The problem with feedback

June 15, 2022
 by Paul McGowan

PS engineer Darren Myers and I were enjoying lunch together at a new Thai food restaurant. The food was lousy but the conversation was stimulating when the subject turned to feedback.

Darren knows my distaste for too much feedback and he wanted to wrangle me in a different direction.

What he pointed out to me made perfect sense. That when feedback is used to correct a problem, it doesn’t sound good and the more you rely upon it the worse it sounds. That much we could certainly agree on. This is the reason so many off-the-shelf op-amps sound dreadful. Their open loop (without feedback) bandwidth rolls off within the low audio band. They need feedback to even work.

Compare that with the opposite: a properly designed audio circuit whose open-loop bandwidth extends well beyond 50kHz and whose distortion products are below 0.1%. Used on its own that’d be a nice sounding circuit. Add feedback and wowsers!

The point is that when we use feedback to fix something that is broken—as opposed to fixing the problem itself—sound quality always suffers.

Used as a Band Aid we’re all the worse for it in the circuit.

Used as an enhancement the purity of music is honored and we all benefit.

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11 comments on “The problem with feedback”

  1. Delusion of the day award 😉
    “but the conversation was stimulating when the subject turned to feedback.”
    Ha, the funniest thing I read this morning, in fact the first thing I read this morning.
    Think about it. I doubt many non audiophiles would agree with that statement.
    Even amongst them I reckon the number would be less than 50%.

  2. I’m more worried about the state of Thai food in Boulder, Colorado. The last thing you need is a bit of feedback from a plate of Pad Thai. Obviously the choice of restaurant was a Pho Pas, as Paul is won’t to say, on the subject of which I had a very nice spicy chicken bun hue before a dance show last night.

  3. First, let me say that circuit design is not something I am good at. With guidance I can understand how a circuit works and I have done some casual reading about amplifier circuits. I know there is controversy about feedback and how much is too much. In my very humble opinion, the best power amps are not built with op amps and when it comes to feedback just a touch, not too much, is best. As always in audio YMMV.

  4. Let’s see. Negative feedback back of restaurant food – lousy = not good

    Negative feedback in audio circuitry = icing on cake sometimes =‘s good.
    Lousy circuit or component no matter what you apply = lousy = tai food restaurant

  5. I think it pretty much applies to all things even non-audio. With the exception of the “corrective lenses” on the Hubble, it’s never good ignore fundamental issues and try bandaid fixes.

  6. Yes very true most Op Amps are poorly suited for audio use for reasons Paul stated.
    Signetics brought out the NE5534 (single) & 5532 (dual) Op Amps in early ‘80’s which were specifically designed for audio use – and they could drive 600R output Z (most other devices give up driving less then 2k ohm)
    The [single] 5534 device has pins for dominant pole frequency compensation (permitting use in applications where the feedback loop goes beyond the device’s output pin)
    All the major mixing console makers used the 55xx device in their 100’s right up to the end of the analogue console era around 2006 at least 5 55xx devices being in the signal path of each channel.
    Prior to my retirement 2020 I used to service pro audio kit, the monster mixing consoles of the period generally done (at component level) on site.

  7. I’ve been to a Thai restaurant like that in Longmont, which is no longer in business, Todd’s Thai or something like that. The aging hippie crowd loved it. I thought the food looked like feedback.

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