Trying to identify a circuit -- or at least part of it....

I found an interesting little gadget.  It appears to be a mic preamp mounted on an octal base for plugging into an octal relay socket.
Apparently, someone, somewhere built a modular mixer(?) and this was a spare.  It's clearly labeled as a "mic amp," but there's really nothing more in the way of markings. 

I took the cover off and found a little PCB in there, and I managed to trace out the circuit.  I think I figured out where most of the connections go.  A few of the eight pins aren't used at all.  I think I figured out the input, the output, the power supply, and ground.  But there's one pin that I can't quite figure out the purpose of.  Here's a schematic.....

The intersection if R3 R4 is actually ground. The other currently labelled ground is actually for an AC coupled gain control. Imagine if you just put a cap to ground there. AC gain goes way up.

squarewave said:

The intersection if R3 R4 is actually ground. The other currently labelled ground is actually for an AC coupled gain control. Imagine if you just put a cap to ground there. AC gain goes way up.

Click to expand...

Odd, because (and I should have mentioned this earlier) the reason I labeled it "ground" was because that pin is also connected to the case.  What you're saying about the intersection of R3 and R4 makes perfect sense now that I look at it again, but why would they have an AC coupled gain control shorted to the case?  Maybe the case doesn't actually connect to the ground of whatever this is supposed to plug into.

I donno. That is weird. I guess if the case isn't actually shorted to actual ground then it could still be an effective shield as a low impedance and common mode to the input. But you can't ground the emitter. The thing would drift. There's no feedback. There has to be more to the circuit that's not shown. I don't mean to pee in the punch but that circuit is probably not something you should bother with too much.

Are you sure the emitter of Q1 is not connected to the junction of R3 and R4? It would make more sense if it was.

Cheers

Ian

ruffrecords said:

Are you sure the emitter of Q1 is not connected to the junction of R3 and R4? It would make more sense if it was.

Click to expand...

Bingo.

Altec had plug in octal preamps like this  - the 1588.  They had a tiny transformer and three transistors though.

ruffrecords said:

Are you sure the emitter of Q1 is not connected to the junction of R3 and R4? It would make more sense if it was.

Click to expand...

Actually looking at this again the feedback is wrong too. So emitter should be junction of R3 / R4 and Q2 should be PNP. Then it would make sense.

I double checked the PCB and my schematic is right.  I also checked the numbers on the transistors and they are correct.

I think mjrippe might be onto something.  The top of the cover for this thing has a tiny logo, which I don't recognize, under which are the words "MIC AMP."  Under that is a model number, AL70A.  Maybe this is a transformerless variant of what he posted.

A Google search of those terms turns up nothing.

The two points yu labelled Ground, are they separate pins or are they connected together??
a
Cheers

Ian

You got to do something with that open/floating base.

If Q1 E is grounded, the 3300:180 suggest using the 0.6V at Q1 B to bias Q2 E this way.



Since the new/added Base resistor will Miller-down to 1/16th of marked value, I'd really suspect a large bypass cap too, unless this is a VE mixer (but then gain seems high).

The sum of all the answers so far is: you have not shown enough pieces of the puzzle. It "can" be "something", but only with more bits. "Part of it" isn't enough data.

ruffrecords said:

The two points yu labelled Ground, are they separate pins or are they connected together??
a
Cheers

Ian

Click to expand...

They are connected to the same pin.

CurtZHP said:

I think mjrippe might be onto something.  The top of the cover for this thing has a tiny logo, which I don't recognize, under which are the words "MIC AMP."  Under that is a model number, AL70A.  Maybe this is a transformerless variant of what he posted.

A Google search of those terms turns up nothing.

Click to expand...

Focusing on the historical aspect for a sec, octal plug-in modules were very common in all sorts of gear from many manufacturers. Line amps, mic pres, summing amps, sync & bias for tape units, etc.I have a box full of those things from such manufacturers as Altec, Autogram, McMartin, Ampex, Ann Arbor Systems. Rockwell, Opamp Labs, etc. Snap a picture of the can and maybe we can identify it.

Here's a picture.  Sorry for the wait.  Had a few fires to put out at work.

That’s a new one to me. Was trying to make out the letters on the logo to see if searching by manufacturer would yield something, but “SH”, “SJ” and “JS” (the 3 possibilities to my eyes) doesn’t conjure any audio companies I can think of. I’d be interested to find out who made this.

Maybe someone else will recognize that logo or see some letter combination in it that I don’t.

This one's been bugging me. Here's what I found, just to close the loop on this.

It's a Stancil Hoffman plug-in module for sure. Logo matches:



Documentation is thin, but this SH catalog from 1972/73 shows several modules of Terminal Recorder/Reproducers that use COLOR CODED, octal plug-in modules. This specific model number (AL-70A) doesn't match the plug in modules documented there that I can find, but it shows that such modules were made (in various colors) for Stancil Hoffman gear. 

http://lcweb2.loc.gov/master/mbrs/recording_preservation/manuals/Stancil%20Hoffman%20Price%20List%20(1972-73).pdf

There's probably documentation floating around. Maybe that doc could give you some clues.

it's a start, anyway.

That logo was driving me nuts.

Use this PDF from the Wayback Machine as a reference instead. More models covered, more detail, and in color.

https://ia800804.us.archive.org/21/items/TNM_Various_reel-to-reel_equipment_from_Stancil_H_20170801_0351/TNM_Various_reel-to-reel_equipment_from_Stancil_H_20170801_0351.pdf

rackmonkey said:

Use this copy of the PDF above from the Wayback Machine. Same document title, but this one is much larger, with more models, and in color.

http://lcweb2.loc.gov/master/mbrs/recording_preservation/manuals/Stancil%20Hoffman%20Price%20List%20(1972-73).pdf

Click to expand...

Did you post the right link? This pdf seems to just cover speech recorders.

Cheers

Ian

ruffrecords said:

Did you post the right link? This pdf seems to just cover speech recorders.

Cheers

Ian
.

Click to expand...

Corrected. Thanks for the catch

Search didn't find AL-70, but manually browsing that second document, I found your amplifier. Schematic below, sans part values.