Help with cut off frequency

[hesamadman]

Im trying to adjust some values in a build and I cant quite wrap my head around cut off frequency from coupling cap RC circuits. A low E in the guitar is roughly 88hz. I understand there are a lot of low undertones and such, but what is the range of cutoff to shoot for.


I have been kind of looking at schematics of other amps. If you look at a stage of a jcm800 and ignore the bypass caps- [ .022uf cap followed by two 470k resistors (940k) you get a cut off around 7.7hz)
I had a jcm800 with instead of a 470k voltage divider I had a 1m. So that gave a 2m load on the .022uf coupling cap reducing the cutoff down to 3.6hz. I was told this "had a lot of lows". In my mind I am thinking both of these numbers are well below a guitar frequency and I wouldnt think you would even notice a difference?


If anyone could explain this idea a little better I would appreciate it.

[92Volts]

Coupling caps are needed to block DC voltage from affecting the bias of the next stage. They may be used to alter the frequency response, but aren't always used that way-- in the case with a 7hz cutoff, it wouldn't have an audible impact.

[PRR]

> I wouldnt think you would even notice a difference?

99% of the time, that is the point. You want "no effect in the audio band".

For assumed 20Hz, one 20Hz cut is hardly much effect. But complete audio paths have several, many, even dozens of low-cuts (to block DC). If each one is -3dB@20Hz, and you have a dozen of these, that is -36(!)dB at 20Hz, -18dB at 40Hz, maybe -9dB at 80Hz, prolly -4dB at 160Hz.... that's real lame bass.

In guitar amps we have flat to below 82Hz, full bass. And we have shaved-bass tone where the bottom octave is sloped down, so a given signal sounds more up in the midrange, where the speaker works better and you are not fighting the upper notes of the bass player. A solo lounge lizard may want full bass to fill the room alone; a 4-man band with strong bassist wants the guitarist sounding more in higher registers so shaved-bass may be suitable.

We usually set 1 to 3 of the total possible bass-cuts at or above 82hz to get shaved bass. Not all of them; we don't want the lowest notes to vanish, just not boom excessively.