How does negative feedback work?

One of the topics I see discussed on tube amp forums is negative feedback and what it does and doesn’t do. A lot of boutique amps have a feature where you can change the amount of negative feedback in the circuit, but I see a lot of confusion regarding what this feature does. So, let’s do a quick primer today on negative feedback, and how it works.

Your guitar’s signal enters the amp and then goes through a process of amplification and refinement. The signal that goes to the speaker is very different from the signal that left your guitar, and this is the signal’s most refined form. When you create a negative feedback loop, you are taking this refined speaker output signal and injecting it back into the preamp, creating a loop. This is your negative feedback loop. The result of mixing your output signal back into the amp’s circuit is a reduction in gain an a cleaner signal.

This was a tactic that Leo Fender used when designing the Tweed Champ back in the ‘50s. At that point, people didn’t want overdrive at all, but amp builders were still trying to figure out how to tame the gain while achieving good tone. The Champ is a pretty high gain amp as designed, so eliminating the negative feedback loop would have rendered it useless when it was released.

Negative feedback isn’t the only way to clean up an amp, and it doesn’t have to applied in a global sense (we’ll talk about that another time). But what negative feedback does allow you to do is custom tailor your amp to the gain level you want. The amount of negative feedback is controlled by a resistor that sits between the output signal and the cathode of one of the preamp tubes. The lower the value of the resistor, the more negative feedback is allowed to flow into the circuit. The more negative feedback, the cleaner the amp.

This is probably a good time to explain the Presence control found on many amps because that is another mysterious— though related— topic.

A presence control allows you to vary the negative feedback, but only part of it. Presence works by tapping into your negative feedback with a pot (Presence knob) and a capacitor. The pot works as a variable resistor and the capacitor rolls off certain frequencies. So, the presence control is controlling the amount of a certain (usually high) frequency within the negative feedback loop. Instead of injecting the entire output signal into the circuit, a presence control allows you to add certain frequencies of the output signal back into the circuit.

The overall effect of a presence control is a fine tuning of the EQ, and this is tremendously useful in shaping the sound of the amp.

So, negative feedback can have pretty dramatic effects (changing the amp’s gain structure) or subtle effects (a presence control to fine tune eq).

Not all amps have negative feedback (Fender 5e3), but it can be added to most amps.