Are all amps being built wrong?

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.

@atmasphere 

Ralph, quick question on negative feedback (NF) distortion.  I seem to recall that Audio Research output stage coupling is a combination of "Ultralinear" and "partially cathode-coupled" topology.   I have no idea what that means.  I clipped it from a Google search. 

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?      

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.

@atmasphere 

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. 

Perhaps in the case of "[u]ltralinear" and 'partially cathode-coupled' topology," the physical distance is shorter because the local feedback loop is shorter.  Just a guess.        

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.