Langevin AM GC 17

hello all!

Though I've been following this forum for some time now, this is my first post. To start, thank you all for this wonderfull source of information and inspiration. So far, my diy has been limited to racking a pair of telefunken v672's (with much help from the relevant threads here). With that project succesfully behind me, i'm gearing up to try a few of the gyraf projects...but in the meantime, i recently came into this:



My searches on this forum and beyond have turned up a wealth of info on the Langevin Am16, but next to nothing on the Am17 or this guy, the AM GC 17. Best I can tell, the GC stands for gain control, and judging by the looks of things this is probably a program/monitor amp of some kind. Does anybody here have any further info? schematic? pin out? Is this worth doing anything with...I'd imagine the transformers are worth using, if nothing else?

Thanks!

david

IIRC it's essentially an AM16 with extra output stage. Good for about 57db of gain. No schematics but I think the Audio Cyclopedia has the schems for the non-gain control version.

This is all I have, altough I think someone had a schemo not long ago here.

I have the schem which I can maybe scan later tonight but cant host it anywhere at the moment. anyone?

dave

I can, if nobody else finds theirs, PRR had one, didn't he?

wow-

thanks for the quick replies. y'all rule!

Soundguy- I'd be happy to host the schematic if youd like to mail it to me - "david at assassins.com"

in the meantime ive been studying the various threads on the am16 - judging from the identical language used in both manual descriptions, i'm guessing the 16 and 17 are pin compatible?

david

thanks guys-

the first schem. posted above seems closest to what im looking at here, though there are a few additional capacitors and what i think is a pad before the input - thus "GC" i'd imagine.

im learning a lot trying to figure this thing out - thanks again for the help - now to see if it works!




by the way, i have the accompanying psu for this thing, the ps222, with the schematic if anyone is interested.

david

Great thread, I just picked up a couple Langevin AMGC17 too. Yeah, I too found little info out there re: these. What a coincidence.

I'm not racking mine up quite yet, but am interested to how things go with your modules. I'm staying tuned...

so, i've gotten the gc17 up and running on the bench and it sounds quite good, but.....there are some issues

the gain control functions, but provides an ever increasing amount of hum as one turns it toward maximum attenuation, i.e. zero on the knob. at fully clockwise there is no hum, just some signal noise. any ideas?

also, ive currently got it strapped for low gain, but the psu is getting rather hot. considering that the ps222 provides 3 amps of current, which should be ample for 1 am17, is this indicative of a problem somewhere along the line? the same psu, when used to test out 2 v672's, didn't get nearly as warm.

thanks!

Even at low-power these things will draw 1 amp. Way more current than the am-16.

Have you gone through the power supply? Maybe it's just tired out and needs a capi-otomy?

i think you may be on the right track with having me check out the psu, Miko. i opened it up and things do indeed look a bit scary in there. i don't, however, plan on using this supply if and when i get around to getting this properly up and running, so i'll see how things go with another psu before i try and repair this one.

anyone have any idea what might be causing the ferociuos hum in the gain control?

if you find out, let me know, I have the EXACT same problem with mine, I quickly hooked them up and wrote it off to not have a good ground installed, maybe its something more than that.

dave

this is, if i've looked at it correctly, the circuit that controls the gain (this is my first bit of reverse engineering, so forgive any schematic drawing conventions i've gotten wrong). im not sure of the values of the dual gang potentiometer/rheostat.



is this a type of pad? or something else?

I think its a t attenuator.

dave

cool- now i know what a T attenuator looks like

is there anything in the attenuator itself that would cause the hum? could the potentiometer be somehow faulty?

That can't be right:


As you see: the input signal always flows full-strength to the transformer, no attenuation.

Trace the wires again. "Verify" that the input pins go right to the transformer without flowing through any other part.... I suspect you will find they don't, and that will clear-up what is really going on.

I suspect it could be a T-pad, but it could be a O-pad, or something else.

Don't worry how ugly the sketch is; your first dozen or hundred tries are always pretty confusing. When in doubt, draw it as "picture" not as "schematic", and just be sure the wires go to the right pins on the various gizmos. Then we can try to untangle it.

PRR- thanks for the encouragement! i got so caught up in trying to draw it well, that i didn't stop and think about what i was looking at, once i was finished . a dozen or so scetches later, this is what i have:

What´s the value of the dual pot?

im measuring 40K ohms across it - there is no indicated value on the pot itself.