Topic: Capsule amplifier in electret microphone

[Roger Gustavsson]

I was not doing the simulation myself. It was made by a friend from another forum. He is not using SPICE as far as I know.

 

[SparkE!]

The problem with the high distortion turned out to be a problem between my ears.  I needed to reduce the time step size in the simulation. Once I did that, I found that the distortion is negligible.  When I do the FFT, an artifact of the turn-on transient is what sets the noise floor of the plot and all of the harmonics are below that.  From what I can tell, the distortion has to be below 0.05%, but I don't yet know how far below.  Whatever the case, it's clear that the microphone distortion will be dominated by the response of the mic element itself and not by the amplifier.

[SparkE!]

OK, I finally got it figured out.  I needed to use both a smaller time increment and a longer sample.  The simulation took about 10 minutes to run and the distortion appears to be on the order of .007% which is clearly negligible compared to the distortion that will be introduced by nonlinearities in the mechanical deflection of the mic diaphragm itself.  Unintentional coupling within the layout will have a larger effect than that!

I knew that as a differential amp, it had to have a lower distortion than a single ended amplifier, but it surprises me at how low the distortion turns out to be.

[Roger Gustavsson]

You are still talking about your "NakMod.jpg"? What are you using for Q1 and Q2?

[Nick's Picks]

man...
I love people who know more than I do.


+T guys...

[SparkE!]

You are still talking about your "NakMod.jpg"? What are you using for Q1 and Q2?

Yup, that's what I'm talking about.  Q1 and Q2 are modeled right now as 2N3906 transistors.  Nothing special required since they are just emitter followers.  You could probably choose something quieter.

[Roger Gustavsson]

I have found a simular circuit. Please, see below. I have adapted it for my Nakamichi capsules. Will it work? Are the 10 kOhm resistors necessary? What happens if I lower the supply voltage to 7.2 or 9V?

[SparkE!]

It'll work just fine.  You need to just build some circuits and try them.  I keep expecting you to come back to the list with some actual information and you just keep coming back with more questions.

The 10k resistors could probably be omitted, but they don't hurt anything.  They may actually lower the harmonic distortion a little, but it would not be a dramatic decrease.

[Roger Gustavsson]

Yes, I will build this circuitry... The JFET and the 1 GOhm resistor will sit very close to the capsule. The rest fill fit on a tiny board, just 25 x 12.5 mm. Like my attached drawing. I take your word that it will work, as I cannot confirm it with meassurements. It is also difficult to find the JFET and the highvoltage resistors but I have them now.

[Roger Gustavsson]

I just realized that it will be a lot easier to stay with a 2-pole connection between the microphone capsule and the main body, just like in the original Nakamichi CM-300. That will not let me use the Hicks' circuitry. Went back to the P93 designed by Rod Elliot. Are there any way I can use it with a 2-wire/2-pole connection? Maybe like the Kudelski (Nagra) supply? Not with +62 V but +9 or so...