DIY RF Condenser Mics

[rogs]

If there was no resonance, I wouldn't be getting hundreds of millivolts RF at the gate.

The amplitude of the RF at the gate can be quite large - even without considering any resonance.
It will depend on the difference in value betwen the capsule capacitance and C4..... In other words, the amount of 'bridge' imbalance.
It is the change in amplitude level at the gate that determines the signal output, not the level of RF present.

 

[shot]

Sadly, removing those cans from an assembled through plate PCB, without causing damage, I've found to be almost impossible.

I must admit that I had been removing cans more than ten times from the same board!
The trick was to use hot air gun! I was just heating it's pins until I was able to pull out the can using minimal force. After that I had to clean pcb thoroughly, then rewind a new core, solder it in, try to calibrate, dissapointment, unsolder all again... repeat... repeat... repeat...
So it's possible!
But very time consuming



Luka

 

[rogs]

I must admit that I had been removing cans more than ten times from the same board!
The trick was to use hot air gun! I was just heating it's pins until I was able to pull out the can using minimal force. After that I had to clean pcb thoroughly, then rewind a new core, solder it in, try to calibrate, dissapointment, unsolder all again... repeat... repeat... repeat...
So it's possible!
But very time consuming



Luka

I have manged to succeed removing cans from th PCBs .. although it's not easy with a through plate boards that has a ground plane!
It usually wrecks the can - or the board -- or both! - and as you say, it's very time consuming....

What I have been trying is to build the oscillator and inductor assembly onto a good old fashioned 'breadboard'... OK, not ideal for 10MHz RF circuits - especially from a 'capacitive leakage' point of view - but it is still possible to calibrate and test different arrangements, by just plugging the cans in and out.
Although the capacitance leakage between the 'breadboard' tracks serve to mess up the calibration somewhat, it's useful to remember that if you only change one component at a time, whatever capacitive leakage is included in the whole assembly remains the same.
Any measured differences must relate to that single changed component.

All a bit crude of course, but quite effective - and certainly better than constantly removing the cans from the PCBs, when simply trying out new ideas!

 

[global-records]

I connected this stock capsule (65 pF, 1 ohm resistance from the ring to the center of the membrane) to the modified rogs circuit, where I replaced T2 and C8 with an inductor with 11 turns, the core turned out to be approximately in the central position (inductance about 2 µH). Also, I bought a 6.5-30 pF smd variable capacitor in parallel to C4=55pF (the one I found in the nearest electronics store). And it works pretty well at 10 Mhz. And it feels like it works a little better with the cap and screen removed. If the capsule is of an even smaller capacity, then the quality factor should be higher, but I did not check. In general, my plans are to reduce the capsule gap to increase sensitivity. I wonder if the diaphragm is glued to the ring?
voice test

 

[rogs]

I connected this stock capsule (65 pF, 1 ohm resistance from the ring to the center of the membrane) to the modified rogs circuit, where I replaced T2 and C8 with an inductor with 11 turns, the core turned out to be approximately in the central position (inductance about 2 µH). Also, I bought a 6.5-30 pF smd variable capacitor in parallel to C4=55pF (the one I found in the nearest electronics store). And it works pretty well at 10 Mhz. And it feels like it works a little better with the cap and screen removed. If the capsule is of an even smaller capacity, then the quality factor should be higher, but I did not check. In general, my plans are to reduce the capsule gap to increase sensitivity. I wonder if the diaphragm is glued to the ring?
voice test

Sounds pretty good! ....... Can you post a schematic of your current circuit ? ... I'm still trying to work out the details of exactly how you're connecting the FET gate to the bridge....

 

[global-records]

 

[rogs]

View attachment 140928

Looks interesting .... What is your reason in replacing T2 with a simple inductor?... If you wind it onto a similar former, I can't see the benefit of not creating a second transformer.... it's only a few more turns onto the former.

Although the Spectrum coils are not perfect for the task, I would think that trying out different winding ratios for a second transformer might result in a higher 'Q' - with potentially more noise free signal gain than from a single inductor ? ..... I may be wrong of course!

 

[Gerard]

Fascinating to read of others who have found the project interesting.... Thanks for the link.
That forum seems to include some experiments with actual hardware, and not just with 'sims'
Interesting to read of the continuing experiments with various alternative transformer options..
This is one area where 'sim' software can be difficult (too many unknown variables!).
I was glad to read that there has been some more successful builds of variations of the project...

Interesting comment in https://forum.vegalab.ru/showthread.php?t=95404&p=3243405&viewfull=1#post3243405 and https://forum.vegalab.ru/showthread.php?t=95404&p=3243445&viewfull=1#post3243445. He suggests that some capsule diaphragms have a high resistance coating but others have a low resistance coating, and that, while a high resistance coating is immaterial in a conventional hi-Z mic circuit, it may be detrimental in an RF circuit. Is this another variable we should consider? How do we even measure it, without risking damaging the diaphragm?

A hi-Z circuit relies on a constant charge on the diaphragm, so (effectively) no current flow. Therefore, a high resistance diaphragm is not an issue. But for an RF-polarised diaphragm, the charge changes at 7-10 MHz. Could it be that some of the cheaper capsules have diaphragms that are not truely metallised, but have a high resistance conductive coaring? These would be unsuitable for use in an RF condenser mic. Maxim87 refers, in https://forum.vegalab.ru/showthread.php?t=95404&p=3260713&viewfull=1#post3260713, the comment above yours, rogs, to trying to silver his own diaphragms (before being shot down by Expresso with comments about sulphide formation on reactive silver).