northsiderap

Figuring Out a Vark Discrete Preamp / Discrete Opamp Card
« on: March 01, 2006, 12:44:13 PM »
I have four of these cards, and I intend to throw them into a rack box with some transformers out front. I believe they are +/-15V.  I didn't measure the input stage trim pot yet, and I only used transistors that were as close as my simulator would allow... Also, I have the OP transistors wrong, as I couldn't find a place to look up "TO-8" pin config. I just assumed it might be an emitter follower, but since it is supposed to be biased into Class A with a +/- PSU it might be a comp. symm.


Pins:
1) Out
2) V-
3) Ground
4) Input 1
5) N/A
6) Input 2
7) V+

Please correct me if I am wrong on anything here...  I am probably going to make it into a mic amp with something between a 1:4 and 1:10 input.
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northsiderap

Actual Part #s
« Reply #1 on: March 01, 2006, 03:25:48 PM »
I thought I'd post the actual part #s  too.

Stage 1: 6571
Stage 2 & 3: 6566
Stage 4: 40314 & 40319
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Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
Figuring Out a Vark Discrete Preamp / Discrete Opamp Card
« Reply #2 on: March 02, 2006, 10:32:16 AM »
Pin-out looks OK except that P2/P3 are likely both negative supply. You'll need to know which is the inverting/noninverting input though. Would need a few more minutes to get that than I have at the moment.

I'm not sure about compensation here. It could be that this opamp is not unity gain stable.

Q7/Q9 form a complementary emitter follower, and--as you said--Q7 is drawn wrong, as is Q5 and probably other parts.

Samuel

northsiderap

First Draft
« Reply #3 on: March 02, 2006, 11:48:43 AM »
The first draft of a circuit is always a little rugged for me - - It usually takes 2 or 3 before all the bugs are ironed out.

I've changed Q5 and Q9 to PNP... I changed the input stage to 2n5088 in my simulator to simulate a Melcor 1731. I still can't get past the first differential input stage. Where is the base bias coming from? Right now I'm feeling like I have to build an external bias circuit to get the base of stage 1 where I want it to be.

It would take some time to hash through this, but I"m starting from left to right.

Stage 1) Differential input (with no base bias?)
Stage 2) Direct couple differential input
Stage 3 & 4) Class A Complimentary Symmetry



I don't see a base biasing scheme here OR on the API 312.
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Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
Figuring Out a Vark Discrete Preamp / Discrete Opamp Card
« Reply #4 on: March 02, 2006, 01:57:09 PM »
It really helps if you draw the schematic clean and nicely. Don't want to praise my work, just what I have at hand: [removed], e.g. place the components such that DC current flows either from left to right, right to left or top to bottom, and with as few turns as possible--that is supply lines are straight lines from left to right or right to left and not a labyrinth.

I can't help you much more in correcting the schematic other than saying that the circuitry around Q5/Q6 doesn't make any sense to me as shown. You'd better have another look at the board. Check the Melcor DOA though--it could be similar.

Quote
I changed the input stage to 2n5088 in my simulator to simulate a Melcor 1731.

I don't know how you are simulating, but you cannot simulate an opamp without external circuitry, i.e. feedback or carefull DC trimming. I usually connect a 1 gH inductor between output and inverting input and a 1 gF cap from inverting input to ground. This settles nice DC conditions while leaving the opamp essentially open-loop at AC, that is you can easily simulate o/l gain.

This means of course that you have to know which input is which; it doesn't look to me as if you've figured this out already. At least I cannot tell because I don't know if Q5/Q6 should form an emitter follower or a third amplifier stage.

At the current level of circuit simulation you don't have to worry about transistor types. Just use 2N4401/2N4403 or your favourite small signal BJT everywhere. Once you get an output sitting around 0 V and a reasonable o/l gain you can change them for a realistic stability analysis.

Samuel


 

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