Opamp booster problem/question

[rubberlips]

G'day guys,
A quick question. I've built up this booster buffer board to put inbetween my preamp and power amp because the output of the preamp isn't enough to run the poweramp to full.
http://img.photobucket.com/albums/v680/rubberlips1/outputbooster.bmp

Anyways, it does work, but when I turn the booster trim up to about 1/3 turn (about 30K), there's no signal at all. Turn it back slightly and it boosts the level about 3 times which isn't too bad.

I've included a bit of the output of the preamp, the booster I made and some of the input to the poweramp.

Does it have something to do with the gain of an opamp when you're using the noninverting terminal?
Any ideas?

Pete

BTW the opamp is a TL071 and the power rails are +/-15V

[Rob Strand]

First check the DC level at the output of the opamp as you rotate the gain pot.  There may be a problem with the pot, or the opamp, or the power supply.

Also check the orientation or any electro's - if you are using any.  Done tule out one could be stuffed, say the 22uF on the input.

Putting that aside try adding a small cap from the opamp output to the -ve input, ie. across the outers of the por - around 33pF.  That will prevent any oscillation issues.

Also try adding a 220 ohm to 1k resistor between the opamp output and the 4u7 cap; this can also prevent oscillation problems.

There's nothing wrong with that type of ckt driving the power amp - done it many time....

[rubberlips]

First check the DC level at the output of the opamp as you rotate the gain pot.  There may be a problem with the pot, or the opamp, or the power supply.

Rob there's a big voltage on that output, about 12V and when you increase the trimmer it goes to the rails, but....

Also check the orientation or any electro's - if you are using any.  Done tule out one could be stuffed, say the 22uF on the input.

Bloody caps back to front , I'll fix that and se ehow it goes. That caps actually on the output of preamp.

Putting that aside try adding a small cap from the opamp output to the -ve input, ie. across the outers of the por - around 33pF.  That will prevent any oscillation issues.

Also try adding a 220 ohm to 1k resistor between the opamp output and the 4u7 cap; this can also prevent oscillation problems.

There's nothing wrong with that type of ckt driving the power amp - done it many time....

I'll see how the first two go and then try the other suggestions.

Thanks

Pete

[rubberlips]

Rob,
Gave it a whirl and fixing the cap up has fixed that DC offset problem with the opamp. Now that I can crank the trimmer, gee there's some background noise with this.
I still think there's some sort of ground loop with this setup I have. The only way around it I can see is make the poweramp and preamps seperate. Running it off a different power supply seems the only thing that fixes it

Pete