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What is wrong with negative feedback?

I am not talking about the kind you get as a flaky seller, but as used in amplifier design. It just seems to me that a lot of amp designs advertise "zero negative feedback" as a selling point.

As I understand, NFB is a loop taken from the amplifier output and fed back into the input to keep the amp stable. This sounds like it should be a good thing. So what are the negative trade-offs involved, if any?
solman989
kijanki's avatar
kijanki
4,477 posts
 
Mapman - Moderation is the word. Instead of feedback or no feedback we can settle for moderate feedback. Sound coloration comes from dynamic nonlinearity of the system with the feedback. Amplifier itself is far from being first order low-pass filter and feedback creates loss of stability - hence dynamic nonlinearity. Part of the problem is speaker - being complex load. That would suggest to me that things are really complicated and listening instead of reading will bring better results. Speaker choice and synergy with amplifier appears to be very important.
clio09
3,374 posts
 
Roger Modjeski has some interesting design philosophies in general. Topics like feedback certainly stir the hornets nest within him. Just don't get him going on cables. At that point it becomes a swarm.

I have to say that the RM-10 manual is one of the best audio reads I've experienced. The amp itself is excellent too, even with 14db of negative feedback.
spectron
130 posts
 
ULTRAFAST NEGATIVE FEEDBACK
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When an amplifier has difficulty in delivering required voltage and current many forms of distortions will occur. For example, in transistor amplifiers the increased current drawn by speakers will cause a small voltage drop across the source--i.e., the amplifier itself--which will heavily contribute to the unpleasant so-called "transistor sound.” Many transistor amplifiers use global negative feedback to reduce distortions and widen the bandwidth.

The crucial factor in negative feedback is transit time, the amount of time it takes from when an error is detected at the input until it is corrected at the output. For example, a typical transistor power amplifier has three primary sections: a low-noise high-gain differential input stage, feeding a differential-to-single-ended conversion driving a high-current output stage. Each of these three stages is designed for low distortion and noise, but those attributes typically come at the sacrifice of speed.

The typical transit time of linear amplifiers is about 2000-3000 nanoseconds, which is too slow for effective implementation of global feedback and error correction. This lagging results in ringing artifacts and enhances ODD-order harmonics which are particularly annoying to the human hearing so even the smallest amounts of these distortions are highly noticeable. Long delays in feedback also introduces transient and phase discrepancies, susceptibility to transient overload and vulnerability to disturbances at the output such as reactive speaker interactions.

In contrast, many switching amplifiers don't use low-distortion circuits. Instead, they use much faster digital logic circuits. For example, the Spectron Musician III transit time is much less then 200 nanoseconds. Such an ultra-short transit time allows the amplifier to correct for many small errors; and the control loop can follow the input much more accurately. These characteristics result in a more detailed, transparent sound with less noise and louder yet cleaner musical reproduction.
ngjockey
1,042 posts
 
Best thread EVER.
atmasphere's avatar
atmasphere
12,161 posts
 
If you can increase the speed of the amp (decrease the propagation delay) theoretically you could speed it up to the point that the odd-ordered enhancement is pushed well outside of the audio band.

Our amps are also pretty fast- 600V/usec is a typical risetime, and we only have one stage of gain. But IME this is still not fast enough, so we resort to other means of getting rid of distortion: class A operation coupled with fully differential balanced operation, which cancels even ordered harmonics not just at the output, but throughout the amplifier. This leaves us with the 3rd harmonic, which is controlled by using only one stage of gain. Odd ordered harmonics are exacerbated by noise problems in the ground and the power supply, so we use star grounding (a lot easier since most of the grounds are balanced) and separate power supplies for the driver and output sections, which also reduces IM distortion.

Distortion has the property of masking detail in addition to adding loudness cues, so if you can get rid of distortion you get greater transparency and greater smoothness at the same time, provided your techniques for getting rid of distortion don't enhance the 5th, 7th and 9th harmonics. IOW real reductions in distortion have real, immediate sonic benefits that anyone can hear: extreme detail accompanied by smoothness are the hallmarks to look for.
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