Preamp Problem

[dpresley58]

I built a preamp for use with a bass guitar from the Anderton book's design section on building your own preamp. Simple non-inverting design with a TL071, built on perfboard using an 18v supply. I put a switch in the power supply ground to shut it off. The board ground is used for the supply ground as well, and then taken to a common ground with the pickups.

The gain wound up at about 4 and all was well - clean tone, plenty of headroom. So I started to box it up and turned the switch off to start doing so. After about a second or so of silence, a -loud- tone started going through the amp. Not squeal; tone.

I can't figure out why this is happening. Any ideas?

[Mark Hammer]

Sounds like something might be touching the case now that it's installed.

[dpresley58]

Thanks, Mark. I'll check it out, but I don't think that's the problem. I wrapped the underside of the perfboard in tape to ensure it wasn't, but who knows..? Worth a shot to make sure nothing is touching, though.

Any other ideas?

[soulsonic]

Maybe the input and output wires are too close together. Since it's non-inverting, the jacks should probably be separated by at least 3 inches and should definitely use shielded wire. The tone could be caused by positive feedback coming from the output back to the input - that's basically how many oscillators work.

[dpresley58]

No jacks involved, but a good thought. The preamp is about an inch square and mounted inside the bass' control cavity. The original unit was about as big as a postage stamp and some oscillation did happen. That's what led to another version; larger and with the input/output on opposite ends of the board, and the bipolar supply wires on still another side.

The odd part about this problem is that the tone starts generating -after- the circuit's power is cut. As stated, the unit is powered by a bipolar 18v, with the switch killing the ground between the two batteries.

I'm stumped.

[anchovie]

Replace the power switch with a DPDT and use the other set of lugs to ground the hot lead to the jack when the power is off. I've no idea what is causing the power-off tone, but this would at least stop it from being heard.

[dpresley58]

Good idea, if I'm interpreting you correctly... You're saying the output should be cut off the same time as the power? (didn't understand the reference to "hot lead") I haven't let this obnoxious tone keep going long enough to find out if it'll eventually die out, but this leads me to wonder if the battery isn't being drawn from regardless...