faulty opamp ?

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zilver

Member
Joined
May 15, 2006
Messages
5
Hello,

I have a soundfield microphone, and the processor box has a small problem :
After powering up (it had been off a few weeks) there was a lot of noise and distorted sound.
After switching on and off the problem was gone, but the next day it did that again. It seems that it disapears when the unit has been on for some time...
I found the opamp that seems to be the trouble. When the unit is in "faulty" mode it measures about dc 15 volts at its inputs.
Also when you touch the opamp it crackles louder.
Would you think the interstage capacitor is too old or the opamp is broken ?
My guess is the capacitor because i cannot get the unit too fail anymore since it has been on a few days in a row.

Very annoying because this way it cannot be repaired.

What would you suggest to do ?

BTW the unit is smd so that's another problem. It's easier to swap the capacitor than the opamp.

All suggestions are very welcome.
Thanks in advance, Hans
 
If it crackles when touched, my guess would be bad soldering - try re-soldering the opamp and related parts.

But yes, it could be the coupling caps as well.. Or even a dirty switch - Soundfields have a history of getting scratchy switches in the control box..

Jakob E.
 
I would check the solder connections at the chip and caps than have the problem.

edit must have posting at the same time.
 
[quote author="gyraf"]If it crackles when touched, my guess would be bad soldering - try re-soldering the opamp and related parts.

But yes, it could be the coupling caps as well.. Or even a dirty switch - Soundfields have a history of getting scratchy switches in the control box..

Jakob E.[/quote]

I didn't write precise enough : It crackles constantly, but this crackling gets louder when i touch the opamp.

Also, when not in "faulty mode" there's no dc Voltage at the inputs.....

Hans
 
Whuch model soundfield? -I've worked on a few in my time...

Soundfields.jpg


I've had a couple of faulty op-amps (833, 339 or 399... it's so long ago I don't remember...) in the MkIV, but never in any other versions, so perhaps yours is a MkIV?

Other than that, there are ways to narrow it down to see if the problem is occuring at the A-format (mic output) level, or the B-format level...

More info, and I can probably steer you towards an answer, I hope!

http://img.photobucket.com/albums/v379/SSLtech/Soundfields.jpg
Can you explain what you mean? -Which inputs? Opamp inputs or mic inputs...? Do you have a schematic? -Let me know the model and I'll see if I have one.

Keith
 
It's the sps 422 (the newer model, but it has no B format outs like the newest).
The DC is at the inputs of the faulty (?) opamp.
I didn't come to listening through the signal path, because the unit now keeps on working.
But i have it opened with a headphone amp and a test pen lying next to it in case it breaks down again.

BTW, the mic sounds even better (in mono mode) when you listen to only one capsule (without all the mixing in the unit).
Maybe i solder a direct out for speach recording....

Hans
 
As long as the DC is the same on both inverting and noninverting inputs, there may not be a problem, but you could of course put in a nice quality (turned/machined pins, not pressed or stamped) socket (it's not surface-mount, is it?) and try some replacement IC's... Look for cracked traces or intermittent solde joints from any rough handling...

The single capsule outputs will basically be like a Calrec CM1050. The microphone front end has the fixed-gain preamp and a balanced driver in it, which is different to the CM1050... I think the CM1050 is much better for this use :wink:

Anyhow, don't forget that the delay compensation and equalisation happens in the processing section, which probably accounts for a lot of what you are hearing.

-You want to trade for a pair of CM1050's? :wink:

Keith
 
[quote author="SSLtech"]As long as the DC is the same on both inverting and noninverting inputs Keith[/quote]

It is/was not the same

[quote author="SSLtech"] (it's not surface-mount, is it?) Keith[/quote]
It is :sad:

[quote author="SSLtech"]
Anyhow, don't forget that the delay compensation and equalisation happens in the processing section, which probably accounts for a lot of what you are hearing.
Keith[/quote]

I know, but my english isn't good enough to write that down short en simple.

[quote author="SSLtech"]
-You want to trade for a pair of CM1050's? :wink:
Keith[/quote]

No, sorry i'm very fond of the microphone and was only thinking that if i use it mono (i do that sometimes) it would sound better if i made a direct out from one of the capsules.

Thanks for the reply,

Hans
 
What type (model) is the op-amp? if the DC at the two inputs is different, then you are either correct, and the op-amp is not 'servo-ing' correctly, or it is possibly being fed a signal outside of its range of correction -either following a faulty stage or sometimes because the output is loaded down and unable to swing "beyond the rails" in order to feed-back the required correction.

Either way, good luck with it... -Where are you? -I just sent an ST250 to Belgium... Surface mount isn't all that scary, certainly you need decent soldering skills, good eyesight and patience, but not much more.

Keith
 
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