[odmanca]

I did a search and found no conclusive answer.

It seems most DAC-AH owners who have performed the passive mod find the result satisfactory in terms of eliminating the graininess in treble, however, I found very few comments about improvements in other aspects such as sound stage, timbre, etc.

Also the mod invariably involves replacing the stock coupling caps/resistors with higher quality components, has anyone had the chance to do a more thorough comparison on the result of the passive mod against the buffered version with better op amp and comparable coupling caps?

Thanks in advance.

 

[Kim Hardee]

I compared the passive bypass mod to a mod involving upgraded opamps (opa627) and the same coupling caps as the passive bypass (2.2uf Auricaps).

I could hear more detail with the passive bypass. The vocals in particular just sounded more "real". There is initially the perception of slightly less bass, but I'm pretty sure that's due to the extra detail. Overall I definitely prefer the passive bypass. In fact, I ended up removing most of the components on the opamp side of the board to make more room for the bypass caps.

I think it really matters what type of coupling cap you use. Some prefer paper-in-oil caps, but so far I prefer the Auricaps.

 

[odmanca]

Quote:

Originally Posted by Kim Hardee I compared the passive bypass mod to a mod involving upgraded opamps (opa627) and the same coupling caps as the passive bypass (2.2uf Auricaps). I could hear more detail with the passive bypass. The vocals in particular just sounded more "real". There is initially the perception of slightly less bass, but I'm pretty sure that's due to the extra detail. Overall I definitely prefer the passive bypass. In fact, I ended up removing most of the components on the opamp side of the board to make more room for the bypass caps. I think it really matters what type of coupling cap you use. Some prefer paper-in-oil caps, but so far I prefer the Auricaps.

Thanks for the info/update. I did read your posts in the other threads but at the time you still had the stock coupling caps for the op amp outputs. So I presume you kept your second DAC-AH at 6v but with the passive mod?

 

[regal]

I found that the passive mod was a little lean with a slightly higher noise floor compared to using OPA627's . It may depend on your amp and cable length.

 

[Kim Hardee]

Quote:

Originally Posted by odmanca Thanks for the info/update. I did read your posts in the other threads but at the time you still had the stock coupling caps for the op amp outputs. So I presume you kept your second DAC-AH at 6v but with the passive mod?

I just received my second DAC-AH. I haven't decided yet if I'm going to convert it to 8V operation or not. I will definitely do the passive bypass. I already have the LM7808s and heatsinks so I may go ahead and convert to 8V. The biggest hassle in the 8V conversion is ventilating the case, which I think is a good idea, although it may not really be necessary.

I suggest temporarily trying the passive bypass and upgrading the opamp output caps before performing a permanent mod. There seems to be a difference of opinion on the bypass, although it seems that the majority of owners prefer the sound of the bypass. I'm listening through HD600s, which can be a little laid back. I suspect that someone using Grado headphones or bright speakers might prefer the opamp output. I think the bypass is more accurate, though. I've measured the frequency response, distortion and noise of both options, and they are virtually identical.

 

[regal]

Fixing the output resistance give the greatest improvement for this DAC (or going to 8V.) Remember without it the DAC will clip.

 

[odmanca]

Quote:

Originally Posted by regal Fixing the output resistance give the greatest improvement for this DAC (or going to 8V.) Remember without it the DAC will clip.

Thanks and yes, I am so glad that Kim Hardee posted the fix; the clipping has been driving me nuts

. As a matter of fact Kim just sent me the explanation on how he derives the correct value for Rload (R35/R36) upon my inquiry.

 

[izquierdaste]

I just added a second set of rca jacks to mine so that I will have the choice between active and passive within the same box. I am not sure that I will be able to fit the top back on, but that is a fairly minor concern for me. Anyways, I am burning in a new set of bypass caps (auricaps), so I won't be able to make a comparison for a few days.

 

[odmanca]

Quote:

Originally Posted by izquierdaste I just added a second set of rca jacks to mine so that I will have the choice between active and passive within the same box. I am not sure that I will be able to fit the top back on, but that is a fairly minor concern for me. Anyways, I am burning in a new set of bypass caps (auricaps), so I won't be able to make a comparison for a few days.

Thanks and please do keep us posted.

 

[izquierdaste]

As an update, I think that I have about 60 hours on the auricaps now. I have to first mention that I don't think the use of dual sets of rca-jack outputs is a good idea. I pulled out the socketed opamps today and the sound quality seemed to increase fairly considerably for my passive output. Hence, I would recommend choosing either active or passive output, but not both. (Note I never connected both outputs at the same time so that is not the problem). Nonetheless, I do think that the passive bypass with auricaps is the way to go. The clarity is great and it seems to have less of a noise floor than with the opamps. I would say that the opa627/vitamin q output is still a big improvement over the stock electrolytics. The opamp output has slightly more bass and is punchier relative to the bypass, but it loses some of the clarity and is not as black during quiet sections. Overall, there is not a huge difference but the bypass is better IMO.

If I were to buy a new dac-ah, I would definitely just get a stock version and go passive. Actually, if I had bought one new in the first place, I would have probably chosen that route in the first place, but I found mine of the FS forum. I definitely would not spend $375 on the new pacific valve version. Though I have not heard it, I doubt that an active output section can result in a huge increase in sound quality relative to a bypassed version at less than half the price.

Sorry I do not have the goldern ears to give a better review.

 

[regal]

Quote:

Originally Posted by izquierdaste The opamp output has slightly more bass and is punchier relative to the bypass, but it loses some of the clarity and is not as black during quiet sections. Overall, there is not a huge difference but the bypass is better IMO. .

Did you fix the output resistance? I found that after doing it the opamp gave the bass punch + had the clarity.

 

[izquierdaste]

I did adjust the trimpot and resitors with the active output. I still think there is more clarity with the bypass. I should note that I used auricaps for the bypass and have vitamin q for the opamp, so the auricaps probably have more clarity. I do agree that the opamp section is punchier and has a bit more bass. However, I have a fairly warm tube amp anyways, so that is not critical for me. The opamp section just doesn't have as much clarity and is not as black during quiet sections. I think it is great either way. I just like the bypass a little better for now. Maybe I'll switch back in a couple weeks just for fun.

 

[pelayostyle]

Im going to have to agree with izquierdaste on this one. The bypass mod, and other mods, i made provide ALOT more clarity in the sound. Its like a night a day difference on how much better it sounds now. Not only has the "graininess" gone but the midrange, mainly vocals, are authoratative. "Real" like Kim said is the best way to describe it. At this moment im listening to my Radiohead - OK computer album and the vocals are amazing. The guitar work in karma police sounds way better now with this mod then it did with the opamp output.
At first i did lack some bass but i now have about 150 hours on my dac with this setup and bass has become strong and tight. I also experiance the blacker background on silent parts like izquierdaste does.
To sum it up, the bypass just sounds better in every aspect of the sound spectrum.

 

[Kim Hardee]

Quote:

Originally Posted by izquierdaste I did adjust the trimpot and resitors with the active output. I still think there is more clarity with the bypass. I should note that I used auricaps for the bypass and have vitamin q for the opamp, so the auricaps probably have more clarity.

I originally added a second pair of RCA jacks so I could directly compare the bypass to the opamp outputs. I tried Vitamin Q caps and Auricaps in both outputs. The Auricaps provide significantly more detail than the Vitamin Qs, and the bypass has more detail than the opamp output with the same caps. My opinion is that the extra detail gives the illusion of slightly less bass. The measured frequency response of both the bypass and the opamp output are flat in the bass region, assuming the caps are large enough (about 2uF).

I can see, however, how some may prefer the sound of the opamp outputs if the rest of their system is on the bright side, especially if they are using Grados.
I definitely prefer the bypass with my HD600s and with my speakers (Axiom M3Ti).

 

[odmanca]

Thanks everyone. I just did the I/V resistor and coupling cap mods last weekend. I will probably stay with the active out for a few weeks before trying out the passive/bypass mod.