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Are all amps being built wrong?

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 an interesting read: https://www.current-drive.info/
rudyb
holmz's avatar
holmz
2,328 posts
 
Speaker drivers are designed to work best when fed with a voltage source.

I am only sure that this is true is the impedance is flat.

Since the equation for magnetic flux is based upon current, then one could argue that it is the current that is producing the force in the motor.

Speakers which means drivers and crossovers are also designed to work properly with a voltage source.

There are advantages and disadvantages of driving a speaker driver with a current source or a hybrid but to make it work the overall speaker must be designed from the ground up for it.

^That^ is a statement, and sounds like an opinion.

No DSP is not a substitute. It has to happen at the driver/amp interface.
Anything that linearises and equalises the output makes for a more linear system. (Yeah it’s a tautology) However if the speaker was a constant impedance and the, then it would play flatter. You can alter the incoming signal’s voltage so that it plays flatter out of the speaker as a system. One can do this for both current source or voltage source amplifiers and end up with a flatter speaker response curve.

atmasphere's avatar
atmasphere
11,616 posts
 
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.

bifwynne
2,206 posts
 
@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?      
atmasphere's avatar
atmasphere
11,616 posts
 
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.
bifwynne
2,206 posts
 
@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.        
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