Experience with this DIY DAC ?

[RichLund]

Originally Posted by PreSapian

thanks, Bill. so are those four all there is in the analog circuit? also my dac came with 13v+13v EI transformer. but i realize that the dac only requires 7-10v on the digital circuit as it is clearly stated on the board... i have the opa1798 version though. would that be why it's different?

p.s. still waiting on the info of how to bypass the op-amps on 1798 board. also, i apologize for acting as though this is a FAQ thread. perhaps i should've come here once it was 600 pages long.

The PCM1798 still needs both 5V (DAC analogue section) and 3.3V (DAC digital section) supplies to operate like any DAC chip. The LM317 regulators need a few volts headroom (dropout) to operate properly so as long as you give them at least 9V supply thats fine. Anything above that is dissipated as heat. You could give them 25V and they would only regulate to 5V but get very hot.

[RichLund]

Originally Posted by sendler

Would running a pair of PCM1794a in mono mode double the available current output? If so this might bring the required transformer gain down to where you might get by with 1:2 depending on how much level your amp really needs to achieve a high listening level opening up more options for affordable transformers.
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I don't think it works like that. Using PCM1794 in dual mono doesn't double the output, it just uses one chip for each channel. Each channel still has the 7.8mA output current that the 1794 provides.

[sendler]

Originally Posted by RichLund

I don't think it works like that. Using PCM1794 in dual mono doesn't double the output, it just uses one chip for each channel. Each channel still has the 7.8mA output current that the 1794 provides.

The chips are usually used with L+, L-, R+, and R-. By selecting a mono mode, which is one of settings available in the registers, you can get L+, L-, L+, and L-. You may need to add a small, well matched series resistor, like 2 ohms or smaller (maybe none), to each output to keep one or the other from power hogging, but the signal will be well correlated in the audio band so you should be able to parallel each pair for 15.6ma which should get the transformer requirement down to 1:2. The SPDIF can simply be shared (re-terminate to get 75 ohms if you like) between two boards with one chip set to left and the other set to right channel.

[RichLund]

Ah, fair play Scott, I stand corrected. In which case this would be very interesting.

I take it you would need to custom design a PCB to mount the two chips (vero board?) If you can work this out, definitely share the layout.

Rich

[sendler]

Originally Posted by RichLund

this would be very interesting.

I take it you would need to custom design a PCB to mount the two chips (vero board?) If you can work this out, definitely share the layout.

Rich

We should just be able to work with two stock Gigaworks boards to start. Either the big dac boards, or the the newer small dac boards if you don't want a DIR9001 or upsampler option. Just split the coax spdif with a "Y" cable as I have done with two DCXs without consequence and do the mods to set the registers to mono and create the parallel, transformer I/V mod. This should also work with the PCM1798 to get the output up and suitable for the 1:4 Lundahls that Rakk dac uses with a stereo 1794.
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K & K Audio - Lundahl Transformers, audio DIY kits and more
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Too bad these Edcor trannies are a little too small. 80% nickel core and center tapped for only $26 vs $130 for an amorphous Lundahl.
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EDCOR - Browse Series

[sendler]

Originally Posted by RichLund

I take it you would need to custom design a PCB to mount the two chips (vero board?) If you can work this out, definitely share the layout.

Rich

It might even be pretty easy to mod the stock PCM1798 module boards to stack one on top of the other in order to try mono mode with a passive I/V. Soldering another set of sockets onto the top of one board would be a piece of cake.

[sendler]

Originally Posted by sendler

It might even be pretty easy to mod the stock PCM1798 module boards to stack one on top of the other in order to try mono mode with a passive I/V. Soldering another set of sockets onto the top of one board would be a piece of cake.

Then my next thought is. Do we need to select mono mode at all or would stacking two stock 1798 (or 1794) boards double the current output with no other mods?

[hoverdonkey]

Long time no speak. At last I'm back onto my DAC project. Before I get on with more mods (power supply), I want to make sure what I already have is correct and would be grateful for some info;

1) What are the audible symptons of magnetised output trannies?

2) Is my anti-ringing filter ok? I have 1000pF cap + 1K resistor in series, soldered across RCA + & - terminals (circuit diagram is here - hoverdonkey's DAC - DIY DAC).

Thanks

[hoverdonkey]

I have pooled together some of the datasheets etc relating to this board.

Enjoy;
Intro/Datasheets - DIY DAC

[Bill Fuss]

Hey, how ya doin?

Looked at your dac, was really surprised. My GA80080s are open frame, that's weird.

The filter's still good. If you drop down to around 250 ohms on the primaries it will give you a little more bass reach if you need it.

Nice collection of info, did you see the posts about the incorrect voltage feed on the upsampler board?