[tilt]

Quote:

Originally Posted by shlomomofo and with a 6V supply, for the sake of completeness.... 6V-1.2V-1.8V = 3V peak-peak 3V/(2.3e-3*8) = 163 ohms I/V 1.8V + (3V/2) = 3.3V midpoint 3.3V/163 ohms = 0.0202A = 20.2mA 20.2mA "plus our 1.15mA DAC digital silence current" = 21.35mA 21.35mA/2 = 10.68mA = 0.01068A 2.2V/0.01068A = 206 ohms not sure if any of this is correct, as sam seems to be getting only ~2V at Vref with ~1000 ohms of resistance... (right sam?)

hmm

 

[tilt]

Quote:

Originally Posted by Samgotit I replaced the trim pot with a 1k bourns. I adjusted the resistance to give me a Vref of 2.20V. I cannot tell a difference in sound. Sadly, this is all I've had time to do. I really need to get to the bigger stuff. Shlomomofo did the calculations earlier for replacing the Vref trim pot with a resistor. I came up with the same numbers. I Do NOT know if they are right. Cheers, Sam

Next time you're in there, messure the resistance across your vr1

 

[d-cee]

hi all,

I've received my DAC-AH (green PCB - everyone else on this forum seems to have red PCB)

anyways, i'm waiting for some sprague paper and oil capacitors to arrives and i had a couple of quick questions

i understand the polarity of electrolytic capacitors is important, however, i am under the impression somehow that paper and oil caps don't have this issue - please correct me if i am wrong to save this exploding in my face =S

and lastly, if i perform the passive output mod will it require me to de-solder the opamps in there or can i leave them be?

thanks

 

[Samgotit]

i Quote:

understand the polarity of electrolytic capacitors is important, however, i am under the impression somehow that paper and oil caps don't have this issue - please correct me if i am wrong to save this exploding in my face =S

They are non-polar AFAIK.

Quote:

and lastly, if i perform the passive output mod will it require me to de-solder the opamps in there or can i leave them be?

Take them out and replace with sockets in case you latter want to try opamps, but I preferred it passive.

 

[mrarroyo]

Quote:

Originally Posted by Samgotit i They are non-polar AFAIK. Take them out and replace with sockets in case you latter want to try opamps, but I preferred it passive.

Which OP Amps can I use w/o having to modify the circuitry? Thanks.

 

[Samgotit]

Quote:

Originally Posted by mrarroyo Which OP Amps can I use w/o having to modify the circuitry? Thanks.

Never did try opamp rolling, but most single channel, unity gain stable opamps that can take up to 15V should be ok. This is mostly a guess. There could be other variables to consider and this shoud be backed up by someone with more experience. Here's the link to the schematic if someone wants to help verify this:
http://www.audiodesignguide.com/DAC/...ircuit_new.JPG

One example is, of course, the OPA627, which works fine. A few others that look good: OPA227, AD8610, and so on.


Here's the data sheet on the NE5534 which is the stock opamp. You can go from there:
http://www.ampslab.com/PDF/ne5534p.pdf

 

[izquierdaste]

I have a dac-ah with opa 627's on the way. I am thinking of replacing the stock coupling capicators with sprague vitamin q 12uF 200V. The schematic says 22/40 phl. Would the 12uF capacitors be enough? Any help would be great. I don't want to do the passive bypass yet, so the capacitors would still be connected to the opamps

Thanks,
Brian

 

[d-cee]

Quote:

Originally Posted by Samgotit Take them out and replace with sockets in case you latter want to try opamps, but I preferred it passive.

i am assuming from this statement that i must remove the opamps completely or put in a socket for the passive output to work?

 

[ozshadow]

tag for later reading

 

[Samgotit]

Quote:

Originally Posted by izquierdaste I have a dac-ah with opa 627's on the way. I am thinking of replacing the stock coupling capicators with sprague vitamin q 12uF 200V. The schematic says 22/40 phl. Would the 12uF capacitors be enough? Any help would be great. I don't want to do the passive bypass yet, so the capacitors would still be connected to the opamps

See this:
http://www.ecp.cc/cap-notes.html
http://tangentsoft.net/audio/input-cap.html

22/40 phl - I still have no clue what this means.

Quote:

Originally Posted by d-cee i am assuming from this statement that i must remove the opamps completely or put in a socket for the passive output to work?

Yes.

 

[shlomomofo]

hmmm... my dac-ah worked ok after doing the bypass, and i did not take out the opamps.

 

[Samgotit]

Quote:

Originally Posted by shlomomofo hmmm... my dac-ah worked ok after doing the bypass, and i did not take out the opamps.

You're probably right shlomomofo, I answered based on tilt's pics on the first page where they are missing. I didn't recall that the pads for Lout and Rout are not used in the bypass (mine is now disassembled). My apologies.

To the person with the green board: It may be interesting to see what voltage your regulators are supplying the DACs. Many of us with the red PCB had 6V regulators where the schematic called for 8V.

 

[d-cee]

Quote:

Originally Posted by Samgotit You're probably right shlomomofo, I answered based on tilt's pics on the first page where they are missing. I didn't recall that the pads for Lout and Rout are not used in the bypass (mine is now disassembled). My apologies. To the person with the green board: It may be interesting to see what voltage your regulators are supplying the DACs. Many of us with the red PCB had 6V regulators where the schematic called for 8V.

how would i find out? mine is a 220/240v australian spec one

edit: also, does anyone else notice that the heatsink closest to the centre gets really hot? but the other two stay cool/warm?

 

[Samgotit]

Can you take a photo of you're board.

The heat sink closest to the middle, near the row of resistors, is for one of the LM7808 (or LM7806, whichever you have). It feeds four of the eight DACs. The one next to it is also a LM7808, feeding the other four, and theoretically should be about the same temp if the boards have the same layout.

To measure the voltage of those regulators you will have to remove the board and measure at the solder joints. The middle pin is the ground. (See this for the pin out: http://www.fairchildsemi.com/pf/LM/LM7808.html click "data sheet". Measure from the middle joint to the output. The output will be the pin closest to the middle row of resistors. You should get either 6V or 8V DC. Again, this if for the red pcb, so please take a picture of yours to be sure they are the same.

 

[TheMarsVolta]

From reading through all the DAC-AH mod threads, here's an attempted summary of mods people have mentioned, sorted from my rough understanding of cost-benefit.

Comments from those who have done these mods would be great, I'm getting some parts together as I anxiously wait for my DAC to arrive in the mail.

1. Bypass the active output stage into passive output, using blackgate or paper-oil caps and higher quality resistor for I/V section.

Question - if the active stage is bypassed, is the +/- 15V section of the power supply still needed? (for the ground channel?)

2. Upgrade power supply caps

3. Replace the 8 DAC chip film caps with something better quality - such as PP

4. Upgrade regulators

5. Add heat sinks to TDA 1543 chips

6. Upgrade RCA and power connectors

Getting more exotic:

7. Stack the TDA1543 (and adjust power supply voltage etc. accordingly)

8. Does having the power supply transformer so close to the PCB cause measurable noise? would moving the PS external to the DAC be worth the effort? I noticed DIYCLUB sells the bare case for $16, it could be used to house the transformer only.

9. Reclocking unit