experiencing stability problems in my simple mic preamp

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Hello!

I'm experiencing stability problems with my simple microphone preamp. The preamp is meant to be very very simple and small. I tried several dual op-amps (unlike my circuit the schematic is single channel for easier review) and they all got the same stability problem but to different extents. It only appears on one channel (on the circuit's picture it's the right channel): the output lowers it's volume and distorts, while there are sounds of several frequencies appearing. Touching the circuit or loading the mic with a loud signal can make the operation normal (on some op-amps (OPA2228 is most stable here)), but the problem may come back after some seconds.

What could be the reason? I tried those op-amps: OPA2228, OPA2227, MC33078, LT1124, LT1126, Max412, RC4580
The gain can be set by selecting R2 via jumper for a gain of about 28, 80 and 180. (on the upside where there's a blue shunt). The circuit was tested in an unshielded case. Oh, and yes, most resistors are SMD and soldered below the board.

Any input will be much appreciated!
Cheers!
Dominique
 

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i would look at joints, endcaps, resistor bodies... under high magnification and verify correct connectiivy ( and the important lack of adjacent shorted paths) with ohmmeter

hand soldered smt parts can be damaged in several visually subtle ways, plated terminations detach, ceramic bodies crack, excess flux chars and forms shorts/leaky paths

high z inputs can drift and give intermitent operation

input rf filter cap per greg's suggestion may help stability but 100 nF is too large for the feedback cap - requires unity gain stability, extends response to frequencies where your proto's impedance may not be so well defined, parasitic C should be fine at the high gains used, at most add a few pF directly across the op amp pins
 
When I have designed mic preamps (and I used to design mixing desks back 30 years for PA, theatre, and broadcast) I have always used a feedback rolloff to bring the large gains back to 2-5 times the minimum stable closed loop gain. This ensures any stray out of band pickup is not passed on to subsequent stages/amplifiers. It worked well.

In this case 100pF //25K will rolloff from 63KHz and -1dB at half that. Ideally then a, say,470ohm in series with the feedback 100pF would limit minimum gain to +14db (x5) and contain the hf output stage loading to 600ohms. A larger than 470, say, 1K would limit to above 1K with >10K ext load.

Cheers,
Greg
 
at high freq the feedback C forms a divider with the - in terminal C, typically single digit pF, so a high freq gain of 5 will limit feedback C from a fraction of a pF to just a few pF

at Av=180 bandwidth is only 44 KHz with 8 MHz op amps like the 227 or 2134 - genuinely faster (possibly decompensated) op amps or composite amps are really needed at this gain level to permit active filter feedback rather than gbw to define band edge

a low 1st order roll off can have audible consequences (0.1 dB amplitude response variation can be detectable double blind)- the in band response can be flattened by going to a higher order butterworth filter alignment
 
Agreed JCX.

I would be using an LT1028 (75MHz GBW) for this and I'm sure Dominique would rather have -0.23dB at 15KHz (about avg limit of hearing) and no feedthrough hf gain - if not he could reduce the feedback C to 47pF! I doubt he wants to bother with an extra active filter stage.

Didn't realise we were finessing a prod'n design - just looking for things that may affect his oscillation or stray pickup problem.

Cheers,
Greg
 
thank you for all of your suggestions!

I tried a bandwidth reducing cap, but it didn't help!
Then I realized I only checked my circuit with my eyes. When I did with my meter, I found a bad soldered connection at an SMT resistor - while it looked perfect by view!

Now my circuit works great and I'm sorry I made you think that hard, but I saved this thread to disk to get back to your suggestions when I possibly get real stability problems!

Best wishes to all of you!
Dominique
 
mastertech said:
dominique learn to solder properly!!!

I'm still trying to improve, but I'm really glad I am able to solder these things at all! I made a photo and finally, it's not THAT bad :)

amplifierguru said:
Well that's great it's sorted Dominique! I hate those SMD resistors - especially the REALLY small ones! Fly specks.
Cheers,
Greg

Me too, 1206 size is still ok, but as I tried to make a very small circuit, everything was pretty near to each other!

Cheers!
Dominique
 

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