What is wrong with op-amps?

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#3,581
Shouldn't it be possible to build a no feedback amp and a high feedback amp of the same gain and phase and see what the differences are under a variety of stimulations? It seems pretty straightforward and trivial. A differential amp or even a transformer primary across the outputs should show what is making them sound different I would think. Unless observer effect (https://en.wikipedia.org/wiki/Observer_effect_(physics)) prevents the magic from coming out.
 
#3,583
I have never designed a completely open loop power amp to my satisfaction. I have enough trouble making phono and line amps open loop, but I have managed to do that well enough. Still, I strive for maximum open loop linearity, wide open loop bandwidth, and moderate global feedback for AUDIO LISTENING.
Once, about 35 years ago, I designed a HIGH negative feedback amp with approximately 100KHz open loop bandwidth, using 12dB/octave compensation for a special oscillator driver for Sound Technology. So far as I know, they put it into one of their analyzers, but even though it was made with the same care as my best efforts, I would not necessarily use that design for audio playback. It only had to do one frequency at a time, at a constant output level, not anything like music.
 
#3,586
cbdb said:
Find the YouTube video of Rupert Neve talking about his old consoles, the input strips are still a favorite of many top recording engineers. He says that there sound is due to the transformers. Recording engineers understand that as soon as you put up a mic your using an "effects box".( look at the top recording mics, almost none of them are flat, they change directionality with frequincies, there's proximity effect, etc. ). You move the mic, change its angle, yo adjust the effect. If this wasn't true a studio would only need one kind of mic, a test mic:flat and consistant with angle. So these engineers consider every piece of gear as an effects box because they all effect the sound.
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Totally irrelevant to sound reproduction. I myself was running a laboratory where designed sound effects, and still volunteer providing sound reinforcement and recording. Yes, it is a combination of an art and a science. Please don't forget a "science" part, otherwise it would be like a hockey game in total darkness.
 
#3,589
#3,590
john curl said:
Still, I strive for maximum open loop linearity, wide open loop bandwidth, and moderate global feedback for AUDIO LISTENING.
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I use maximal possible feedback when there is no way of clipping. In the stages that are supposed to clip I use local feedback. Also I like nested feedbacks, carefully dosed, to make amps misbehave less nasty, being as clean as possible.

As I pointed already many times, any design is optimisation.
 
#3,592
#3,593
john curl said:
I certainly think that 'feedback works', but that does not refute that feedback may well have unpleasant side-effects that can sometimes be worse than most of the advantages that feedback gives. We KNOW that negative feedback can cause: added higher order distortion, TIM, and PIM. This can be proven both by mathematics and measurement. It kind of reminds me of the ads on TV for various medicines, where a superficial problem like acne is proven to be reduced, but the medicine has side effects that can sometimes discomfort or even kill you. Was the medicine (feedback in this case) worth the trouble that it might cause?
This is why Charles Hansen and I try so hard to reduce or even eliminate global negative feedback. We find that our designs have a tendency to sound better running open loop, IF we can get the circuit linear enough without adding a feedback loop. Why, I can only guess.
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Of course feedback has downsides. The rational thing to do is to design in such a way that you can reap the advantages and eliminate the disadvantages rather than chucking out the baby with the bathwater.

Jan
 
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#3,597
john curl said:
It only had to do one frequency at a time, at a constant output level, not anything like music.
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That is a common anthropomorphic error. Because we find a music waveform more complex than a sine wave or square wave we assume an amplifier does too. Actually, to an ampliifer a square wave is much harder to cope with than music, because music is bandlimited.

Simply feed in two sine waves at similar but different frequencies and identical amplitudes. Nothing could be much simpler than that? Yet that waveform envelope varies from peak to zero like full wave rectification; the envelope has infinite bandwidth!
 
#3,598
davym said:
Feedback reduces distortion, except in this thread.

So, op-amps do have more than one downside after all.

1 - They are painful when stood on.
2 - They are painful when sat on.

This thread turned out to be quite a revelation after all then eh 🙄
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Only for the old style DIP devices, SMD SOIC and QFN /QFP packages are pretty painless when sat/stood on, I have tested this myself and sat on a random op-amp in a SOIC package for over an hour without any complaint from my derriere...😉
 
#3,599
jan.didden said:
That maybe true but don't forget that for each 'opponent' in a discussing there are 10''s or 100''s of lurkers who are not directly involved and much more receptive to rational arguments. So it is still worthwhile to debunk the BS.

Jan
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I'm not so sure it works that way. Bystanders end up seeing two increasingly polarized and increasingly antagonistic viewpoints. Various things could happen, such as sympathy for the underdog, as well as polarization of the bystanders as they take sides.

It often tends to turn into a debate with each side trying to win their now polarized positions, rather than a mutual search for truth.
 
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