Are all amps being built wrong?

Ralph, thanks for the info. I recall that many years ago you explained that global NF creates temporal intermodular (TIM) distortion because of the infinitesimally small amount of time that it takes for the signal (after phase inversion) to loop back from the output stage to the input stage. That small time delay causes TIM distortion, ... if I recall your explanation correctly.

@bifwynne Not TIM, but HD. You can look at it in terms of propagation delay (easily known and seen in any class D amp) or you can look at it in terms of phase shift. Either way it means that as frequency goes up, the feedback is increasingly erroneous. Eventually you arrive at a point where feedback causes oscillation since it is no longer negative. Put another way, you have two qualities in any amplifier: its gain bandwidth product which describes how much gain is available at a given frequency to support your feedback, and the phase margin of the amp, which is the frequency above which the amp will oscillate if feedback exists above that frequency. (This is why the Futterman amp could go into oscillation if presented with certain loads, since that load would affect the feedback and allow the amp to exceed its phase margin.)


So one conclusion you can draw from this is that at low frequencies you can have a lot of feedback. Its relatively easy to have 60dB at 10Hz; the real question is how much do you have at 10KHz or 20KHz?? Because most amps simply lack the gain bandwidth product, their feedback value falls off with frequency (meaning that distortion also increases with frequency, resulting in brightness and harshness). To support 60dB at 10KHz you're going to need open loop bandwidth of nearly 100MHz (and yes, that's an 'M')... right away you can see that no amp made can support a claim like that. This is why THD is usually measured at only 100Hz, where things are 'safe'. But that practice sweeps the dirt under the carpet because its between 3KHz and 7KHz where the ear is most sensitive (Fletcher Munson) and at these frequencies the amp will simply have less feedback.


Something also going on is that due to non-linearities existing at the feedback node there will be bifurcation of the audio signal created by the feedback. This results in a noise floor composed of harmonic (which are much higher ordered) and inharmonic (intermodulation) noise rather than just hiss as seen in a zero feedback circuit. Norman Crowhurst wrote about this in the 1950s; this is pretty well-known. IME this kind of noise floor is harder for the ear to penetrate while the ear can typically hear about 10dB or so into a 'hiss' noise floor. I suspect this has a lot to do with how well the amp can portray low level detail.


At any rate, if you really want the amp to sound musical, you need two things; the first is that you'll want to see is that the distortion is the same at all frequencies in the audio band. With traditional solid state this is very hard to achieve using feedback. Its also hard to do with tubes. One way around this is to build a wide bandwidth circuit that has good linearity and no feedback. That's been our solution for the last 46 years (as it is for Ayre and a few others). Of course you pay a price for this (as you do for anything); in our case it means that matching the amp to the speaker requires more attention.


Another solution is class D since you can set up a class D amp with so much feedback that it oscillates (because its phase margin is exceeded) and then use that oscillation as the switching frequency. Now you can have enough feedback that even though the bandwidth of the amp may not be that wide, it can have consistent feedback (and thus low distortion) at *all* frequencies with enough of it that phase shift can be as well controlled as a wide bandwidth amp of no feedback at all. This much feedback can also be enough to clean up the mess that feedback normally creates when there isn't enough of it.


The other thing you need to have is a proper distortion signature (something most people call the amp's 'sonic signature'; quite literally this is the difference we hear in all amps) that prevents the amp from sounding harsh due to unmasked higher ordered harmonic content.

Does that type of feedback create the same distortion as the type of NF used in solid state amps or is the distortion still there but tamed to some extent?    

@bifwynne 

It does not seem to but I've not done a lot of research on the topic. But its well-known that ultralinear (if set up right) gives the same linearity as a good triode; it can be treated in terms of linearity as if a triode is in the circuit. The cross-coupled cathode feedback has a similar property of being more of a local feedback rather than loop feedback. So IMO its not harmful.

Speaker drivers are designed to work best when fed with a voltage source.

I wish it were this simple. The problem has been that the technology hasn't been up to the theory.


To get a tube amplifier to behave as a voltage source you need feedback. But if you don't have enough feedback, one consequence is that higher ordered harmonic distortion will be generated. As seen earlier in this thread, there are very few tube amps that actually had enough feedback.


Higher ordered harmonic distortion is audible as harshness and brightness. You can have a THD of 0.005% and it will be audible. The reason is the ear is more sensitive to higher ordered harmonics than anything else because it uses them to sense sound pressure.


Because most solid state amps need feedback to be linear, they too have this problem (and this is why solid state amps have a reputation for brightness). The Voltage Paradigm relies, for the most part, on amps having feedback (or otherwise a very low output impedance).

Because the ear converts all distortion into some sort of tonality (for example, the 'warmth' of tubes is caused by the 2nd and 3rd harmonic) it also gives that tonality extra attention- so much so that it will favor it over actual frequency response. Add to that the simple fact that no loudspeaker is actually flat and it becomes possible to have a loudspeaker that isn't designed to be driven by a voltage source.

Such speakers were common in the 1950s and 1960s. To identify them, look on the back for a level control or switch that allows you to adjust the midrange and/or tweeter. There are such loudspeakers made today as well. Look on the back of a Sound Lab and you will see that its quite adjustable, to allow it to be compatible with amps that have a variety of output impedances.

Speaker drivers are designed to work best when fed with a voltage source.

**Most** are but some are not. ESLs for example...

Of course it means bias stability demands attention, but i solved that decades ago.

:)

United States Patent Office. 2,773,136. Patented Dec. 4, 1956. 2,773,136. AMPLIFIER. Julius Futterman, New York, N. Y.

Thanks!

One of my business partners at the time had West Sound Lab monsters and used, of course, my amps. Sounded terrific (all speakers are, of course, to someone’s taste). no issues whatsoever. Just sayin’
Edit, after some coffee, do remember back (oh, 300 posts ago, i get it :-)) that my designs are not zero output impedance. Since they do not use global feedback, there is always *some* meaningful output impedance, but also, pretty much zero chance of instability. Both often issues with large 'stats.

I forgot that the Sound Lab has adjustments on the back to allow for more bass and less treble. Anytime you see that sort of thing on the back of the speaker, its an indication that the speaker does not conform to the voltage rules. It has the adjustments to allow the unknown output impedance of the amplifier to work with the speaker (IOW they are not there to adjust the speaker to the room). So that was not the best example on my part.

Running zero feedback is a nice way to insure stability. What is the output impedance of your amp?

@clio09 I'd love to see it!

In an effort to solve the oscillation issue in his first prototype and which you noted, one thing Roger found is Futterman himself omitted some things from the H3 schematic, specifically the lack of notation for the ferrite beads

@clio09 

IMO this was intentional. I have a lot of anecdotal evidence from customers that say that the Futterman amps held together while amps made by others using the Futterman circuit didn't. However Harry Pearson recounted an incident where Futterman brought one of his amps to Sea Cliff for audition but before it could be entirely set up, one of the amps went into oscillation and failed (this was in response to a letter to the editor from Harvey Rosenburg in the late 1990s). But all you have to do is leave out one bead by accident and you're sunk. Futterman made most of his amps himself to my understanding.


60dB of feedback in a tube amplifier is an impressive feat! Normally you have such prodigious issues with not exceeding the phase margin of the amp (OTL or not) that most would not attempt such a thing. If 60dB is correct I'm quite impressed (and stand corrected)! The ones I've seen did not have any such value- I doubt that they even had 60dB of loop gain (that's the gain of the amp plus the amount of feedback).  But some of the Futterman amps have impressively low output impedance figures, such that they would have easily behaved as a voltage source, even though they made more power into higher impedances.


Kron-Hite made laboratory amplifiers in the 1960s. In their manual for the amp (which used KT88s) they claimed 80dB(!) of feedback. Its hard to imagine how they pulled that off- that amp was quite stable. I had a pair of them for a while in the late 1970s and they compared very favorably to an ARC D-75 that a friend of mine had.

Julius Futterman solved the tube amp feedback problem by eliminating the output transformer. His OTL amps can have 60 db feedback with low output impedance and low harmonic/IM distortion. Yet remain 100% stabile into complex loads!

This statement is false. No Futterman amp ever had anything like 60dB of feedback!! The most any had was more like 20dB, and because the amp had very wide bandwidth, oscillation (caused by its phase margin being exceeded by the feedback) was sometimes an issue. So to your closing comment here- they are well-known to **not** be stable into complex loads.


If you doubt any of what I'm saying, just for the record I've been manufacturing OTLs longer than anyone else on the planet (over 46 years); I'm not being hyperbolic.

Aside from what i already wrote, here's also the practical issue. A low impedance, hgih current amp will drive anything.

This isn't quite correct, just so you know. Try putting your amp on a Sound Lab ESL sometime. That speaker is 32 Ohms in the bass and maybe about 2 Ohms at 20KHz (depending on the position of the Brilliance control). Most voltage source amps tend to sound too bright on this speaker; they struggle to make power at bass frequencies. Its MO isn't based on the Voltage Paradigm. Anytime you mix the two paradigms (Voltage and Power) you are at risk of a tonal anomaly.


SETs and other zero feedback tube amps are not meant to be used with difficult speakers (and I argue that no amp should be used with such speakers since the last thing you want to do is make any amp work hard for a living- it will make more distortion which is audible). So no amp can really work with all speakers.

He did but it wasn’t absolute. He added a little resistance so it behaved more like a tube amp. This means it was LESS like an ideal voltage source and MORE like a current source, but even then there are limits to how well this can ever be done or how badly it would sound. Tube amps are still mostly voltage sources and deviate only due to the relatively small output impedances. I say relatively because a single multi-way speaker may vary from 3 to 30 Ohms. An amplifier with a 2 ohm output impedance won’t be near an ideal current source. For that you’d need say 300 to 3,000 ohms of output impedance.

A perfect current source amp would perfectly track the impedance curve of the speaker, which would sound awful.

@erik_squires Most of this is not quite correct. If you add a resistor to the output of a solid state amp it will indeed simulate some sort of tube amp that uses feedback. Most transformer coupled tube amps with 15dB of feedback will act as a pretty good voltage source. You might have to play with the taps on the output transformer. Between Voltage source and Current source there is Power Source, which is how a tube amp will behave if it has no feedback or if it has voltage and current feedback of equal amounts.


Most tube amps do behave as voltage sources. You need current feedback to get the amp to behave as a current source. The only amps commercially available that ever did that had variable damping controls and were made in the 1950s. Fisher made a few as did EV. I'm not sure how many others. But the damping control really allowed you to set up the speaker incorrectly since it did cause the amp to misbehave it you had the control set incorrectly- which was usually towards the 'Current source' side of things.

If the amp has a 50 ohm output impedance it can act as a current source. Some speakers can sound amazing if driven by a current source- Nelson Pass did exactly that with some OB speakers he had a RMAF about 10-12 years ago. They played bass far better than you would have expected considering how small the OB speakers were! But for the most part the approach is impractical.

The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?

Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)

So ... why are amplifiers not built as a high impedance current source?

This is mostly incorrect.


The voltage rules haven't been around forever; MacIntosh and EV were proponents of this in the late 1950s but it took until the 1970s before the industry had really switched over from the older power rules. The idea was plug and play with no adjustments on the loudspeaker. You can read more about it at this link:
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
Any time you see adjustments on the back of the speaker like a tweeter level control, the speaker is designed around the older power rules- the control(s) allow the speaker to be adjusted to the voltage response of the amplifier and are not there to adjust to the room.

Of course a current source amp would be even more sensitive to impedance variation than even a power source amp! But there are speakers that worked best this way; most of them are open baffle with very tight suspensions on the drivers.


The problem for many years with the voltage rules has been the to get the amp to be a voltage sources (especially tube amps) you need feedback, and the simple fact has been that most amps made in the last 70 years that have employed it simply do not have enough. As a result the feedback that they do have has caused all of them tube or solid state to be brighter than they should be, since the feedback wasn't enough to allow the amp to clean up the distortion added by the feedback itself.


Since the ear perceives distortion as tonality, and since the distortion added by feedback (while suppressing the innate distortion of the amp otherwise) is entirely higher ordered harmonics, the ear perceives that as brighter and harsher even though the amp might be flatter frequency response.


That is why there are still power source amps around today (SETs being an example). Tube amps that don't use feedback (and so are not as harsh and bright) will act like a power source.


The trick with feedback of course is to have enough, which is usually over about 35dB or else don't use any at all. Its not really been possible to put that kind of feedback in an amplifier until recently. The typical brightness of solid state (which is why tubes are still being made) is caused by insufficient feedback.