which diy bass preamp?..

[Marcos - Munky]

Ressurrecting this topic again.

Today I got a request from a fried for a bass preamp, and this one was the very first thing that come to mind. I built a similar one to use with guitar and I know it sounds great, so it should sounds great for bass too. And this one have almost all features my friend is looking for.

The only thing my friend asked for and this one doesn't have is a "mid shift" pot. I mean, a pot to shift the center frequency of the mid pot. I know it can be done by switching the caps, but for some reason my friend prefers it as a pot rather than a switch. Any way this can be done?

[11-90-an]

My idea would be something like this...



A midshift pot would act as some sort of capacitance control... i don't know the exact values that would work but i guess trial and error would be needed as I didn't test this or calculate anything at all...

Again, untested, but just my idea...

[iainpunk]

My idea would be something like this...



A midshift pot would act as some sort of capacitance control... i don't know the exact values that would work but i guess trial and error would be needed as I didn't test this or calculate anything at all...

Again, untested, but just my idea...

i doubt that pot will do what you want it to do...
injecting more resistance in an already high impedance circuit ducks up more than the frequency you want to target.

change the treble pot to 1M
and place parallel to the treble pot a 1M pot wired as a variable resistance in series with a 100k resistor, this will sweep the mid frequency, and have about the same degree of interaction with other pots as the other pots have on each other.

cheers, Iain

[Marcos - Munky]

Didn't came to mind I'd have to play around the treble pot to get a midshift. Just to be sure, like that?

[iainpunk]

Didn't came to mind I'd have to play around the treble pot to get a midshift. Just to be sure, like that?

yup!
you might want to add a buffer since it has really high output impedance now.

cheers, Iain

[Marcos - Munky]

Sure, I'll add a jfet buffer. Just another question, should I still use 470K for the volume pot with this high output impedance?

[Rob Strand]

i doubt that pot will do what you want it to do...
injecting more resistance in an already high impedance circuit ducks up more than the frequency you want to target.

A better way to implement the intent of the idea would be to have the 100nF permanent and have a pot in series with the 220nF in parallel with 100nF.    Would need to test to see if it's worth a pot.

There's many ways to bend tone controls like that.  Often the result is quite subtle.    Maybe look at the Blackstar.

[Marcos - Munky]

I did a quick search and found another possibility: to make the resistor variable, like this: https://i.imgur.com/iYkXZqK.jpg. It'll affect the highs and lows, but the results seems to be interesting.

[iainpunk]

Sure, I'll add a jfet buffer. Just another question, should I still use 470K for the volume pot with this high output impedance?

i recommend placing the volume pot after the Jfet buffer as to not load down the tone stack. or if you don't mind crackle while turning, use it as the Jfet pull down resistor, in that case, 10k is fine.

cheers, Iain

[Rob Strand]

I did a quick search and found another possibility: to make the resistor variable, like this: https://i.imgur.com/iYkXZqK.jpg. It'll affect the highs and lows, but the results seems to be interesting.

Do a search on "slope resistor".

[Marcos - Munky]

Found this page while searching for slope resistor: https://robrobinette.com/How_The_TMB_Tone_Stack_Works.htm.

To be honest, the friend could just set with switchable caps and we were good to go lol. But I think the preamp will end up very versatile. Now, just have to set a few details with the friend and etch the board.