Dang it! Just when I thought I was getting this...

[David]

... 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?

[Triffid]

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?

[Mark Hammer]

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.

[David]

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.

[David]

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?

[Peter Snowberg]

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

[David]

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...

[David]

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.

[smoguzbenjamin]

Hell, it works, you're happy, and you've learnt something. Sounds like a successful build to me, whatever the difficulty level

[Brian Marshall]

One thing i aways do on Bread board is forget to ground my input and output jacks....  

always smack my self on the forehead