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Bass guitar preamp

radix

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Joined
Aug 1, 2021
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I need a preamp for my bass guitar. I have to return my Line6 StompXL box and wait for the RMA. I was using that to up-convert to line level. So I need instrument level to line level (about 33dB gain, as measured on the input of a Motu ultralite mk5). Here's a little circuit I think will do the trick. Any suggestions or pointers? I'm running this into a denon AVR and my HT setup for now, until I build my bass cabinet. I could buy the Behringer BDI21 DI for $30, but this seems more fun.

I'm guessing that a 56k input impedance is OK for a passive bass pickup. or do I need a very high input impedance, like a mic capsule?


schematic.png


I prototyped this out with parts I had on hand. This has a fixed 27k resistor, not the 50k pot. And i was not so precise with all the resistor values. I also didn't have a 20uF cap, so I used 2x 10uF in parallel. When I get the case, I'll add a 1/4" instrument in and RCA out, plus the gain control.

IMG_0044.jpeg


Right now, the power supply is my bench unit. I'll need to do something with say a 12VDC power supply and I guess a virtual ground regulated supply. I was thinking of a LM317 set at 10V then use cross-coupled 7805/7905 with 1R output resistors for the virtual ground. Or maybe a DC-DC 12V to +-9V, then 7805/7905. Or even just the LM317 @ 10V w/ a voltage divider. It seems like the split rail power supply is the most complex part. hopefully the case will help with the 60Hz too, though I doubt anything could be worse than the alligator clip spaghetti I tested with.

IMG_0041.jpeg



bass-preamp-thd.png


bass-preamp-thd-vs-freq.png
 
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