... I get dumped back at the starting line again!
Carp! I'm trying to breadboard a preamp. I'm using a TL071, with bias provided courtesy of a 386 per R.G. Right now it's a buffer (no gain), and it hisses like crazy! Could that be because the input coupling capacitor is a ceramic? If so, is the appropriate capacitor actually one of those "Chiclet" caps?
I have found that I get a lot of hissing if my ground connections aren't just right, from input and ouput... maybe check those?
a) Do you NEED to use a 386 for providing a Vref? Most simple op-amp-based preamp stages are perfectly happy to use a traditional equal-value-R divider.
b) Bifets CAN be quieter when the resistance values tied to it are high, but are not ALWAYS quieter. See the op-amp comparison in DEVICE at http://hammer.ampage.org (scroll down a few pages to near the end)
c) A preamp for what? If it is a preamp for a direct guitar signal, then you don't even need the input cap since there is no risk of DC voltage coming in that needs blocking. Anderton had a nice revised guitar preamp design in DEVICE also that took advantage of this aspect for lower noise.
Mark:
No, I don't need to use a 386. The only reason that I did is that I didn't get 4.5 volts out of my 1M resistor bridge voltage divider. I figured that had something to do with breadboarding not matching the real world. I ran a 1M from +9V to a row on the breadboard and another 1M resistor to ground. The input coupling cap and pin 3 from the op-amp also connected here. Nothing happened. Next I tried just feeding 9V to the bridge and checking the voltage between the bridge and ground. I got something like 0.2V. Obviously something wrong here. That's when I switched to biasing via a 386.
This is ultimately intended to be an acoustic guitar preamp.
I very carefully scanned DEVICE issues 1 through 5 looking for that comparison in the front half of each issue. I'll look again.
Gentlemen:
I thought I had read on this forum that ceramic caps are a potential source of noise. Would I be better off with tantalum or film as an input capacitor?
I would quickly blurt out film caps to that question, but it depends on the value of the cap. If you wanted a 1uF or less then film is the only way to go for me. If you want a larger cap then tantalum might be more of what you're looking for because of size and availability. Both are low noise in comparison to ceramic.
Take care,
-Peter
The schematic specified a 0.1. I'm getting so much hiss it swamps the input. Not a good thing for a buffer. No point in amplifying that...
LORD is it nice when something works!!!!
Though I was feeling incredibly frustrated and very stupid yesterday, I kept on plugging. This morning, I got a working preamp (which makes me feel really dumb because I've done this circuit before!!).
It helps if you:
Make bloody sure your input impedance is high. Turns out the "input" or "pulldown" resistor I was using was only 150K. Misread the stupid color bands. :oops: I need to take a page out of Pete's book and measure all the time!
Don't try to measure Vbias on a voltage divider all by itself.
Right now, I feel a lot better. My goal (maybe I should call it a quest because I'm beginning to feel like Don Quixote) is to make a minimal parts count acoustic guitar preamp that has more gain and sounds better than the stupid Barcus-Berry preamp with one op-amp and a volume control that I paid about $150 for in 1985! :evil:
Next on the list are to:
Put a passive tone control on it -- maybe the BMP control
Put some power filtering on it so it doesn't squeal when I power it from a wall wart
I know this sounds incredibly lame to those of you with high build rates, but just let me gloat for 5 seconds.
OK, I'm done.
Hell, it works, you're happy, and you've learnt something. Sounds like a successful build to me, whatever the difficulty level ;)
One thing i aways do on Bread board is forget to ground my input and output jacks....
always smack my self on the forehead