All OpAmp mic design (no FET at first stage)

[kingkorg]

Just to reiterate. This was just a exercise of mine to get the most out of the opamp already used without using internal pad. Also, since we use dedicated tube mic PSUs, why not one for fet/opamp mics?

Second was i have a Neumann sdc capsule that can take 150db spl by specification from Neumann and i want to stick it on top of a snare without thinking what could happen under recording and if internal circuit will cause non-linearities i might not be able to catch immediately.

This kind of thing is not necessity by any means.

Also well worth knowing your typical mics can easily put this kind of voltage at your fet gate (tube grid) on plosives if pop-filter is not used.

 

[Paul W]

It's funny because vintage circuits change performance on high transient sources like drums a lot and for a long time I saw people on here being like "vintage circuits can't affect sound that much because they only affect much higher levels than a capsule would output" and then Kingkorg proved that capsules are actually really hot in terms of voltage. Of course tube and transformer microphones sound different on drums. The transients are huge! They're distorting!

Can somebody explain how transformers distort transients? Is it saturation, or slew rate limiting, or both, or what? And how does it differ from amplifier saturation and/or slew rate limiting (or whatever else matters).

Should we think a classic circuit does a particularly good job of shaping transients? Or should we engineer some distortion pedal-like saturation circuit into microphones, or what?

 

[rogs]

Most AD converters in audio interfaces these days run on 3.3, 2.5 or most recently on 1.8 volts. And convert nicely slightly less.

No point in bringing 26dB snare into the DAW either. We are long past tape days when we had to fight high noise floor.

I think setting gain and trims to suit your source is still often required even with 24 bit A/Ds, to ensure optimum performance?

That's not something you need to worry about at all with 32 bit float recording.
Not really a standard format as yet, but I can see it growing in popularity?

As I mentioned above, I use a Zoom F3 which only records as 32 bit float.
I find it very useful to record bat ultrasound, which has a notoriously unpredictable dynamic range.... No more worries about accurate gain setting in the dark with 32 bit float!

The Zoom utlises the AK53888 A/D converter, with 2 x 24 bit converters per channel, configured for 32 float recording.

I use an ancient version of Adobe Audition as my DAW, and that is happy to accept 32 bit float files.
Once they're inputted into the DAW, its's simple enough to adjust the gain and carry out any processing on the 32 bit files, which can then be converted to 24 or 16 bit files, for onward use.

With my cheap Behringer UMC404 audio interface capable of handling 24 bit 192 KHz sample rate formats, it's astonising the quality that can be obtained from really not very expensive kit these days.

Doesn't help at all with rubbish source material though!

 

[rogs]

Further to my earlier comments in THIS POST I've built a transformer coupled version of the OPIC.45 mic.
That's a multipattern OPIC mic, built into a U87 style body, and fitted with one of Ari's 'flat 47' capsules.

Some notes here: https://www.jp137.com/lts/OPIC.45TX.pdf

Initial tests are very encouraging .... still need to check things like the values of the resistors in series with the transformer windings (intended to minimise any reactive loading 'nasties' ).

And I've yet to discover how the transformer will react to high level transients.... They will presumably saturate the core in some way?
How that 'distortion' will actually sound, I've yet to find out?.....

I think I maybe entering the snake oil filled world of finding 'trendy' words to describe distortion!

 

[kingkorg]

They will presumably saturate the core in some way?
How that 'distortion' will actually sound, I've yet to find out?.....

I think I maybe entering the snake oil filled world of finding 'trendy' words to describe distortion!

Probably the best way to avoid snake oil territory is to inject the signal using REW to analyze the saturation profile.
I presume saturation will mostly affect the low end.

 

[homero.leal]

Of course Ricardo, this is the final schematic of the circuit:

View attachment 141833I redrew the schematic on Eagle CAD, since I like PCBs from Eagle, so that's what I used.


I could not find an OPA134PA on the component database, so I used a generic DIP-8 layout and then changed the label to OPA134PA.

All of the polarized caps are electrolytics. All of the non polarized caps, are ceramics.

Resistors are precision 1% 1/4W.

One curious or strange detail. I bought some special low impedance electrolytics. As I read that these are the best for singal coupling, I used this caps for signal coupling on XLR 2 and XLR3, and they worked... but after doing a test recording, I saw a strange signal behavior on the DAW track. It seems as a very low frequency oscillation:

After replacing this two caps for a pair of nichicon audio grade electrolytics, everything worked fine as on the previous DAW image.

Regarding voltage measurements, I got:

XLR2 18.95V
XLR3 18.97V
Zener(+) 11:31V
OPA134 PIN 2: 0.577V
OPA134 PIN 3: 5.54V
OPA134 PIN 6: 5.57V
OPA134 PIN 7: 11:14V

I was hoping that OPA134 PIN 2 will be also aprox 5.5V, but that was not the case, don't really know why. Anyway, the mic is working.

Thank you again Ricardo for your advice and help!

HL

I have reuploaded the schematic just for reference. Please note that it uses an OPA2140, which is closely related to the OPA1642.
OPA2134 also works, replacing R3 and R4 for 2k resistors, and R6 for a 220R resistor.
Regards!