A nice dac to complement the Aleph-X.

[LBHajdu]

Now that Aleph X projects are well on there way I think a nice dac and passive preamp are all that’s needed to finish the set. When I say dac, all that’s truly needed is the I/V stage as dac schematics are plentiful on the web. My personal favorites are the PCM65, PCM 1702 and for 24bit PCB1704. The I/V for the Aleph X has to be balanced have a low HTD and for my personal tastes have a 2 volt swing for a passive preamp. Here is what I have come up with to get the ball rolling, I’ve taken Jocko Homo’s I/V stage from the Easy-to-build I/V stage thread and made it balanced. The constant current sources share the LEDs or zeners if you like so the noise they make should be the same on both halves. Then, I added the last stage from the Nelson Passes D1 for a nice voltage swing. Following the Nelson Pass philosophy the I/v stage has only 2 semiconductors and one resistor and one cap in the signal path.

I have not built this dac, and it may indeed prove not to work. I am no electrical engineer of I have noooo idea how to even start calculating values for something like this. Because I mostly cut and pasted, most of the values should be right, but I wouldn’t be surprise if the whole thing has to be recalculated and re-biased to make it work. This is just my idea of what the perfect dac to drive an Aleph X amp would look like. It’s my hope that someone will calculate values for this thing and try it or improve on it.


Leve

[uli]

Hi,
why not take a 9610 instead and put a feedbackresistor from the
drain to the dac´s output, acting as the c/v converter and to
control the conversion factor? You even can swap that cap
in the signal path (don´t like any cap in the signal path).

U

[LBHajdu]

I don’t really know which is worst, feedback, or a cap it the signal path. It may help to make that 1.5k resistor a constant current source. I don’t know if that would stop DC. If you wish take the picture and pencil it in so people can see what it looks like.

Leve

[Mark A. Gulbrandsen]

Hey, When you guys get this all figured out let me know. I need a balanced out DAC!! So you design it and I'll be the first to build it. Wish I was the engineer type and could help with that, but I'm not....
Mark

[uli]

Hi,
reason for a (very tiny) amount of feedback is that you have to control VERY precicely the I/V factor, especially when going symmetric! both halves of the circuit should be matched far beyond 0.1%, with a feedback of lets say 6dB you are able to match. Otherwise you are a victim to broad tolerances concerning
ß of the first transistor and transconductance of the fet!
IMHO it´s impossible to get symmetric performance without control.
I try to draw and simulate that circuit (UltiCap) and tell you
what happens
Uli

[uli]

Drawing is attached, if you use the 10k pot swap the 1k65 0.1%
with 2k5 0.1% for a maximum output of 2V pk. with PCM1704
Uli

[OliverD]

Uli, you need to add a resistor from the anode of the LED to ground to make some current run through it. For a standard LED, 1k might be a good value.

[LBHajdu]

Uli,
Thank you for your simulation. I see you’ve made some changes. I like the idea of using matched transistors, however there very hard to find. Digi-Key had them; however I don’t think they stock them any more. I will try to find my self an alternative source for these. I don’t really understand what happened to the constant current sources at the top. They seem to have become ways of adjusting the voltage. In Jocko’s “Easy-to-build I/V stage” thread somebody suggested replacing the top constant current sources with a 2.49K resistor and a small cap Jocko said that the distortion would be at least 20 dB higher, I don’t know why, take a look at the thread it’s in the Digital section of this Forum. What happens with the dc offset in reference to ground? That’s still there right?


Let me post a picture of your I/V stage to make it easier for people to see without having to download. I had to make it a little smaller, but you can still see all the values.

Leve

[tschrama]

Where is Jocko Homo?!!!!!!!!!


Dammit I would be very pissed if the guy that got me this 'design-the-perfect-I/V-stage' desided to ship the DIY-scene and instead is selling sony's and technics in a local Low-Fi shop.




gr,
Thijs

[uli]

oops,
that resistor should have 12k!
trimmers are for output nulling without influence on gain!
Uli