EEVblog - 2014-05-03
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? Forum: http://www.eevblog.com/forum/blog/eevblog-611-electret-microphone-design/ All the microphone series videos are here: http://www.youtube.com/playlist?list=PLvOlSehNtuHv98KUcud260yJBRQngBKiw EEVblog Main Web Site: http://www.eevblog.com EEVblog Amazon Store: http://astore.amazon.com/eevblogstore-20 Donations: http://www.eevblog.com/donations/ Projects: http://www.eevblog.com/projects/ Electronics Info Wiki: http://www.eevblog.com/wiki/
Thanks Dave, Doug, This series is absolutely amazing!
I would love to see more in depth / master-class type stuff on your channel. I'm learning some really useful stuff here.
Great series, Dave! Even though you say Mail Bag is everyone's favorite, practical electronics lectures are far more interesting and useful. That's why your channel is one of my favorite ones. I missed fundamentals videos so much)
The microphones that don't have leak resistors don't use a normal JFET, they use what the friendly people at Toshiba call a FET-IC. They have parasitic diodes fabricated on the surface of the transistor in order to provide a leak resistor as well as a source bias resistor. Sadly this does not improve linearity or noise performance, but it sure is cheap to make!
Hi Dave - Thanks very much for taking the time to share your expertise through your numerous videos. In a correspondence with Alan (w2aew), I believe he mentioned that you have done work in the area of underwater acoustics. I spent about 45 years working on underwater acoustic signal processing algorithms. Now that I’m (mostly) retired, I’m trying to learn more about the hardware side of underwater acoustics. Recently I’ve been trying to come up with a 40 dB gain preamp for a hydrophone as related to some student mentoring I’m involved in and I’ve been focusing in on designs that employ JFET front ends. I’m curious to know if you’ve produced any videos that may be helpful to me. Thanks again! - Jim
Doug Ford is a brilliant presenter and lecture; I look forward seeing Doug becoming a T.V presenter of a science program one day on T.V, can't wait for that to happen.
Nice job Dave! Your vids are always educational and very entertaining!
This is just awesome, great teachers both of them. Thank you very much.
Very informative. Enjoyed it greatly Dave. I'm surprised they aren't more views than they are on this. Thanks for sharing these series of vids.
Hi Dave, I did a rerun off some your vid's, kinda missed this one.. But this set is one of your best guest appearances !!
So informative an clear on info! Fun to see the interaction of you both...
Would love to see you guys extend this series with a discussion on Piezo Electric Oscillators as microphones, especially since many people are using them for acoustic guitar pickups.
This series is fantastic. Thanks.
Thanks! I found the part of choosing the bias resistor for the JFET in cases of low supply very useful! Good advices!
This is absolute gold. Thank you so much!
09:24 This part is GOLD IN MY EYES. This microphones series are marvellous.
Genial, simplesmente!
I like this very much...even if I will never build a microphone, it gives me knowledge about electronics...Thank`s
Hi, I am using an electret with a sensitivity of -38.44 dBV to make sound measurements at a distance of 1m. I decided to add an amplification circuit with a gain of 125. So in the end to convert to SPL, I first used my sensitivity which is 11.967mV/Pa (in SI units) to convert the measured voltage to pressure. P=V/S, where V is the voltage before amplification (i.e. output value divide by 125) and S is sensitivity. In the final step, I then convert this to dB SPL using a reference pressure of 20e-6 Pa and the formula dB SPL=20log(Pa_rms/Pref). However, with this formula, I get values ranging around 100 dB, I believe that I am supposed to be getting values around half that, what I am doing wrong? any help would be appreciated.
Part 5? Didn't see that until the video was over. Part 1 here I come! Thank you, both!
Nice! I just took an Analog electronics, electronics and circuit analysis class. I had to design an electret microphone amplifier for a project . It was interesting to see how a professional would have designed it. You should do more of these professional designer guest appearances.
thanks guys, this content is amazing!
Those cheap mics can be modded (or at least some of them) to get access to the source pin and use the fet in common drain connection (repeater). Look up "Linkwitz Mod"
Please show more things like this!
This explains exactly why the microphone in the cheap-ass headset I bought does not want to work with my semi-professional mixer. It gives me no output voltage and when measured, it likes to draw a fixed current of about 200 µA, no matter what voltage I apply. (Well, I'm glad I did not go higher than 10 V.)
Looks like I've got to build myself a small pre-amp.
No, I am not going to sing :) I just want to be able to talk to the musicians over the monitor channel in case of emergency.
The electrets without gate resistors remind me of grid-leak biased vacuum tubes - unstable as hell!
Just a comment on the dimensions, its not -45dB/Pa, dB is the logarithm of the ratio of two units, in this case -45dB where 0dB=1V/1Pa. Thus adding the /Pa again after dB is wrong...Just nitpicking, still thumbs up!
If you have current modulation per pressure, then why not use a transimpendance amplifier?
I love the way you guys chat together. Hilarious :D
I hate it haha. But it's very informative so I suffer through it.
Outstanding ! :-)
At 6:35, you brought out a device. How well does that work? Can I have the circuit diagram of that device? Please?
Do you know of any digital microphones within the range of 10Hz (20Hz at most) to 10KHz (the upper limit is not important to me)?
Ah, now I see why a microphone project I attempted last fall failed... those bias currents.
Really very interesting.
...and now I know what the experts know bout electret mikes. Came here to have a question answered, left with answers to questions I'm yet to ask.
Thank you Doug
i just built a pcb and i got my mic to work but im getting a bit of a low hum or feed back in the back ground it was really loud then i started to an some resistors to the input of the pcb and it smoothed out but i still cant figure out how to get ride of it so i can use the mic to make music idk if i should change quality of wire from the mic to pcb or from pcb to xlr out put or if i need to add something balance out the pcb some how
im using all 5% resistor seems like i should be in better condition then i have another pcb i was working on i bailed on it i had it working where you could here my voice but as i spoke i got back ground rumble totally different than the one im working on now and it was odd if i left the mic alone but on it was quiet but as i talked it would get more feed back and more so i got sick of playing around and went back to the other design i was working
i just need some one t ling me up with the front half of a circuit so a 34mm capsule will work and im using 2n5457 jfet
Who uses Jfets, when Mosfets are so cheap nowadays?
Doug Ford is quite an electronics guru: search the web for his article "the secret world of oscilloscope probes"
The choice for a JFET has nothing to do with the price (vs a MOSFET) but all to do with noise, MOSFETS are notoriously more noisy than JFETs and bipolar transistors, they have significantly more 1/f noise, i.e. low frequency noise which will be in the audio band ;-)
Best cheap gigaohm resistor, is no resistor at all!
Amazing
Talking of Mics... there seems to be gate on this that isn't usually there.
I had to add this, but on the part that computer audio inputs have, Apple doesn't supply a voltage on their audio in, so you have to use a powered mic. Not sure why though...
Well, if we go back even further, even older Macs did supply voltage on the audio input port. It was needed to power the Apple PlainTalk mic, which used a proprietary extended jack. Its longer tip reached the DC voltage contact in the socket. Regular ⅛″-inch jacks could also be used in a normal fashion.
Not sure about ten-year-old machines, but all my Macs—the oldest being from 2008—can power electret mics (with an adaptor, as I mentioned).
P.S. Ironically, I just made a simple adaptor for my iPhone to work with a RØDE Lav mic a week ago. Works great.
@Ast A. Moore An iPhone isn't a computer, it's a phone. Yes, there's a distinction. They're not Macs.
A Mac Mini (first or second gen Intel) does not provide a mic supply. They made them up until 2010. Adapters for machines with Plaintalk connectors (not present on my G5 and Intel Macs, my G4 has no input at all) do not count. That's like claiming (as Apple do!) that their portable devices are compliant with the common external power supply specification.. with an additional adapter which costs as much as (MORE than most, actually) a power supply. Defeats the purpose, does not count.
Macs historically did not provide a mic power supply for using normal microphones. Modern ones may be different.
For all intents and purposes, an iPhone is a computer. To get back on topic, though, the adaptor that I made also works with fairly modern Apple laptops (as I mentioned, down to at least those made in early 2008), and I assume some desktops, as well.
The earbuds that come standard with every iPhone use a small electret mic capsule and can be plugged into a Mac. The computer then supplies power to the mic’s FET.
@Ast A. Moore iPhone is not a Mac. It does not support the same external interfaces, and is not expected to. iMac 2010 here. Line-in on a 1/8" TRS No mic power. Just like my last 2 iMacs, and every Mac since the G4/5 series. Now more recent laptops, I am not sure about. But my 2001 Titanium does not even have an input at all. No internal mic no line in. The idea was that USB would provide better input support than any internal audio hardware would. That didn't work out so well for portable situations.
This is way off-topic, but the iPhone does share some external interfaces that are also supported on a Mac. Case in point: Apogee Jam. It’s a class-compliant audio interface. Works without additional drivers on iOS devices as well as on Macs.
P.S. Just to clarify, you need to use a TRRS jack and the Headphones port, not the Line In port on a Mac. Apple’s TRRS headphone jack pinout is as follows:
Tip—Left Audio Out
Ring—Right Audio Out
Ring—Ground
Sleeve—Mic In (plus plug-in power)
Some strange sound compression weirdness going on (or is it noise gate ?) that's very distracting, it feels like my ears are shutting off every time it goes quiet.
I ran this one through The Levelator, which usually does a pretty good job.
I might not have noticed any issues, because I edited this one in my temporary (see twitter photo) computer setup, and the room acoustics are terrible.
@EEVblog I noticed it in a cheap caravan, in France.
Mike is noisy, disable the Line-In port.
for the life of me I don't understand why the gain goes down as the bias resistor goes down, increasing Vds. I've simulated as such but still don't understand the why. anyone?
See, the FET inside is essentially a current source. Now note that we're speaking of VOLTAGE gain, and output voltage will be, of course,
Vout = Id * Rd,
where Id is a drain current (which is dependent on a transconductance, which is constant) and Rd is a bias resistor.
So now you see - the lower the resistor, the lower the output voltage...
Well, more correctly, the output voltage will be
Vout = Vbias - (Id * Rd),
but this does not change the principle. :)
No biggie, but guy says JFET and draws MOSFET.
His name is really Doug Ford? Mayor Rob Fords brother? lol
da dove viene quest'uomo?
Nice =D ps: first
yee second
Click here where? :P
Antonio Barba - 2014-05-05
Doug is really good at teaching, keep goin on!