Alice OPA: Adding harmonic coloration?

[Wordsushi]

Dude! Thank you for posting the pics. That is fantastic.

Hmmmm..... So, this is getting interesting. I dug around and found this schematic, which at first glance looks like it may match that "Alice with Lamp" board you just posted.

 

[Fastunov]

Personally, I really lack attenuators (-10, -20) and filters (high frequencies, low frequencies) in the Alice scheme

 

[kingkorg]

Dude! Thank you for posting the pics. That is fantastic.

Hmmmm..... So, this is getting interesting. I dug around and found this schematic, which at first glance looks like it may match that "Alice with Lamp" board you just posted. View attachment 98009

That is frankentube. I have built several of those. Nice, but issues with microphonics. If i had to build them again, i'd just use aaa batery for the filament. Those 2665 chips are really tiny. Not diy friendly.

 

[mhelin]

There is an interesting circuit presented on Elliot's ESP page

https://sound-au.com/articles/jfet-design.htm
which has a chapter

6 - Active Opamp Load JFET Amplifier

where it is claimed that the circuit presented "sets up the JFET to operate as a 'true' square-law device, and it has only 2nd harmonic distortion" and sounds good at least on guitar (well, there was 3rd harmonic <100dB level). Maybe worth experiments, just replace the TL081 with OPA164x.

OT: Found that also TI has their own "OPA Alice" application note there:
https://www.ti.com/lit/an/sboa320a/sboa320a.pdf
Additionally this paper explains why OPA164x's have less common mode distortion in non-inverting configuration (like in the first buffer stage in OPA Alice) compared to other FET input opamps:
https://www.ti.com/lit/an/slyt595/slyt595.pdf

 

[Script]

However, that circuit from sound.au also produces intermodulation distortion which, as the article also says, is quite objectionable (the more the louder it's driven ?!).

Looks similar to this animation for third:
https://en.m.wikipedia.org/wiki/File:3rd_order_intermod_animation_(thumbnail).png

 

[rankot]

For a very short time, the Alice opa board with a 6418 lamp was sold on aliexpress. I managed to buy two boards, then the sale was stopped.

If you're interested, I can take a close-up photo of everything

Looks interesting! How does it sound? What kind of capsule does it need to work?

 

[abbey road d enfer]

Looks interesting! How does it sound? What kind of capsule does it need to work?

The schematic shows a FET-less electret capsule. It could work with an externally biased condenser capsule, but you would need to provide bias via a 1Gohm resistor. If you take it from the +27V point, the capsule would be about half-biased, resulting in about 6dB less sensitivity. In order to bias the capsule correctly, you would need a voltage multiplier circuit, which is a standard appointment of mant DIY mics.

 

[Mellow Audio]

Not at all, I'm at the beginning of this particular learning curve. Thank you.

We all are at one point or another lol

 

[EatsMicrophones]

@Wordsushi

Found this project a while back. Might be of some interest to you. OP1641 board (inspired by Jules' work) but transformer.
http://www.halfshavedyaks.xyz/pcb/OPATX.php
Ignoring the ratios of transformers, wouldn't a transformer interacting with an opamp have less of an impact than a traditional fet? Hope I'm wrong.

 

[abbey road d enfer]

@Wordsushi

Found this project a while back. Might be of some interest to you. OP1641 board (inspired by Jules' work) but transformer.
http://www.halfshavedyaks.xyz/pcb/OPATX.php
Ignoring the ratios of transformers, wouldn't a transformer interacting with an opamp have less of an impact than a traditional fet? Hope I'm wrong.

In order to minimize distortion, teh drive impedance must be the lowest possible.
The same xfmr, driven by an opamp will result in less distortion than driven by an FET, which has a high impedance output.
However, considering the low level there, the difference would be minimal.

 

[StanislavTugushev]

The Alice OPA circuit is pretty incredible. So clean. Super quiet. Great, great stuff.
And it made me wonder: how difficult would it be to add some level of saturation/harmonic coloration?

My couple of cents.
1. Just keep this "Great, great stuff" great. Really. Add "coloration" in post-processing or in case of live production further in the chain. Add tube cascade (after the microphone), exciter, fuzz, distortion, vocoder, whatever you like, but outside of microphone circuit itself. This way, if you need "So clean. Super quiet" sound - you will always be able to have it. You can always spoil it later.
2. You do not need any odd harmonics. While even harmonics (2nd in particular) add "warmth" to the sound, the odd ones (3rd, 5th, etc.) add dirt.