All OpAmp mic design (no FET at first stage)

[rogs]

Rogs does it yet again!
Have you considered opa1655 because of it's even lower noise? I forgot to include it in my last Mouser order

I did look at the OPA1655 and the noise figure seems to be even lower than the 1641 ..... They don't always make reading data sheet noise figures very easy, and often quote different parameters! .
Whether it would make much difference in most 'real world' applications, I'm not sure?
Certainly it's ambient noise that dominates in the sort of environments I use my mics....

It doesn't state on the data sheet front page bullet points that the 1655 amp is unity gain stable. (They make a special point of mentioning that in the OPA164* data sheet ) It may well be of course?.... Just needs to be checked for unity gain buffer use!

The 1655 draws twice as much current at the 1641 .... still not that much, but it's not a plus point.
The input impedance is a lot lower on the 1655 - only about 100M in differential mode. That may be a bit low?

The input capacitance is also better on the 164* series according to this comment from a TI engineer posting on a different forum:
"The OPA1641 would be slightly better. It has the most stable input capacitance vs. common mode voltage that I've measured on any op amp. When we developed the OPA1656, this was also a major design target for us, and the designer and myself spent quite awhile in the lab with previous CMOS audio op amps (specifically OPA1652 and OPA1688) hunting for sources of input capacitance variation. The end result was slightly better than the previous parts, but we still met some limitations from the CMOS process itself. The dielectrically isolated JFETs on the input of the OPA1641 are superior in this regard."
Looks like the genuine JFET input of the 164* series may win out there?

Probably all just marginal differences in reality of course?

 

[soliloqueen]

a lower current version of the 1641 does exist in the OPA145 (it's specified for different use cases, but electrically that's more or less what it is), but it's 50% more expensive. not really much use for it considering it has slightly higher noise and distortion and phantom power already supplies enough power for 2 channels of the 164x, but i thought i'd share

 

[rogs]

a lower current version of the 1641 does exist in the OPA145 (it's specified for different use cases, but electrically that's more or less what it is), but it's 50% more expensive. not really much use for it considering it has slightly higher noise and distortion and phantom power already supplies enough power for 2 channels of the 164x, but i thought i'd share

My original OPIC multi-pattern mic used 3 x 164* op-amps .. (1 x 1642 and 1 x 1641). That was to enable it have a level adjustment, to allow matching of the output levels of wayward dual sided capsules....
That did of course drop the available phantom power volts available to power the devices, which reduced headroom as a result.
It was also fractionally more noisy - altough that wasn't really too much a of a problem, in practice.

When I did the OPIC.45 multipattern version that was designed to use one of my Arienne Audio 'flat 47' capsules, so I didn't need to worry about balancing output levels, and could resort to using a single OPA1641.

I think the next version might use the OPIC 45 circuitry, plus a transformer coupled output.
That might well produce the best mic I've done yet?......

• Single OPA1641 for minimum noise and maximum headroom.
• 'Flat 47' capsule for a lovely smooth FR, and simple pattern control.
• Transformer coupled output for a little extra noise free gain - and a respectable CMRR.

I don't actually need any more mics of course... I just can't resist experimenting!

 

[tk@halmi]

With the help of an opamp, ironically, it is also possible to build auto biasing (servo augmented) discrete JFET amplifiers, and eliminate the need for manual tuning.

You still need to know the target device parameter range, and set sensible values for source and drain resistors, but you would not have to tune for individual 2N4416 devices, for example.