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Old 4th February 2014, 09:15 PM   #1
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Default DIY Class A BJT Guitar Preamp problems

Hello,

I am currently building a preamplifier for guitar, as you will see from the schematic in the end it will use a valve and solid state pre blended together. However currently only the solid state pre, summing mixer and output are built, so the valve section can be completely discounted.

I have the circuit built on breadboards and have it running off a +-17VDC PSU. All the DC voltages in the preamp section have been checked on my multimeter, and have checked op amps for any DC offset, all seems good. The circuit basically seems stable.

However! I have tried plugging a guitar in and plugging the output into a small solid state guitar preamp and all I'm getting is a high pitch frequency out of the amp, that can be varied if the output volume pot is turned- I would guess this must be a circuit oscillating somewhere I would guess. Maybe the input impedence is too high?

I am also having trouble with the circuit picking up interferance from radio frequencies, I'm thinking maybe this is a sign of bad earthing, or a loop somewhere. Also when I touch the pots or guitar strings it can create hum, again this would seem like an earthing issue to me. The PSU maybe?

Any thoughts are very much welcomed, I know a little more than the basics but by no means an expert!
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Old 26th February 2014, 05:31 AM   #2
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I'm surprised no one has replied. Unfortunately, the schematic you posted has problems in almost every area. The mixer and tone circuitry looks ok at a first glance, but I see problems with just about everything else. The worst error I can see is that the paralleled transistors have no DC path to V- or even ground on their bases. Despite that, I can't understand why parallel transistors are even used there. Paralleling helps with noise or heat/current but neither reason makes much sense in the middle where paralleling is used.

Also, breadboards can become unstable and/or rf sensitive with ease. A proper prototype often relieves woes that were intractable on a bread board.

I hope you the best with this project, though. The first thing to add is a resistor to ground or V- from the bases of those ssm2220's. 10k-50k-ish seems about right.
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