Electret Microphone Stereo PreAmp - Strange Behaviour - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 07:33:42 am
content: Hello guys,I am trying to make a stereo microphone preamp to use with my pc, based on the following schematicI found on the net.SchematicI wired up on the breadboard the first channel and it works perfectly! I am using the very fast LM4562opamp which made all the difference. I've tried the LM358N which is useless with a lot of noise andterrible sound, then the TL072 which way better and finally the LM4562 which is perfect even with thevery cheap electret capsules I am using. (They will be upgraded soon).Then, I tried to wire on the breadboard the second channel as well and a strange behaviour occurred.(Strange, because I cannot explain it...) Below is the actual circuit. The other channel is mirrored andthe only difference is the green 10uF cap to ground which is from a different brand .BreadboardIf I connect a signal generator common to both inputs the amplification works fine with a flatfrequency response from 300Hz to 20kHz. (When I connect the signal gen, I remove the electretbiasing 10k resistors.)20KhzNo signal degradation or attenuation up to 20kHz500HzSame signal level down to 500Hz with only some ramping but that's a different story.(Result of high pass filtering? any ideas?)So everything ok with the signal generator.The problem now occurs when I connect the microphones on the circuit.Specifically, the first channel(left on the photo) always works ok,but when I connect the second microphone here is what happens:(Both multi turn pots are set to zero ohms for the highest amplification)OscillationI get a huge 3V oscillation on the second channel! When I start increasing the resistance of the secondchannel pot the oscillation stops at around 520 Ohm. After that, the circuit looks that it works ok butactually the second channel amplifier attenuates high frequency signals. The first channel works fine.Vowel soundLow frequency vowel sound is not attenuated.Hissing sound... but a higher frequency hissing sound is attenuated quite a bit.(Left 2nd channel is yellow and Right is blue)(The other thing that I observed is that the second channel is a lot noisier and if I touch the 1k5resistor that goes to ground I get a lot of noise. Nothing happens if I do the same to the firstchannel.)I have tried to swap microphone capsules, use different ones, swap the op-amp with different types,swap the green capacitor, but the same thing happens with consistency all the time. (I've poweredthe circuit from a battery with the same results.)So, can anybody think of a reason for this circuit behaviour?Any help is appreciated.Thank you.George.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2015, 07:39:56 am
content: Place the 47uF decoupling capacitor right across the IC power pins, not dangling somewhere on the board. It must be right at the pins, with short leads.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 07:46:32 am
content: It made a difference but a very small one. Signal attenuation of the second channel is a littlebit lower but the problem continues.

author: rob77
author_rank: Super Contributor
timestamp: April 06, 2015, 08:02:44 am
content: i would try to build it on a piece of veroboard or protoboard (socket the opamp of course), breadboards are sometimes unpredictable.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 08:06:46 am
content: I know, I have had bad experiences with breadboards previously... but this time it does not looks that theproblem is from the breadboard because the stereo circuit works perfectly when I connect the signalgenerator.

author: ciccio
author_rank: Frequent Contributor
timestamp: April 06, 2015, 08:38:14 am
content: I don't see the feedback caps (2 pF on the schematic) in the breadboard.My experience is that in parallel with 33 Kohms the capacitor should beat least22 pF (for suitable HF response).In professional audio "standard" feedback values are 22 Kohm //100 pF...You do not need a 2.5 MHz bandwidth (33 Kohm // 2 pF).Uncompensated op-amps are evil in audio, even if the data sheets do not show them.Best regards

author: rob77
author_rank: Super Contributor
timestamp: April 06, 2015, 08:54:56 am
content: Considering an average breadboard, you have approx 2.5pF between every 2 adjacent rows. - that's why i suggested to use a different build technique.

author: kg4arn
author_rank: Unknown
timestamp: April 06, 2015, 09:36:39 am
content: I agree with the others. The breadboard is not the place for the 4562 with its 55 MHz GBW. It's going to be very very difficult to stabilize the cicuit.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 09:51:47 am
content: Ciccio, you are right they are missing and this is because when I tried the suggested 2pf ceramicsthey made no difference. After your suggestion I've tried two 33pF caps and they made a lot ofdifference. The noise problem and the oscillation stopped completely! The amplification increasedslightly.Only thing that remains is that I still cannot match the frequency response of the two channelsbut its way better than before! Maybe the breadboard indeed interferes with the circuit here.At least I can order the new microphone capsules now...Thank you guys for your help!

author: joseph.anand
author_rank: Regular Contributor
timestamp: April 06, 2015, 09:52:55 am
content: I have actually "breadboarded" 4 channel microphone preamplifiers using 4 dual opamps (the second opamp in each was used to adjust levels), but none with such a high GBW. However you have basically taken a design for a mono amplifier and replicated it for multiple channels.Consider doing the following: Instead of the 10K resistor Use two 4.7K resistors (Say Ra, Rb) in series with one terminal of Ra connected to Vcc, the other end of Rb connected to the microphone. Now connect a 10-25uF cap from the series junction of Ra and Rb to GND.Alternatively source the biasing for each of the microphones from a voltage divider network (separate ones for each microphones) and ensure that you use a 10-25uF cap across each of the lower resistors in the network. I'm too lazy/busy to draw a schematic, but I think you should be able to get a picture of what I'm trying to say.ps: I'm unable to see your schematic or any of your posted images clearly, and clicking on them takes me to the homepage of postimage.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 09:57:50 am
content: Josheph I will indeed try your suggestions later today, thanks!QuoteWhich browser are you using?Firefox displays everything fine.

author: mikerj
author_rank: Super Contributor
timestamp: April 06, 2015, 10:18:25 am
content: Quote from: kg4arn on April 06, 2015, 09:36:39 amI totally agree. The biggest mystery here is why the first channel doesn't oscillate, rather than why the second channel does.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 11:45:15 am
content: Oscillation solved. It works very well with full dynamic sound and very little noise which I thinkit will be taken care off by the new mic capsules.From my little search I found that the best are:1) Primo EM173 (Low noise and most sensitive)2) Panasonic WM-61A (Pretty good and flat to the point they might be lying...)You can also apply the Linkwitz mod.3) PUI Audio AOM-6738L-R (Balance between price and specs)I think I will try the PUI Audio ones.If someone knows better ones for a reasonable price please let me know.Something else.Do you know if there is a way to remove this annoying peak when turning on and off the preamp?Thanks!Josephdo you mean something like the following ?What are the advantages of this biasing method? Less noise?Thank you.

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2015, 01:51:19 pm
content: The on/off peak is due to the capacitors charging and discharging of the DC blocking capacitors. The easiest way to get rid of it is to use a relay/analogue switch and timer, so the output is only connected to the circuit, after the DC bias point has stabilised.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 02:02:12 pm
content: I see.Is that the reason why many audio amplifiers have relays?

author: ciccio
author_rank: Frequent Contributor
timestamp: April 06, 2015, 03:43:49 pm
content: Audio amplifiers use relays for two reasons:-the relay has a delayed on to avoid the on pulse that can damage speakers, and has an immediate off to avoid the off pulse (usually they are powered not from the main amplifier's power supply, but directly from secondary ac).-the relay has a control circuit that allows it to switch-on only in absence of significant dc on the output terminals, to avoid damaging speakers in case of a defective amplifier channel.

author: ConKbot
author_rank: Super Contributor
timestamp: April 06, 2015, 03:52:28 pm
content: Yes, for click suppression, and for general protection circuitry (which turn on/off click suppression is a part of). I.e. dc bias protection, short circuit protection. A good plated relay contact is hard to beat in linearity and power handling for the cost.

author: hgg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 03:57:27 pm
content: I see.Thank you for explanation.

author: joseph.anand
author_rank: Regular Contributor
timestamp: April 07, 2015, 11:10:31 am
content: Quote from: hgg on April 06, 2015, 11:45:15 amno.Something more like the followingAdvantage is less cross channel interference. I prefer using A for my audio designs.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2015, 12:21:41 pm
content: Joseph thank you very much for the schematic!I will try it.Isn't the 100K resistor a possible noise source? Wouldn't the second circuit be more noise free?(I am trying to make it as noise free as possible. Just ordered the Primo EM172 capsules)

author: hgg
author_rank: Frequent Contributor
timestamp: April 28, 2015, 03:27:46 pm
content: Hi,Just a quick question on the mic preamp circuit:In the circuit above the volume is reduced by the blue 10k log resistor while in my circuit I am usinga potentiometer in place of the red 27k resistor to reduce the gain and a fixed 10k in place of the blue pot.I was thinking of using the same configuration as above.Do you think that the output 2.2uF capacitor together with the blue 10k pot will work like a high passfilter? So when you lower the volume of the output signal you will have a reduced bass response.Is this assumption correct?Thank you.George.

author: Zero999
author_rank: Super Contributor
timestamp: April 28, 2015, 10:36:30 pm
content: Yes, it will be a high pass filter. The cut-off frequency can be calculated using the following formula:FC= 1/(2pi*RC)Don't forger that changing the value of the red resistor will also affect the bandwidth of the circuit, as it's in series with a capacitor.

author: JuKu
author_rank: Frequent Contributor
timestamp: April 28, 2015, 11:13:15 pm
content: > Do you think that the output 2.2uF capacitor together with the blue 10k pot will work like a high pass filter?Yes, as already explained.> So when you lower the volume of the output signal you will have a reduced bass response.No. Turning the pot does not change the resistance of the pot, assuming reasonably high impedance on the other side.Another use for relays in audio is signal source switching. Relays don't leak signal through like electronic switches do.

author: hgg
author_rank: Frequent Contributor
timestamp: April 29, 2015, 04:10:49 am
content: Juku, Hero666 thank you for your replies.Quote from: Hero999 on April 28, 2015, 10:36:30 pmIn what way though? High pass filter again?So how can you go about solving this problem?What about if instead of the blue 10k pot I use a larger value like 100k?Maybe this way the filtering effect will be much smaller since the resistances are greater.

author: flynwill
author_rank: Regular Contributor
timestamp: April 29, 2015, 02:07:30 pm
content: Quote from: hgg on April 06, 2015, 02:02:12 pmYes, exactly.Let us know how those PUI mics work out for you. The spec sheet isn't very encouragingwith a quoted S/N of 60 dB (Meaning a self-noise at about 34 dB SPL). However theyalso spec it at 1.5V it should get a lot quieter running of 9V like I presume you are doing.That Panasonic capsule was the gold standard for cheap electret capsules, but it is nolonger being made.

author: hgg
author_rank: Frequent Contributor
timestamp: April 29, 2015, 02:19:21 pm
content: Hi,I finally bought the Primo EM172 Capsules which turned out to be very nice indeed.Extremely sensitive and the noise decreased substantially. I am recording voice1 meter away from the microphones with excellent clarity. I am now doing some testswith different kinds of capacitors (electrolytics vs polypropylene) to see if they reallymake any difference in audio quality. I will power the preamp with 6V from 4 1.5V AAbatteries. The Primo capsules need 5V. I will also try a 9V battery and a lower gainif that further decreases the noise.

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2015, 06:25:40 pm
content: Quote from: hgg on April 29, 2015, 04:10:49 amIt's a non-inverting amplifier.http://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifierThe formula for calculating the cut-off is the same as above.I don't think it would be a problem.