Simple open-source condenser studio microphone with a modern twist

[Spirit532]

Everything is on github, not going to duplicate the description here, just going to say that this was my first time using a hex inverter as a charge pump, maybe there's people here who aren't aware that this is a cool trick to get high voltages with low part count, price, and noise.

https://github.com/Spirit532/studio_mic

Critiques welcome, I suppose.

[JohanH]

Interesting, thanks for sharing. I've thought of getting a (better) mic, but it would be a great idea to build one instead.

Just a comment about ceramic capacitors. Any class 2 capacitor will have loss of capacitance at higher voltages. It doesn't help with higher voltage rating. Larger package and higher temp dielectric helps somewhat (so X7R/X8R in larger case). But class 1 capacitors are best, C0G(NP0). They will have virtually no loss of capacitance at higher voltages. They are more expensive, though, and isn't available in higher capacitance values and all package sizes.

[Spirit532]

Any class 2 capacitor will have loss of capacitance at higher voltages. It doesn't help with higher voltage rating.

It's good that you pointed that out!
I'm aware of this, but the note I made about capacitors is more to do just with the voltage rating. 100V caps will typically be X7R(at least the ones I've looked at briefly), which have good enough capacitance for what's happening in the circuit. Derating by even 70% is acceptable, but it's the unexpected behavior(failure, really) if you go too close to the rated voltage that's the biggest issue.
Since it's aimed at less experienced people and perhaps non-EEs, I just chose to generalize, if a bit wrong.

[TimFox]

The 1 nF and 10 nF capacitors should be available in NP0/C0G.
100 nF C0G capacitors are rather large.

[Spirit532]

The 1 nF and 10 nF capacitors should be available in NP0/C0G.
100 nF C0G capacitors are rather large.

Those are ones that really matter the least, so going for price is more important.

[MasterT]

I wander what is the voltage comes out of mic, that you could afford translate signal over diff pair w/o amplification? SNR is not in consideration?

[Weston]

I saw this on twitter earlier today, really cool work! Good documentation too. With a few changes you could probably have an ionization chamber radiation detector 

You probably don't need to worry about NP0 capacitors for the charge pump, but class 2 capacitors have a strong temperature coefficient too. If C19 is not NP0 you could see changes in the low frequency cutoff of the microphone with ambient temperature.

I would be a little bit worried about the charge pump injecting noise back into the 12V power rail with the fast transitions of the hex inverter. Have you scoped out the 12V power rail for switching noise?

[Spirit532]

I wander what is the voltage comes out of mic, that you could afford translate signal over diff pair w/o amplification? SNR is not in consideration?

SNR is not an issue, unity gain is typical for mics. You have to remember that you're doing a massive impedance conversion, the mic capacitor charged to 84V going from >xxGOhm to ~1G, with a FET input that draws no current, you get quite a high signal.
In this configuration, the mic can pick up me whispering across the room at max gain of the preamp(I'm using a UMC204HD), with the only noise I can hear being wind from the open window and my (almost silent to the ear) PC fans.

Everything I'm saying is handwavy because I don't have an isolated scope with high impedance inputs to actually poke at it.

I saw this on twitter earlier today, really cool work! Good documentation too. With a few changes you could probably have an ionization chamber radiation detector 

You probably don't need to worry about NP0 capacitors for the charge pump, but class 2 capacitors have a strong temperature coefficient too. If C19 is not NP0 you could see changes in the low frequency cutoff of the microphone with ambient temperature.

I would be a little bit worried about the charge pump injecting noise back into the 12V power rail with the fast transitions of the hex inverter. Have you scoped out the 12V power rail for switching noise?

C19 also doesn't particularly matter, and if the capacitance drops, all the better. You should only start to see frequency response issues when you go up about an order of magnitude, I just chose 1nf because it would be a duplicate part in the circuit, fewer values to buy.
The capsule also has a highly nonlinear response below the "normally audible" range of ~20-30Hz, and varies quite a bit unit to unit, so even if I wanted to measure its effects, I probably couldn't without a full anechoic chamber and calibrated freq response metrology mics.

Noise in the power rail is a valid concern, but filtering thankfully works both ways(the high impedance power path has its advantages), and since there's basically no inductive component to the charge pump(*ideally), you'll only really get noise at the oscillation frequency and higher(*ideally), which is at least 10x higher than the audio range, and doesn't matter at all.

(same handwavy response as above, don't have the right scope setup to poke at it)

[JohanH]

I tried to find some information about these clone microphone capsules. According to some sources they are based on a design similar to the original K67, even the edge connected CK12/RK12 clones. Anyway, some of them are rumored to be quite bright sounding with several dB peak at around 10 kHz, similar to the original K67. So I got thinking, would it be clever to include a switchable notch or low pass filter in the circuit, and what would be the best way. Now all of this is speculation as I don't know much about the capsules.

[Spirit532]

Unfortunately you really need a calibrated transducer and an anechoic chamber to test any of this, or at least a calibrated transducer and a good measurement of the room. Not to mention a good handful of these capsules, and while cheap, they're not cheap cheap.
The edge terminated capsules seem to fare a bit better and have a lower response at the high end, but they all use 3um mylar instead of 6um, so they will tend to ring at higher frequencies more than the old ones. I haven't found a good source of 6um mylar, and my sputter coater isn't quite ready yet, so I can't start building my own

[jonpaul]

sorry cannot decode any of the files on GitHub.

Please post a simple JPG image of SCH, PCB and device.


Jon

[Spirit532]

sorry cannot decode any of the files on GitHub.

Please post a simple JPG image of SCH, PCB and device.


Jon

That's all on GitHub.
The schematic is the first thing on that page... The rest is a KiCad project.

[JohanH]

Unfortunately you really need a calibrated transducer and an anechoic chamber to test any of this, or at least a calibrated transducer and a good measurement of the room. Not to mention a good handful of these capsules, and while cheap, they're not cheap cheap.
The edge terminated capsules seem to fare a bit better and have a lower response at the high end, but they all use 3um mylar instead of 6um, so they will tend to ring at higher frequencies more than the old ones. I haven't found a good source of 6um mylar, and my sputter coater isn't quite ready yet, so I can't start building my own

You are probably right, you would need some good measurement equipment.

Just another detail I wondered about. Do you have the microphone metal chassis connected to ground in the circuit, or is it floating?
This is unverified, but I read that some Neumann microphones had the chassis connected to circuit ground with a 10 ohm resistor.

[Spirit532]

Just another detail I wondered about. Do you have the microphone metal chassis connected to ground in the circuit, or is it floating?
This is unverified, but I read that some Neumann microphones had the chassis connected to circuit ground with a 10 ohm resistor.

It's connected to the XLR (&cable) shield, which I believe is connected to ground at the audio interface.
That would essentially make the whole thing a diffpair coax, which is probably the most optimal configuration.

[jonpaul]

Rebonjour a tous, BRAVO to the OP on the project.

The advent of ECM since 1970s has greatly changed the mkt for true HV condenser, eg Neumann/Valves.

We have a fine Akai ECM mic bought at a fle in Paris for $7.



Is your design  built and tested or just a schema?

Happy to comment an markup esp on the VM 100V, phantom, EMI and Hi Meg R.

Whose capsule using?

Let me know what is your project stage.

Amicalement,


Jon

[Spirit532]

Is your design  built and tested or just a schema?
Whose capsule using?

Scroll down on the github page. It's a finished build. Pictures are there.

[jonpaul]

github is a pita, way too many oddball extt files.

For non gît hub folks, PLEASE  just post your design, schematic and project pix as images direct.

j

[JohanH]

Jean-Paul,

There is literally a web page on the github site, when you scroll down. Or direct link here: https://github.com/Spirit532/studio_mic#schematic

The schematic is also in pdf format. Here's the direct link https://github.com/Spirit532/studio_mic/raw/main/StudioMic.pdf

I found a youtuber that has made pretty much the same microphone interface, but he uses a two membrane capsule to get two signals for stereo or mixing (he has made multiple variants):



He demonstrates the ready microphone. It sounds good (as good as you can expect through youtube and random headphones). There is a web page for the same build, where you can see schematics etc.:
https://www.instructables.com/True-Condenser-OPA-Mics/

And another web page describing the amplifier/impedance converter part with schematics:
https://www.instructables.com/OPA-Based-Alice-Microphones-a-Cardioid-and-a-Figur/

[jonpaul]

Rebonjour cher  JohanH, and Sprit 532: Many thahks for the alrts to github, links, etc!

Bravo for the mic projects!  Very fine! I was unaware of these DIY attempts to recreate the classics 1940s..1970s condenser mics with valves eg Neumann, Sennheiser,

Am retired Audio designer since 1970s, ( Sequerra, Dolby Labs, THX, Lucasfilm)  sat on AES digital microphones standards committee, AES-42 in 1980s, invented the first digital mic.

Thus,  some experience and a lot of interest  in microphones!!

For the schema, 2018 KiCAD, before I can make a cogent comment,  a few questions please!

0/ Test setup, equip used?

1/  Freq resp,  weighted SNR and DYN R  ?

2/ EMI susceptibility ?  XLR output EMI  filter needs work.

3/ What is source, spec  for the condenser capsule?

4/ What is source/spec of the 100M Ohm R13, 101 ..110 SMD resistors?

 5/ How is performance vs modern ECM mics?

Amicalment!

Jon

[Spirit532]

2018 KiCAD

2023, latest

0/ Test setup, equip used?

None, besides a Keithley 2000.

1/  Freq resp,  weighted SNR and DYN R  ?

No clue. I'm accepting donations of free anechoic chambers and calibrated transducer kits
SNR probably between "a lot" and "tons", dynamic range is between "plenty" and "good enough".

2/ EMI susceptibility ?  XLR output EMI  filter needs work.

None. Runs just fine without earthing, near lots of electrical noise. EMI filter doesn't need any work, simpler is better.

3/ What is source, spec  for the condenser capsule?

China. Spec is "sounds good". It's an AKG CK12 clone, with thinner mylar.

4/ What is source/spec of the 100M Ohm R13, 101 ..110 SMD resistors?

Whatever cheap stuff I could get at the local store. +-5%, doesn't matter much. Maybe a dB of gain here or there, the audio interface can compensate. Not aiming for Neumann quality or Neumann pricing. This is cheaper to build than some electrets are to buy.

5/ How is performance vs modern ECM mics?

Significantly better. Many times higher sensitivity, many times lower SNR, much flatter(but still not flat) frequency response.
This is expected of any large diaphragm condenser, modern or old. Electrets are good and more robust, but by no means better performance.

[JohanH]

4/ What is source/spec of the 100M Ohm R13, 101 ..110 SMD resistors?

Jon

Jon, is there a reason for wanting to have some better spec resistors in this place?

I'm thinking a long string of resistors is good against leakage, better than a single 1 Gohm resistor (unless through hole), so quality shouldn't matter. A guard ring around opamp inputs could help if PCB becomes dirty with time etc.

[EC8010]

All large capsule microphones have a bit of a peak around 10kHz or so. It's as well to remember that recording microphones are not the same as measurement microphones and are chosen for whether or not they favour their subject. I wouldn't worry about the peak.

The capsule is likely to have a source capacitance of around 30pF or less. That makes it a high impedance source and construction quality of your electronics becomes paramount. Notably, cleanliness. Everything has to be squeaky clean, and bare fingers shouldn't go near high impedance parts or noise will result. And don't breathe on it (water vapour in exhaled breath). The main issue with a 100M surface mount resistor will be surface leakage. When I used a 1G surface mount resistor, I routed a large hole under its body so that dirty defluxer etc couldn't be trapped under the resistor and increase leakage. You'll find that's quite a common technique where low leakage is needed.

[Spirit532]

The main issue with a 100M surface mount resistor will be surface leakage. When I used a 1G surface mount resistor, I routed a large hole under its body so that dirty defluxer etc couldn't be trapped under the resistor and increase leakage. You'll find that's quite a common technique where low leakage is needed.

The 1G assembly out of 100M resistors is fairly long and realistically, it doesn't matter if there's a small amount of DC leakage or not, because it's all dissipated. The signal is AC coupled by nature, and I've not noticed any significant performance differences between a completely clean, degreased and DI rinsed board and one that I touched with my fingers.

A lot of the audio stuff is way overblown in importance, as is natural with audiophools.

[NiHaoMike]

Since the microphone element itself is enclosed behind a shield, wouldn't it make more sense to connect its casing to the high voltage supply and then the signal output go to the amplifier with some bootstrapping tricks to eliminate the need for such high value resistors or a low leakage coupling cap?

[Spirit532]

The outer housing is the shield, and with the not-insignificant capacitance I wouldn't.

[RFDx]

Very nice build. Thanks for sharing.

Is there a specific reason for the bias resistor R13 to be only 100 MOhm? R13 sets the lower cutoff frequency. Why do you need a 1GOhm bias resistor in front of the noninverting op-amp? Btw. the ground in the +/-6V supply is "artificial". There's nothing "virtual" about it .

[Spirit532]

Is there a specific reason for the bias resistor R13 to be only 100 MOhm?

Nope. Just off the top of my head.

Why do you need a 1GOhm bias resistor in front of the noninverting op-amp?

Avoids DC charging of the input. Given the extremely high(many GOhm, and not particularly well characterized) input impedance of the opamp, it can cause a serious charge to build up on the input and just rail it. I've tried it without the resistor, and found that this happens.

Btw. the ground in the +/-6V supply is "artificial". There's nothing "virtual" about it .

https://en.wikipedia.org/wiki/Virtual_ground

[Marco]

The analog ground connection should ideally be in the middle of the bias string, the extra parasitic capacitance imbalances the input.

[Spirit532]

The analog ground connection should ideally be in the middle of the bias string, the extra parasitic capacitance imbalances the input.

Can you illustrate what you mean by this?

[Marco]

GNDA has 47uf capacitance to GNDREF, so for AC that's what it is. Which means the bottom opamp is doing nothing, it's single ended in disguise.

[Spirit532]

It's not doing nothing, it does provide a differential signal(that I've measured), but there is an imbalance due to the way they're linked. Solving this would require adding a buffer, which would likely blow the power budget. It's already stretched pretty thin with the high impedance takeoff.

[Marco]

Voltage wise it's creating a differential signal on the XLR connector, but it's creating it from an imbalanced connection to the capsule. A voltage differential signal an sich has no real value at mic level, it's not like the mic level inputs will saturate, it's cargo cult differential. If you made the top opamp a simple unity gain buffer, removed the bottom opamp and just connected the bottom coupling capacitor directly to GNDA you would have superior noise.

If GNDA was connected in the middle of the string of bias resistors instead of the bottom, the connection to the capsule would be balanced and the differential amplification would have some justification.

[Spirit532]

It's a differential signal going down the cable for common mode immunity, there's absolutely no point in doing differential pickup from the capsule.
All audio interfaces expect a differential input by default. That's what the second opamp is for. Yes, you can ignore that and take it single ended from ground.

[Marco]

It's not the differential signal an sich which gives you noise immmunity, it's the matched impedance and a differential input on the receiving end. You can have matched impedance on a single ended output just fine.

That's why this type of differential amplification is cargo cult, it looks at completely differential signal chains and confuses an unimportant artifact (the signal being differential to a reference voltage) and confuses it for being essential. It ain't, all you are achieving by recreating the artifact is increasing amplifier noise.

[Spirit532]

It's not the differential signal an sich which gives you noise immmunity, it's the matched impedance and a differential input on the receiving end. You can have matched impedance on a single ended output just fine.

That's why this type of differential amplification is cargo cult, it looks at completely differential signal chains and confuses an unimportant artifact (the signal being differential to a reference voltage) and confuses it for being essential. It ain't, all you are achieving by recreating the artifact is increasing amplifier noise.

It's partially true, but having it be even slightly mismatched differential will give an edge over completely single-ended. It matters less when the whole thing is shielded, and it is, but as this is an opensource project I'm expecting other, less experienced people to run into blunders like running cheaper unshielded cable, plugging it into questionable gear and such, in which case it does actually help.

[Marco]

Better make those 47 Ohm resistors very high precision then. In audio range the output impedance of the opamp is a bee's dick.

PS. AFAICS with something modern like an OPAx828, a 0.1% difference between the 47 Ohm resistors causes more impedance mismatch across the audio range than dropping the bottom opamp.

[RFDx]

Why do you need a 1GOhm bias resistor in front of the noninverting op-amp?

Avoids DC charging of the input. Given the extremely high(many GOhm, and not particularly well characterized) input impedance of the opamp, it can cause a serious charge to build up on the input and just rail it. I've tried it without the resistor, and found that this happens.

What I was wondering was why do you need 10 x 100 MOhm in series? You would get the same result with 200 MOhm for R13 and the opamp bias resistor.

https://en.wikipedia.org/wiki/Virtual_ground

That's right. A node that has the same voltage potential as GNDA but is not connected to it would be a virtual ground. The inverting input of U1B is indeed a virtual ground. The non-inverting input of U1B, which is connected to the (real) GNDA from the rail splitter, isn't.

[Zipdox]

Have you looked at the JLI electronics mic kits?
https://www.jlielectronics.com/diy-accessories/opa-alice-single-channel/
https://www.jlielectronics.com/diy-accessories/dc-dc-hex-inverter-pcb/
How does it compare?

Also, have you considered using two 6V zener diodes in series to create the virtual ground and 12V supply? That would result in a lower power-on time and reduce component count a bit.

[Valueduser]

This is seriously cool, thanks for sharing!  I can't wait to make one.

[Md tarique anwar]

Certainly! When a simple open-source condenser studio microphone with a modern twist, you might consider.