How to keep noise down on opamp stage

Started by nisios, September 18, 2007, 03:54:28 AM

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nisios

Well....

Using a gain of 200 i get a lot of noise.

How can i keep it down...or is it impossible with this kind of gain?

Dividinding the work by two stages whould improve it somehow?

R.G.

Getting low noise with high gain is HARD. You must do everything right to achieve this.

The input impedance must be appropriate to the source impedance; the input biasing must be done correctly, and with low noise resistors; part types must be selected with attention to using low noise parts, and the inputs must be protected against transients that can damage them. You may even have to employ input guarding PCB layout techniques.

That being said, it is likely that you have one or more mistakes that make most of your noise.
(1) choose a low noise opamp; the TL07x series is quite good for being so cheap. There are others which are lower noise, but harder to find and more expensive. Using a 741 will make your noise horrible.
(2) use so-called "noiseless biasing"; I have posted this here many times
(3) use metal film resistors for freedom from excess noise
(4) use the lowest-value resistors you can; thermal noise is proportional to resistance
(5) protect your active devices with impedance and diodes so the inputs are never broken over EVEN ONCE; see "When good opamps go bad" at GEO.

It is possible to have low noise at a gain of 200; however, a gain of 200 will multiply both your signal and the noise by 200. The signal will presumably be clipped; the noise will not. So this kind of circuit will inevitably have a poorer signal-to-noise ratio than one which is not clipping because you're throwing away the signal above some level by clipping it.

Dividing the work in two sections will make noise worse. For low noise, no matter what  you do, the first stage determines the signal to noise ratio. Get as much quiet gain in the first stage as possible.

Not knowing what your circuit is makes any more guidelines impossible.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

darron

try looking into the n35534ap opamp also maybe.
Blood, Sweat & Flux. Pedals made with lasers and real wires!

nisios

I cant seem to find noiseless biasing....i just find people saying they have implemented but not how.

Im using bypolar supply so....i dont know.....

This is the schematic:

Do you think i can lower resistors in the feedback even more and go to the less than 1k area?


GibsonGM

Why not run this bad boy on 9 volts, and use a voltage divider to provide a bias voltage?  Maybe I'm high, but it looks like you're actually running the power to this in the "old fashioned way"....by using 2 10K resistors across a 9v supply, with a big cap to ground between them, you can set the bias point (commonly called Vref or Vbias) to operate around 4.5v, which is added to the input to provide a 'center point' for the signal to ride around.   I believe this may be what R.G. referred to.   Look up the schematic for a Tube Screamer and note how that's done - it's much quieter ;o) 
There are good articles searchable on Google, try "single supply opamp biasing" and you'll find some good stuff!
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nisios

This circuit sounds a lot better with at least 18 volts headroom.

Inicially i tried the voltage divider aproax because its allways better to only need one battery to operate a stompbox but the saound was very dull compared to the achieved when i used the power this way.

And i might be wrong but i acctually think bypolar supply is quieter than the voltage divider method.

If not, i can allways make it 18v with a voltage divider(i think this is a bit silly but one never knows).