"Flatline" crackling

Started by rosssurf, April 17, 2006, 02:45:45 PM

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rosssurf

When playing thru my new Flatline compressor build, it is very staticy. loud signals will come thru but with static. Quieter signals do not come thu very well. The Vol and Sustain pots do seem to be functioning correctly. Why is this? what have I done wrong?

When looking at the recording wave fom on my home recording software ( Audacity) there seems to be more signal above the middle line and not that much below it. I think these lines are supposed to be a mirror image of themselves.

Transmogrifox

6 bucks says you connected the V/2 reference to battery negative and defeated your virtual ground.  16 bucks says that (if that's not true), I come out 10 bucks ahead if there is a virtual ground or similar bias problem.  Put a multimeter negative lead on the battery negative connection.  Probe with the positive lead to the 10k/10k/10uF voltage divider center.  See if that's actually at 4.5Volts (or near it...1/2 battery voltage).  If that's true, then check the voltage at the + terminal on both op amps.  If the input to the op amp at the input is not near 4.5 volts, then there's the explaination of your problem. 

Whether this is the problem or not--replace that 10Meg resistor to the (+) terminal of the op amp with a 470k or 1Meg (max).  Solder a 10nF cap in parallel with the 1nF (if easier)...or just desolder the 1nF and replace it with the 10nf.  That 10Meg will cause all kinds of unnecessary noise, and may be the source of your problem as the op-amp's input bias current is dragging it either to the 9V rail or the 0V rail.

If you're using a BJT input amplifier (and not the specified TL072), this is certainly your problem.  The 10Meg will only work with a CMOS or JFET input amplifier.

The moral of the story is:  make sure the + terminals of the op amps are biased at 4.5V.   In fact, make sure all op amp terminals are at about 4.5V (or real near half whatever the battery voltage is).  Please see the sticky for information about troubleshooting, measurements and presentation of your problems.  It is reaalllly really nice on  the posts where people have used that format and itemized their problem down the list.  It makes it really easy to pinpoint a problem.

I hope what I have given you helps you work it out, though.  It's a pretty simple circuit without much that can go wrong so I don't think it'll take you too long to figure it out.

trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.