found a TL081/LM741 preamp schem....

[onboard]

...drawn by Bob Blick that may be inspiring. A trim pot or off-board pot at the 1M resistor between pins 2 and 6 would be cool, or a volume control at the end. Or splicing the whole thing or parts into any one of a zillion other circuits  :wink: Did anyone build Donnie James' low battery indicator yet btw? I'm still working on messing with this preamp...

[onboard]

The truth is, I tried building this and it's dead but was embarrassed to ask for help de-bugging it since it looked fairly simple. Posting the schem first to see if there was any interest seemed like the thing to do.

Maybe I used the wrong LED's, and the voltage at the IC is wrong. Or just a stupid mistake somewhere on the board. The chip in mine is an NTE857 which crossed to an LF351 which is supposed to be an upgrade to the noisy LF741. The pinouts are all the same. I can post IC pin voltages if anyone's game.

[Robin]

Hmmm. I can't remember seeing series LED's in a voltage divider before. What are you getting for a bias voltage at the + input pin? And the rest for that matter. What value are you using as feedback resistor Rg?

Rob

[onboard]

Hey Rob, thanks!

Rg is a 1M for max gain. Pin voltages are (with my battery dying..)

1) 145.5 mV
2) 3.79 V
3) 3.48 V
4) 0.10 mV
5) 144.6 mV
6) 3.79 V
7) 7.92 V
? didn't test

So 3.48 at the +input pin. All the ground points and +9V points are showing good. I did notice that if #4 pin was not grounded, I get unity gain for about two seconds, then the signal fades to nothing. My first build had 2, 3, and 6 pins in the 7V ranges (I think) and I thought that was way off so I scrapped it and am getting 3-4V range now with this second board. I really ought to learn the math....

-Ryan

[David]

Ryan:

That was the first circuit I tried to build about 2 years ago.  I failed utterly to breadboard.  The bit with using LEDs to bias is odd, but I guess it would work.  Anyway, I'd like to know if you pull it off.

If you get pi**** off with it, there are MUCH easier preamps to build.

[onboard]

Uhhhh, I didn't realize my #8 in the pin list would show up as the sly guy .

David- I thought the same thing about the LED's for bias. Looked cool to try though.  Maybe I'll ask Bob....then again, maybe not. I'll keep putzing w/it...

[Robin]

Well, the inputs differ by a few hundred mV (do you know how to use pins 1 and 5 for offset null?) so I'd first try to replace the LED's with a 2.2K resistor just to see if it works. These are pretty low values for a divider though, I think.
Did you breadboard this or make a board? If you can, try replacing the opamp with a known working one. Check the datasheets but most single opamps have the same pinouts. Half a dual like a TL072 or a *4558 should work, too.
Check GEOFEX for the Audio Probe. It's very easy to build and will allow you to trace the signal path from front to back (input to output?). Check that site and here for debugging tips, also.
That said, the LED's may be cool looking but I think you'll find many simple and probably superior preamps by searching here and checking the links. Gus's NPN Boost beginner's project forum posted here makes a good, if not pristine, front end and many FET, MOSFET, bipolar and opamp buffer/boost/preamp circuits work great as building blocks for your own pedal design.

Rob

[humbuck]

Dose this work with the UA741 opamp?

H.

[onboard]

Thanks for the tips Rob. I built it on perf-board, so... The opamp is new out of the package, so my guess is it's functioning. No, I don't know about using pins 1 and 5 for offset null. Greek to me. I'll try the 2k2 in place of the led's and see what happens, but it looks this circuit is headed for the scrap heap.

humbuck- try a web search for semiconductor cross-reference, that's how I arrrived at the NTE857 from the LF741. A quick call to a Parts Express tech helped too, they were out of both the LF741 and it's upgrade, LM351.
There's also a downloadable IC database out there with most known types (transistor, opamp, fet, jfet, mosfet, etc.) and their pinouts, substitutions, and other info. I can't find the link off-hand but if I do I'll post it in a new thread.