Noisy mixers.

[Andi]

I've been having trouble recently with mixer circutis being rather noisy - in the sense of a constant background hiss. I've built a number to the same schematic but the last few have had this issue. I've tried changing op-amps and tweaking the bias network to a slight improvement but the hiss is still there to some extent.

If anyone could direct my thinking in the right direction I would be very grateful.

VCC/2 is generated via a pair of 10k resistors on another PCB.



Thanks, as always, in advance.

[grapefruit]

There are a few things that should help.

1. Use a low noise op amp like an LM833 (or better)
2. Decrease your input and feedback resistors to 10k and 47k. Do you really need 5x gain. Maybe you could make RF 22k...
If you need a high input impedance for the returns put buffers in there. 10k -50k would be a suitable value for the volume pots.
3. Use metal film resistors.

Someone correct me if I'm wrong here, but R12 could be lower too. If you make the input resistors 10k make R12 10k too.
Also, the 2.2M pulldown resistors after the volume pots are redundant. The pots have a direct path to ground anyway.

Cheers,
Stew.

[RedHouse]

And just to expand on what grapefruit was saying to you andi, the idea here is to lower resistances in your circuit.

Higher resistance networks bring in noise currents.

For example in an M72 console:


the "line" inputs on a single channel board use NE5534A chips, the support components are very much lower in resistance than your schema, try dropping some of the values as grapefruit said, here is a simple example of a low-noise studio line input module:



to upgrade it to a "Neve" psuedo-spec, just plop a good .01uF Poly across each of those 47uF Electro's, and use a better IC.

In the following pic, the amp on the left is lower-noise than the amp on the right, they both have the same gain and use the same chip:




The 4558 chip is a low performance chip (it's hissy, deal with it) or to get better, just go to a better chip for instance the NE5532/34 chips are tried-n-true, or you can go even better with the OPA chips, or even better with THAT chips, it all dependes on your wallet as you can go from a 25¢ 4558, to a $1 5532, to a $20 THAT chip as your budget allows. One thing to keep in mind is that your signal needs to be of sufficient strength to keep above the noise floor, your hiss could be from inadequate preamplification.

Don't expect the 4559 to be a microphone preamp, use something better like a old LF411 or LF353 or something, then go into a 5532 (or 4558 if you absolutely have to)   .


Oh and Yes, the 2M2 resistors after the pots are un-needed here.

[Andi]

I had suspected the bleed resistors weren't needed - thanks.

1) I'm not actually using 4558s - they're just the dual op-amp in my cut-down Eagle library, but that is good advice.
2) I'd never even thought that dropping all the resistances would help - I will try that today. I'd already dropped the gain to 2 which helped, but still gave plenty of boost if needed.
The pots are 47k at the mo, so I'll leave them be.
3) Already using metal films.

I'll get onto changing those resistors now.

Thanks enormously chaps - it really is much appreciated.

[Andi]

I did it - 10k mostly, 22k feedback. The noise floor is definitely a lot lower - thanks again.