Pronomic SCM-1

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Khron

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Those output transistors, have you replaced them? Or were they just hand-soldered (or hand-reworked) at the factory?

That source resistor on the edge of the board makes (more) sense,  since the bias varies with each individual JFET...

JuanV said:
Received, inside photos.
I'll do better photos later with Nikon.

 

e.oelberg

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ln76d said:
Nitro solvent is much easier and safer to remove logo ;)
true :)

I did a couple of test with the mic but I didn't compare, usually i get annoyed of the sound of mics after a while. For instance  MBHO or Rode or Oktava or this very same mic with the old capsule. But with the new capsule I must admit, I like  them very much. They sound like no need for any eq to my ears.
 

RuudNL

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I finally measured the "Banzai" capsule and the (new) Pronomic capsule.
The frequency response matched almost exactly! ( A centimeter difference in position can already make a difference.)
So I have the impression that they are made by the same manufacturer.
 

Icantthinkofaname

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JuanV said:
So update pronomic mic with Banzai PCB (not available i guess) can do some improvements ?
I think the capsule is the important part. So buying a similar capsule to the Banzai one and installing it on an older Pronomic SCM-1 would be an improvement. The PCB would probably be less so because it seems that the capsule was the problem (or rather a bright installed in a circuit with no HF de-emphasis). Because the new SCM-1 is very balanced/neutral it's probably that the Banzai capsules are designed to be really flat sounding. Unless the new mics have a new circuit with HF de-emphasis but then why change the capsule?
 

RuudNL

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Circuit of the "old" and "new" Pronomic microphones is (almost) the same.
The old ones had a trimpot to adjust the FET bias.
But both microphones are based on the "Schoeps" circuit, with a DC/DC converter for the polarisation voltage.
If Banzai is planning to sell the capsules separately (I think he mentioned this already), this capsule would be a great update for the "old" Pronomic or Takstar CM-60 microphones.
 

JuanV

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I thought replace pronomic PCB with Banzai PCB with transformer - different circuit. Without changing capsule. But i do not know if banzai circuit is better than pronomic.
 

RuudNL

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The Banzai circuit is the original Neumann KM84 circuit.
Personally I wouldn't go through all this trouble (expensive transformer!), because the stock Pronomic SCM-1 is good as it is.
(Anyway for me...  A transformer doesn't add quality, I prefer a transformerless circuit.)
 

e.oelberg

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I agree with Ruud. The mic sounds just fine. Btw a Schoeps also sounds fine and  has no transformer. One of my mics needed a pcb wash and I will bias the FET if I have time. I guess  the Banzai capsule and the new pronomic are identical ...
 

homero.leal

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RuudNL said:
The Banzai circuit is the original Neumann KM84 circuit.
Personally I wouldn't go through all this trouble (expensive transformer!), because the stock Pronomic SCM-1 is good as it is.
(Anyway for me...  A transformer doesn't add quality, I prefer a transformerless circuit.)

RuudNL,

Some time ago I published a Transformerless KM-84 schenmatic on the MicBuilders forum, if you want to give it a look.

https://groups.yahoo.com/neo/groups/micbuilders/files/Homero%20Leal%27s%20Files/Transformerless%20KM-84%20Refresh/


It works pretty well. I also have some PCBs left for BM-800/700 or MXL mic bodies,  if some want to try it.

Regards!

HL
 

RuudNL

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Sorry, I can't find it.
It says:

"Oops!
You need to be a member to perform this action. "

Well, I am a member (and logged in), so I don't know what went wrong.
 

RuudNL

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Thanks.
Some remarks about a couple of  things that come to mind at first sight:

- I don't really see the point of using 3 resistors (R17, R18, R19). I suppose two 10 M.ohm resistors would do the same.
- The filtering for the polarisation voltage is a bit extreme.  0.1 uF + 1 G.ohm will cause a very long charge time!
(Something in the order of 100 seconds or more than 1.5 minute!)
- Personally, I don't like OpAmps in microphone circuits, but it has been done before, so it will probably work.

For the rest it is basically a KM84 circuit with an OpAmp buffer, so I don't expect real problems.
I suppose this will be a "high output" microphone, because in the KM84 a step down transformer is used, but in this design there is even 6 dB gain after the FET. But maybe this is already compensated by C14.
 

Khron

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Just to chip in real quick, i'd personally lean towards using a dual opamp instead of two singles :)
Less board space, less traces to route, and potentially even lower current draw, as well as cost.
 

homero.leal

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Yes Khron, I agree... originally, schematic had a dual unit OpAmp... but this design requires a very low quiescent current OpAmp.

I found the OPA145 that uses just 0.475mA per unit, but I could not find the dual version of it. So I had to stick with the single unit version.

To save space, I placed one OPA on the top, and the other at the bottom of the PCB.

I also considered OPA1692, but it's very tiny, may be for a next version.

If you have more options on low current Opas,  that could work, plz let me know!

Regards!

HL





 

homero.leal

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RuudNL said:
- I don't really see the point of using 3 resistors (R17, R18, R19). I suppose two 10 M.ohm resistors would do the same.
- The filtering for the polarisation voltage is a bit extreme.  0.1 uF + 1 G.ohm will cause a very long charge time!
(Something in the order of 100 seconds or more than 1.5 minute!)

Yes RuudNL, you are right.

RuudNL said:
I suppose this will be a "high output" microphone, because in the KM84 a step down transformer is used, but in this design there is even 6 dB gain after the FET. But maybe this is already compensated by C14.

Yes, that's the reason C14 is 39p on my actual build.

Regards!
 

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