Roger, I recommend you try some measurements on this bit yourself. Its obvious to me that you have not. Its simple enough to do. Bascom is a nice guy and I've always had respect for him, but he screwed this one up. People are human.
Its a common question as to what about the wiring in the wall and such? Part of that is what substations are for; to make sure that power at a local level is the correct voltage. In tandem with that are the power transformers mounted on power poles that serve local nodes. The one on the pole in my back yard serves 5 houses. The AC power sent to that transformer is at a higher voltage; this is done to minimize current loss thru the power lines. As you might know, a different waveform is employed on high voltage transmission lines (like the ones which come from the Hoover Dam; that is why you can hear them on the AM radio if you get close to them) for this same reason. This of course is in a nutshell.
Once in the house, voltage drops can indeed occur, but the Romex found in most homes is very high performance. The problem is how to get the power out of the wall, since Romex as you know is solid core and very stiff; thus being illegal for use in a power cord. IOW the real challenge is how to build a power cord that does the job properly and still be flexible and legal. Its doesn't have to be expensive- it just has to be right. That is why its worth it to replace a power cord, even though there is wiring in the wall going all the way back (in effect) to the Hoover Dam.
We employ a simple bias mechanism that allows our amps to autobias and so respond to changes in wall voltage. It won't do for example to have the AC voltage to be very high and possibly damage power tubes! So the amp reduces the bias level when the AC line voltage is high and increases it as the line voltage drops.
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@almarg
No. If that were the only factor transistor amps would be more sensitive than tube amps in this regard, and I don't see that happening (although transistor amps don't have a filament circuit to cool off). I suspect Kirchoff's law might be in play there. But we use triodes and they are usually more susceptible to the actual B+ level due to reduced plate resistance and lower mu.
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I would like to know how to measure that. 30% is hard to imagine. I test
amplifiers all the time with ordinary power cords and have never seen
this. This reminds me too much of Bruce Brission's white paper which was
a total embarassment to his distributor who promptly dropped his line. Set up the amp on a variac with an appropriate speaker load and sine wave input. Measure the voltage drop across the power cord with the amp at full power. A 3 1/2 digit DVM is sufficient for this. Then boost the variac voltage to compensate for the voltage loss. You may have to do this several times because as you increase the output power of the amp, the load on the variac increases too. This came to our attention with our MA-1 amplifier back about 1998 or so. Bascomb King had measured the amp as only 100 watts where we get 140. I was wondering where the measurement error was and found it easily enough. We spec the amp at 120 volts (which is what Bascomb claimed he had) since these days that's more common than 117V. We normally set the incoming voltage by measuring it at the IEC connection, since there is typically a voltage sag in our variac. We have IEC connections on all of our products since we know that audiophiles will want to use or audition different power cords from what we supply; the idea that power cords make an audible difference is nothing new- its been around for decades (this is also why our MA-2 employs *two* power cords per chassis, to reduce the effect of the power cord). So the power cord isn't regarded as part of the amplifier in our test. Bascomb King didn't do it that way- thus the difference. By our measurements he was seeing about 117 volts at the AC input of the amp. So it follows that the heavier the load on the power cord, the more audible the differences between power cords will become. This ignores the higher frequency aspects of any power cord's current delivery of course, but cynics of the fact that power cords can make a difference are usually silenced by the simple revelation that they never bothered to measure the power cord's voltage drop! It is easier to measure the effects the cord has on the product with which it is being used. For example on our MP-1 preamp, which has heavily regulated power supplies, even though we can measure differences between power cords, we can't measure any differences in the performance of the preamp, nor can we hear any differences. So we can also conclude that regulation in the circuit plays a role here as well. |
The bass is markedly more pronounced from my tube amp (60 Watt push
pull, KT88, “some” local negative feedback) than my SS CA-2300. It’s
also seems “looser” or less defined.
Nonetheless, it seems
reasonable to my novice brain that if an increase in speaker impedance
increases the sound level “heard”, that the bass should sound louder
where the impedance spikes. No? If the tube amp is able to act as a voltage source, then it should not make any more bass than the solid state amp. In your speaker, there are two bass impedance peaks, which represent the box resonance with the port. If the amp is behaving as a voltage source, it will make less power, not more, into these peaks. Otherwise the speaker is a fairly benign load for a tube amp- our amps would do fairly well on that speaker (seems to me we have customers with them too). Solid state amps often have what is called 'tight' bass, but I've yet to encounter tight bass at any show I've attended. IMO/IME tight bass is a symptom of an over-damped speaker. Here is an older article, written by the head engineer at Electro-Voice back when the industry was trying to sort out the voltage rules (EV and Mac lead the charge on that one): http://www.dissident-audio.com/Loudspeakers/CriticalLSDamping.pdfAs you can see from the article, not all speakers need high damping factors and there are some that need really low damping factors. That is still true today and is why equipment matching is still an on-going conversation! |
Guess that explains the bloated bass when I run 802D3s on my tube amp. No- it doesn't. If the amp is able to act as a voltage source, then it can make enough energy to be flat on the 4 ohm load of your woofer array. Since you say the bass is bloated, its doing that, so the bass bloat is something else. One possibility is that like at amps, yours is making more distortion into 4 ohms. The extra distortion could easily be perceived as bass bloat, since the 2nd harmonic is likely predominant. It could also be the power cord, as a cord that limited the amplifier's ability to replenish its power supplies might come off with more distortion as well. This effect is quite measurable- I've seen power cords rob a tube amp of nearly 30% of its total power! |
@ramtubes
Hello Roger, welcome to the forum. Thanks for your posts!
We used to make an autoformer we called the Z-Music autoformer, which had taps for 4,3,2 and 1 ohm loads. With it, Steven Stone of TAS was able to listen to his Apogee Full Range loudspeakers with tubes for the first time. Although the Full Range had a very low impedance (not much more than 1 ohm) they were otherwise easy to drive as they had no serious phase angles and were reasonable efficiency.
I think you and I are on the same page here- if an amplifier is acting as a voltage source, it really doesn't matter if it has an autoformer in its output section, especially as in the case of the Mac, which includes the device in its feedback loop. If its a voltage paradigm device, it will make constant voltage into all (practical) loads by definition.
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@73max Just for the record, I've nothing to gain if you buy a Mac with an autoformer in it :)
However, you if you can find a set of ZEROs (which, so far as I know, are the only outboard autoformers available for this task), I think its a very good idea to give them a shot if you feel like it.
One thing George is ignoring here is that in a Mac, the autoformers are inside the feedback loop, which makes a difference on how the amp behaves. As others have suggested, audition is the arbitrator.
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@73max The amount of feedback is usually given in the spec sheet. Bubble still up, your having illusions, or didn’t do your homework. Really?? Your prior statement which I quoted suggested that you don't understand how feedback works. And here you are doubling down. We make zero feedback amplifiers because of some of the problems that Nelson also points out in the article I linked. In other threads, @georgehifi , you are adamant that Nelson is the guru, but in this case you don't seem to want to acknowledge him. You can't have if both ways. Here's a little primer on how it works with feedback- you can use little or none (which means you have to rely on other means to suppress distortion). If you use a moderate amount of feedback, Bad Things happen- its use will be a degradation. But if you use a lot of feedback (20db or more) then it use is far less audible (it can start to compensate for the distortion it introduces) and so can be pretty effective. Of course, this is the tip of the iceberg; I don't like feedback because even in high amounts it makes the amp brighter than real life (so its a coloration). Since we make tube amps, I also want soft clipping, and feedback pretty well means that soft clipping isn't going to happen. Here is another excellent article on some of the nuances of feedback design in an amplifier: http://www.normankoren.com/Audio/FeedbackFidelity.htmlWhen you are done with Part 1, read Part 2 as well. All the math and proofs are presented. |
This statement ignores just how much loop feedback is needed to fully
compensate for this sort of thing. We all know what too much feedback
has the reputation to sound like. 20 db is plenty. The problem area with feedback is **moderate amounts** between about 4 db up to about 15 or so. Over that and the feedback is usually sufficient to make almost any amplifier act like a voltage source. IOW George, the quote above was completely misleading. If you doubt me, read Nelson Pass' article on distortion and feedback: https://www.passlabs.com/press/audio-distortion-and-feedbackAny of the amps that you recommend run 20 to 30 db of feedback! In this case you're trying to play both ends- when all the amps in question run large amounts of feedback. Consider your bubble popped. |
an Autoformer would work, because it’s one of the only speakers that presents a very benign 3-4 ohm impedance load. This statement ignores the fact that loop feedback compensates for this sort of thing. Atmasphere, regarding your link:
1. In general do reactive speakers match better with voltage paradigm amps?
That has to do with the intentions of the speaker designer. In a general fashion, another way to answer this is that most speakers with highly reactive loads are Voltage Paradigm devices and so the answer would be 'yes'. 2. What happens to linearity with a voltage paradigm amp? Does the sp
increase where the ohms dip (ie watts increase)—with an increase in
distortion—and decrease where the ohms increase? Linearity usually refers to distortion... to the latter question, the sound pressure should stay constant, while the power input fluctuates. 3. Same question as above, but with power paradigm amp. What happens to
sp where ohms dip or increase...is it the opposite of voltage paradigm
amps?
If the amp is on a speaker that is designed using Power rules, the output will vary only a little since the designer isn't expecting the amp to throttle its output power back on higher impedances. A good example of this phenomena is an electrostatic loudspeaker, whose impedance is based on a capacitor rather than the impedance of a driver in a box. The idea here is that distortion is kept down, with a preference for lower distortion rather than perfectly flat frequency response, since **the latter doesn't exist** despite what speaker or amp is used. Does this mean Macs are voltage paradigm amps? Doesn’t this conflict
with the way autoformers function? Seems they would be power paradigm,
especially given their multiple output taps. A little over my head here,
but learning, so please forgive my ignorance! Mac amplifiers have always been voltage source amplifiers since the 1950s. It does not conflict with the use of an autoformer; such use is not the defining aspect. Most transformer coupled amps are voltage sources too. The taps are used to optimize the interface between the output devices and the speaker to minimize distortion. To make a power paradigm amp you have two pathways- either no feedback at all, or current feedback and voltage feedback of equal amounts. Since all forms of loop feedback are known to add higher ordered harmonic distortions as well as intermodulations, zero feedback is preferred, if adequate means are employed to otherwise suppress distortion in the amplifier. In this way the result can be considerably less colored, despite likely not having perfectly linear frequency response, due to the way the ear perceives distortion. Put another way, the ear hears tiny amounts of higher ordered harmonics with striking ease. So if an amp has 0.001THD, but all the distortion it has is the 5th harmonic and above, it will sound bright and harsh. **That** is a coloration, and not a particularly pleasant one. This is why tubes vs transistors has been such an on-going debate and why tubes are still around. |
What isn’t addressed is, what happens “electrically and sonically” with Autoformers when 8 ohm speakers, like 802D3s, dip down to 3 ohms or exceed 20 ohms???
With typical output transformers (common with tube amps) speakers with wild impedance swings (and phase) present a challenging load to the amp and fidelity can suffer.
Can the same be said of autoformers? Or, are these electrically different enough from transformers as to not present the same issues? With either an autoformer or output transformer the operation is similar, so we don’t have to separate them. Transformers **transform** impedance. To do this, it goes both ways- if a lower impedance as you suggest above is on the 8 ohm tap, a lower impedance is thus also seen by the output devices. This is exactly the same as if there were no output transformer at all- think about it- if an amp has no output transformer, it too will see a varying load. How the designer set up the amplifier with the transformer plays a big role. The reason you do this BTW is to reduce distortion- all amps have higher distortion playing lower impedances, so if you can raise the overall impedance seen by the output devices distortion will be reduced. This is why Mac uses autoformers. Now there is the issue of the varying load, and a lot depends on what the designer of that load expects. You might want to read the article at this link: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpESLs have an impedance curve that typically varies by about 10:1 over their range. Yet it works better for them if the amp can make constant power rather than constant voltage. A lot of box speakers don’t behave this way- for them, constant voltage over the range is more important, this in order to control the normal resonance and resulting impedance peak usually in the bass region near the speaker’s cutoff. To accomplish the latter, enough loop negative feedback applied will cause any amplifier, tube or solid state, output transformer or not, to behave as a voltage source and the resulting output will be constant voltage. This is why this comment: Large impedance swings and severe phase angles do exactly what you’d expect; they produce dips and humps in response. McIntosh is about the only solid state manufacturer to use output coupling transformers. If you’re driving benign, modestly reactive loads, all is peachy. But you throw some significantly reactive speakers at it, not so much.
- is only partially true- if the amp is acting a voltage source, the above statement is false. Specifically in the case of MacIntosh, the statement is false; Mac led the way in the late 1950s towards developing the idea that speakers be ’voltage driven’ (IOW act like a voltage source) and I don’t think they’ve backed away from that, since most of the industry has followed their lead. |
