Schematic Decca M50 FET

[RuudNL]

It seems that Decca has modified many Neumann M50 microphones.
They removed the AC701 tube and converted the electronics to a FET circuit, so that the microphones could be used with normal phantom power. Although I am not sure if this was a very wise decision, I would be interested to see the schematic they used for the FET version. Any ideas where this can be found?

 

[Khron]

Just speculating here, but... Any chance it might be similar to the KM84, perhaps?

 

[RuudNL]

I don't think so.
On the picture I once saw, there were a couple of transistors or FETs.
Also resistor values were different. (390 K.ohm etc.)
Here is a less detailed picture with only 1 transistor/FET visible, I can't find the 'better' one anymore.

I just found better pictures.
The 390K, 75K (and 2.7K?) are still there.
Do I see a BC264 (FET) and a BC109?
(By the way: a pretty ugly construction, if you ask me...)

 

[wlinart]

I've collected a few pictures of the decca m49 and decca m50, even different versions of both. I'll send them to you.

 

[RuudNL]

That would be very nice! Many thanks in advance!

 

[wlinart]

Already send them (to your email), i thought i had more of them, but it seems i lost them. If i find them back, i will send those too.

 

[Gus]

I seem to remember Oliver posted a schematic of the solid state conversion at a forum years ago(early 2000's?)
I think it was for a KM54 not the M50 I would guess it is close.

Maybe a search could find it?

 

[RuudNL]

Already send them (to your email), i thought i had more of them, but it seems i lost them. If i find them back, i will send those too.

Unfortunately nothing arrived here. (I also checked the spam folder.)

 

[wlinart]

Unfortunately nothing arrived here. (I also checked the spam folder.)

Strange, maybe the mail was too big (around 11Mb i think). Will try again.

 

[Vac11]

It looks like there are some photos in the GS threads (decca M49 though):
https://www.gearslutz.com/board/high-end/534844-new-neumann-m49.html
https://www.gearslutz.com/board/high-end/1137832-m49-decca-fet-flea-m49-ac701.html

 

[bremusound]

Here is a thread with a reply from Oliver Archut

https://www.tapatalk.com/groups/prorecordingworkshop/tube-mics-and-fets-t18604308.html#.Um-rbFMliM0

He said the circuit is similar to the AKG C414

Somewhere else i read that Mick Hinton is the man behind the Decca FET mods

 

[gyraf]

neumann m50 - decca fet modification - 2 types

image from ??

 

[ln76d]

I just found better pictures.
The 390K, 75K (and 2.7K?) are still there.
Do I see a BC264 (FET) and a BC109?
(By the way: a pretty ugly construction, if you ask me...)

Yes it looks like BC109 but the FET (pic is really blury) looks to me much more like BF254

 

[mrclunk]

I've seen a couple of different versions of M49 at least.
One with BC264 fet and single BC109 and a version with two BC109s.
Circuit is similar to 414 without DC/DC

 

[RuudNL]

Yes it looks like BC109 but the FET (pic is really blury) looks to me much more like BF254

No. It is a BC264. A BF254 is a NPN transistor, not a FET.

 

[Khron]

Is it... ironic that those old circuits look as messy as they do (tons of flux residue etc), but nowadays we obsess(?) over the hi-Z area being pristinely clean etc? 

 

[ln76d]

I've seen a couple of different versions of M49 at least.
One with BC264 fet and single BC109 and a version with two BC109s.
Circuit is similar to 414 without DC/DC

Then definately better
Just speculating from what i see on blury pics. Just realised that what i wrote have no sense, because i was thinking about BF245 and BF254 is just some BJT Sorry for confusing

Looks like correct pinout for BC264.
From left 3 (gate), 2 (source), 1 (drain).

Gate goes to high ohm resistor, drain direct to BC109, but i wonder what for is such small capacitance on the source, and where it goes (ground or some feedback? ), looks like 1p2.

 

[ln76d]

No. It is a BC264. A BF254 is a NPN transistor, not a FET.

Yes sorry Rudolph,  just realised my fault, which about i wrote in upper post

Is it... ironic that those old circuits look as messy as they do (tons of flux residue etc), but nowadays we obsess(?) over the hi-Z area being pristinely clean etc? 

You would be truly surprised what you can find in many iconic microphones, starting from the boards quality for example.
In some (as even here) gate and and hiohm resistors aren't isolated from the board, just simple connection on the board trace.
"Tons of flux" also

 

[RuudNL]

i wonder what for is such small capacitance on the source, and where it goes (ground or some feedback? ), looks like 1p2.

On another picture I see that this small capacitor is 8.2pF.
One side of this capacitor is connected to the capsule, I suppose it is a feedback capacitor.

And yes, I agree... what a mess! (But apparently this is what vintage lovers like!  ;D )

 

[bremusound]

Saturn-Sound converts those back to AC701. Some months ago i asked if they could give me the Decca FET schematic, but didn't get any answer. Maybe someone else has more luck?

http://www.saturn-sound.com/services/decca_m50_fet_conversion_to_ac701.htm