#1
Hey guys!

I was hoping for a little help here. I've been unsatisfied with the tonal varieties I can get from a standard onboard 2/3band preamp for my basses, and I was looking into building a low pass filter preamp to replace it.

Thus far, I have a couple of designs. In the first, we have a passive blend, going into a simple pair of low pass filters, going into a buffer with mid switches going into a master volume preamp.



I prefer the second at the moment. It's only the filters and buffer, but I plan on putting the blend after the filters and first buffer to isolate the pickups and avoid loading and interference between them. There is also a resonant frequency boost wired into this filter. Whether or not I use it remains to be seen.



However, what I think will work in theory may not necessarily be the best/most efficient way of doing things.

This is where I come to you for help! If you can give me any advice or guidance, that would be amazing!
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#2
http://www.geofex.com/article_folders/foolwfets/foolwfets.htm Something cool to look at, if you do a search for JFET SRPP and JFET Mu-amp, you should be able to find more.

You might wanna just try out some different cap values for the input low pass filters, it's cheaper to order a handful of caps than it is to order two caps, then decide you want another two caps.

For the second scheme:

Your buffer at the end of the second schematic, you've got 2k for the source, and 6k8 for the drain, you could throw a cap across the 2k, and get some gain back (You've got a gain of 2/6.8 atm). You could switch it with a Pchannel, and throw that 3M towards 9v, this would give you a gain of 6.8/2, or even more simply, just switch the resistors, a 2k for the source, and 6k8 for the drain.

P2 and P3, you don't need 2 pots, maybe change the P2 to a 22k or 27k fixed resistor, and wire the wiper of P3 to the connection of P2 and the 3k3.

Where you've got Vbias, you haven't set up an established Vb, Rather than 1M tied to 1M at the input, take 10k resistors in series, the center tap the Vb, use a capacitor (10uF-100uF doesn't matter too much) from the tap to ground. Use a 330k-470k resistor from the + of the first opamp to Vb, and tie the other Vb's directly to it.

You have a lot of neg. feedback for so little gain, not necessarily a bad thing, just something to bear in mind.

Unless those 10n resistors are there for low-pass filtering, you may want to alter their value, using f=1/(2*pi*10*10^-9*53300)=2986Hz~3kHz

Disconnect C8 from the output, you're killing your signal with it, have C8 connect from the 9v to the -v (Gnd). Increase R13, to something like 1M-10M. You can exclude R5, it's giving positive feedback (very bad) from the 4th opamp to the 3rd.

Add a 100-1k resistor from the output of pin 14 to C6, R10 isn't needed, P4 biases the gate with respect to ground.

Have R3 just connect to the wiper of the 100k trim pot rather than shorting the wiper to the one end of the pot.
#3
That's a lot of info to get my head around . Thanks though! I'll look through it and make changes!

Just in case you wanted to see, what I've done here is just mixed these two diagrams (probably incredibly crudely, in fact, I know it is).



and



(and the board version of the first)



In your opinion, do you think pursuing the second design will work better for an onboard preamp than the first?

Edit: Oops, forgot to put the power supply section into the second design... .
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Last edited by Nutter_101 at Apr 28, 2011,
#4
I think that you should buy parts for both, and test them both, see what you prefer. I haven't really built either type, the first one just didn't have much for me to comment on as far as schematic errors. For the JFET gain recovery, you won't need the additional cap from 9v to ground since it'll be filters from the Vb bus; also, R4 doesn't need to be included.
Attachments:
NutterFilter1.pdf
#5
Quote by Nutter_101
That's a lot of info to get my head around . Thanks though! I'll look through it and make changes!

Just in case you wanted to see, what I've done here is just mixed these two diagrams (probably incredibly crudely, in fact, I know it is).



and



(and the board version of the first)



In your opinion, do you think pursuing the second design will work better for an onboard preamp than the first?

Edit: Oops, forgot to put the power supply section into the second design... .
Hey, did you test this layout?