Bass preamp - need more headroom

[marcelomd]

Hi,

I designed a bass preamp with a mixer for an external effect (dirt, obviously) and a 4 band equalizer. Sounds awesome.


For the volume control I used the feedback resistor of the mixer. Volume boost AND cut!

Problem is I'm running out of headroom from a 9V supply. Any moderate boost in volume or eq will clip.

Things I thought about:
- Charge pump/higher voltages;
- Attenuation at the input;
- Forget the gain and use a passive volume control somewhere else;

Did I miss something? Any suggestions?

Thanks!

[iainpunk]

what are you planning to drive with this? a power amp? does that power amp have its own gain control?

maybe a charge pump or batteries to up the voltage to 18v or 27v?
different op amps? some op amps have a closer output swing than others! (closer to the supply rail that is, ofcourse)

cheers, Iain

[pruttelherrie]

- attenuation right before U1B so you have at least a buffered signal that gets attenuated
- extra resistor in the feedback loop of U1B so it's not cut/boost but cut-a-lot/cut-less

[marcelomd]

I'm building one to use with my power amp, which has a gain control and supplies 24V. Which solves everything except my first opamp being 16V only (TLC2272).



I thought about making it more like a real pedal later, to put on the board. Buffers for the effect loop, no weird power supply requirements, maybe a balanced out, etc.

[PRR]

It can't overload unless RV2 is turned too high, and that would output too much signal for an Instrument Input.

Go through the circuit for DC sanity. Are half-bias opamp outputs really at half bias? Are polarized caps pointing the right way?

[marcelomd]

It can't overload unless RV2 is turned too high, and that would output too much signal for an Instrument Input.

Go through the circuit for DC sanity. Are half-bias opamp outputs really at half bias? Are polarized caps pointing the right way?

RV2 was supposed to give some overall gain.  From RG's article, Panning For Fun, 51k should give more or less unity gain. Anything more than that is too much for the following stages. I guess I was too optimistic about what one can do with 9V. I guess I'll just stick a resistor there and move the volume elsewhere.

The bias voltages are within 2% of 4.5%, so no problem there, I guess.

Question: Are there any 10u capacitors I can lower the value or remove? Such as C15?

Thanks a lot!

[11-90-an]

Are there any 10u capacitors I can lower the value or remove? Such as C15?

I think you can replace all the 10u caps with 1u, but then since I've never tried running a bass through my ckts, I don't really know... maybe leave the output cap as 10u...

[iainpunk]

Question: Are there any 10u capacitors I can lower the value or remove? Such as C15?

depends on the impedance following it!
http://www.muzique.com/schem/filter.htm
keep in mind that the corner frequency is already subtracting 3dB, thats about 29,3% loss. so keep in mind that the corner frequency should be an octave lower than the lowes note on your bass, (41 Hz for a 4 string's low E and 31 Hz for a 5 string's low B, which translate in 20 Hz and 15 Hz cutoff points)

cheers, Iain

[marcelomd]

Since I don't know how to calculate the impedance of a Baxandall style mid control... I resorted to LTSpice 

Changing all 10u to 1u decreased the bass about 2db at 20Hz. Changing to 2.2u decreased the bass less than 1db. Looks good. And a lot smaller. Maybe I can fit a film capacitor there.

C15 can be removed too.

Thanks!

[Vivek]

Impedance of a Baxandall is very low

And changes with pot settings and frequency

I would design assuming that the impedance is 1.5k ohms