The evils of feedback Part 2

July 12, 2022
 by Paul McGowan

3 comments on “The evils of feedback Part 2”

  1. I’ve written this before, maybe even here but I had one interesting experience decades ago with the affects of negative feed back.

    It was late night in My 1979 at the old Chicago CES show in the Precedent Audio Room of Murray Zeligman. We were using a very interesting modular speaker with transmission lines on both the bass and mid range using modified KEF drivers. The amplifier was an Audionics BA150 designed by David Berning who was there. The BA150 was a hybrid(solid state driver stage only) class B amp using horizontal output tubes in triode rated at 150 watts(it actually put out about 225 watts. But what was most interesting here is that it had a switch with 4 or 5 stops to vary the negative feedback from about 15 dB to 0 dB.

    We began with 15 dB and stepped down to 0 dB. And each reduction made it sound better to me, more open, more detailed and more dynamic. I was naturally puzzled since reducing feedback led to more harmonic and IM distortion and poorer frequency response due to increased output impedance reacting with the speaker crossover. Finally expecting no real reply I asked if improved measurements led to poorer sound what the heck was going on. Was there any measurement that correlated with better sound, expecting no real reply. But the answer was yes. Linearity(if 1 volt in gave 5 out then 2 volts should output 10 volts, etc.) correlated with better sound.

    Just for historic interest both negative feedback and positive feed forward were invented by a man named Black at Bell labs to increase the clarity of long distance transmission(which was very narrow bandwidth). He preferred negative feedback because it was simpler to implement by a simple loop in the circuit from the output to the input while feed forward needed a second amplifier to feed the correction signal from the input to the output.

  2. Thanks Paul for your insight. I remember an article about transient intermodulation distortion T.I.M by Matti Otala . is it not what you are in a simplified way are alluding to. And is it not related to phase and gain margin as applied in control systems. Is it not then a stability problem?

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