Converting a DAC-in-the-Box to I-out? - diyAudio
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Old 26th May 2007, 11:44 AM   #1
diyAudio Member
 
Join Date: Apr 2006
Location: London
Default Converting a DAC-in-the-Box to I-out?

A recent thread discussed modding the DITB with 12v regualtors for it's analogue stage.

I mentioned the idea of reducing the voltage seen by the OP275 opamp (which was admittedly slightly off-topic), and hence the line-out voltage. The DITB (IIRC) swings 2.4v full-scale, so if one reduced this to, say, 1.0v, would this not go some way to ameliorating the problem of the marginal +/- 5v supplying the opamp?

I guess this might be a problem if one was intending to use passive pre-amplification, but the reduced output would actually be an advantage for me.

In connection with this, I mentioned a different modification (which could atually be made along with the 12v mod) - which involves using the DAC-chips' I-out (current-out) instead of V-out - apparently the latter uses an integrated op-amp on the DAC itself.

I found it at this web-page, although the gentleman doesn't go into any detail about the actual surgery involved. But he contends that I-out produces a cleaner signal.

I'm not that experienced with PCB-modding, but I do some electronics tinkering - I wonder if anyone could confirm the best way to go about the I-out conversion?

I assume it would be along these lines;

1 - drill two holes in each of the two tracks from the DAC's V-out pins to isolate them from the rest of the board (which would also allow reversing the mod with a soldered jumper if needed)

2 - Possibly on the underside of the board, solder a resistor directly from the I-out pins to ground (I assume there are no tracks to them, but havn't checked yet, would be boon if there are) - although I'm not sure where on the board would be the best place for the ground end - and a jumper to the holes further 'downstream' of the two previously made in each of the V-out tracks

'Jon' used 900ohms, and I thought I read that this reduced the voltage seen by the OP275, but in fact he doesn't say, so I was thinking that a pair of resistors configured as a voltage-divider might be needed if the aim is to achieve this.

Does this sound feasible to Those Who Know at DIYAudio?

Also, could anyone give me an idea how much current the DAC(s**) would end up pushing, and whether heat-sinking them might be desirable or necessary?

**BTW, I have 2 DITB's - the older one uses the dual-channel AD1864 DAC (which I guess makes it very old), the other a pair of single channel AD1860's - the latter came as a surprise because I thought the 1864/1865 was always used, but in fact you can see they both use exactly the same board.

ciao,
A.
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