6mm Electret & reusing BM800 PCB test

[KZhack14]

I recently got my hands on some 6mm electret from a China seller that has below listed specs:

Sensitivity: 52dB
Impedance: 2.2 KΩ-30%
Flat rate: 20 ~ 16.000HZ
Operating voltage: 10V (TVU)
Reference operating voltage: 4.5V
Current consumption: 0.5mA (max)
Sensitivity attenuation: ≤-3dB at 2-1.5v

Now as far as I understood electrets, most of them should have a built in FET that will already charge & polarize their diaphragm and will work as long as you supply power to the right leg. However my test with the below circuit (and another simple condenser circuit, both circuit works with the default electret that came in the cheap BM800 mic) yielded nothing no matter which leg I connect to, so either I'm misunderstanding something, or this is actually a non-polarized capsule that requires an external FET to work: (If this SDC could work, I plan to put some of them into a Simple P48 circuit.)

(This is the XJM0013A electret ciruit, and my frequency measurement matches with the findings in this blog - A Few More China Mic PCBs Measured, also measured an output voltage of 7.7+, so the zener diode is possibly a 9v - measured direct probing @ 8.8V)

So as I've a few more experiment/test to play with, I hav some questions regarding them below:
1. How do you identify electrets which need external FET(or not for the matter, the electret model no. here is CWFBI0350 / 7D0266B)?
2. On a side note, I plan to modify the XJM electret circuit shown above (by changing the zener to possibly a 11V or 10V to increase the operating voltage closer to 10V for most electret capsules), will this also increase the noise floor alongside the sensitivity?
3. In electret circuits like these, if I liked to clean up the signal by adding additional HPF stage, should I implement it before or after the FET (Can the same be applied the same way and be as effective in a simple P48 circuit?)
(Mic Circuits From Simple to Professional Source pic taken from here too)

4. Also regarding the simple circuit found above in some of the cheap BM800 mics, since they are unbalanced, is it possible to make them a pseudo-balanced circuit by adding something like a RF filter like described in this (Mic Circuits From Simple to Professional) from pin1 to pin 3 of the xlr?


Thanks a bunch in advance, and thank you for your patience & attention.

P.S before anyone asks, I'm having various sized electret capsules (9mm, 16mm, 25/26mm) coming soon, I'm planning to make a male XLR mount for them so that I can combine the electret circuit easily withn anyone of them once I mount the PCB into a pencil sized shell)

 

[k brown]

I can't answer all your questions, but I can clear up a few things for you.

The FET in a microphone that the signal passed through is functioning as an impedance converter (and also often a phase splitter as well), and has no role in polarizing the diaphragm; that's done with separate circuitry in externally polarized capsules; electrets hold a permanent charge.

Virtually all capsules with FETs built in are electret; the only exception I know of (there may be others) is the Schoeps V4 U, which has small circuit board on the back of the capsule that contains the FET and a few related SMD parts.

But, there are a great many electret capsules that have no built-in FET. Primo makes one (I think only one), and Transound/JLI has many. Various 'no-name' Chinese makers also offer LDC electrets that have no FET.

FWIW, the best electret SDC options out there right now are the Primo EM200 (or EM204) for cardioid, and the Primo EM273 (or EM272) for omni. Primo no longer lists the EM200/204 for some reason, but micbooster.com (FEL Communications) still has stock remaining.
https://micbooster.com/22-primo-microphones

 

[KZhack14]

So best I can do now is assume this capsule is a no-FET capsule and try it out with an FET?

 

[k brown]

If you're referring to the capsules shown in 1st post, it lists 'operating voltage 10v', so they must have FETs, as electret capsules w/o FET require no voltage.

The vast majority of these small, inexpensive electret elements have FETs.

 

[KZhack14]

How strange then. If they already have FET, in theory it should work with the above circuit... either that or they're duds

 

[ccaudle]

If they already have FET, in theory it should work with the above circuit...

I would not assume that unless you trace out the circuit and verify. And that link you provided basically says most of those circuits are incorrectly designed and barely work at all but the vendors never noticed (PNP transistor installed in a circuit designed for NPN), which suggests incompetence in the extreme.

This description on that web page of the BM800-style circuit:
"The modded ZRAMO is a BM800 style PCB with added power filtering and the JFET reconfigured as a source follower."
says that the circuit has a JFET. If the circuit has a JFET, that means it is designed to work with a capsule which does not have a JFET. The capsule you have almost surely does have a JFET inside, so you would have to do some modifications to get that capsule to work with the circuit you have in hand.

 

[KZhack14]

Hmm ok got it. Gonna grab myself some more different cheap capsules to test out first then

 

[KZhack14]

So I got my hands on some more samples of BM800 locally (less than $7-10 per pc, I got them for the bodies only) And they came with the below pcb board (Audio improv's blog showed their general schematic, which looks exactly like this):



Now I feel like it would be a waste throwing them away, I thought it might be a good idea to combine them with some modifications so that I can reuse it as a balanced pcb to test some electret capsules. (Drawn circuit below vs the doubling circuit from audioimprov):



A few questions that I would like some insight on:
1. The original circuit used quite a large resistor (2M) between the PNP transistor & capacitor to Ground. Can I use a much smaller resistor (10k+/-) to turn it into a ~16hz HPF w/o affecting anything like the power fed to the capsule?
2. I see in Helmke's Alice Circuit that drain load resistors on both legs of the capsule go to ground. Can I safely reposition the 1.5k drain Resistor on the +ve leg of the electret to ground? Or rather, can I just maintain the original connection of the drain resistor to pin 3 of the parallel circuit? (also thinking of adding a LPF stage after pin 2 & 3 to further clean up the output stage)

EDIT: Redrawn the PNP Transistor symbol as pointed out kindly by @Khron. Didn't realize that until I searched a bit and found it looked like a BA SOT-23 transistor variant

 

[KZhack14]

So I had some time again today to play around with the circuit above. Drawing inspiration from Henry Spragen's analysis & PZM Modifications balanced electret circuit by Christopher Hicks, I boot strapped a couple of components to finally arrive at this circuit:



Previously before bootstrapping the additional components I found my experience with the circuit similar to AnalogSteph's analysis of the BM800 circuit, where there was a drastic LF Roll off from around 500-600Hz (persumably due to distortion) and it just made whatever electret I tested on harsh sounding - peaky in the 3k-6k region. I figure adding a filter Cap between input stage & ground should at least clean up the whole thing, so I went ahead and inserted a 3300uF 16V electrolytic cap as seen in pic.

Sounds much better now, though I did notice HF roll off around 12khz onwards (not sure if its due to capsule type). LED is there for phantom power indicator & for bling )

While testing capsules I also found that this particular 16mm electret: does not work with the previous pencil condenser circuit in my earlier post nor the circuit above as well. Using a 2SK170BL I had on hand as a common source solved the issue. I thought I might as well give it a try with the 6mm capsule in my 1st post here too, but although its able to pickup me blowing directly into the capsule, any other sound source is either too weak or inaudible, just plain noise when I turn up the preamp gain.

Any ideas as to why it may be so?

 

[Khron]

Are the transistor you used, NPN or PNP after all?

I found this as per the reference you gave:
http://www.uneeda-audio.com/pzm/pzm_ch.htm
Under "Method 2", it says:
"2) Component EBC is a PNP bipolar transistor"

https://www.shutterstock.com/image-vector/npn-pnp-transistor-symbol-260nw-1948398364.jpg

 

[KZhack14]

Are the transistor you used, NPN or PNP after all?

I found this as per the reference you gave:
http://www.uneeda-audio.com/pzm/pzm_ch.htm
Under "Method 2", it says:
"2) Component EBC is a PNP bipolar transistor"

https://www.shutterstock.com/image-vector/npn-pnp-transistor-symbol-260nw-1948398364.jpg

Its a BA SOT-23, so should be a PNP, if the datasheet i found online is correct?

Also, 2SK170BL should be a NPN according to specsheet yes?

 

[k brown]

170BL is a FET.

 

[KZhack14]

170BL is a FET.

Thank you for correcting me.

Anyway, since its established that I need an FET to pair with the small electret, any recommendations for the type of transistor i could use besides 170BL?

 

[k brown]

I believe 2SK660 was specifically designed for use with electrets.

 

[KZhack14]

I believe 2SK660 was specifically designed for use with electrets.

I'll check it out.

Also, for the output stage transistors. I have couple of old S9013 type NPN transistors. Can I use them to make another condenser circuit?

 

[KZhack14]

Was looking around online for some electret circuits that would utilize NPN transistors as the amplifier stages and stumbled upon this circuit from EEWEB




Was wondering if the circuit adaptation to a phantom powered mic input is as simple as swapping some components as above, and just pulling phantom power straight from pin 2 & 3 of the XLR? (perhaps still need some impedance matching?) The brown lettered components are what I had lying around.

 

[Mixermend]

You should be able to get this working with a few changes.
Reverse the polarity of the two 10uF output coupling capacitors (consider increasing the value to 47uF?). In the feed to the electret capsule drop the voltage supply rail with a series 6K8 resistor, with a parallel 9V1 zener diode, to reduce the mic capsule feed voltage. The 1K0 feeding the capsule should be increased in value, 2K2 or 4K7 would be a good start when experimenting.
The two 6K8 Phantom resistors can probably be reduced in value (pair match them accurately for best balance) - again experiment to achieve a suitable voltage across the 220uF capacitor.

 

[KZhack14]

You should be able to get this working with a few changes.
Reverse the polarity of the two 10uF output coupling capacitors (consider increasing the value to 47uF?). In the feed to the electret capsule drop the voltage supply rail with a series 6K8 resistor, with a parallel 9V1 zener diode, to reduce the mic capsule feed voltage. The 1K0 feeding the capsule should be increased in value, 2K2 or 4K7 would be a good start when experimenting.
The two 6K8 Phantom resistors can probably be reduced in value (pair match them accurately for best balance) - again experiment to achieve a suitable voltage across the 220uF capacitor.

Got it thanks. Actually, I'm trying to make it work w/o the use of zener diodes. If that's the case, should I instead use another series Resistor (6.8k) & parallel filter electrolytic Cap on the DC rail towards Q2 collector leg to divide the voltage?

(I have some Red & RGB LED on hand, maybe I could stack a couple of them in series to get up to 9v? instead of zener)

 

[MicMaven]

Good Day Mr. @KZhack14

I am curious. What did you finally do with this project? Did you abandon the little circuit board in your opening post? Did you use the modified circuit you found on the Internet? Which capsule is in the final product?

I ask because I have some of those little boards you first mention, and wonder how you came out with them. I have had OK (not great but useful) results with them and various 16 mm FET electret capsules and a 25 mm JLI/Transsound JLI-2590A capsule. The latter capsule has somewhat limited frequency response (80 -10,000 Hz) but it works well enough for spoken word, radio communications, and computer speech recognition.

What finally worked for you? Thanks. James/K8JHR

 

[KZhack14]

Good Day Mr. @KZhack14

I am curious. What did you finally do with this project? Did you abandon the little circuit board in your opening post? Did you use the modified circuit you found on the Internet? Which capsule is in the final product?

I ask because I have some of those little boards you first mention, and wonder how you came out with them. I have had OK (not great but useful) results with them and various 16 mm FET electret capsules and a 25 mm JLI/Transsound JLI-2590A capsule. The latter capsule has somewhat limited frequency response (80 -10,000 Hz) but it works well enough for spoken word, radio communications, and computer speech recognition.

What finally worked for you? Thanks. James/K8JHR

Hello @MicMaven;

Haven't been updating the thread for a while because I've been setting up and running a small PA rental as a side business, so the little board was on the back burner for a while. I did not abandon the project, but I will most likely house it in a BM800 shell with a spare 25mm capsule that I had lying around and call it a day. (might even attempt a cheap figure 8 mod using a pair of 16mm capsules, haven't decided yet)