New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

[olafmatt]

"Higher" noise floor with 22uF is because I forgot to set the full-scale level of the ADC. So the whole dBu scale should read 14 or 18dB less, as in "more negative" (it's been a while since I made these plots so can't remember which ADC I actually used).

[ilya]

I've just stumbled upon this:
https://audioxpress.com/news/acoustics- ... ugust-2022

Article by Michel Nieuwenhuizen looks a lot like Waynes work.

[mediatechnology]

I've just stumbled upon this:
https://audioxpress.com/news/acoustics- ... ugust-2022

Article by Michel Nieuwenhuizen looks a lot like Waynes work.

Thanks for sharing that!
It sure does look similar.
I see he references Chalupa which is a seminal work but then AC-couples the output - as I do - to avoid burning off the CM voltage.

[olafmatt]

Had a look at the article. He is floating the frontend and then uses a 220n cap to avoid the level shifting. The floating frontend is a SSM2017 with a DC servo (470k/470n) sampling the output and then injecting correction ("balanced", by also inverting the servo output) into the input through 100k resistors. So fc varies with gain.

As reason for building it he cites the Bateman "capacitor sound" articles and that he wanted to get rid of electrolytic caps in the signal path.

[mediatechnology]

This looks remarkably like my original flawed 1510 design.

IIRC the problem is the Vref connection of the SSM2017 is filtered which limits common mode rejection - its not referenced to the ground at pin 1. I used a first-order filter so a third order might improve it dunno.

In later designs I moved to the 1570 and carried the fully balanced output (with the CM gain unity) to the "dry" side so CM rejection could be realized by referencing back to signal ground pin 1.

So fc varies with gain.

Yes and possibly resonant second-order.

[ilya]

This discussion has sparked a new wave of my interest. Might get to building the Wayne version at some point.

[mediatechnology]

I think I read that the CM filter is about -90 dB at 60 Hz so my earlier comment does not apply.

The author makes an interesting point about impedance-balanced outputs that only have one leg driven producing CM voltage that's half the differential. That explains the need for a steep CM filter.
Thinking about it if the input contains a differential (floating) pad the CM voltage could easily exceed the differential.

He seems to think that since they're aren't any caps in the input that lower current protection diodes could be used but that ignores any stored charge in the microphone due to its output coupling caps.

The first page of the pdf is pretty pixelated so its a little hard to read. I found the link to buy the issue for $9 and may order it today.

[olafmatt]

The first page you can maybe read by just looking at an enlarged version of the image used on the audioXpress website:



In the description for the photo it says that the author is using this with home-built microphones. So lets hope those don't have any electrolytic caps in them.

[ilya]

I’m thinking about this flying concept and applying it to some other preamp. I was planning to build Cohen type preamp at some point anyway. And this whole flying rails concept makes the project much more fun.
I suppose that THAT chips are based on the same topology, so there should be no problem with its discrete implementation. Take some 4401/03 matched transistors (or newer ZTXs), or even MAT02, which are already matched.

I know THAT chips are SOA, but I’d hate soldering QFNs and spending time programming code for microcontroller.
What you think?

[mediatechnology]

I've been wanting to use the ZTX851 or ZTX1053 in a mic preamp.
Both are exceptional.