Identifying an old preamp/lineamp card?

[jazzcrisis]

I came across an old DIY mixer, probably from the 70s. It was built by some engineering professor/pastor for use in his church. Inside is a jumbled point-to-point mess, and it doesn't work, but it contains several of these cards, which seem to be preamps. All they have inscribed on them is "A50." No luck with google.

I can infer that they run on 15vdc, and that the transformer is probably an output tranny, because its pins go directly to an output jack.

Does anyone recognize these??

 

[jazzcrisis]

Here's another shot:

 

[PRR]

I know the cards, but the name is slipping my mind. Maybe Langevin.

There ain't much there. Surely you can trace it in a half-hour.

If it is what I think it is, those are INPUT transformers. Note the shielding.... outputs don't need shielding. The circuit is a variant of the standard 2-transistor phono preamp, but flat EQ. IIRC the power was 30V-40V, and the biasing would be in trouble at 15V. The load for preamp cards was 10K; output cards have a large 600 Ohm resistor.

 

[emrr]

Langevin.  Schem in Audio Cyclopedia. 

 

[jazzcrisis]

Whoa! Awesome. Thanks for the help!

It's cool that they're Langevin cards. The AM16 is my all-time favorite preamp, so I'm going to try racking these things up.

PRR, regarding the voltages + transformer--you may be right, though it could be the guy who built this mixer did it haphazardly. This section of it didn't work, and it''s a real mess inside. (Lots of expensive attenuators and switches to pillage though, as well as big daka knobs.)

 

[analogguru]

I highly doubt that this are Langevin cards.  Maybe they are from an electronic magazine or from a kit.  The transformer is located on the input.

 

[PRR]

> doubt that this are Langevin cards

I agree the build-quality does not look like what I remember.

> The AM16 is my all-time favorite

Totally different designer; or the same designer with very different design pressures.

 

[jazzcrisis]

Yeah, I'm not sure they're Langevin either. I looked around the web I found some Langevin cards that look similar, but fairly different (some pics on Orphan Audio's site).

And yeah, you guys are right about that being an input transformer, but in the mixer I pulled them from, the transformer pins were wired up to an XLR labelled "Output," so I just assumed. No wonder the thing didn't work.

My next step is to check out the audio cyclopedia for the schematics emrr mentioned and see if they match up. I think I know enough about them now to hook them up to see if they're worth a damn, but I'm still interested to find out their origin. I'll keep you folks posted! Thanks for the help.

 

[emrr]

Yep, same form factor as the Langevin, but not a match, sorry.  I haven't seen anything else in that layout. 

 

[crochambeau]

Yeah, I'm not sure they're Langevin either. I looked around the web I found some Langevin cards that look similar, but fairly different (some pics on Orphan Audio's site).

The cards from my AM-301 look closer than what I saw on Orphan Audio, but don't quite line up perfectly either. The 4102 cards have the Langevin logo etched into the copper on the trace face (and lack the massive ground plane):