OpAmp based Electret PC microphone input circuit hissing like crazy - Page 1
Forum section: electronics / projects

author: ptwz
author_rank: Newbie
timestamp: July 27, 2023, 02:58:07 pm
content: Hi there, I'm currently scratching my head.I'm designing an interface for PC headsets. The microphone parts gives me trouble.There is noise (hiss) with a level of about 200mV. It is the input stage, as replacing it with a BJT circuit reduces it to about 1mV.Replacing the OP-Amps with "Audio grade" ones does not significantly change anything. But how come I get such horrid noise?Could any of the seasoned audio electronics engineers help me out?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 27, 2023, 03:25:03 pm
content: I'm assuming you have bipolar +/- 9V supplies. It might be oscillating. I'd make sure you have bypass caps near the opamps and maybe put couple k resistance in the non-inverting leg of the first opamp.

author: ptwz
author_rank: Newbie
timestamp: July 27, 2023, 04:41:03 pm
content: I've been down this rabbit hole. (Un)fortunately it doesn't oscillate. In the layout I took quite some effort to keep all bypass caps close. Low impedance probing the supplies didn't show any signs of anything oscillating.Taking an FFT of the preamp stage noise shows a nearly perfect 1/f slope at first, levelling to a uniform distribution at about 1kHz to 16kHz.

author: Faranight
author_rank: Unknown
timestamp: July 27, 2023, 05:25:14 pm
content: Maybe a shot in the dark, but can you try a different opamp?Just saw you already tried a different opamp.Does it still hiss, if you disconnect the mic?Could you try replacing the TL431 with some other power supply i.e. a linear regulator and see, if that helps?

author: wasedadoc
author_rank: Super Contributor
timestamp: July 27, 2023, 06:09:42 pm
content: Quote from: Faranight on July 27, 2023, 05:25:14 pmIn the first post the second schematic which gives massively less noise has exactly the same TL431 configuration supplying power to the electret mic.

author: Faranight
author_rank: Unknown
timestamp: July 27, 2023, 07:09:55 pm
content: I'm just trying to eliminate any remote possibility that the noise might be originating from the microphone power circuit.

author: floobydust
author_rank: Super Contributor
timestamp: July 27, 2023, 07:12:46 pm
content: Check your TL431, some brands of them go unstable and oscillate with certain capacitance range across them. 10uF is likely doing it.

author: ptwz
author_rank: Newbie
timestamp: July 27, 2023, 07:34:26 pm
content: Just checked, no oscillation to be seen on the TL431. Also should be stable according to the stability criteria diagram in the respective manufacturers (ST) datasheet.

author: magic
author_rank: Super Contributor
timestamp: July 27, 2023, 07:40:47 pm
content: I will repeat the question how is the opamp chip powered?What does "200mV noise" mean exactly?Does it go down when RV1 is turned all the way to ground?How about shorting MIC to MIC_GND?editC5 is pointless. Its reactance at audio frequencies is higher than output impedance of TL431. It won't reduce output impedance of this voltage rail (nor is there any need for such reduction) and it won't make a meaningful lowpass filter against noise (unless additional resistance is inserted between TL431 and C5).

author: iMo
author_rank: Super Contributor
timestamp: July 27, 2023, 08:49:35 pm
content: As Magic indicates - a resistor is needed there, some 431 may even oscillate with larger capacitancies wired at the output (see the datasheet).

author: blackdog
author_rank: Frequent Contributor
timestamp: July 27, 2023, 09:03:52 pm
content: Hi,Look in the datasheet of a TL431, what is the Ri of this circuit, it is on average 0.3 Ohms.Now what is the crossover point of 0.3 Ohms and the 10uF capacitor?So that is around 53KHz!The only thing the capacitor C5 does, is to make the TL431 start up slower, but this capacitor does nothing in terms of the hefty noise coming from these TL431 references.Make a double RC filter after the TL431 and make sure the crossover point is a lot below the audio spectrum.Say below 5Hz, that does give a bit longer startup time, but low noise is more important.Look also what the manufacturer indicates as "Pull Up" resistance of the microphone, if you take it too low, your signal noise distance will also be smaller.Kind regarts,Bram

author: moffy
author_rank: Super Contributor
timestamp: July 28, 2023, 12:15:22 am
content: Your noise appears to have two sources:1) The TL431 pdf page 16:https://www.ti.com/lit/ds/symlink/tl431.pdf?ts=1690440364327&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTL4312) The TL084 page 20:https://www.ti.com/lit/ds/symlink/tl084.pdf?ts=1690422866538&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTL084When you replaced the TL084 with a lower noise opamp the TL431 noise dominated. So your solution should be to heavily LPF, as previously suggested, the output of the TL431 and use a low noise opamp.

author: ptwz
author_rank: Newbie
timestamp: July 28, 2023, 07:12:05 am
content: Quote from: magic on July 27, 2023, 07:40:47 pmI use a Murata DC/DC module including their recommended filter circuit. The supply line is rather quiet after filtering. Ripple >1mVppQuote from: magic on July 27, 2023, 07:40:47 pmRMS according to my Scope. Probing with a coax cable soldered directly across SW1.Quote from: magic on July 27, 2023, 07:40:47 pmTo nearly zero (<=1mVpp)Quote from: magic on July 27, 2023, 07:40:47 pmNearly zero again.Quote from: magic on July 27, 2023, 07:40:47 pmThis was the point that fixed the behavior. The TL431 did not oscillate but seemed not having a an inline resistor beween TL431 and C5 indeed made a lowpass >50kHz. Bodging in a 100 ohms resistor brought it down 159 Hz, lowering the noise floor significantly. I will use 220R in future when reworking the boards.Thank all of you for pointing me in the right direction!