Merlaud Cassettes, unbalanced output acts strange.

Hi there!

About 2 years ago I bought an old Merlaud mixer (french from the 70s).
All modular, 4 channels summing (2 to 1) and (2 to 1).
After struggling to get a decent output from the summing modules, I finally decided to size down the frame, keep the PSU and the 4 input cassettes wired straight to jacks instead of the busses.

I know it's not a cool thing to do, impedances are probably screwed, but here's the catch:
If I plug the ouput into the unbalanced input of my Fostex 6301B powered monitor, I get a healthy level, it seems fine.
If I plug this into my Avid interface, the level is crap, I actually need to plug it into a micpre with about 30dB gain to get something useful.

Could it be just an impedance problem?

I haven't traced the merlaud module but here's the Fostex manual with schems:
http://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CCcQFjAA&url=http%3A%2F%2Fwww.fostexinternational.com%2Fdocs%2Fdownloads%2Fpdfs%2F6301b_service_manual.pdf&ei=BmhQUO3LIoa-0QWv54CQBQ&usg=AFQjCNGZ6wXmgNRt1D-10kU8S76tnmryGw

..could very well be that your fostex likes -10dB and your Avid wants +20dB - adds up to some 30dB difference...

Jakob E.

Hmm makes sense.
So I need to add extra amplifiers in the mixer.
Can I amplify 30dB AND buffer the ouptut with a single opamp?
I like when things are basic

It operates on a single 35V rail by the way.

30dB is 31x amplification. Possible with e.g. NE5534. you'll need a ½V+ as reference to have them work on single +35V.

This looks like it could do the job for 2 channels:

How about this? (without eh diodes and cap in the feedbackloop of course)

the opamp-based circuit will probably be fine - use e.g. LM833 or NE5532 for opamp.

Transistor circuit is less usable - quite high (and variable) driving impedance - good for guitar amps, but not for high-level converters.

Jakob E.

Ok, i'll try the IC circuit.
I cant find the values intended for that schematic, i'll need to calculate them.
The only 5532 circuits I have to compare have dual rail PSU, how does this work?