api mic pre amp - low gain fault

[axevictim]

There are lots of excellent 2520 clones available, pricey compared to a 8-pin DIP, but not when compared to an SSL channel.

All of the projects I've built or seen that use DOAs (like the 2520 or JE990) have them socketed. I'd be surprised if yours are not also socketed.

Aha!
Maybe mine is too then.
I didn't try tugging on it and didn't see a socket surrounding the boxes so assumed they were soldered.
As I said, I ran out of time earlier.
Maybe I'll pull it from the rack again and check now...

[MarkusBass]

I didn't try tugging on it and didn't see a socket surrounding the boxes so assumed they were soldered.

On the photo you posted previously you can clearly see that ICs are is sockets in this preamp. But I'm not sure whether switching the ICs will help though. Such problems as you have are rather caused but failed soldering of resistors or by switches. As other guys say, checking the preamp with signal generator could quickly help you to solve the problem.

Mark

[axevictim]

On the photo you posted previously you can clearly see that ICs are is sockets in this preamp. But I'm not sure whether switching the ICs will help though. Such problems as you have are rather caused but failed soldering of resistors or by switches. As other guys say, checking the preamp with signal generator could quickly help you to solve the problem.

Mark

Ok,
In the meantime I have opened it up and here is what I found
Yes they are socketed.
And now I've swapped over channel 3 with the one in faulty channel 4 and the results haven't changed.
I guess that's good news inasmuch as the Api op amps are fine.

I suddenly had a brainwave and wondered if it was just a faulty led array on channel four but having monitored through an armed channel in Logic Pro there is definitely a pre amp problem.
It also does not appear to be linear!
When I set the pre amp gains at noon and set the test tone to give a zero db reading on channel 4 there is a huge discrepancy in their relative outputs

However when the pre amp gains are set to max and the test tone signal is turned down to give a zero db level on channel 4 they each give a very similar output into my soundcard.


The settings are shown in the top of the jpgs

And those results are shown by levels my soundcard are seeing AND what is shown on the Api led readings on each channel.

Any clues there, in the non linear nature of the gain differences?

[Leo_Gnardo]

When I set the pre amp gains at noon and set the test tone to give a zero db reading on channel 4 there is a huge discrepancy in their relative outputs

- - - -

However when the pre amp gains are set to max and the test tone signal is turned down to give a zero db level on channel 4 they each give a very similar output into my soundcard.

The settings are shown in the top of the jpgs

And those results are shown by levels my soundcard are seeing AND what is shown on the Api led readings on each channel.

Any clues there, in the non linear nature of the gain differences?

That's why an early thought was, did someone replace parts, a gain pot for instance? Maybe put in a linear instead of audio taper, or just a different "audio" taper? Could even be a mis match of pot tapers, straight from the API factory. It's nice when the knobs all line up. Sometimes they don't.

[axevictim]

That's why an early thought was, did someone replace parts, a gain pot for instance? Maybe put in a linear instead of audio taper, or just a different "audio" taper? Could even be a mis match of pot tapers, straight from the API factory. It's nice when the knobs all line up. Sometimes they don't.

Indeed. I am a trifle anally retentive when it comes to knobs all lining up.

However, on your suggestion, when I opened up the unit I checked for any differences between channel four and the others and couldn't see any that leapt out at me.
The pots all look the same to me. In fact, if you zoom into my photograph you will see that the pots are all labelled identically.
I guess it could be a mismatch in tolerances between the pots during the manufacturing process.
Could they really be that far out though?

Maybe it doesn't really matter.
I only noticed because I was trying to set up a stereo pair on 3 & 4 and realised the balance was out of whack.
I'd have assumed for something fairly high end and basic, like mic pres they'd have at least got something as straightforward as matching levels correct.
Or is Api's quality control that dodgy?

[eschertron]

Any clues there, in the non linear nature of the gain differences?

It might be interesting to do a simple comparison of preamp gain (pot) position vs output. The levels are equivalent at the fully CW position, so does the level discrepancy become apparent 'just off' from full, or does it scale gradually as the pots are turned down?

[g1]

However when the pre amp gains are set to max and the test tone signal is turned down to give a zero db level on channel 4 they each give a very similar output into my soundcard.

With a constant tone into the bad channel and a good one, you should be able to measure what is coming out of the gain pot with your meter. This will tell you whether the problem is the pot or not.
Is there a schematic or even a block diagram you can post? It would sure help to at least isolate the faulty area of the bad channel.

[mhuss]

The schematic is dead simple, basically an input xfmr, an op amp, and an output xfrm.
http://www.hansenaudio.se/API%20312%20mic%20amp.jpg
To this likely add phase invert and pad switching.