Measuring the level of feedback in an amp
 

Measuring the level of feedback in an amp

First, measure the input sensitivity of the amplifier with no feedback.

This is the input level of signal needed to drive the amp to full output and is usually done with sine wave at 1KHz. Increase the level from a signal generator into the amp until clipping distortion is just visible on a scope monitoring the amp output. Check the level of the sig gen signal at this point.

Then measure it again with feedback.

The difference, expressed in dB, is the level of feedback.

Using WAD`s 300B PSE as an example,
it has 380mV sensitivity without feedback and 1000mV with, so the level of feedback is determined by;

log(1000mV divided by 380mV) x 20 = 8.4dB