Homero,

That's a really cool project and a nice tribute to Pat.?

Would love to hear a comparison to a regular mic.

Thanks,
-Scott

On 4/12/22 20:22, Homero Leal wrote:

HI folks!

As some of you may know, one of the dreams of our friend Patrick Davis (aka. P@T, Enjoybiking) was the idea of designing a mic that could have the "tube" sound, but that could also be very cheap to build.

We discussed the topic many times, and the idea of having a solid state tube emulation was a really fascinating option that was always on the table.

Some time ago I found a paper from Dimitri Danyuk called "Triode Emulator", and in this paper he proposes very a simple way to "emulate" triode behavior using a JFET with very specific values for drain and source resistors.

Later, I found this page:



Which provides a way to calculate (look at the end of the page) the required drain and source resistor values using some JFET parameters like:

• VP
• IDSS

And also the input voltage of the circuit (VCC).

So, I decided to try a design using the Dimitri Danyuk approach, with the values calculated from the Fetzer Valve page.

You could find the schematic here:

/g/MicBuilders/files/Homero%20Leal%27s%20Files/Pat%27s%20Mic/Pat%27s%20Mic.png

Circuit design uses a Charge Amplifier input stage, a JFET middle stage (providing the "valve" tone"), and finally a impedance balanced output stage with a Sallen Key HPF at about 19.XHz.

Also, a custom voltage regulator is used, using a BC549C NPN and a zener, very similar to the one used on some very known chinese mics. This VR provides about 9.4V for VCC.

I used two drain resistors and two source resistors, so it could be easier to approach the right values calculated by the Fetzer Valve page, by using the combination of two resistors.

As the JFET stage may have an amplification level on the signal, I also used two capacitors for the charge amplifier, this way you could reduce output level by placing an additional capacitor (some math required).

I know the design may have some room for improvement, but I can confirm I have working mic with this design, and it seems to my untrained ear, that it has a very sleek and warm sound, very similar to the sound of the valve mics.

For the build, I used a cheap Zramo donor mic from Amazon, which that very nice 26mm electret capsule we have talked about before.

I decided to name this schematic? "Pat's Mic"... in memory of our friend and former member of this group, P@T?(Patrick Davis, R.I.P.)

This post is for your review and consideration, and please if you have any ideas for improvement, don't hesitate to let me know.

Thank you, and kind regards!

HL

-- 
---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
"I have ceased distinguishing between the religious and the secular,
for everything is holy"  - Joe Henry

Any recording tests you could share?

Why do you need RD1 and RD2, wouldn't be one RD enough? Same for RS.

Would we need a HexInverter or something else for the polarisation Voltage?

With the OPA and the Fet and T1 could there be a problem with the current from the Phantompower?