All OpAmp mic design (no FET at first stage)

[kingkorg]

I've used both the OPA1641 and 1642 in my various projects here: OPIC Impedance Converter
Unlike the design linked to in the 'instructable' above, I don't regulate the supply to the op amp, and get a little extra headroom as a result.
Using the single amp 1641 draws half the current, so that helps a bit as well!
The audio output is single sided - although the output is still impedance balanced - so there is no additional noise introduced by the resistors in the inverting second amp used for a differential audio output.
I've found the various configurations I've tried all work pretty well .. low distortion and low noise....

@rogs I tried dropping you a DM, bot i'm not allowed for some reason.

I just built your opic circuit on a perfboard for an extra Akg electret i have. Just as i was finishing the build i realized the capsule is coupled to half rail instead of ground on one end. The only reason i can think of is not to present half rail dc voltage across the capsule which might interfere with polarization charge within capsule. Is this the only reason, or am i missing something?

 

[rogs]

@rogs I tried dropping you a DM, bot i'm not allowed for some reason.

I just built your opic circuit on a perfboard for an extra Akg electret i have. Just as i was finishing the build i realized the capsule is coupled to half rail instead of ground on one end. The only reason i can think of is not to present half rail dc voltage across the capsule which might interfere with polarization charge within capsule. Is this the only reason, or am i missing something?

The main reason for biasing the input that way was to try get away with only using a single 1G resistor, and keeping the circuit as simple as possible!
It will depend on the polarisation of the electret capsule whether that bias voltage is additive or subtractive....and that will of course affect the sensitivity a little.
Again, how much will depend on what the value of the 'baked in' polarisation voltage is, for any individual capsule.

When it came to using the same circuit with an LDC capsule - but without a voltage multiplier - then the loss of some 12v out of 48V could well be significant, so I included a second 1 G resistor to allow the op-amp to be referenced to 'half rail', and the capsule to be referenced to ground. That works pretty well.

This op-amp has quite a low noise figure, so even with only 48v as a polarisation voltage it's possible to get pretty good results. Especially with low noise transformerless mic preamps -- like those found in many of todays' budget audio interfaces.
It's a simple circuit. No FET bias worries, low distortion, decent headroom, and pretty low noise. The audio is single sided, but the line is still passively impedance balanced to help optimise the CMRR.
No reason that input configuration - with the capsule referenced to ground - shouldn't be applied to a FETless electret as well, as far as I can see, if the extra sensitivity is important?

I find it works pretty well.
(Some notes on the input with the extra 1G resistor here: http://www.jp137.com/lts/OPIC48.pdf )

 

[kingkorg]

The main reason for biasing the input that way was to try get away with only using a single 1G resistor, and keeping the circuit as simple as possible!
It will depend on the polarisation of the electret capsule whether that bias voltage is additive or subtractive....and that will of course affect the sensitivity a little.
Again, how much will depend on what the value of the 'baked in' polarisation voltage is, for any individual capsule.

When it came to using the same circuit with an LDC capsule - but without a voltage multiplier - then the loss of some 12v out of 48V could well be significant, so I included a second 1 G resistor to allow the op-amp to be referenced to 'half rail', and the capsule to be referenced to ground. That works pretty well.

This op-amp has quite a low noise figure, so even with only 48v as a polarisation voltage it's possible to get pretty good results. Especially with low noise transformerless mic preamps -- like those found in many of todays' budget audio interfaces.
It's a simple circuit. No FET bias worries, low distortion, decent headroom, and pretty low noise. The audio is single sided, but the line is still passively impedance balanced to help optimise the CMRR.
No reason that input configuration - with the capsule referenced to ground - shouldn't be applied to a FETless electret as well, as far as I can see, if the extra sensitivity is important?

I find it works pretty well.
(Some notes on the input with the extra 1G resistor here: http://www.jp137.com/lts/OPIC48.pdf )

The only reason i asked is capsule shielding situation i have here, because of the dimensions i just have to connect one side of the capsule to the ground. So i was just wondering if there was something more i wasn't seeing. Awesome, elegant and simple circuit! I'll use it for kick drum mic because of the high headroom.

 

[Gus]

vf14 inspired
tape over the opamp
Built with parts I had. Yes cheap block caps
You do need a good transformer

 

[kingkorg]

The main reason for biasing the input that way was to try get away with only using a single 1G resistor, and keeping the circuit as simple as possible!
It will depend on the polarisation of the electret capsule whether that bias voltage is additive or subtractive....and that will of course affect the sensitivity a little.
Again, how much will depend on what the value of the 'baked in' polarisation voltage is, for any individual capsule.

When it came to using the same circuit with an LDC capsule - but without a voltage multiplier - then the loss of some 12v out of 48V could well be significant, so I included a second 1 G resistor to allow the op-amp to be referenced to 'half rail', and the capsule to be referenced to ground. That works pretty well.

This op-amp has quite a low noise figure, so even with only 48v as a polarisation voltage it's possible to get pretty good results. Especially with low noise transformerless mic preamps -- like those found in many of todays' budget audio interfaces.
It's a simple circuit. No FET bias worries, low distortion, decent headroom, and pretty low noise. The audio is single sided, but the line is still passively impedance balanced to help optimise the CMRR.
No reason that input configuration - with the capsule referenced to ground - shouldn't be applied to a FETless electret as well, as far as I can see, if the extra sensitivity is important?

I find it works pretty well.
(Some notes on the input with the extra 1G resistor here: http://www.jp137.com/lts/OPIC48.pdf )

Just tested the mic, with that 3$ electret from Ali. The thing is in the noise range of my NT2-a. Which is pretty amazing for an opamp and a 3$ elwctret capsule

 

[MicUlli]

An all OpAmp mic design has benefits and drawbacks.

Benefits:
Ultra low THD in the output stage
High output level
Very high power supply rejection

Drawbacks:
Excessive white noise (5nV/sqrt(Hz) versus 2,5nV/sqrt(Hz) in a good JFET design)
Excessive current shot noise (2..3 times more than in a single JFET stage)
High input capacitance versus a well designed JFET stage

The designer has to be aware of these facts...

 

[rogs]

An all OpAmp mic design has benefits and drawbacks.

Benefits:
Ultra low THD in the output stage
High output level
Very high power supply rejection

Drawbacks:
Excessive white noise (5nV/sqrt(Hz) versus 2,5nV/sqrt(Hz) in a good JFET design)
Excessive current shot noise (2..3 times more than in a single JFET stage)
High input capacitance versus a well designed JFET stage

The designer has to be aware of these facts...

My primary reason for trying an op-amp in this kind of project was simplicity.
An ideal project for a novice DIY mic builder, for example?
I have tried various 'Schoeps style' clones in the past and had quite acceptable results... So I wondered just how an op-amp 'front end' would compare, and again found the results quite acceptable.

I'm sure there are ways of improving the nuances of very high spec impedance converter circuitry, but whether these make very much difference outside the scientific - or very high end audio - world, I have my doubts.

The input capacitance of the OPA1641 is around 5.5pF - (fairly linearly over the entire range, it would appear from fig 36 on the data sheet (see HERE ) ) - That does not seem an excessive figure?
In this configuration, there are no additional resistors in the signal path, so that helps minimise further noise generation.
Depending on the noise figure of the following mic preamp - which will need to provide any additional gain required of course - I've found that in the environments I'm using my mics, the external ambient noise always dominates.
So noise figures lower then the typical 5nV/sqrt(Hz) quoted for this device probably wouldn't make any significant difference in many 'real world' usage situations...

As you say, there are ways of improving aspects of impedance converter circuitry, but I would suggest that we are well into the realms of the 'law of diminishing returns' there.

I would suggest that this simple (and cheap!) circuit 'punches above its weight', as they say?..... I've certainly found it to be quite useful.

 

[Gus]

I think this could be a good first build.
Add a good capsule and grill enclosure

 

[neowalla]

I built these opamp mics from Jules Rykebush and they sound great.

True Condenser OPA Mics

True Condenser OPA Mics: Let's build a couple of exceptional quality condenser microphones. These are true condensers as they are externally biased. They use an Operational Amplifier, (Op-Amp or "OPA") based impedance converter circuit and a hex inverter voltage m…

www.instructables.com

 

[MicUlli]

My primary reason for trying an op-amp in this kind of project was simplicity.
An ideal project for a novice DIY mic builder, for example?
I have tried various 'Schoeps style' clones in the past and had quite acceptable results... So I wondered just how an op-amp 'front end' would compare, and again found the results quite acceptable.

I'm sure there are ways of improving the nuances of very high spec impedance converter circuitry, but whether these make very much difference outside the scientific - or very high end audio - world, I have my doubts.

The input capacitance of the OPA1641 is around 5.5pF - (fairly linearly over the entire range, it would appear from fig 36 on the data sheet (see HERE ) ) - That does not seem an excessive figure?
In this configuration, there are no additional resistors in the signal path, so that helps minimise further noise generation.
Depending on the noise figure of the following mic preamp - which will need to provide any additional gain required of course - I've found that in the environments I'm using my mics, the external ambient noise always dominates.
So noise figures lower then the typical 5nV/sqrt(Hz) quoted for this device probably wouldn't make any significant difference in many 'real world' usage situations...

As you say, there are ways of improving aspects of impedance converter circuitry, but I would suggest that we are well into the realms of the 'law of diminishing returns' there.

I would suggest that this simple (and cheap!) circuit 'punches above its weight', as they say?..... I've certainly found it to be quite useful.

Yes, of course, simplicity rules It was not my intention to run you down. For hi cap hi output capsules (for example 80 pF, -34 dBV) there is really no necessity to think it over. But if you are using an sdc capsule (35 pF and -40 dBV) things change significantly...