By anders pearson 01 Jul 2023
Reddit seems to be circling the drain right now so I’ve been going through my comment history there and archiving the ones that seem worth preserving. This one in particular has (IMHO) a decent explanation of some concepts around impedance matching applied to guitars, guitar pedals, and amplifiers that I haven’t seen summarized like this elsewhere, so I thought I’d post it here to make it easier to find. The comment was in response to a question basically asking “what is the difference between an overdrive pedal and a solid-state preamp?”
(I will also add a caveat that while I was once an EE student and studied this stuff, I haven’t really done any of that in about 20 years, so I may be out of date or fuzzy on some of this.)
A preamp generally serves several purposes. One is the EQ and distortion/overdrive. Importantly, though, it also turns an instrument level signal into a line level signal. A major part of that is getting the impedance right. To connect two pieces of gear with a minimum of signal loss/loading, you do “impedance bridging” which basically involves using a low impedance output (“source”) to drive a much higher impedance input (“load”); at least 10x ratio, usually higher. A passive guitar pickup will have an output in the 6-15kΩ range. A guitar amp’s input impedance is usually in the 1MΩ range. An amp is usually designed in two stages, the preamp and the power amp (though each of those might have multiple internal stages). If an amp has an effects loop, the loop is usually between those two stages. There aren’t hard and fast standards, but the common approach is to have the preamp stage produce “line level” output with a source impedance of 100-600Ω and the power amp input to also take “line level” input with an impedance of 10kΩ (though higher is also very common).
If you take your guitar and try to run it directly into the power amp stage, you’ll be potentially running a 10-ish kΩ source into a 10-ish kΩ load. If you’re lucky, the power amp input impedance will be higher (up into the 100kΩ range), but there’s no guarantee. If you run your guitar straight into a power amp or into the effects return of an amp, you’ll probably get a sound out, but it’s likely to be very quiet and probably not sound very good (important to note though that some amps convert to instrument level signal for the loop). That’s because the pickups are being loaded more than usual (which you’re mostly going to hear as a loss of high frequency) and because the power amp is expecting a much lower impedance source connecting to it.
Overdrive and distortion pedals don’t really have any kind of standard for input/output impedance either except that they are usually designed to go in between a guitar and an amp input, so they’re expecting to see a 6-15kΩ source connected to their input and a 1MΩ load connected to their output. A reasonable design would be for the pedal to have a 1MΩ input impedance and a 10kΩ output impedance. 1MΩ or higher is about what you’d expect from a pedal with a good buffer in it. There’s no real rule on the output side though and pedals can be all over the place. If a pedal is designed to be a “preamp” though, you would expect it to have a much lower output impedance, 1kΩ or lower.
Some distortion pedals have output impedances in that range. Eg, everyone’s favorite Boss Metal Zone has an output impedance around 1kΩ. That means that if you plug it into a line-level input like a power amp input or a line-level effects return, it will do a pretty decent job. Plenty of other pedals have much higher impedance outputs though. Eg, a Tube Screamer is more in the 10kΩ range. If you plug that straight into a power amp input, it’s not going to be ideal.
So, in a sense, yes an overdrive/distortion pedal can be essentially the same as a solid state preamp. But not always.
The other part of it is that since the preamp stages of most guitar amps add a significant amount of “flavor” in terms of distortion and EQ, overdrive and distortion pedals are usually designed with that in mind. Even if the impedance is right for using it as a preamp, if it wasn’t designed with that in mind, you might not like the results.