eliminating noise in mixer?

[slotbot]

Hi

A while ago i built a simple mixer similar to the one fouind on the schematics page. basically an op amp mixer.

Even with all the volumes down (off) there is still a bit of white noise coming through the line. I want to make another one and would like to avoid this problem. any suggestions? i think in the one i made i used a tl062 dual op amp. are there better op omps that would elminate the noise or addition of filter caps or???

any help is appreciated.

scott.

[Joe Kramer]

Hi!

That TL062 is the immediate culprit--a noisy op amp for sure.  A quick fix would be TL072.  A much quieter but more power hungry fix would be 5532.  Here's info from a previous post of mine:

The TL062 has a noise figure of 42 and a slew rate of 3.5, the TL082 has noise of 25 and slew of 13, and the TL072 has noise of 18 and slew of 13. The 072 will be quieter than the other two, but draw more juice than the 062 and about the same as the 082. I'd get them all, put in a socket, and pick the one that sounds best to you.


Op amp specs to look for:

Current draw or Icc, expressed in mA, the lower the better for battery powered effects.

Noise figure, sometime NF or Vn, expressed as nV/Hz, usually the lower the better, but it depends on gain and impedance.

Slew rate, or SR, expressed in V/us (volts per microsecond), the faster the more "transparent" or "neutral" especially regarding high freqs, the slower the "darker" or "warmer."

And of course the last spec is: sound! In the end, ditch the specs and go with your ears!

Hope this helps!

Joe

[slotbot]

hi

ok i changed it to a tl072 but i am pretty much getting the same result. i will possibly draw up a schematic and post it maybe that will help more.

its not radia interferance or anhting its liek a white noise. i dont care so much about fixing this particular unit but i am making another unit with a mixer as part of the deal and i dont want there to be this noise in that one.

thanks

scott.

[Joe Kramer]

Hi!

If it's not the op amp, it may have to do with your gain structure and impedance.  There's a happy medium for input impedance where it's high enough for guitar and low enough not to add too much noise.  But too low and you'll lose highs and volume and have to crank the gain (more noise), or too high and you'll get more noise again.

If it's a typical mixer with two inverting op amps in series, it's a good idea to add a small amount of gain at both stages rather than a lot in only one of the stages.  Also, the second stage can have a lot lower impedance (around 10K) because it's being driven by the first op amp.  This saves somewhat on noise.

One Last point: with mixers in general, it helps to make the individual inputs as high as possible (within reason), and the master output as low as possible.

Hope this helps.

Joe