New Nady SCM 900 design?

[Guest]

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In a moment of weakness (and curiosity) I ordered and received a Nady
SCM 900 from Musicians Friend for $39 with free shipping. It appears
that the microphone that I received is a transformerless design and is
different from the singer-side picture that appears in Scott Dorsey's
article for Shanghai microphones. The singer-side board has 3
transistors instead of 2. Two of the transistors appear to be emitter
followers and are connected through coupling capacitors to pins 2 and 3
of the XLR connector. Has the microphone circuitry been redisigned such
that Scott's upgrade would not provide a benefit?

Thanks

JWVM

 

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<jwvm@umich.edu> wrote:
>In a moment of weakness (and curiosity) I ordered and received a Nady
>SCM 900 from Musicians Friend for $39 with free shipping. It appears
>that the microphone that I received is a transformerless design and is
>different from the singer-side picture that appears in Scott Dorsey's
>article for Shanghai microphones. The singer-side board has 3
>transistors instead of 2. Two of the transistors appear to be emitter
>followers and are connected through coupling capacitors to pins 2 and 3
>of the XLR connector. Has the microphone circuitry been redisigned such
>that Scott's upgrade would not provide a benefit?

There was some discussion of this here a while ago. This design was
developed by Marshall Electronics as the MXL57, I think. I'm not
positive about that number. Anyway, as soon as that design made it to
Shanghai, everybody else started making it, and they turn up now and
then inside some of the Shanghai-style microphones from various factories
there. Sometimes you'll see a case of mikes and a dozen will have one
board and another dozen will have a different one.

Anyway, my board will sound better than this board, but this one is
a whole lot better than the original transformer-type electronics. It
does use a DC-DC converter to crank the voltage up for the capsule,
which means the capsule-to-capsule variations are exaggerated with it.

The circuit that Brent at Marshall used was indeed a Schoeps-like circuit
like I used, although he skimped a bit on capacitors to keep the cost down,
and used the DC-DC converter.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

I gave in to temptation and also bought one of these Nady SCM 900
(Musician's Friend, 40$ with shipping), and find it also has the
non-transformer coupled output stage. Has anyone traced out and drawn
up the schematic for this circuit? I'll try to find the time, and post
it (is this legal?) if not. Wondering if a few parts substitutions will
lead to a near-optimal exploitation of the capsule.

dhs

 

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<dhs@mit.edu> wrote:
>I gave in to temptation and also bought one of these Nady SCM 900
>(Musician's Friend, 40$ with shipping), and find it also has the
>non-transformer coupled output stage. Has anyone traced out and drawn
>up the schematic for this circuit? I'll try to find the time, and post
>it (is this legal?) if not. Wondering if a few parts substitutions will
>lead to a near-optimal exploitation of the capsule.

I have drawn it out, and it's nothing too exciting. The problem with
parts substitions, though, is that the board is very crowded and there
is really no room for much else.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

 

[Guest]

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It looked (don't have it here) like an FET stage followed with a
differential driver (presumably unity gain followers), and the FET is
marked 'K170' which I think means it is a 2sk170 -- not a bad part. If
they worked out the gains to be dominated by the input device, then one
could not do much better, it seems (and one could find another lower
noise FET to drop in). What am I missing -- the quality of the
capacitors? that could lead to difficulties indeed.

Forgive me, as it may be elsewhere, but what is the story with the
DC-to-DC converter? is 48 V not enough to get a big enough signal to
the FET, so they boost that up? Does the higher voltage lead to an
attractive (and non-linear!) force between the diaphragm and the other
side of the capacitor, screwing up the response (the mention of a 6 kHz
peak, which is maybe the lowest drumhead resonance of the diaphragm)?


dhs

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Oops, just found and read the article you wrote on replacing these
amplifiers. Nice! I think they may have done a cosmetically good copy;
have not had a chance to listen to it yet. Still curious about the 'HV'
for the capsule, though.

dhs

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

<dhs@mit.edu> wrote:
>It looked (don't have it here) like an FET stage followed with a
>differential driver (presumably unity gain followers), and the FET is
>marked 'K170' which I think means it is a 2sk170 -- not a bad part. If
>they worked out the gains to be dominated by the input device, then one
>could not do much better, it seems (and one could find another lower
>noise FET to drop in). What am I missing -- the quality of the
>capacitors? that could lead to difficulties indeed.

They are all cheap electrolytics, and there is no room for anything else
in there. Life is like that.

>Forgive me, as it may be elsewhere, but what is the story with the
>DC-to-DC converter? is 48 V not enough to get a big enough signal to
>the FET, so they boost that up? Does the higher voltage lead to an
>attractive (and non-linear!) force between the diaphragm and the other
>side of the capacitor, screwing up the response (the mention of a 6 kHz
>peak, which is maybe the lowest drumhead resonance of the diaphragm)?

The capsules these mikes use are copies of capsules intended for higher
voltage operation. If they were made properly, polarizing them at a
higher voltage would result in flatter frequency response (because the
attraction of the diaphragm to the backplate would increase, thereby
raising the resonant frequency), and higher output (because the charge
would be increased).

In reality, because these capsules aren't so consistent and aren't always
so well-made, raising the frequency response is as likely to be bad as it
is to be good. And because the backplate machining is not always so good
and the tensioning is not always so good, sometimes the capsules just fail
at the higher voltage. Marshall was pretty good about getting better
consistency out of the capsules they used with these electronics, but the
other vendors? Who knows?
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."