Matching the tubes made the process a lot easier. So regardless of the type of tubes used, and even though Ralph doesn't require the tubes to be matched, based on my experience I would suggest matching them if you had a means to do so.
@atmasphere - Hi Ralph, I know we have had conversations about the 6AS7GA tubes and your comment above echoes the advice you gave me. However, Roger has a nice stock of those tubes and he asked me if I wanted to try them in the M-60s. So I took it over to the shop and he put in matched quads (2 matched per monoblock). Now I have no idea if the grid heat sink is warping and I wouldn't even begin to know where to look to check, but I have to say these have been the best tubes that I have had in the amp. The Chinese were not very reliable, even after preconditioning, the Russian were much better, and the RCA on par with the Russian. So far I think it has been a good 6 months since those tubes have been in the amp and no issues with arcing or lint shorts (Roger's term) or other type of failure. In addition, Roger didn't precondition the tubes. Now as to lasting 10,000 hours I guess we will see. Obviously still too early to tell.
The thing that concerns me about 6AS7s, it that they emit cathode sparks
in my Tektronix 530 curver tracer at zero grid.That is consistant with
your statement of cathode coating failure as it appears the sparks are
bits of cathode coating. They actually look like sparks off of a 4th of
July sparkler.
The 6AS7G begins to develop grid current at about -15V or so; 0 volts on the grid is pushing them pretty hard (a bank of 12 power tubes at 0v on the grid can pop a 15 amp fuse rather quickly BTW). Despite that our driver circuit can push them to +15 volts on the grid while maintaining linearity. Here are some tips for reducing arcing: Precondition the tubes by putting filament current on them for at least 72 hours if you have the Russian variant (6H13C). If you have the Chinese (6N13) or American (6AS7G) tubes you will want to precondition for 96 hours. If B+ is applied before preconditioning or at any time during the process, you may consider the process ended. We built a jig for just this purpose. Preconditioning reduces premature arcing and can double the life of any power tube. The Russian and Chinese tubes hold up much better in this regard. The 6AS7GA and its 6080 variant to the GA should be avoided for use in our amps- the grid heatsink is much smaller and tends to warp with grid current.
Ralph, Thanks for your answer. I think most of us develop amplifiers in real space verses cyber space. SPICE, by its name, was indeed designed to help people design ICs not tube power amps.
The thing that concerns me about 6AS7s, it that they emit cathode sparks in my Tektronix 530 curver tracer at zero grid.That is consistant with your statement of cathode coating failure as it appears the sparks are bits of cathode coating. They actually look like sparks off of a 4th of July sparkler.
I know of no tube amp designer who uses SPICE. can you name a few?
Nope. Maybe one? I know I don't! Victor Khomenko was trying to model our MA-1 in Spice many years ago before he became part of BAT. He called me up saying that the amp didn't run in Spice, but I had to point out to him that the amp actually worked despite what Spice came up with :)
As you can imagine, the ratings of the tubes tend to vary with load- and the amp will draw less power and run less heat if a higher impedance load is used. Also, as you've pointed out, the class of operation is affected. In our case the amp is biased to be class A2 on the proper load, but will be class AB2 if the load impedance gets low enough. We don't get crossover artifacts on that account though.
As far as high current spikes, at full power the tubes still have some dissipation left over, so they can handle spikes, however as the load impedance is reduced, eventually this will cause the cathode coating to fail.
Since the RM-200 produces about the same power on both taps, what possible reason is there for NOT hooking up a nominal 8 ohm speaker to the 4 ohm tap? Thanks---Eric.
This is something that needs clarification. The reason we have taps on tube amplifiers is to get the full rated power into different loads. That means when we test an amplifier it produces full power into a load of the same resistance as the tap. Indeed hooking a 8 ohm load to the 4 ohm tap results in reduced power. Usually though only a 30% reduction, not 50% due to other factors (reduced loss in the output transformer, power supply and tube saturation voltage). So a typical 100 watt amp puts out 100 watts into any matched tap and somewhere around 75 watts into a tap mismatched by one step. In this case the tube are loafing along, distortion is reduced and damping increased. But this requires that the load does not go significantly below the tap impedance.
However going in the other direction where the load is lower than the tap impedance bad things happen. In that case the amplifier puts out less power, works harder and the tubes get overly hot to the point or radically shortening their life.
What an RM-200 does is to go into AB2 mode in the above case The tubes stay happy and the reduced load gets extra power in the same way a transistor amp gives more power into a lower load.
10,000 hours is typical- so we warrant the power tubes for a year on this basis, and always have.
Ralph, What is your dissipation figure as a percentage of rated? How do you figure in high current peaks when the amp is used at full power?
I agree about the feedback 'used properly' (which many designers do not) comment. Proper application of feedback is tricky to say the least, and may not have been possible until the age of personal computing, due to the number of variables involved. Here is a nice primer on the topic:http://www.normankoren.com/Audio/FeedbackFidelity.html
I have spoken with DImitry and a group of people trying to improve the tube models for SPICE. He put the parameters of the RM-10 into their SPICE program and got results that did not even come close to what an RM-10 actually does, and this is only for the midrange where the output tranformer is considered perfect. He did not attempt any feedback analysis as the output transformer is almost impossible to model where it matters.
I know of no tube amp designer who uses SPICE. can you name a few?
one thing I greatly value is a collegial, challenging, supportive and sharing attitude in the user AND supplier community....I did not know Roger did the bias forming....also..so I will add to my list of collaborative yet competitive firms RM, Vandersteen, Audio Quest, Aesthetix, Brinkmann, ......I am sure the list goes on....
If Ralph (Atmasphere) picks this post up, ... what is the tube life is
in your amps. Btw Ralph, …. Roger mentioned a couple of times that
negative feedback, *if used properly*, is not all that bad a design
feature. Not taking a position, just passing along what RM said.
10,000 hours is typical- so we warrant the power tubes for a year on this basis, and always have.
I agree about the feedback 'used properly' (which many designers do not) comment. Proper application of feedback is tricky to say the least, and may not have been possible until the age of personal computing, due to the number of variables involved. Here is a nice primer on the topic:http://www.normankoren.com/Audio/FeedbackFidelity.html
In addition to Brooks, John Ruttan at Audio Connection used to sell
Vandersteen and MR combos and IIRC correctly it was the RM-10 and 2s. Given the new cost of the RM-200 a used one for under $3k is quite a bargain. It should also be noted that the RM-200, unlike most vacuum tube amplifiers which lose power while the speaker load impedance drops, increases output power as the speaker load becomes more challenging.
It is stable down to 1 ohm and includes a 1 ohm tap.
While light loading sounds better to me, Roger would also be the first to say use the tap that sounds best to you. There is no reason to not experiment. It's a cheap and easy tweak.
Speaking of Richard and Roger, Roger says the idea to install the Capacitor "Forming" function into the MK.2 version of his Music Reference RM-200 amp was a suggestion from Richard. Brooks Berdan sold a lot of RM-9's and RM-200's to owners of Vandersteen 2's and 3's, a great matchup.
For the price of retubing an ARC Reference 150, you can just about buy a used RM-200! The four output tubes run in the RM-200 (producing 100w/ch) last about five times longer than those in the REF 150. How many 150 owners have compared their amp to the RM-200? I’m guessing none. You can buy a used PAIR of RM-200’s for less than a single used Ref 75! There’s one on AudiogoN right now for less than $3k, a ridiculously good deal. I have all the power I need from the one I already own or I’d buy it.
i am rebuilding a set of ESL63.... to learn but also listen to.... will start out with a rebuilt MC240 ( which probably has the best transformers available in 1961 ! anyway, my mentor Richard Vandersteen always says run it on the best sounding tap” but i can see both a Ralph and a Roger amp in my future....
bdp24, … see my post to Ralph and Roger about the same question. Theoretically, the 4 ohm taps should work better. In my case, I subjectively think the 8 ohm taps simply sound better and I do not know why.
Roger, I'll ask the following for myself, but your answer may well be of interest to others. You rate your RM-200 Mk.2 amp at about 100w/ch at 8 ohms, and, unlike a "normal" tube amp, about the same at 4 ohms. The advantage of using the 4 ohm tap is less distortion, lower output impedance (higher damping factor), and longer tube life. With a tube amp which produces less power at 4 ohms than at 8 (typically half as much), the user forfeits power to gain the advantages afforded by using the 4 ohm tap ("light loading"). Since the RM-200 produces about the same power on both taps, what possible reason is there for NOT hooking up a nominal 8 ohm speaker to the 4 ohm tap? Thanks---Eric.
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