Topic: Capsule amplifier in electret microphone

[Roger Gustavsson]

I am new to this forum. Just found out that there are quite a few threads on the Nakamichi microphones (=Teac, Tascam, Altec, all manufactored by Primo). I would like to modify my own set of CM-300. The output level is too low in my set up. The output transformer reduces the output from 8 to 1.6 mV. Please, look at the schematics below:

A is the original circuitry of the CM-300.

B is a revised circuitry with a raised supply voltage as suggested by Stephen Sank.

C would be the front end of B. Without transformer not the very best for driving long cables. What difference will there be between C and D?

What about E, can it be used? Sure, also problematic with long cables. F is taken from the homepage of Scott Helmke. It uses a P48 phantom power supply. As I would like to run my microphones on internal batteries maybe a P12 would be suitable? Is my schematic G correct? The components are sometimes added, for what reasons? What about the values of the drain and source resistors to suit the 2SK118-O with an Idss=1mA?

Roger Gustavsson

[SparkE!]

Schematic E looks OK to me.

Schematic F is exactly what Scott Helmke uses and other DIY folks have used that schematic with good results with the Transsound TSB-165 mic elements.

Schematic G would need some sort of path for current to flow from the collectors of the PNP transistors.  In Scott's schematic, the current would flow through the zener diode.  The way you have it connected, it would put something close to 48 V across your JFET and it probably wouldn't like that too much.

Before building anything, you might want to try modeling the circuit with Spice.

[Roger Gustavsson]

So E would also work but with limited capability in driving long cables?

About the schematic G. There will not be 48V anywhere, the phantom supply voltage is just 12V in scheamtic G, that is why I deleted the 12V zener. I think I forgot a resistor, like the 330R in Scott's schematic. I am only a layman in electronics, so all help is welcome!

I have no wish at all to run on outboard phantom power supply. Therefore, a lower supply voltage would make things easier, 9-12V. I also would like to use the 2SK118 JFET that is already there and get rid of the tiny output transformer.

Roger Gustavsson

[Roger Gustavsson]

Well, Team Nakamichi seems to concentrate more on how to modify the microphones to "actives" and/or how to run them on phantom supply. The "actives", actually not any more active than the originals, are not suited to drive long cables. I think it is the possibility of "stealth" recording that attracts many. I am not doing such recordings so it is of less importance to me. Phantom supply is not very diffcult at all (I have a schematic from Stephen Sank) but it is a no help if your equipment does not have it. All you need are some piece of wire, two resistors and a zener diod.

I am usually running two Nakamichi CM-300 (ORTF or A/B) into the Nakmichi MX-100 (on batteries) and then to a Digigram VX Pocket V2 soundcard in my laptop. The Digigram soundcard does have a micpreamp but it is too noisy at higher gains with the CM-300 while recording acoustic music. The original CM-300 needs a gain of 500 in my setup. The MX-100 will do that but I would like to do it without the help of the MX-100.

I would certainly prefer to shorten the route for the weak signals from the microphones to the soundcard. Skiping the output transformer will give about 14 dB more output and a gain of 100 would do the trick. For better "transport", and some amplification, of the signal a Chargeamp à la Scott Wurcer can be used just near to the capsule and it will run on an internal battery (there are various alternatives to the original battery). It means soldering surface mount devices, not very easy for an amateur like me but I have the components... The Helmke way seems to be a bit easier with its "normal" components. Less gain but probably okay for my soundcard. The P93 by Rod Elliot can also be used. The gain is raised in my schematic.

[Roger Gustavsson]

I have made some changes to my schematics. Would like to ask for some comments!

[SparkE!]

You still have no DC path from the collectors of your PNP transistors to ground in Schematic G.  That one will not work.  Schematic E will still work and it would be the easiest to implement.

[Roger Gustavsson]

You still have no DC path from the collectors of your PNP transistors to ground in Schematic G.  That one will not work.  Schematic E will still work and it would be the easiest to implement.

Here we go again with a new schemtic.... Are there any components that can be taken out?

[Roger Gustavsson]

So, with all this hi-tech digestion of drawings with little squiggly lines on 'em  ... is there a chance that someone holding a pair of Leggedy moded Naks could find that there is a way to get them to really "put out"?

  I was rather... no, was really disappointed when I ran them the first time at moderately high SPL (read, louder than my stereo) and that they really didn't come up until I was at 9.5 (to 10) on my UA5 gain.  further control between 9.5 and 10 is nearly impossible to manage.  Here's a coupla' Ts for the entertaining dialog... carry on.

I am not familiar with Leggedy's modification. Is it just adding a phantom supply? A phantom supply just replaces the internal battery. The output of the CM-300 is low. Removing the balancing output transformer will raise the output by five times (14 dB) but driving long cables will be diffycult.

[Roger Gustavsson]

Then you have about 5 times (14 dB) more level than I have! The Edirol UA-5 seems to have a maximum gain of 50 dB (317 times). Sure you will have to crank up that level control. Depending on what you are recording, somewhere between 20-40 dB of gain is probably needed. Are you having problems with the background noise level at high gain settings? That is what I have with Nak300 into my Digigram soundcard. Running the Nak300 with MX-100 is without problem.

Roger 

[SparkE!]

You still have no DC path from the collectors of your PNP transistors to ground in Schematic G.  That one will not work.  Schematic E will still work and it would be the easiest to implement.

Here we go again with a new schemtic.... Are there any components that can be taken out?

I'd start by getting rid of the 12 V zener and replacing it with a wire.  Then I'd move the wire that goes to the 330 ohm resistor over to connect directly to the 12 V battery.  Then I'd add two resistors, each 10k or so, between the base and emitter of each of the PNP transistors.  (You had these added in a previous version of the schematic.)

[Roger Gustavsson]

Thank you, SparkE!

Here is a new schematic. Is it correct? Are the components in red needed?

[SparkE!]

Thank you, SparkE!

Here is a new schematic. Is it correct? Are the components in red needed?

Yes, the components in red will reduce the signal-induced power supply ripple in the second amplifier stage.

Now you don't have any DC path to the emitters in your second stage, though.  Do you have any circuit design experience?

[Roger Gustavsson]

I am a complete layman in electronics.... I want to run the F circuitry on batteries, 9-12V. That would be equal to P12 phantom power? Should the two 6k8 resistors be changed to 680R? What about that 12V zener? I do not know if anything else has to be changed to suit the 2SK118-O.

[SparkE!]

How about this?

[Roger Gustavsson]

Great!

Why R4/R5 = 10k? With C4/C5 lower cutoff frequency = 34 Hz. Should I increase the resistors or the capacitors for a lower cutoff frequency?