Help me mod my DAC-AH. Capacitor, bridge and other questions. - diyAudio
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Old 6th March 2005, 06:45 AM   #1
bg4533 is offline bg4533  United States
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Default Help me mod my DAC-AH. Capacitor, bridge and other questions.

I purchased a DAC-AH a few weeks ago and would like to mod it. The sound is decent, but I imagine it can be better. My only big complaint is that the highs are a little sibilant and lack definition. Not horrible, but my EMU1212M soundcard has better highs. My other complaint is that the noise floor is a bit high. There is always a little bit of background hum. I imagine this is caused by the transformer being so close and there isn't much I can do without recasing the DAC.

Here are a bunch of pictures I took of the DAC-AH insides. I am currently using it, so I didn't want to take it out of the case yet. http://brian.osupages.com/audio/pictures/dacah/

Currently it has OPA602AP opamps in it. I plan on removing these, and throwing in sockets. The opamp has 30V, so I am slightly limited here. I will probably just go with OPA627s. I will experiment a bit though. Would I be better off just bypassing the opamp? What is the best way to do this?

The DAC currently has Philips electrolytic 22uF coupling capacitors. I would like to replace these with film caps. There isn't a ton of room here. Do I need caps this large? Would 1-2uF be enough?

There are an assorment of other capacitors in the DAC. The brown ones are Elnas that say "For Audio". I think these are RA2s. The green ones are Sanyos. They don't have a model on them. I thought about replacing all of the caps with Blackgates, but that would get quite expensive and I would have to use less capacitance. How do Panasonic FMs compare to the caps that are in there quality wise? Would it be good to put in larger capacitance FMs?

I asked the DIYClub people where I bought this from a few questions about the DAC and they told me "you can also change those rectify by some high speed soft recovery rect diode (BYV26 series), it will up grade the performance of the DAC a lot." Any help with what I should do here? What will this improve?

The TDA1543s get fairly warm. I saw a few people have heatsinked these. Will this give any noticable increase in performance?

Any other mods I should do?

I want to keep this a fairly low budget mod. I already have a nice assortment of opamps to try out in the DAC.

Thanks for the help!
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Old 6th March 2005, 06:33 PM   #2
bg4533 is offline bg4533  United States
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I came across a schematic for the DAC: Link

I also found a post about it at DIYAudio: Link

On bypassing the opamp Supra says:
" It's quite easy- I just cut the link to pin 3 of the opamp. There's some spare holes to connect a pair of 220uF Bi Polar Blackgates in Super E style from the output of the DAc (after the 270 ohm I/V resistor) direct to the output socket. Disconnect the wires from the opamp to the output socket, put a 100K resistor across output socket. That's it. Email me if you need further help."

I sent him an email, but I am not sure he is around here much anymore. I understand what he said except about the 100k resistor. What is the purpose of this? Does it go from the RCA signal to RCA ground? Is it inline with the coupling cap? Something else I missed? Is the value of the resistor important?

I am thinking bypassing the output stage is basically a free and easy mod. It takes the analog caps, rectifier, opamps, etc out of the questions. If I don't like it, it should be reversible. If I do like it maybe I will rip the transformer out and come up with a better way of supplying the 9V and 10V power.
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Old 7th March 2005, 09:25 PM   #3
vmac011 is offline vmac011  Serbia
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I have bypassed output capacitors (22uf) altogether on my DAC-AH. That's because I have 4u7 poly as an input cap to my amplifier.
Also, try to keep toroid away from DAC, preferably in a separate enclosure. 270ohm passive I/V works just fine considering there is enough current output from 8 x TDA's. For short interconnects, replace OPA602 with OPA627 - they are pin compatible. Otherwise keep 602 since it handles capacitive loads much better (up to 1500pf). Sonically, the difference between 602 and 627 is negligible - it's not worth swapping.
I agree with you that analog stage is average. There is room for improvement.
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Old 7th March 2005, 09:39 PM   #4
bg4533 is offline bg4533  United States
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For now I am removing the opamp and dropping in a socket. I am going to try bypassing the analog stage and see how it does. Given the layout and extra holes in the board it is easy and reversible. I will compare the bypass with different opamps (OPA627, AD843 and a few others). The DAC sits on top of my headphone amp which also acts as a preamp. It has no input caps. My interconnects are under 18" long.

I got a response from Supra. The 100k resistors he mentions are just for load. They go from signal to ground.
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Old 19th August 2007, 01:35 AM   #5
recca is offline recca  United States
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I've read some posts on AA regarding battery powering the DAC-AH but I haven't found out how it was done exactly. Does anyone have an idea how this is done? Also, can I completely forego the caps if I have input coupling caps on my Trends TA 10.1?

Thanks!
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Old 13th January 2008, 11:16 PM   #6
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My guess is the socket you added will contribute/create oscillations with the fast opamps you suggest as replacements. Best with no socket.
BTW... OPA602's are one of my favorites.

=RR=
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Old 14th January 2008, 05:21 AM   #7
glt is offline glt  United States
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I have 627's on sockets, no oscillations...
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Old 14th January 2008, 08:43 PM   #8
traf is offline traf  United Kingdom
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I made quite a few mods to my old DAC-AH (well 2 of them actually).

1. A good start is to remove the bridge rectifiers and replace them with equivalents made up using Schottky diodes. This provides a bit more focus and clarity to the sound.

2. Bypassing the active stage is a no brainer and quite simple as previously discussed.

3. I used Auricaps as DC filter caps and found these to be very transparent, partly offseting the otherwise slightly soft sound.

4. On both my DAC-AHs I removed all the TDA1543 chips and replaced them with sockets, then put piggy-backed TDA1543s (with a small blob of heatsink paste in between) back in so they had 16 DAC chips instead of 8 fitted. This is about the limit for the power supply arrangements - I seem to remember that the a resistor value needs to be changed when doing this.

Now, having done this the sound definitely improved BUT one of the units ended up getting rather hot and after about 20mins audible distortion resulted. Even leaving the top cover off didn't help. Having scratched my head as to why only one unit suffered this problem, it transpired that the 2 power supply regulators feeding the DAC chips (two groups of 4 chips) in one unit were 8V (7808), but only 6V (7806) in the other. The 8V unit got hot, the other didn't. Prior to the mods both units sounded the same to me.

hth
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Old 16th January 2008, 03:56 AM   #9
JandG is offline JandG  United States
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I run the AH with Mudorf silver imput cap, ditched the bridge & also use socketed 627ap. I also installed a 2nd set of rca's & then did a simple dirrect tap from 270 ohm I/V w/ 2.2 Auricaps. leads are long enough to use just the cap leads them selves. That is it, then I listen to which ever version I want, this has been 2 years of no problems. I prefer 627 to 602.
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Old 16th January 2008, 06:57 AM   #10
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Quote:
Originally posted by JandG
I prefer 627 to 602.
Can I be constructively argumentative here?
I have my doubts that you could hear a difference between the two chips, or the diodes too. Perhaps the only way to tell would be an A/B test, and for that, you'd need two units...one with the changes, and one without, and careful level matching. Your "memory" of how it sounded before the mods is not valid, even 5 minutes later, and I'm sure it took you guys a lot longer to perform these mods.
(Check Rod Elliot's site, there is an A/B test box over there.)

Having said that, I give more credit than the normal DIY'er/audiophile, to the subjective circumstances of the musical experience. I see nothing wrong with performing mods that in theory, are supposed to improve the sound. I've always believed..."If you think it sounds better, it will sound better."
I'll call this my "Think-Will" theory.
.......And I do it often in my own DIY work.
What does bother me, is when someone exaggerates the quality and quantity of a mod/tweak. Myself, having built two of a same circuit, and swapping in/out "near quality" opamps...and performing an A/B test....I can attest (to my ears), that little or no difference can be heard.

=RR=
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