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Amb alpha20 preamp - anyone built one?

J

James Evans

Bedroom Bodger
I'm looking at playing around with a high input impedance low gain, low output impedance preamp and came across the amb alpha 20. I already have a +-24v psu based on one of their sigma22 boards so it would seem the ideal match. Just wondered if anyone here has built one and any impressions/inputs on sound?

The α20 line stage amplifier

The α20 line stage amplifier
www.amb.org

(the website is incredibly slow to load here...)
 
It's "just" a diamond buffer with a few additions, but it is a good circuit, the J-FET input doesn't hurt the SQ one bit and you can bias it deep into class A for headphone use if you want. I made my own PCB version (but also have a pile of the original boards) - see https://theslowdiyer.wordpress.com/2020/02/28/jisbos-alpha20-buffer-clone/

I only really bench tested mine and confirmed they work, but not really enough to justify posting subjective sound impressions. If you want to experiment with these simple discrete circuits I think you could do a lot worse than start here. Other candidates would be the B1 by Nelson Pass which I personally like a lot (there is a DC-coupled version which I have also made a board layout for) and Keantoken's error correction buffer (which I did a board for as well, but never really tried).
 
Thanks. I have built a couple of DCB1s in the past but never really liked them in my system at the time. Much has changed since then though! I'm only pondering the idea of an active pre/buffer as I'm playing around with a Bisesik output transformer pcb on my pi stack and it has a relatively high output impedance, which I'm guessing is not ideal when using a passive pre (10k khozmo), hence thinking about having a play around to see how it affects the sound. No desperate need though.
 
The B4 from this parish might also fit the bill. Somebody might have some boards left?
 
Yeah I was wondering about the B4 as well. The appealing thing with the a20 is the potential to vary gain. The transformer hat has relatively low output as well, around 400mV for output impedance of 1.2k so a bit of gain wouldn't go amiss. Those two values scale, so you can see how the output Z gets high when getting near 2v output. I'm trying to avoid opamps as the damn thing replaced the opamp I/V hat so it seems madness then sticking opamps after it (I was looking at the neurochrome buffer as well)
 
"Modern" opamps (OPA16xx and others) are really very good when properly implemented, but fully understand why you would prefer a discrete circuit :)
 
The transformer board actually has spaces to add jfet matched pair buffers on the outputs, but in a much more stripped back fashion than the a20, another reason I'm leaning in that direction as it feels "related" :)

The Ian Canada I/V board this replaced was using OPA16xx opamps actually
 
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