EEVblog #611 - Electret Microphone Design - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: May 03, 2014, 12:12:50 pm
content: Part 5 - Doug Ford explains the design of Electret Microphone circuits, and all the associated traps:Transconductance, correct biasing, and temperature effects, leaky JFETS, non-linear clipping & headroom, current noise and voltage noise, and pseudo bias resistors.Also, the non-intuitive concept of higher resistance = less noise, and how this applies to electret biasing.And what is a soggy circuit board and why is it important?

author: electronics man
author_rank: Frequent Contributor
timestamp: May 03, 2014, 01:30:18 pm
content: Very nice video very useful

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 03, 2014, 02:34:22 pm
content: When I've taken electret mics apart, I've wonderd about the lack of a bias resistor - I wonder if they use a special FET with some resistance built in?

author: Legion
author_rank: Frequent Contributor
timestamp: May 03, 2014, 03:07:32 pm
content: When you guys are talking about how they don't use bias resistors anymore and then about how a high value gate resistor reduces noise it got me wondering, when does "high resistance" become "open circuit"? If the answer is "when current stops flowing", what constitutes a stoppage of current flow? Does 1 electron/second still count as "current"?I guess what I'm getting at is, is it even meaningful to talk about a 100 teraohm resistor? Or is it an open circuit at that point?

author: ejeffrey
author_rank: Super Contributor
timestamp: May 03, 2014, 03:37:06 pm
content: You can have a resistance as high as you want as long as it is 'ohmic.' i.e., the current is proportional to voltage down to arbitrarily small voltages. This depends on the semiconductor physics of the material, not the value of the resistance. If you take a bunch of megaohm resistors in series for instance, you can get as large a resistance as you want. Insulators on the other hand, are simply not ohmic, at low voltages zero current flows. At high voltages current flows due to various mechanisms like field emission and electrical breakdown. Also, while the bulk of a material may not conduct, you can still get conduction along the surface due to water films. So when you see a specification for 'insulation resistance' it doesn't mean that the material acts as an ohmic conductor, but is rather a specification for maximum leakage current at some specific voltage due to those secondary mechanism.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 03, 2014, 03:52:34 pm
content: Those cheap microphones can easily be hacked for better performance.http://sound.westhost.com/project93.htm

author: jaxbird
author_rank: Frequent Contributor
timestamp: May 03, 2014, 03:58:51 pm
content: Big thanks to Doug Ford for this excellent seriesVery much enjoyed it. Doug is clearly very knowledgeable, experienced and a great communicator.Hope we get to hear much more from DougFor those interested in the complex design aspects of audio, I can recommend having a good look at thewww.linkwitzlab.comwebsite.Although I'm sure Dave would instantly label Mr. Linkwitz as an audiophoolHe has actually been designing instruments for HP for about 40 years or so. And he has received a lot of recognition together with Mr. Riley for their work in filters.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 03, 2014, 05:07:13 pm
content: Quote from: mikeselectricstuff on May 03, 2014, 02:34:22 pmA VERY common transistor used in cheap electret mic capsules is the 2SK596. It has built-in bias resistor AND protective diode as you can see within the dotted line portion of the circuit below...http://www.openmusiclabs.com/wp/wp-content/uploads/2011/03/2SK5961.pdf

author: calexanian
author_rank: Super Contributor
timestamp: May 03, 2014, 06:01:21 pm
content: I think we need an extended series of how things really work for the wanker musician and audiofool. It will end the ever present questions from said people when they learn I am in electronics, only to tell me how wrong my explanation is! Evidently these people think physics can lie!

author: jaxbird
author_rank: Frequent Contributor
timestamp: May 03, 2014, 09:37:14 pm
content: Quote from: calexanian on May 03, 2014, 06:01:21 pmMaybe you can be a bit more specific on what you have in mind? is the Doug/Dave presentation bogus?

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 03, 2014, 09:48:57 pm
content: The characteristics of the capsule itself are an important factor, of course. But the mechanical design of the complete microphone head, even the pattern of the grille, etc. have major effects on the performance of any microphone.And then there are decades of "tradition" and "superstition" in the entertainment biz about what microphone models are good for various applications. And there is a HUGE influence where wanna-be musos use a particular microphone (or guitar or whatever) because their favorite rock star uses that product.Rode is a relative newcomer on the scene of pro microphones. But they have made some significant contributions to the body of the world's better microphones. I am still waiting for Dave's promise of the discussion of designing very low-noise circuits.And I found the discussion of selecting the load resistance and supply voltage very interesting. But I'd like to know why some of the microphones at the very tip of the pinnacle of the very best microphones on the planet (DPA high-voltage mics) take 140V power supplies, even though they are electret and presumably don't even use such a remarkably high voltage to bias the capsule, or even to operate a firebotle (as they are solid-state).

author: jaxbird
author_rank: Frequent Contributor
timestamp: May 03, 2014, 09:57:46 pm
content: In my experience, all my quality microphones need the 48v phantom voltage from my mixer.That has not really covered by Doug in this series so far.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 03, 2014, 10:12:11 pm
content: 48V was the "standard" established back in the early 1960s by Neuman when they made the first solid-state mics for Norwegian Broadcasting Corp. Apparently NRK already had a good 48V supply (for their emergency lighting) and they just used it to power the microphones.Relatively few modern microphones need the full 48V to meet their published specs. A great many (especially electret) will operate perfectly fine on 24V or even 12V. Of course, the non-electret mics that operate on phantom voltage typically use the full voltage (nearly 48V after bypass/filtering) as the bias voltage across the condenser capsule. Here is the classic "Schoeps circuit"...This is Scott Dorsey's modification for the budget-priced true condenser mics, notably the Oktava products from Russia.The 1/2/3 pins at the right are the XLR output connector. R5 and R7 collect the phantom voltage directly, and filter/bypass it across C5, then persent it directly to the capsule through the R9, 1 gigohm resistor. The two terminals at the left go to the mic capsule, of course. Q3 provides the necessary impedance conversion, as well as the "phase-splitting" so that proper anti-phase outputs are presented through emitter-follower transistors Q1/Q2 to the balanced mic line on XLR pins 2 and 3.

author: jaxbird
author_rank: Frequent Contributor
timestamp: May 03, 2014, 10:28:33 pm
content: Quote from: Richard Crowley on May 03, 2014, 10:12:11 pmThanks for the nice schematicI'm sure my mics would work at a lower voltage with minor modifications, I was mostly wondering where the 48v came from. I'm surprised that we must blame the Norwegians for itWell then I guess the other Scandinavian Bruel and Kjaer have some blame as well.Cheers

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 03, 2014, 10:34:37 pm
content: Quote from: jaxbird on May 03, 2014, 10:28:33 pmWell DPA (Danish Pro Audio) are an offshoot from B&K because many recording engineers (particularly those of us who do classical music recording) started using B&K lab measurement mics, and DPA took over and made a range of products more suitable for audio recording (vs. laboratory measurements).

author: jaxbird
author_rank: Frequent Contributor
timestamp: May 03, 2014, 10:47:14 pm
content: Quote from: Richard Crowley on May 03, 2014, 10:34:37 pmHave to admit I have no knowledge of DPA, but I do remember buying a B&K mic with psu and certificate at some obscene amount many years ago.I can however appreciate a good classical recording with a full ~110dB dynamic range any time of the day

author: calexanian
author_rank: Super Contributor
timestamp: May 03, 2014, 11:14:35 pm
content: Quote from: jaxbird on May 03, 2014, 09:37:14 pmOh no. Quite the opposite! This is how things need to be explained. Doug is dead on all of these things. His few minutes of explaining how things work is worth a hundred "Expert" opinions of how things "Sound" Objective, not subjective. Musicians and audiophiles are more into the psychological value of a piece of equipment than any quantitative measurements or performance. I find that if you explain how something works rather than explain how it "sounds" the person will be able to understand why things are the way they are and can figure out on their own what is quality and legitimate and what is bullshit.

author: jaxbird
author_rank: Frequent Contributor
timestamp: May 03, 2014, 11:44:34 pm
content: Quote from: calexanian on May 03, 2014, 11:14:35 pmNice, can't disagree with thatIt's often a fine line between bullshit and legitimate, the bullshitters are unfortunately often clever enough these days to include some truth in their bullshit.But personally my stance is that, hey if anyone feel their gold/silver/super pure copper whatever cable etc. makes them enjoy their music more, then who am I to argue? it's not like there are not enough warnings/ridicule already online, so let them have their fun, it's their money to spend on whatever they wish.

author: EEVblog
author_rank: Unknown
timestamp: May 03, 2014, 11:55:02 pm
content: Quote from: jaxbird on May 03, 2014, 09:57:46 pmI thought we did that already?If not, it's covered in material I haven't released yet. I definitely remember Doug covering that.The problem with this series is that all the raw content is all a bit higgledy-piggledy. I have to edit together what segments I can in a somewhat organised fashion. I hope I don't miss anything because of that. 48V phatom power is in there somewhere...

author: calexanian
author_rank: Super Contributor
timestamp: May 03, 2014, 11:55:26 pm
content: Quote from: jaxbird on May 03, 2014, 11:44:34 pmI understand. Hey, whatever makes you happy after all!

author: calexanian
author_rank: Super Contributor
timestamp: May 03, 2014, 11:59:46 pm
content: Quote from: EEVblog on May 03, 2014, 11:55:02 pmI don't think any equivalence of quality to having or not having phantom power was really stated, or can be.. They are just simply different methods of operation. This relates more to the virtues of dynamic vs. condenser type microphones.. Not really in the scope of this series. This is more informational by means of how they work, not how they sound or any relationship to quality or usefulness. Your mileage will vary!~

author: pgross
author_rank: Contributor
timestamp: May 04, 2014, 01:18:15 am
content: Kudos to Dough and Dave for the mic seriesQuoteUsing 140V, (actually 130V in the DPA Mic. example) instead of 48V, will provide higher SPL handling capability of the built-in mic amplifier. For special purposes; like measurements, high SPL sound recordings in the 154-168 dB range (depending of capsule), that extra headroom might come in handy. The DPA "High Voltage" mic series must be connected to a dedicated high voltage mic. amplifier - HMA. Beside the 130V, this HMA also features a 200V supply, used for powering external polarizing condenser capsules.Typically, it is necessary practice to attenuate the output by 20-40 dB on the HMA. Otherwise the input of a recorder or mixing console will be overloaded when the mic is exposed to these very high SPLs./ Peter

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 04, 2014, 02:48:46 am
content: Quote from: pgross on May 04, 2014, 01:18:15 amYeah, that is the standard response. But I'm not buying it. No other part of the audio chain requires more than 30-50V max (from - rail to + rail) even for +24dBu line level. What exactly is the point of generating perhaps "line-level" signals (which are perhaps 1~2% of 130V) and then knocking it back down to "mic-level"? Something is just fundamentally goofy here.

author: pgross
author_rank: Contributor
timestamp: May 04, 2014, 02:54:17 pm
content: QuoteThe 130V supply is derived – I think, from the older days where the high voltage was needed as anode voltage to amplifiers with tubes.In more contemporary semiconductor designs, this higher voltage is needed where the std. P48 Volt becomes inadequate – theoretically at least.Let say, that we have a capsule with a sensitivity of 10 mV/Pa. If this capsule is exposed to its stated 168 dB absolute max SPL handling with THD < 1 %, the output voltage is 50.12 V peak: 94 dB – 168 dB = 74 dB. Where, inv log(74/20)10-3= 50.12 V.A microphone connected to a IEC 61938 compliant power supply with a current draw of 2.5 mA will sink the phantom voltage to 31V.In other words, the voltage provided will compromise the signal handling capability of the amplifier, resulting in audio distortion.Common practice is to attenuate (PAD) the mic output signal by 10–20 dB. Of cause it dosent help much if the amplifier is already distorted.On some mics, a PAD is placed in the front end circuit, unfortunately this is the worst place to attenuate the signal with respect to SNR.For most users, the “high voltage” principle is merely a marketing ploy in order to justify esoteric audio gear – you know, the average Joe equipment can’t possibly satisfy the discerning audiophile…/Peter

author: azi
author_rank: Contributor
timestamp: May 15, 2014, 02:34:13 am
content: Quote from: mikeselectricstuff on May 03, 2014, 02:34:22 pmI think it's very unlikely that the FET has a built-in resistor or ohmic resistance for creating gate bias. Semiconductor manufacturing is very good for making diodes and transistors, but horrible for resistors. Unless things have changed since I was doing chip design (a long time ago), the way you make a resistor is with a wire of polysilicon, which has a resistance of about 20 ohms per square. The 2SK596 datasheet (see the link in Richard Crowley's post), the input resistance is listed as 25 Mohms. That would require a very long wire.Also on the datasheet, there are both a "protection diode" and "resistor" shown connecting the FET's source to its gate. I *suspect* that these are shown as a disguise for using the reverse leakage of the diode and/or the gate's p-n junction to bias the gate to 0 volts. The two devices might be manufactured together into an odd kind of jFET with an extra area of p-n junction continuing outside the gate, onto the source of the device, to increase the reverse leakage sufficiently. (However, this is a guess and the manufacturer may have found another way to make a "resistor"! Unfortunately, I lack sufficient test equipment to test my hypothesis. Mike, Dave and others can try it if they want. ;-)I got some CUI electret capsules in the mail today from Digi-Key and took one apart. What I found matched the pictures on this web page almost exactly:http://www.openmusiclabs.com/learning/sensors/electret-microphones/I thought that might help others who are following this thread. It is the same website that hosts the 2SK596 datasheet.It's pretty easy to remove the transistor with intact leads if you want to play with one and explore this topic further.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: May 15, 2014, 02:45:13 am
content: Quote from: jaxbird on May 03, 2014, 09:57:46 pm48V phantom supply is a real misnomer. It's not really a voltage source.The 48VDC is the open circuit voltage, however this is in series with a 3K4 resistance in the console, with a short circuit current of about 14mA.Many high-end mics (Schoeps, DPA, Neumann etc) have their own phantom power supplies, that supply whatever is required by the mic to operate (essential for tube mics).

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 15, 2014, 03:48:50 am
content: P48 phantom power is well defined in DIN 45596, followed by IEC 61938.It is indeed a 48V voltage source with 6.8K matched (within 0.4%). Provisions are made for 24V and 12V options with lower-value resistors.Pro, externally-biased condenser mics use their own custom, mains-supplied, external power supply boxes, they don't operate on P48 phantom power.For example, the Rode Classic II that was mentioned in the video...

author: townkat
author_rank: Newbie
timestamp: September 16, 2014, 09:52:11 am
content: hi everyone,i have some unclear things about this subject which i'll try to expose here,i have a Samson c01u usb electret wich i am analyzing for, maybe, improving the noise floori noticed that it does have the (50)megohm resistor connected between fet gate and a voltage divider mid point (unlike in the video where it is connected to ground?..), so my understanding is that the megohm resistor purpose is to limit the current from the voltage divider matching it to the current from the electret capsule. is my understanding corect? (is this called impedance matching?)and the 2nd,i have seen on internet that some electret pre-amp schematics also have an ac coupling capacitor between the electret capsule and fet gate, what can you say about this? (does it have any effect, is it useless, can you also explain why etc..)these are probably beginner questions but i tough they are still more suitable asked here than in a new topic in the beginner forumthank you

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 16, 2014, 01:44:37 pm
content: It is almost impossible to have this discussion without referring to a circuit diagram somewhere. Do you have a schematic diagram you are looking at?50M ohms seems quite low for an electret condenser capsule circuit. If it is connected over to a voltage divider it seems likely it is biasing the FET at midway between ground and the supply voltage. There isn't any current here to "limit". A very high value resistor is used to minimize draining away the audio signal.The electret capsule output is very high "output impedance" which is why the FET is used because the FET has very high "input impedance". The FET is doing the impedance conversion with a very high input impedance and a relative low output impedance.There are so many different circuits for this, it is impossible to make generalizations about all of them together. In the circuits I've seen, the series capacitor between the mic capsule and the FET input was used for high-pass (low-cut) filtering and/or as an attenuator (to allow the mic to be used in high loudness situations).Improving the SNR of that mic is an interesting project and it will be particularly challenging where you are starting from. Because that mic has both a conventional electret mic circuit plus the USB ADC and interface, etc. it will be somewhat different than most of the condenser mic circuits you have seen. Conventional mics are designed to differentially (balanced) drive a long, low-impedance mic cable. But since the ADC is right there in the mic body, they have a rather different design parameter. We don't really know how much preamp they used to get a signal level (also unknown) appropriate for their ADC. Again, a circuit diagram would practically be required to work on that mic. Most condenser mics are simple enough that you can draw out the diagram yourself. But that would be almost impossible for a USB condenser mic like that.

author: townkat
author_rank: Newbie
timestamp: September 16, 2014, 03:12:16 pm
content: someone made some good pictures of the boards of c01u, (and some explanation)http://jumperone.com/2011/10/samson-c01u-teardown/is this enough or a diagram would help further? i can also take more pictures if neededthe preamp is built into the adc chip (24+24db programmable gain amplifier)all the noise is coming from the small board as i bypassed the megohm resistor with a small cap and the noise was gone (but also the signal)what i am not sure yet is if the noise source is the 50megohm resistor or the capsule itself

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 16, 2014, 04:50:09 pm
content: Quote from: townkat on September 16, 2014, 03:12:16 pmWe can't really have any useful discussions about the circuit design or operation without looking at the circuit diagram.Randomly poking around blink is not only a waste of time, but very likely to be fatal to the circuit if you poke the wrong place.QuoteThat severely limits what you can do to quiet the audio path. This kind of "all in one" microphone was not designed for high performance. It was designed for convenient operation.QuoteWhich is exactly why it is such a large value resistor, to avoid draining off the very miniscule audio signal.Putting almost ANY value capacitor in that spot is guaranteed to short out ALL the audio signal and leave you with nothing.QuoteIt is probably a safe differential test to simply disconnect the capsule and see how the circuit behaves by itself.Then you will hear only the circuit noise, and nothing from the mic capsule.VERY IMPORTANT NOTE!!!: That "solder blob" where the capsule connects is the VERY MOST SENSITIVE part of the whole circuit.The reason it is floating like that is because the most minuscule, invisible substance could cause major problems and quite possibly render the circuit non-functional.I can't stress how important it is to treat that node with extraordinary care and make sure it is scrupulously clean.Even left over solder rosin residue that is completely invisible under a microscope can severely damage the audio signal.We use special no-residue solvents and methods to clean that extremely high impedance node to eliminate any trace of leakage.Messing with that node at all is a very bad idea and to be avoided if at all possible.

author: townkat
author_rank: Newbie
timestamp: September 16, 2014, 09:15:25 pm
content: some more pictures and articles on this subject herehttp://www.noisyaudio.com/tag/samson-c01u/the guy there suspect the a/d chip for the noise but the experiment i made with the megohm resistor ac shorted seem to indicate the resistor or the capsule..i attached a schematic i made of the small boardthe text on fet was not fully readable, what i could read seem to be k.. j? (it may be K596), then on second line j83mi could not find a microphone symbol so i used a led one

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 17, 2014, 01:19:24 am
content: The transistor is a 2SK596, a special FET designed for electret condenser microphones.http://www.onsemi.com/PowerSolutions/product.do?id=2SK596SAs we suspected, the 50M resistor is biasing the FET from the voltage divider formed by the 39K and 40K resistors.The resistors are not equal because note that the transistor actually has a resistor inside.The node where the 50M resistor, the business-end of the capsule, and the FET gate is the super-high-impedance node.You won't be able to measure any voltage there because it is extraordinarily high impedance and your meter loads it down too much.There should be a wire going from the back of the mic capsule to the high-impedance node.What happens if you disconnect that wire at the back of the capsule? (Avoid touching anything to that high-impedance node!!!!)Or if you connect your capacitor across where the capsule would be?That would show you how much noise is coming from the circuit (FET + preamp/ADC/USB).

author: townkat
author_rank: Newbie
timestamp: September 17, 2014, 09:05:25 am
content: test:computer gain was set to maximum (teoreticaly adc preamp is at +48dB)original,noise floor -49dBwire from capsule disconnectedhuge buzz -11dB(if i was touching the mic case the signal was lowering to -42dB)wire from capsule disconnected and 0.1uF (104) capacitor connected between the disconnected wire and capsule ground (the other wire still conected to capsule)noise floor -56.5dBall readings where made with microphone assembled to benefit from the case shielding

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 17, 2014, 12:12:10 pm
content: How much of the "original noise floor -49dB" was ACOUSTIC noise from the environment?It isn't fair to ascribe that to the microphone when it is only doing its duty by picking up ambient noise.If you don't have an anechoic chamber handy, you could do the next best thing (for home experiments).You could take all the bedding in the house and pile it on one bed to block as much ambient noise as possible.Then you could wait until 3am on the most quiet night of the week in your neighborhood.Then bury the microphone inside the pile of pillows, blankets, etc. to do the experiment.So the difference between -49dB and -56.5dB is (roughly) the contribution of the mic capsule.Some people claim the capsule is a TransSound TSB-160A and it looks very similar from the photos.You could buy a pile of them and select one with the lowest noise. This appears to be one source....http://www.ebay.co.uk/itm/Trans-Sound-Unidirectional-TSB-160A-Unidirectional-mic-capsule-Quantity-of-1-/221541095942Of course for the same $$$ you could have simply bought a quieter microphone and let the manufacturer sort the capsules for you.But I would agree with some of the comments that the most likely source of the mediocre performance is that AK5371 chip.It is rather pushing it to even use such a low-end chip in that kind of microphone product.This is clearly a low-end circuit design and practically impossible to "improve" without simply replacing the entire circuit with a better design.

author: townkat
author_rank: Newbie
timestamp: September 17, 2014, 04:40:49 pm
content: the mic was wrapped in a blanket and between pillows in the first test were the capsule could pick up ambient noise, also the room was quiet at the moment of measurement with the mic conected to a laptop as the desktop pc was raising the noise with 3dB, the blanket and pillows reduced the noise with about 13-15dBthe normal gain needed is about 30dBthe noise could also be reduced by 1-2dB if the official app is used which commands the amplifiers separately (a little bit of gain staging)i find it quite usable anyway just that i was wondering if there are any improvements possible(i wouldn't change the capsule with one of the same model)thank you very much for your help

author: JustSquareEnough
author_rank: Unknown
timestamp: April 23, 2018, 08:14:45 pm
content: I posted this in the youtube comments but figured I'd have a better chance here:for the life of me I dont understand why gain goes down as the external resistor goes down which has the effect of increasing Vds, correct?I've simulated as much and can see that the amplitude of the output wave does in fact go down as the external resistor gets smaller (1k vs 5k for example) but I don't understand why.anyone?

author: JustSquareEnough
author_rank: Unknown
timestamp: April 23, 2018, 09:04:43 pm
content: Quote from: JustSquareEnough on April 23, 2018, 08:14:45 pmresearching my own question, common source amplifier configuration (https://en.wikipedia.org/wiki/Common_source) we see from the voltage gain equations, Rd being in the numerator now makes sense mathematically as to why as Rd gets small gain goes down. However, it still doesn't make sense intuitively, to me yet, why when Ids is fixed and Vds is increasing due to Rds decreasing gain goes down?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 23, 2018, 09:06:26 pm
content: No transistor (inclucing the one inside the electret capsule) is perfect. They all have some absolute limit of how much current they will pass, AND they all have an intrinsic series resistance which limits how much current we can see externally.But then the capsule isn't supposed to be a power or current device. It is supposed to be a signal or voltage device. If you need more current use a buffer stage. That is how most electret condenser mics are designed. For example here is a circuit using a typical electret capsule, and then a pair ofresistorstransistors as an impedance buffer to drive the low-impedance balanced typical microphone circuit: