Analyze a Mic Pre Circuit

[northsiderap]

Here is an image of the bumblebee that my computer says will not fly.

Thought I'd share the schematic to see if anyone thinks there could be improvements in the circuit.

 

[Larrchild]

if that is a 22 ohm resistor in series with an 800uf cap to ground across the first transistor's base, you are going to have a very dull unbiased amp stage=)

 

[northsiderap]

That resistor is supposed to be selected to match the gain of each successive preamp.Sorry, left that out.

It's kinda fuzzy but that's a 300uF cap...

 

[Larrchild]

Ok, help me here. I see a mic transformer driving a 2uf cap to the base of Q1. But what I can't see is how you are forward-biasing Q1.

1.There are no resistors to either Vcc or gnd attached to the base junction to bias it.
2. There is in effect a 300uf cap across the base.

Why not...and Why?

 

[Samuel Groner]

As Larrchild already said, Q1 does not get any base current and the value of R12 is orders to small.

I suggest that you have a look at the GainBloak--pretty similar to what you want.

Or check a design by Brad: simple 5V mic pre.pdf

Samuel

 

[northsiderap]

Yes, I was hoping some of these questions would come up! I do not understand yet why it works.

I already have 8 of these racked ... I've been using them to tame my bright condensers.

The transformers are Beyer TR145/BV 35570

 

[PRR]

> have 8 of these racked ... I've been using them

The schematic is obviously a paste-up job. How do we, or you, know it accurately reflects the actual amplifier?

Does turning R11 do anything?

Is there any DC voltage inside except >+22V or ground?

 

[PRR]

D'oh! You left out a ~100K resistor between Q1 Base and the top of R6. Or maybe R12 is 22K and on the other side of R6. Anything along those lines gives the ancient and excellent 2-transistor Phono amp (plus buffer), widely used by Neve and nearly everybody else.

Sumthin odd about Q4, but I'll go to bed while you re-check your reverse engineering.

 

[northsiderap]

The schematic is directly copied from the factory schemo, and I started the thread very open ended to get very general comments first.

I had been wondering if there is a schematic error, and the base of Q1 is biased between R5 & R6. Then that 300uF becomes Q2 emitter bypass cap, and that 22R* becomes the feedback R. Luckily, PRR's post popped up as I was editing that into my previous post.

What IS happening with the output stage? I've been wondering myself, enough so to make it one of my main motivations for posting this schemo in the first place. Sofar in my limited time and budget I've only really messed with DC class A and have not done much PP & Comp Symm.

Circuitmaker still hates it though.

This amp looked suspiciously like the highly-touted Yamaha PM - 1000 mixer pre schemo... Circuitmaker works with that one.

*Individually selected per card.

 

[Samuel Groner]

The schematic is directly copied from the factory schemo.

Don't assume that the factory schemo is correct. Or you did some errors while copying it. This is never going to work as shown.

What IS happening with the output stage?

Q4 should be a PNP.

Samuel

 

[TedF]

The general shape of the amplifier looks like a very conventional descrete mic amp of the early 70s; before engineers started 'improving' things with active collector loads and fancy feedback. But on closer inspection there are so many obvious errors; any one of which would prevent it from working at all, that it can't possibly be a reflection of real equipment.
It's well worth having a look at Douglas Self's site:-
http://www.dself.dsl.pipex.com/ampins/discrete/twoq.htm
:grin:

 

[northsiderap]

I kid you not... I've checked and re-checked, but the schematic above matches the factory schemo. :?:

????

I'm thinking about popping a card out and tracing the PCB now...

 

[TedF]

It's certainly worthwhile to do a trace from the actual board....
Whoever drew the circuits is no engineer!

 

[magicchord]

Yeah, Q4 should be a PNP.

Try connecting the R12/Q1 junction and the R5/R6 junction together. The mfr schematic probably left out the "dot".

 

[bcarso]

A 2N3638 is a PNP, so Q3 must be a different part number. Agree with PRR that you need a resistor between the input base and where it ties now, and magicchord that they left out a connection dot. Then the rest of it looks more or less o.k. if a little suspicious as far as specific resistor values.

The 2N4010's are also likely an error---that is an obscure part and not suitable for this application. A complement to the 3638 which may be the correct part for Q4 would be the 2N3643. Today I would replace NPN's with 2N4401's and the PNP with a 2N4403, or if you like metal cases the 2N2222 and 2N2907, respectively.

 

[northsiderap]

I..

1) Attached that cap & resistor between R5 & R6

2) Changed Q4 to a PNP

Now it makes sense.

I assume the schemo is from the factory... It is about 22" x 11" and looks like an original runoff. It's sounding like someone else must have made this up...

Questions:

Anyone have info on the Beyer TR145/BV 35570?

AND

should I throw some Neve type output xformers after this to balance it even though all of my patch cords between my pre's and my recorder are under 3 feet?

 

[Samuel Groner]

Now it makes sense.

There is still something wrong with it. You need the ~100 k ohm resistor for Q1 base PRR mentioned. Otherwise the input impedance will be way too low.

Should I throw some Neve type output xformers after this to balance it even though all of my patch cords between my pre's and my recorder are under 3 feet?

If you want to save that iron for another project (not sure it's suitable for that one anyway), you could use an impedance balanced output.

Samuel