Valve DAC from Linear Audio volume 13

I was seeing problems that aren't there. I just looked up the 3A5 datasheet ( http://www.r-type.org/pdfs/3a5.pdf ) and apparently the CD enhancer II circuit is designed for a 0 V input common-mode voltage. Apart from adding those 180 ohm resistors to ground, you don't need to do anything special to interface the valve DAC main board with the CD enhancer II circuit.

(For the record, using termination resistors to a positive supply at the DAC output would have been impossible anyway because of the way the mute circuit is designed.)


Ahh cool. Do I still need the reconstruction filer to precede it?
 
Raj1, you will need the two filter capacitors on the main board, but you can probably do without the rest of the reconstruction filter. That is, I understand from the website that you linked to that the CD enhancer II itself is meant to be used as a reconstruction filter for sigma-delta DACs.
 
I assume I need to keep C28 & C29 (and their equivalents on the other channel), and can do away with the DC blocking caps C31 & C32?
 

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Raj1, you will need the two filter capacitors on the main board, but you can probably do without the rest of the reconstruction filter. That is, I understand from the website that you linked to that the CD enhancer II itself is meant to be used as a reconstruction filter for sigma-delta DACs.

It's a simple little circuit and it'll be interesting to hear about the results Raj1 gets with it. I'm planning on building a second Valve DAC myself and it is on my list of 'possibles'.

A question I have though is whether the rationale of the DAC output, required gain etc. discussed in the article holds true with the Valve DAC; I have no measure of the output beyond the simple observation, based on the setting of the volume control of my headphone amp, that it has a 'healthy' output level. I've not thought about it beyond that so some more consideration required.

I see on the website there are several versions of the circuit using different valves.
 
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diyAchim - 1 PCB
Raj!1 - 1 PCB (UK)
dubulup - 2 PCB (USA)
ericoto -1 PCB
HenSch - 2 PCB (Germany)
mterbekke - 2 PCB (Netherlands, confirmed)
rfbrw - 1 PCB
Pistollero - 1 PCB (Portugal)
luchoh - 1 PCB (USA)
pgour - 1 PCB
agent.5 - 1 PCB (USA)
spm - 1 PCB (UK)


Total = 15

Excellent progress Ray, many thanks. We're following your (Marcel and your) lead, who would've thought when you were building yours alone, that we'd come to this. Great :)
 
Hi Ray,

AFAIK, the output voltage of the DAC is multiplied by the Mu of the tube and then divided by the turns ratio of the output transformer. I’m sure Marcel will correct me if I’m wrong.

-Raja

Yes, but I'm wondering if the output level of the Valve DAC (before the reconstruction stage) is higher than the DAC chips the circuit was originally designed for?
 
Neglecting transformer losses, if any, the peak output voltage is about 2.5 mA times the parallel connection of three things: the anode resistors of the upper E88CCs, together about 11.2 kohm, the termination resistor, and whatever load you get from the amplifier that's connected to the output. With an 806 ohm termination resistor and a 10 kohm amplifier input resistance, that's about 1.75 V peak or 1.24 V RMS.

Apparently the CD enhancer II is meant for lower levels, that's why I recommended using two times 180 ohm = 360 ohm termination.
 
This has got me thinking that, when I do a second build, I may just take the outputs from the main board to some XLR connectors and have the output secton (filters) in a separate box so it's easy to plug in different solutions.

The other possibility is, because I have both Valve DAC and DSC2, along with a couple of conventional balanced output Delta -Sigma DACs, with just a little reconfiguration I might be able to try mixing and matching DACs and output stages.

I wonder if I could use the CD-Optimiser circuit with my DSC2?

Hmmm...
 
Should work with the DSC2, as well. From what I can see, there is ample overhead in the transformers, so the circuit should work with a variety of differential line-level outputs. TBH, I do prefer DC-coupled designs, so tend to opt for DACs that allow me to use this output stage.
 
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You could argue that its not DC coupled because it uses a transformer on the output. I used to worry about eliminating caps but less so now I've built various fantastic sounding projects that include them, Valve DAC being a case in point.

One of the quirks of my system is my use of output-transformerless SE amps but my recent source projects have both had transformers on their outputs.

Agree the circuit you're using should be OK with DSC2 - might just give it a try - the CD-Optimiser article mentions that it's possible to use 6SN7s and I have a nice NOS pair of RCA 12SN7s sitting patiently in a drawer.