100% nfb, amp still makes sound?

[Max999]

Hi all,

In an amp I am experimenting with ( see schematic) I have removed the 47K NFB resistor connected to the presence pot and made a direct connection.

Why does this amp still makes noise? Is it because the 5K presence pot is always shunting some signal to ground?

When applying the B+ after warmup there is some low level hum for a short period that could be motorboating. It disappears after a minute. Can this be an effect of all the NFB I am using?

 

[Bigun]

Unless I've misunderstood your question, 100% feedback will result in a gain of unity - roughly speaking. So signal in = signal out.

 

[cbdb]

So you havent removed the resistor youve just made it vey small. This has increased the feedback signal to full output. You need to study a little feedback theory, easiest to look at opamp circuits. The easiest way to look at it: the amp will try to keep the voltage at the - input the same as the + input ( or close, input /gain = output, so with lots of gain the input is very small so - almost equals the +. ). Since youve shorted the output to the negative input the 2 will have the same voltage, and since the - input follows the + input ( or close) the output = + input, or gain of 1.

 

[JMFahey]

One problem is that with so much feedback the amplifier usually becomes unstable or even oscillates.
I´m amazed it hasn't done so yet.

 

[PRR]

Stock the gain is about 11. Your mod makes it gain of about 1. So it doesn't go to ZERO, it goes to much-less.

And yes, stability margin is eroded and the motoboating is not a surprise.