Simple open-source condenser studio microphone with a modern twist - Page 1
Forum section: electronics / general

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 18, 2023, 01:00:19 pm
content: 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_micCritiques welcome, I suppose.

author: JohanH
author_rank: Frequent Contributor
timestamp: March 18, 2023, 01:38:56 pm
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 18, 2023, 01:43:47 pm
content: Quote from: JohanH on March 18, 2023, 01:38:56 pmIt'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.

author: TimFox
author_rank: Super Contributor
timestamp: March 18, 2023, 01:49:22 pm
content: The 1 nF and 10 nF capacitors should be available in NP0/C0G.100 nF C0G capacitors are rather large.

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 18, 2023, 01:50:39 pm
content: Quote from: TimFox on March 18, 2023, 01:49:22 pmThose are ones that really matter the least, so going for price is more important.

author: MasterT
author_rank: Frequent Contributor
timestamp: March 18, 2023, 03:02:24 pm
content: 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?

author: Weston
author_rank: Regular Contributor
timestamp: March 18, 2023, 06:50:07 pm
content: 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 detectorYou 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?

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 19, 2023, 01:34:33 am
content: Quote from: MasterT on March 18, 2023, 03:02:24 pmSNR 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.Quote from: Weston on March 18, 2023, 06:50:07 pmC19 also doesn't particularly matter, and if the capacitance drops, all the better. You should only start to see frequency response issues when you goupabout 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 onlyreallyget 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)

author: JohanH
author_rank: Frequent Contributor
timestamp: March 20, 2023, 09:08:20 am
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 20, 2023, 10:14:18 am
content: 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 notcheapcheap.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

author: jonpaul
author_rank: Super Contributor
timestamp: March 20, 2023, 11:00:58 am
content: sorry cannot decode any of the files on GitHub.Please post a simple JPG image of SCH, PCB and device.Jon

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 20, 2023, 11:04:43 am
content: Quote from: jonpaul on March 20, 2023, 11:00:58 amThat's all on GitHub.The schematic is the first thing on that page... The rest is a KiCad project.

author: JohanH
author_rank: Frequent Contributor
timestamp: March 20, 2023, 02:11:49 pm
content: Quote from: Spirit532 on March 20, 2023, 10:14:18 amYou 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 20, 2023, 02:15:50 pm
content: Quote from: JohanH on March 20, 2023, 02:11:49 pmIt'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.

author: jonpaul
author_rank: Super Contributor
timestamp: March 20, 2023, 06:24:23 pm
content: 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

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 21, 2023, 02:15:46 am
content: Quote from: jonpaul on March 20, 2023, 06:24:23 pmScroll down on the github page. It's a finished build. Pictures are there.

author: jonpaul
author_rank: Super Contributor
timestamp: March 21, 2023, 03:39:03 am
content: 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

author: JohanH
author_rank: Frequent Contributor
timestamp: March 21, 2023, 07:13:36 am
content: 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#schematicThe schematic is also in pdf format. Here's the direct linkhttps://github.com/Spirit532/studio_mic/raw/main/StudioMic.pdfI 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/

author: jonpaul
author_rank: Super Contributor
timestamp: March 21, 2023, 12:25:58 pm
content: 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

author: Spirit532
author_rank: Frequent Contributor
timestamp: March 21, 2023, 12:34:53 pm
content: Quote from: jonpaul on March 21, 2023, 12:25:58 pm2023, latestQuote from: jonpaul on March 21, 2023, 12:25:58 pmNone, besides a Keithley 2000.Quote from: jonpaul on March 21, 2023, 12:25:58 pmNo clue. I'm accepting donations of free anechoic chambers and calibrated transducer kitsSNR probably between "a lot" and "tons", dynamic range is between "plenty" and "good enough".Quote from: jonpaul on March 21, 2023, 12:25:58 pmNone. Runs just fine without earthing, near lots of electrical noise. EMI filter doesn't need any work, simpler is better.Quote from: jonpaul on March 21, 2023, 12:25:58 pmChina. Spec is "sounds good". It's an AKG CK12 clone, with thinner mylar.Quote from: jonpaul on March 21, 2023, 12:25:58 pmWhatever 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.Quote from: jonpaul on March 21, 2023, 12:25:58 pmSignificantly 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.

author: JohanH
author_rank: Frequent Contributor
timestamp: March 21, 2023, 02:15:09 pm
content: Quote from: jonpaul on March 21, 2023, 12:25:58 pmJon, 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.

author: EC8010
author_rank: Frequent Contributor
timestamp: July 12, 2023, 09:10:50 am
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 12, 2023, 06:58:13 pm
content: Quote from: EC8010 on July 12, 2023, 09:10:50 amThe 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 15, 2023, 02:49:54 am
content: 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?

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 15, 2023, 02:51:20 am
content: The outer housing is the shield, and with the not-insignificant capacitance I wouldn't.

author: RFDx
author_rank: Regular Contributor
timestamp: July 19, 2023, 05:08:31 am
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 19, 2023, 05:13:50 am
content: Quote from: RFDx on July 19, 2023, 05:08:31 amNope. Just off the top of my head.Quote from: RFDx on July 19, 2023, 05:08:31 amAvoids 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.Quote from: RFDx on July 19, 2023, 05:08:31 amhttps://en.wikipedia.org/wiki/Virtual_ground

author: Marco
author_rank: Super Contributor
timestamp: July 19, 2023, 10:45:47 am
content: The analog ground connection should ideally be in the middle of the bias string, the extra parasitic capacitance imbalances the input.

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 19, 2023, 10:48:32 am
content: Quote from: Marco on July 19, 2023, 10:45:47 amCan you illustrate what you mean by this?

author: Marco
author_rank: Super Contributor
timestamp: July 19, 2023, 10:58:00 am
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 19, 2023, 10:59:29 am
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: July 19, 2023, 11:33:01 am
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 19, 2023, 11:35:14 am
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: July 19, 2023, 11:56:37 am
content: 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.

author: Spirit532
author_rank: Frequent Contributor
timestamp: July 19, 2023, 11:59:21 am
content: Quote from: Marco on July 19, 2023, 11:56:37 amIt'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.

author: Marco
author_rank: Super Contributor
timestamp: July 19, 2023, 12:08:26 pm
content: 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.

author: RFDx
author_rank: Regular Contributor
timestamp: July 21, 2023, 07:11:05 am
content: Quote from: Spirit532 on July 19, 2023, 05:13:50 amWhat I was wondering was why do you need10x 100 MOhm in series? You would get the same result with 200 MOhm for R13 and the opamp bias resistor.Quote from: RFDx on July 19, 2023, 05:08:31 amThat's right. A node that has the same voltage potential as GNDA but isnotconnected 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.

author: Zipdox
author_rank: Regular Contributor
timestamp: March 04, 2024, 10:47:42 am
content: 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.

author: Valueduser
author_rank: Contributor
timestamp: May 10, 2024, 03:56:08 pm
content: This is seriously cool, thanks for sharing! I can't wait to make one.

author: Md tarique anwar
author_rank: Newbie
timestamp: August 13, 2024, 10:16:11 am
content: Certainly! When a simple open-source condenser studio microphone with a modern twist, you might consider.