OPA1612 as microphone pramplifier

[Devicet]

Hi, I'm designing an electrect microphone pramplifier that is supplied by 5V. I decided to use OPA1612 because of it's low noise input levels. I heared that I should be aware about Input current noise density - provide low impedance at input and use proper feedback resistors. Here is the circuit I have designed:



Should I remove R3 to lower input impedance and maybe use second chanel for second stage, so I could lower feedback resistors? I was planning to use second channel as 1/2 Vcc voltage buffer. The line_out should provide consumer line level (max 0.89 Vpp). As you can see I used j-fet in common drain mode for low output impedance.

 

[ccaudle]

Using a FET input amp like OPA1642 in a single stage configuration (i.e. no JFET as first stage) would be simpler and should have close to equivalent noise performance. Search for the term OPIC, similar designs have been discussed multiple times on these forums.

 

[Voyager10]

It can't work with R3 = 16K. If you look at the DC operating point of the op-amp, pin 1 can't go high enough to get pin 2 at 1/2 VCC. (If you remove R3 altogether, the DC operating point will be just fine).

 

[ntch]

What electret is that, and why do you use a jfet and a 1Gohm resistor? Isn't there a jfet in the electret body already?
Why not just go straight into the opamp, i.e. as in https://www.ti.com/lit/ug/tidu765/tidu765.pdf ?

edit: there are suggestions for opamps in the pdf, with some minimum specs.

 

[jp8]

https://www.ednasia.com/amplify-small-signals-in-low-noise-circuit-with-discrete-jfet/

 

[Devicet]

It can't work with R3 = 16K.

You're right. Will it work when R3 is pulled up to 1/2VCC? But still does this resistor make sense? It lower input impedance for OPA1612.

What electret is that, and why do you use a jfet and a 1Gohm resistor?

It is a chinese 25mm condenser microphone without FET. But I can swtich it to something better anytime. Maybe I should measure parastitic resistance and if it's around 100Mohm - 1Gohm I wouldn't put additional resistor here.

I don't want to put it directly to FET opamp. As you metioned there are better and cheaper solutions.

What about modifying my circuit, where 2SK209 is in common source mode and the Rd resisior is 960Ohm, so the output impedance of first stage isn't a lot? I would go into https://www.ti.com/lit/ug/tidu765/tidu765.pdf

The second chanel OPA1612 would have variable resistor in feedback, so I could adjust output signal level.

 

[jp8]

You're right. Will it work when R3 is pulled up to 1/2VCC? But still does this resistor make sense? It lower input impedance for OPA1612.


It is a chinese 25mm condenser microphone without FET. But I can swtich it to something better anytime. Maybe I should measure parastitic resistance and if it's around 100Mohm - 1Gohm I wouldn't put additional resistor here.
View attachment 138503View attachment 138504
I don't want to put it directly to FET opamp. As you metioned there are better and cheaper solutions.

What about modifying my circuit, where 2SK209 is in common source mode and the Rd resisior is 960Ohm, so the output impedance of first stage isn't a lot? I would go into https://www.ti.com/lit/ug/tidu765/tidu765.pdf

The second chanel OPA1612 would have variable resistor in feedback, so I could adjust output signal level.

R3 doesn't make sense. If you want to maintain this circuit concept, just remove it. But I'm afraid the whole concept of your circuit is flawed if your ultimate goal is to achieve the lowest possible noise. That's why I shared that link. R4 and the output impedance of the JFET stage already create way more noise than the OPA1612. The OPA stage also amplifies the noise of the JFET stage. At first sight, that circuit I shared does the same, but there the noise contribution of components inside the feedback loop is reduced by the feedback and noise is mainly determined by the JFET and 1G resistor only and is not amplified by the AC gain of the OPA stage. Actually, the 1G is the main contributor at lower frequencies. See Voyager10's JFET shootout thread where he proved that. You could increase that to say 2G or even 5G. But be sure to use either a teflon turret or isolating slots or guardrings and remove solder flux from the JFET area.

Btw, that capsule is probably the same one that I used recently in a mic build and it sounded quite nice. Also from the side without coloration. Another similar capsule with a mesh and white back side that I tried was just too bright sounding. I received two other 25 capsules yesterday that I'd also like to try. Maybe this weekend.

Jan

 

[ccaudle]

But still does this resistor make sense?

No, R3 serves no useful purpose in that circuit.
Do you want an inverting circuit configuration for some reason? A non-inverting configuration could be simpler and lower noise.
The gain range of the circuit as configured is very small, 40dB-45dB. Is that the intended range?

I don't want to put it directly to FET opamp.

For what reason? For this use case an OPA1642 device would be lower cost, lower noise, and lower distortion than the circuit you have shown.

 

[pasarski]

R3 doesn't make sense. If you want to maintain this circuit concept, just remove it. But I'm afraid the whole concept of your circuit is flawed if your ultimate goal is to achieve the lowest possible noise. That's why I shared that link. R4 and the output impedance of the JFET stage already create way more noise than the OPA1612. The OPA stage also amplifies the noise of the JFET stage. At first sight, that circuit I shared does the same, but there the noise contribution of components inside the feedback loop is reduced by the feedback and noise is mainly determined by the JFET and 1G resistor only and is not amplified by the AC gain of the OPA stage. Actually, the 1G is the main contributor at lower frequencies. See Voyager10's JFET shootout thread where he proved that. You could increase that to say 2G or even 5G. But be sure to use either a teflon turret or isolating slots or guardrings and remove solder flux from the JFET area.

Btw, that capsule is probably the same one that I used recently in a mic build and it sounded quite nice. Also from the side without coloration. Another similar capsule with a mesh and white back side that I tried was just too bright sounding. I received two other 25 capsules yesterday that I'd also like to try. Maybe this weekend.

Jan

I also think these thinner capsules with blue back sound nice. IMO the smoothest version is the one with no mesh but this kind of grate over the diaphragm.

 

[jp8]

I also think these thinner capsules with blue back sound nice. IMO the smoothest version is the one with no mesh but this kind of grate over the diaphragm.

Ah, that's good to know! That is one of the two I just received. The other one I will test, is the one in the bottom-right corner of the picture. The top-right is too bright. For my DPA4055 clone build, I'd actually want an open cardioid pattern instead of cardioid, but let's see how these will perform.

Back to the topic of low noise mic preamp design: to educate oneself on this topic, a must-read (imho) are Scott Wurcer's "Low-Noise Microphone Preamplifiers" papers from the Linear Audio Vol 1 publication, ISBN 1986464318. Available from Amazon.

Jan