Graham's Blog . . .


Zero Negative Feedback

Posted by Graham
July 14th, 2016
Feedback block diagram (Image licensed under Creative Commons)

Right now negative feedback is bad. They're selling amps without it.

As a circuit designer I didn't understand what zero negative feedback meant! Well, I did, and I didn't...

I know what a circuit without negative feedback does - it distorts badly, is noisy, and doesn't have much of a frequency response.

In fact, a solid state circuit without negative feedback has high gain and is unstable. For stability it has to be compensated (negative feedback at high frequency), and when compensated its frequency response might cover the bass frequencies and then droops.

If it uses valves the droop happens higher up the scale. If it uses solid state it droops from a few hertz up to a few tens of hertz. It all depends on the gain of the devices used - but you definitely have no highs!

However, looking at the specs of these products none of it was making any sense. The specifications showed a full frequency response and reasonable noise and distortion - it didn't make sense, so I bought a book.

Yes, after designing audio circuits for 4 decades I found myself having to buy a book on amplifier design so I could read the chapter on "zero negative feedback" (this being something that didn't exist until recent years).

I read it and understood it. It didn't take me long. Turns out that zero-negative feedback is a bit of a lie.

There is negative feedback! It's just that rather than applying it globally (from the output the user sees to the input the user sees), it is applied locally between outputs and inputs of each stage that makes up the circuit.

In analogue signal processing: preamps, mixers, EQ etc, we've had local negative feedback all along - you can't apply global negative feedback here.

In power amplifiers we've had local negative feedback - admittedly with global negative feedback to boot, and that polishes-off the last bit of distortion (reduces it considerably).

Therefore I have to conclude that zero-negative feedback is yet another desperate marketing plea to get you to buy theirs instead of somebody else's.

So who jumped on this latest bandwagon? You can easily see. But count me out - I am much more interested in doing the job my way, the way which appeals to the ears rather than the ego.


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