Variable Mid Shift a la Peavey Sessionbass Head

[Gel Douche]

Hello all,



On a Peavey Sessionbass Head (http://www.peavey.com/media/pdf/manuals/sessionbass.pdf) there is a Mid Shift knob which "selects the frequency band that the mid control cuts or boosts". The knob is marked from 200 to 1k to 2k (presumably Hz?). This knob sounds really cool if you turn it while playing, very wah-like.

Basically, I would like to be able to do this without purchasing this amp. Is there a circuit that can do the same thing? As in an extreme, pot-controlled mid shift I guess (that isn't a wah). Or does it have to be after some amplifier stages or something?

[MarcoMike]

in many bass amps the mids control, with selectable centre frequency, works with a LCR filter. the switch selects a capacitor-inductor combination, which sets the working area. lately I've been studying the ampeg svt3pro preamp, which has that kind of tonestack, you can find the full schematic @ music electronics forum

[Gel Douche]

Thanks for the reply.

Would the circuit work stand-alone, i.e. not in an amp, but just on the unamplified guitar signal?

[GibsonGM]

Well, you would need an input buffer to drive the 'mid stack', and then a recovery stage to make up for the losses in signal level the circuit would cause.  But they are easy to make!  Like, a single op-amp IC would work, with just a few external components...

[Gel Douche]

I see. Wtf are input buffers and recovery stages? Would the signal level loss matter if it was going straight into a high-gain fuzz pedal?

Cheers.

[Rob Strand]

A lot of Peavey stuff used this paramid, it's quite possible they used the same circuit,

PVBassMK3

PDF Page1: Around U3, Input at C23, output at C25

These units varied from 150Hz to 1.5kHz.  They had the sound you described.

To get  200Hz to 2kHz you need to change some caps:
C31   was 3.3nF now  2.7nF; (2n2 actually gives closer to 200Hz to 2kHz)
C32   was 33nF  now  27nF

[Mark Hammer]

More than likely the same circuit.

Note that sweeping a quasi-parametric equalizer in post-production is one of the classic studio tricks to add wah to a track that didn't start out with it.