What kind of preamp circuit is this?

I just bought a custom console and I'm trying to figure out what type of pres these are, if they're a decent design, worth keeping, or if I should consider replacing them.

Here's a schematic I've created. It's not pretty, but I've checked it many times for accuracy:

https://drive.google.com/file/d/0BxiPOzSDUqGKLUp4ejlxWUNfUlU/view?usp=sharing

The audio is being fed from 150:600 or 600:600 Peerless transformers via a switch that selects between MIC, -15dB and LINE. Not sure which is the audio in and which is out.

Watcha think? Should I keep em or dump em? I'm thinking I could replace em with EZ1290's and keep the 150:600 xformers.

PS: I've edited the original subject and post to more clearly differentiate between multiple posts on this console and these preamp cards. I've also removed the pics of the PCB and replaced it with an actual schematic. I'll sort out the PSU issues in other posts and leave the preamp circuitry/design to this one. Sorry if this has caused any confusion.

jdurango said:

I just bought a custom, hand-wired console that's missing it's power supply. I'm trying to figure out the components. Are the mic pres? Line cards? I've included a pic of the front and back of the PCB, which appears to be fed by the input transformers. The transistor is a 3391A.

Click to expand...

Is there actually only one transistor on this card? 2N33991A is actually one of the first really low-noise transistors, but I guess other performance aspects such as THD and headroom cannot be much in comparison with more elaborate topologies. It may sound good, though, but the soft spot could be somewhat restricted.

I've also included a pic of another PCB with a bunch of 741's, which I think are meant to drive the insert ins and outs, aux ins, tape in/out, revb send, etc. ...but I'm not sure. I'm really hoping those 741's aren't part of the mic pre path. Watcha think? Thanks fellas!!

Click to expand...

You'll have to do the hard work and draw the schemo (including identifying where the connections go).

EDIT: As usual pic upload isn't working.

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That is somewhat surprizing; I have no problem uploading .jpg's. Are you sure the file you're trying to upload is physically on your computer?  Are you on Windows or Mac? Have you notified Ethan about this issue?

jdurango said:

EDIT: As usual pic upload isn't working. Her ares links to the photos on google drive:

Click to expand...

Use Photobucket to host you pics and copy the link .
Then Post reply windows here  click  "insert image"  and paste the link into the code.

Much easier and nice to visualize the pics in a thread.

http://s1124.photobucket.com/

Attachments:
Allowed file types: gif, jpg, jpeg, pdf, png, txt, brd, sch, lbr, svg
Restrictions: 1 per post, maximum individual size 2048KB

Just put together a schematic of these cards. It's my first schematic, so it's not very pretty, but I've checked everything and it should be accurate. As you can see, there are two 3391 transistors on the card.

The "switch" refereed to in the labels green, white, red, blk (small gauge), red (small gauge) and shield is the switch that selects between transformer taps for 150:600 and 600:600 and a third. The switch is labelled MIC, -15dB and LINE. I'm guessing 150:600 for MIC, 150:600 with a pad for -15dB and 600:600 for LINE.

Anyway, here's the schematic. Let me know watcha think. Thanks!

https://drive.google.com/file/d/0BxiPOzSDUqGKLUp4ejlxWUNfUlU/view?usp=sharing

jdurango said:

Just put together a schematic of these cards. It's my first schematic, so it's not very pretty, but I've checked everything and it should be accurate. As you can see, there are two 3391 transistors on the card.

The "switch" refereed to in the labels green, white, red, blk (small gauge), red (small gauge) and shield is the switch that selects between transformer taps for 150:600 and 600:600 and a third. The switch is labelled MIC, -15dB and LINE. I'm guessing 150:600 for MIC, 150:600 with a pad for -15dB and 600:600 for LINE.

Anyway, here's the schematic. Let me know watcha think. Thanks!

https://drive.google.com/file/d/0BxiPOzSDUqGKLUp4ejlxWUNfUlU/view?usp=sharing

Click to expand...

My eyes burn at reading your schemo. It doesn't make much sense as it is. Are you sure you have properly identified base and collector of the transistors? IIRC 3391 was one of the first transistors with collector in the middle. Can't you at least find where the power supply rail connects? That would help a lot.

abbey road d enfer said:

My eyes burn at reading your schemo. It doesn't make much sense as it is. Are you sure you have properly identified base and collector of the transistors? IIRC 3391 was one of the first transistors with collector in the middle. Can't you at least find where the power supply rail connects? That would help a lot.

Click to expand...

I know it's not pretty. It's my first schematic translated from a mystery PCB....it was all I could do to trace all the leads and get it correct. I'll triple check, but that should be it.

And yeah I checked the data sheet on the 3391 beforehand. Should be BCE from left to right looking from the top with the flat side facing up.

Also double-checked polarity on electrolytic caps.

abbey road d enfer said:

My eyes burn at reading your schemo. It doesn't make much sense as it is. Are you sure you have properly identified base and collector of the transistors? IIRC 3391 was one of the first transistors with collector in the middle. Can't you at least find where the power supply rail connects? That would help a lot.

Click to expand...

Perhaps you're thinking there's a connection wherever lines cross? It should only be read as a connection of there's a dot, otherwise lines crossing shouldn't be considered shorted. I thought this was pretty standard but I could be wrong..again,its my first schematic. When drawing them manually I usually make a arched bump "over" intersecting lines that aren't intended to be connected together.

jdurango said:

Perhaps you're thinking there's a connection wherever lines cross? It should only be read as a connection of there's a dot, otherwise lines crossing shouldn't be considered shorted.

Click to expand...

No, I'm used to that convention. There are a number of questions: Where is the input signal? I would think it connects to the left side of C7, but no connection is visible there. Also I would think the collector of Q1 goes to the base of Q2, either directly or via a coupling cap, but I don't see it on your schemo.
Now I guess the output is taken at the right side of R8, but the coupling cap C5 goes to the base of Q2! It should go to either the collector or the emitter. That's why I suggested you had the pin-out wrong.

abbey road d enfer said:

No, I'm used to that convention. There are a number of questions: Where is the input signal? I would think it connects to the left side of C7, but no connection is visible there. Also I would think the collector of Q1 goes to the base of Q2, either directly or via a coupling cap, but I don't see it on your schemo.
Now I guess the output is taken at the right side of R8, but the coupling cap C5 goes to the base of Q2! It should go to either the collector or the emitter. That's why I suggested you had the pin-out wrong.

Click to expand...

I'll check it all again, but should be right. I'm assuming one pair of the red (sm gauge), blk (sm gauge), white and green wires are for in and the other pair for out. Not sure which is which though.

abbey road d enfer said:

No, I'm used to that convention. There are a number of questions: Where is the input signal? I would think it connects to the left side of C7, but no connection is visible there. Also I would think the collector of Q1 goes to the base of Q2, either directly or via a coupling cap, but I don't see it on your schemo.
Now I guess the output is taken at the right side of R8, but the coupling cap C5 goes to the base of Q2! It should go to either the collector or the emitter. That's why I suggested you had the pin-out wrong.

Click to expand...

Just checked everything again. Nothing major is off, although R5 and R6 were reversed. I updated the PDF to reflect that, plug tried to make the wiring more clear. Obviously the hvy red i spower, obviously the green, white, sm red, sm blk and their shield are for audio, so I labelled them as such. Hope it's a slightly easier on the eyes. Thanks a ton for the help!

jdurango said:

Just checked everything again. Nothing major is off, although R5 and R6 were reversed. I updated the PDF to reflect that, plug tried to make the wiring more clear. Obviously the hvy red i spower, obviously the green, white, sm red, sm blk and their shield are for audio, so I labelled them as such. Hope it's a slightly easier on the eyes. Thanks a ton for the help!

Click to expand...

Still the part around Q2 does not make sense. It is impossible for the collector to be "isolated" by a 180k resistor.

abbey road d enfer said:

Still the part around Q2 does not make sense. It is impossible for the collector to be "isolated" by a 180k resistor.

Click to expand...

I don't know what to tell you....just checked again....that's how it's wired.

jdurango said:

I don't know what to tell you....just checked again....that's how it's wired.

Click to expand...

Can you posts pix of this area both sides)?

There's gotta be something missing here. The base of Q2 has no DC path to anywhere, so it can't be biased on.

abbey road d enfer said:

Can you posts pix of this area both sides)?

Click to expand...

Did you one better, took hi-res, well-lit pics of both sides and also created an overlay of the PCB traces and made a seperate image with the top of the PCB + the trace overlay....hopefully that helps.

Also, should be noted that I had to disconnect the two 50uF caps to get a clear shot of the traces on the underneath (you can see them off to the side). But they are  wired to connect to the top of R1, as in the schematic....hand-wired, not actually part of the PCB.

https://drive.google.com/drive/folders/0BxiPOzSDUqGKTjNEY2dISnh1cXc?usp=sharing

Thanks very much for the help on this!! It's very much appreciated!

Also should be noted in these pics, the flat side of both 2n3391's is facing DOWN.

jdurango said:

Also should be noted in these pics, the flat side of both 2n3391's is facing DOWN.

Click to expand...

On your schemo, you've missed one very important connection, from collector of Q2 to the left side of C7. Now that makes things much clearer!
Q2 is the 1st stage and Q1 the output stage.
The input signal connects to P5 and the output to P10.
Now you have to work out the switch's action. Use an ohmmeter and make a table of the connections.
The design is quite basic, reminiscent of HiFi phono preamps; good noise performance but not much headroom and dynamic range.

abbey road d enfer said:

On your schemo, you've missed one very important connection, from collector of Q2 to the left side of C7. Now that makes things much clearer!
Q2 is the 1st stage and Q1 the output stage.
The input signal connects to P5 and the output to P10.
Now you have to work out the switch's action. Use an ohmmeter and make a table of the connections.
The design is quite basic, reminiscent of HiFi phono preamps; good noise performance but not much headroom and dynamic range.

Click to expand...

Thanks so much! I'm really glad I did that overlay! That helped me see what you pointed out, and also some other stuff I missed! I'm definitely going to do that right off the bat in the future!

Anyhow, I've updated the PDF....should finally be correct now.

I'm curious how these pres will sound....I'm going for a very oldschool 60's type of sound. I'm hoping the transformers will add some nice color. Thanks again! I really, really appreciate the help!

> things much clearer!

Thanks.

jdurango, draw it so it makes sense at-a-glance.