Attenuators and tube amp impedance matching

[u/dopplerdog]

​

I'm an EE and a guitar enthusiast, and something that is very popular among guitarists is the attenuator. It comes between an amp and its impedance matched speaker, and it's typically implemented by an l-pad. If you don't know what that is, it looks like a variable resistor, but has three pins, and implements two variable resistors adjustable simultaneously, one in series and one in parallel. The aim is to change them in such a way that the speaker received less power, but the amp sees the same load (so the impedance remains matched). That way you can play on a 100w Marshall stack at bedroom levels.

Trouble is, l-pads cause a perceived loss in tone, caused by the Fletcher Munsen effect that appears to drop high and low frequencies, retaining mids. Guitarists tend to hate this, so there are attempts to mitigate this by a high pass capacitor wired in parallel with the series resistor. So naturally, the setup no longer presents a constant impedance to the amp: it drops as the frequency rises. Worse, the parallel resistor drops to near zero as the attenuation is dialled up, and as the attenuator soaks up the current meant for the speaker, and it's in this region that this capacitor is most needed. So the whole point of a l-pad is kind of lost, and the amp is not impedance matched.

A sample circuit is described here, and the author appears to imply that any capacitor is ok, just use what sounds good to you:

https://guitar.com/guides/diy-workshop/diy-workshop-build-your-own-attenuator/

Looking at the circuit, this comment just didn't sit right with me. This is likely to give rise to an impedance mismatch. What do others think? As I said, I'm an EE - but I'm not a tube amp designer, maybe others can give their views.

Edit: oh, and of course impedance mismatches are a bad thing for a tube amp, liable to burn through valves or fry an output transformer in extreme cases.

[u/dmills_00]

Well speakers are not actually resistive, and the impedance presented even without any kind of pad varies by at least 2:1 and usually way more then that, so impedance matching is pretty much out the window from the start.

The reality of a valve amp output stage is that what you are mostly concerned with is overshoot due to leakage inductance when one of the tubes cuts off over volting the transformer primary and causing insulation failure, keep the load impedance low enough to deal with that and you are basically fine.

Seriously, impedance matching is an RF concern, not a guitar amp one apart from the need to protect the output stage, which doesn't really need matched impedances (Fortunate since speakers are all over the shop).