Comparison of JFETs for mic applications

[rogs]

I've been using an OPA1641 in a very similar circuit with my OPIC project for the last couple of years now....
Low distortion, decent headroom and a reasonably low noise figure.
No more deciding on whether to go for low distortion or symmetrical clipping, as you need to do with discrete FET inputs...
The noise level maybe slightly higher than some JFET input circuits, but I've never found it to be a problem.....

 

[Matador]

Here's the circuit

Is it just me or is C5 backwards?

 

[rogs]

Is it just me or is C5 backwards?

No, it's the right way in this configuration...... The +ve end has the full phantom power voltage applied, whereas the -ve side only has the lower op-amp output voltage applied.
The output is biased at the same potential as the non-inverting input, the 'half rail' value derived from the junction of R4 and R5, and applied via R2 and R3

 

[Matador]

Of course...duh!

 

[Somkereki]

OPA1641 Initial Results​

Here's the circuit - just a very simple voltage follower running off P48 power:

View attachment 128224
R6 can be chosen to set the supply voltage, in this case 24V. Supply current is approximately 1.7mA, and there's 41.6V available at XLR pins 2 and 3 for capsule polarisation.

Measurements (all at 1KHz) are as follows:

Value	Cin=1nF	Cin=68pF
Voltage gain (100mV in)	-0.3dB (x0.96)	-1.2dB (x0.87)
Input noise, A-weighted	-122.9 dBV	-119.4 dBV
THD, 100mV RMS in	0.00052%	0.00057%
THD, 500mV RMS in	0.00047%	0.00040%

Caveats​

• The construction might be responsible for some excess input capacitance (I used the IC soldered to a carrier board, plugged into an IC socket, soldered to a perfboard), which lowers the voltage gain (and raises the input noise) for Cin=68pF. I may try an 'air-wired' build.
• A practical microphone circuit would probably use a second high-value resistor at the input, which would increase the low-frequency noise.
• THD numbers really just mean "lower than I can measure" here. See the spectrum below - most of what you're seeing is the signal source (an iPhone!) rather than the amp itself. View attachment 128230

The noise curves look like this - again, it's possible that lowering the input capacitance might bring the green one down by a fraction.

View attachment 128231

Nowadays, it is worth examining the 1/f noise even under a frequency of 20 Hz! Because everything will be digitized in the end. Few people know the effect when high level 1/f noise "phase modulates" the harmonics belonging to the fundamental tone, which causes an unnatural sound. I noticed this a long time ago, in the analog era, although my colleagues told me not to worry about it, our ears can't hear below 20 Hz anyway.

 

[k brown]

Nowadays, it is worth examining the 1/f noise even under a frequency of 20 Hz! Because everything will be digitized in the end. Few people know the effect when high level 1/f noise "phase modulates" the harmonics belonging to the fundamental tone, which causes an unnatural sound. I noticed this a long time ago, in the analog era, although my colleagues told me not to worry about it, our ears can't hear below 20 Hz anyway.

Doesn't need to be done in the microphone, though. Just high pass it somewhere down the line.

Many decades of successful recordings made with mics that don't have infrasonic filtering in them.

 

[Voyager10]

TL071 comparison​

Purely for fun, I put a TL071 into the previous circuit and measured it:

Value​	Cin= 1nF​	Cin = 68pF​
Gain	-0.3dB	-1.2dB
Input Noise, A-weighted	-112.6 dBV	-111.2 dBV
THD, 100mV in	0.00082%	0.0062%
THD, 500mV in	0.0024%	0.031%

Here's the input noise comparison:


and the spectrum for that 500mV / 68pF THD figure looks like this: