LFlab Posted June 6 Report Share Posted June 6 I have done a few other preamps but I couldn't get my design for a dual lowpass filter to work, so I bought one from Underhill. It is a pretty clever modular design, and that gave me an idea for my own preamp. So these are the first modules, a discrete buffer (jfet input) and a discrete summing amp (actually the circuit of a Moog 907 fixed filter bank module). so these are two buffers for a two pickup instrument, and a summer. There are some through hole components, but most is SMD on the underside of the boards. 2 Quote Link to comment Share on other sites More sharing options...
LFlab Posted June 11 Author Report Share Posted June 11 So, the FET input (schematic from Nuts&Volts article by Ray Marston "FET principles & circuits" https://www.nutsvolts.com/magazine/contributor/ray_marston) works, it does need selection of the FET, because of the variability between FETs of the same type, so I now have one channel which overloads quicker than the other, need to replace that FET. Basspre_PCB.pdf Basspre_SCH.pdf 1 Quote Link to comment Share on other sites More sharing options...
LFlab Posted June 11 Author Report Share Posted June 11 The summing amp needs a bit of work, so I think I'll order a revised board after the holidays. I have done a home etched board for this 15 years or so ago, and I seemed to recall it was working properly, but that is not the case. And after simulation it turned out that it did indeed need a resistor on the collector of Q1 to ground. So I did a bit of creative 3d SMD soldering (photo to follow) and now it does indeed work. This a 5.6k resistor mounted on it's end, and then a smd transistor precariously fitted on top. who says you can't prototype with smd Summing_amp_PCB.pdf Summing_amp_SCH.pdf 1 Quote Link to comment Share on other sites More sharing options...
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