What microphone could this be?

[Gus]

I tied to trace a microphone from photos on the web there is one good inside picture at gearslutz  of one side of the microphone PCB
and good photos at recordinghacks of a ST77.
Colors are sometimes hard to to see clearly on my monitor not sure of red and orange on some resistors at times.
transformer is guess
I also assumed the caps values would be close to the 2001, v67 etc.

The schematic is still a guess. I have not looked inside this  microphone.

What do you think I tried to trace from a few photos I saw on the web?

 

[RuudNL]

Funny they use the "Neumann" component values, even the 6.8K and 560 ohms in series...
But I am not sure if the result will be the same, because of the lower drain resistor (20K instead of 47K) and the not bypassed source resistor R4 (3.3K). IMHO the gain of the FET stage will be lower than in the Neumann circuit, meaning less negative feedback.

On the recordinghacks website:
"In a signal injection test of the circuit, we found the circuit to be linear to about 15kHz, then dropping to -2dB at 20kHz. DIY enthusiasts might find this to be a suitable platform for a K47 or CK12 style capsule upgrade."

-2 dB at 20 KHz is less than the correct compensation for a K67-style capsule, so this confirms my thoughts about the lower negative feedback.

 

[analogguru]

Oh thats easy, it is one of those U87-clones and derivates.  Therefore it could be a:
Apex - 420
Apex - 430
Fame - SKEC010-W
MXL - 2001
MXL - V67
SoundKing - SKEC010-W
Stagg - MCO-5
t.bone - SC-450
TSM - MT87
(and others)

or like in this case a Sterling Audio - ST77 FET.

Colors are sometimes hard to to see clearly on my monitor not sure of red and orange on some resistors at times.

Tip: fiddling with the "blue" fader in your graphic program will increase the difference.

 

[abbey road d enfer]

There's something not quite right with the FET's gate bias. With the actual values you have about 10V on the 1G resistor, which would completely saturate the FET (Vds=0). I don't see anything that looks like it in a U87...

 

[RuudNL]

R6/R7 looks indeed a bit strange.

 

[Gus]

Two links I found

https://www.gearslutz.com/board/gear-shoot-outs-sound-file-comparisons-audio-tests/1121079-shootout-u87ai-vs-oceanway-st6050.html 

http://recordinghacks.com/microphones/Sterling/ST77

19 resistors instead of 17 like in the 2001.
I was curious about  what was changed. I did not want to buy one to find out.

The first post is what I think I traced. I should switch the .047uf and .01uf that would be more like what you find in other n-channel, PNP circuits  or make both .047uf.

Anyone want to check what I traced? I could have made a mistake and forced myself to draw the first stage as I think I traced it.
If I traced the first stage correctly it reduces the capsule voltage as well this is not good for SN but it allows a higher sound level for the same gate AC voltage .

I guessed the 6050 was close to the ST77. Interesting in the ST77 picture it looks like a cube shaped film cap is added to the 4.7uf output coupling cap

I will adjust the blue fader next time I look at the photos

I do not know what the JFET number is, I might guess a 2SK30 type because that is what is used in the ST51a.

The CM9711 guess is because it is dual bobbin and a 5:1 ratio.

 

[abbey road d enfer]

Anyone want to check what I traced?

Quite difficult but I see the source of the FET going to a 1k in // with a C, then going to a 10k (?) that goes to ground. The 1G being connected to the junction of the two resistors, which seems more correct to me.

 

[Gus]

Did anyone else take a look at the pictures and compare them to the schematic?

 

[abbey road d enfer]

Did anyone else take a look at the pictures and compare them to the schematic?

I tried, although it's tearing up my eyes, and I could see that your schemo doesn't match the actual PCB's pictures.

 

[RuudNL]

I find it hard to see on the pictures, but I think it is more likely that the top of R7 is connected to the source of the FET.

 

[abbey road d enfer]

I find it hard to see on the pictures, but I think it is more likely that the top of R7 is connected to the source of the FET.

That would certainly make much more sense.

 

[moamps]

Did anyone else take a look at the pictures and compare them to the schematic?

Your schematic is OK, R4 is 11k IMO (brown, brown, black, red, brown), and there is a square red capacitor paralleled with the big one 4u7.

 

[abbey road d enfer]

Your schematic is OK, R4 is 11k IMO (brown, brown, black, red, brown),

That seems much more correct, but still the connection of R7 to B+ is dubious.

 

[moamps]

That seems much more correct, but still the connection of R7 to B+ is dubious.

R7 and R6 form the  voltage divider from a local PS (about 15-20V after R15/C8), so the capsule is polarized with  difference of potentials on JFET's gate (about 7,5V or so)  and  the voltage on the right side of R15.  R2 is not 20k, most probably 2k, so the JFET works in linear regime.
The picture on second link (GS) shows different values for R2, R4,  R5....and Q2 is missing .

Everything is dubious in chinese  "designs".

 

[analogguru]

..... R2 is not 20k, most probably 2k, .....

R2 is definetly 20k (4th ring is red ring).  If you don't believe it, compare it to the pictures/schematics of the microphones mentioned above.  They all use 20k at the drain.

FET probably will be 2SK170-BL, Q1 2SA1015-GR, C1 10n, C4 47n, C10 4µ7+1µ, R16/R17 2k2.

One can see the ingenuity of the design by the paralleling of a 1µF film capacitor to the (oversized) 4µ7 film capacitor at the output (where everybody else uses a 4µ7 tantalum), while keeping the 22µF bypass capacitor (C2)..... of course, because it is not (directly visible) in the signal path.

So the sales argument is:
"....for better sound performance we don't use an input coupling cap and at the output we use a big film capacitor instead of the bad sounding tantalums...."

how cool..... how (£-)sterling-ish...

 

[Gus]

Thanks for the feedback

I was curious about the ST6050.  Because of the name behind it and the price. Why 19 resistors and not one less than 2001,v67 etc.

Kind of surprised how low the capsule is in the grill maybe that is part of the tuning?

I believe someone posted here in the past there is a SMD PNP on the backside of the PCB.

Sometimes I like to trace a circuit from a picture as a puzzle.

It is sometime hard to tell 5 band resistor values in pictures if both the start and stop(brown for 1%) are  brown.

I did think it was odd how another smaller film cap was added to the ST77 4.7uf cap.

 

[abbey road d enfer]

I did think it was odd how another smaller film cap was added to the ST77 4.7uf cap.

Paralleling caps is so common in audiophoolery...

 

[Gus]

If I was to post a nice adjustment(s) to the N channel PNP type circuits in some microphones how soon would it be before someone(s) posts it as their own design?

 

[cyrano]

How much harm would that do?

People are vain. Both ways. Even if you had an original idea, it is possible someone else had that same idea.

In fact, it's even very likely. Biologists have observed animals acquiring a skill. Usually, a disconnected group, far away develops the same idea. Some see that as proof of telepathy...

 

[83artem83]

The ST6050 circuitry is 99% identical to the sterling audio ST77 circuitry. The capsule is the same. The enclosure is the same too, except for the painting, engraving and chrome on the bottom ring.

why overpay