Technical question for the techies . . .

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chessrock

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What is Oscillation?

I was thinking about replacing/upgrading the opamps in a few of my mic pres.

However, I am a bit leery, since I have been warned to look out for oscillation problems.

So anyway, I was just wondering if there was a simple, non-technical way of checking for oscillation once the chip is replaced ? ? Does oscillation produce an unmistakeable sound that I should be able to hear right away? Or is it a subtle thing that I should use some sort of test equipment for . . .

If it is oscillating, how can I correct it? Should I just have a couple of extra replacement amps lying around and try sticking another one in there if the first one oscillates?

Should I just cut my dumb ass out of this and leave the job to a professional? :D
 
This is a good question that I would like to know the answer to as well.
 
I'll email / PM NWSoundman (or Tom Cram?), and see if I can get him to reply.
 
Hey thanks, arcaxis.

My reasoning is strictly for better sonics. I don't know how much perusing you've done on Rec.audio.pro, but Monte McGuire posted instructions for a rather popular mod for the Symetrix SX202 mic pres. Here's part of his quote:

"As stock, the SX202 output stage uses some variant of the 4560 dual bipolar op amp, usually either the Exar XR4560 on older units or the Rohm BA4560 on later units, the Rohm version being especially repugnant. At best, these amplifiers are somewhat slow, and this adds excess distortion above 3KHz or thereabouts. The Rohm devices seem to be marginally unstable in addition to being too slow to pass audio cleanly, and this causes the HD spectrum to be littered with many high
order harmonics. The end result is an overly bright and hard sound, solely due to the 4560.

As of this writing, the best possible replacement chip is the new
LT1469CN8 from Linear Technology. It is a dual, low noise bipolar
input op amp that is remarkable in that it has extremely low
distortion, excellent output drive and a very high gain-bandwidth
product. It is unity gain stable and it can operate from high supply voltages, so it is a simple drop in replacement. The only drawbacks are cost and availability. "
 
ocillation is like when you hit the harmonics on the 5th and 7th fret.. when the strings are out of tune the .. wah_wah_wah that gets faster or slower is the oscillation.

Also If you huff nitrous... oscillation.
 
Hey Arc,


I know this is going to seem like a real obvious question . . .


. . . but from what Monte seems to be describing, the "nasties" he's refering to seem to be coming from the output amplifiers.

So the question is what if I were to just use the input gain and make sure to leave the output gain alone and left at Ze-ro ? ?

:D

Do you suppose the signal would still pass through them and pick up all that high frequency distortion Monte's talking about?

Thanks for your advice.
 
Just make sure you add chip holders rather than directly soldering to cover all the duff components and mess left after a few aborted attempts
 
chessrock said:
What is Oscillation?

I was thinking about replacing/upgrading the opamps in a few of my mic pres.

However, I am a bit leery, since I have been warned to look out for oscillation problems.

So anyway, I was just wondering if there was a simple, non-technical way of checking for oscillation once the chip is replaced ? ? Does oscillation produce an unmistakeable sound that I should be able to hear right away? Or is it a subtle thing that I should use some sort of test equipment for . . .

If it is oscillating, how can I correct it? Should I just have a couple of extra replacement amps lying around and try sticking another one in there if the first one oscillates?

Should I just cut my dumb ass out of this and leave the job to a professional? :D
Click to expand...

In modern Op-Amps, oscillation happens usually in the 100Mhz-200Mhz range. This is in a negative-feedback design. Oscillation happens when the reactance within the feedback circuit approaches zero. In other words, the Op-Amp "runs-away". In a posative feedback design, the frequencies of oscillation are much lower. The good news is that no amp design in audio uses the posative feedback design. You are extremely unlikely to have oscillation by changing Op-Amps. Audio frequencies are really quite low for today's Op-Amps. To check for oscillation, you must use a frequency generator as the input and an O-scope to look at the out put. Sweep the generator form lowest to highest frequency and watch the scope. The "nice" sine wave will look like it is quadruple the frequency when you approach oscillation. The only way to fix oscillation problems are to calculate the feedback resitor/capacitor combonation to make sure that the Op-Amp's "zero reactance" points are well out of range of the audio frequencies. See, that was simple........
 
Re: Re: Technical question for the techies . . .

Oh Yeah, I would suggest if you are going to try switching Op-Amps that you put in IC sockets. This way you can swap Op-Amps quickly in case you have oscillation problems. Just check the manufacturers spec sheets for the old and new Op-Amps. If they are compatable, you are looking for a difference in "slew-rate"=faster, and CMRR = lower. These are the two parameters that will make a real difference to you.
BTW: Oscillation problems are very hard to find in a complex waveform like music. Usually, you will hear what sounds like distortion at the offending frequencies. The waveform generator will nail down the exact frequencies that cause oscillation. If the design is solid, there should be no poles anywhere near the audio frequencies and stepping up to a better Op-Amp should be easy.


Good Luck.
 
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