BIF, 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. http://https//www.stereophile.com/content/devore-fidelity-orangutan-o96-loudspeaker-measurements Thanks for your question, I hope this clears it up for you and others. Feel free to ask for any further clairication. |
C1Ferrari 9-6-2018
Very cool thread ;-) +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. |
@ramtubes
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?
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@almarg 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. |
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. In case you are thinking I made this stuff up, this is really all about history; take a look at this Google search, in this case one for a Fisher A-55: https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+A-55&ie=utf-8&oe=utf...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. |
BIF,
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. |
In case you are thinking I made this stuff up, this is really all about history; take a look at this Google search, in this case one for a Fisher A-55:
https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+A-55&ie=utf-8&oe=utf... 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 http://audiophool.com/Schem_A/Fisher_100_schem.gifOne 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. |
@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! |
@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.
Thanks Roger and Anthony
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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.
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The Class A3 amp sounded good too. 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 |
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. What is bifurcation in feedback?
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 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. |
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. |
@ramtubes, Roger, take a look at the New Sensor specs for the KT-150. I believe it says the plate resistance is 3000 ohms. https://www.newsensor.com/pdf/tungsol/kt150-tungsol.pdfI 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. What did I not understand?? BIF |
BIF, 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.. http://www.tubeampdoctor.com/images/File/data%20sheet%20KT150%20Tung-Sol.pdfOn 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. |
Rodger, Take a looks at the specs for a Tung Sol 6550. https://www.newsensor.com/pdf/tungsol/6550-tungsol.pdfInterestingly, 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. BIF |
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. |
What is bifurcation in feedback? 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. |
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. |
@toshko1
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.
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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. This statement is false. The 7th harmonic of 100Hz is 700Hz and the 7th harmonic of 1000Hz is 7KHz. See: https://en.wikipedia.org/wiki/Harmonic |
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. |
@toshkol Please spell out an abbreviation the first time you use it. What is Le to you? Leakage inductance? Prove you wrong about what? Thats not what I am here for. :) |
Interesting discussion. I’ve always wanted a McIntosh. The big integrated with the 5-band EQ. The only thing ever stopping me was my pocketbook. |
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. |
