Sanity check differential amplifier for DAC

[zigzagflux]

No need for R2 at all, in fact better to leave it off.
Personally I would use a B- closer to 35V rather than 21. More is better, and 20V is kind of the minimum value for low capacitance.

 

[Chris Hornbeck]

Is 50R really a low enough value as an I/V convertor? Seems really large to me, for linearity.

All good fortune,
Chris

 

[Matthew78]

Hi Chris, slow reaction here, but the value of that resistor seems to be a debated topic of itself. The inspiration for this DAC came from JE labs that uses an even higher 90 ohms, but it’s definitely on my list to vary and see how it alters the sound. Especially in balanced mode I should have twice the voltage so lowering it should be viable, thanks for the tip!

 

[Matthew78]

Decided to build a standalone test for the 10m45s, to see if it's working before I connect it to the valves, and it feels like I'm missing something simple here:

I've rectified a 2*18V transformer with 4 diodes and 4700uF to get ~57V DC, however when I connect as above I don't get any voltage across R2, can anyone enlighten me and tell me if I'm doing something wrong? (For now there is no ground connection, could the fact that this 57V is "floating" be an issue? Also I've been wondering if I can just connect the + to the ground at a later stage?)

I've tried 3 different 10m90s, but all with the same result, they came from China although the seller got good reviews. For the last test I used a Vero board as I read they are sensitive to heat, so wanted to remove that from the equation.

Any thoughts are appreciated! thanks - Matthew

 

[Matthew78]

Adding on: I've also tired with R1 bypassed, but still not voltage across R2, any thoughts appreciated before I order some (expensive) but for certain real one from Mouser or so. Thanks!

 

[gerrittube]

Hi Matthew,

Just check (again) you have wired the 10M90s properly.
I bought some from Aliexpress some time ago, but they were all fake ones that didn’t work. I went back to the real thing.

Regards, Gerrit

 

[Matthew78]

Thanks Gerrit, I was afraid I would end up that path, just annoying that the shipping is so much more than the parts itself 🙂 or perhaps a good excuse to buy some more components while at it…