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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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In Reply to: Re: FRENCH ART - more on tweaks posted by Steven S. on October 13, 2001 at 09:05:32:
HI STEVEN,Thanks for your explanation on The "Minimum Output Current" and on the op-amps. I agree with you on the resistance and stability points, the amps you mentioned “should†work as simple plug and play replacement.
Your approach is very interesting and pragmatic, keep us in touch with the results! However, be careful with the double faced board, and with extra heat on the voltage regulators… but maybe the following helps in that matter.
I have just received the schemes from the importer, but these are still not complete and not perfectly readable! However, I guess that’s really the best they can do, so I will have to work on this base.
Regarding the 2 quad op-amps (U1 and U2) on the main DI/O board, my understanding of the scheme is the following :
- U2 is NOT used in DAC mode only. Half of it is there to handle the analog input (Left and Right tube warmth) in ADC mode, the other half to “handle†de 3 diodes “0dB/ WRM/-20dB†in ADC mode.
- U1 is the only one to be concerned in DAC only mode, but only half of it, ie 2 single op-amps in fact. Indeed, it seems half of U1 is there to handle the analog input (Left and Right input level) in ADC mode. The other half is the output stage in DAC mode, ie 1 single op-amp per channel. That’s what we are looking for.That would mean you could perhaps simply take out U2 in DAC only mode, and, as the current supply seem to be in common with U1, get some current reserve when replacing U1 with a more “hungry†op-amp (even if the tracks of the board are thin, they can surely withstand some extra mA).
To be precise, you could also in DAC only mode take out U1 and replace only half of it, as only 2 single op-amps are required in the output stage. That’s the route I would like to go, not using another quad op-amp but 2 race single op-amps, indeed with some pin conversion (AD825).
Why? Well, yes, I believe the LT quad op-amps are surely the best solution as direct replacements, and really very promising. Unfortunatly, here in France, all the interesting chips are very difficult to find cost around 20$, and, as I can not try them and bring them back, that would mean for 2 AD, 2LT, and one BB, around 100$ on testing material. I have to go the right way at once, based on the other’s experience… like yours!
The LT chips might also not be the best achievable solution in absolute. It has a extremly high slew rate, very good, but maybe to high for our requirements. Shouldn’t harm, but that means it is mainly suited for very high frequencies, not for audible low ones. Distortions and harmonics are surely optimized around 10 Mhz, rather then around voice critical 1 kHz. That’s also one reason why some amps are simply better sounding to us regardless their data. On the other hand, would these chips sound best in absolute, then why has (to my knowledge) no company used them (or anybody reported) as they are available as single and dual amps?
I expect from the AD825 the best sides from an OPA627 and an AD827. I have not tried it, but independent reviews (not comments from some tuners that sell them!) from different amps I found where going in that direction, an had the same comments as mine on the other amps I know, so that it means I have chances having similar findings with the reviewers.
Just my 2 cent comments to tell you why I hope much from your comments, and why I won’t sadly be able to go on with the same tests as you.
Just in order to face any situation during your tests, the interesting “half of U1†pins in DAC mode are:
- 4 and 11 (respective + and – power supply)
- 1, 2, 3 (respective final output Left, negative output L entry from DAC (chip), positive output L entry from DAC)
- 7, 6, 5 (respective final output Right, negative output R entry from DAC, positive output R entry from DAC)The other pins are for the 2 single op-amps in ADC mode. This should allow you every equivalent montage.
Good luck Steven!
HI STEVE,
Thanks for your help. Sadly, unless you go for a montage outside the unit, with 2 double triodes (summing and output stage), a triode only output stage is not possible. It is also really not realistic to use the current set up in order to move in that direction. In clear, that means for me I am giving up the tube output stage solution.I love tubes, the Sovtek could have been replaced by a Philips or Telefunken ECC 83… Whereas tube only solution is not always best in a entire hifi system, having one tube stage in my hifi (output, preamp, or amp) sounded nice to me. That’s why I initially looked (by mistake) at the ART : because of its tube.
However, tubes as output in a DAC play less advantages (low current, filter stage…) then in other uses, and I am confident we can achieve (in the case of the DI/O) the same result with the adequate op-amps as with a (rare) state of the art tube stage.
Sounds awful from my mouth? Well, tubes are rarely used as they should. They are often used to tune the sound (ie most 300B montages do emphase the middles only). On the other hand, some research are currently done in order to copy the curves and harmonic spectrum of some tubes with transistor only montages. I now similar from the car manufacturing: nowadays, a FWD can tend rather to oversteer under special circumstances (Alfa 156), a RWD with rear engine car (Porsche 911) rather to understeer. Unbelievable but true. It is mainly a question of suspension set up (ie front and rear roll center, gravity center, polar momentum), but we are nearly able, on easy street and even on though race conditions (Peugeot in rallies), to get the required road behaviours. I think, in our specific case, we should achieve outstanding results using the right op-amps (see above)… yes, that’s my way now!
Another subject : the fact that some units do sound better without their casing despite the loss of a Faraday cage is IMHO linked to vibrations! I had good experience with damping (or freeing) both from inside and ouside casings of CD players. That should allow you to run the units closed without vibrations. For me, however, not a point on my ART as stated.
O course I will keep you informed with my next mods.
Enjoy music,
Claude
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Follow Ups
- Re: FRENCH ART - more on tweaks, STEVE and STEVEN - Claude G 10/14/0113:11:48 10/14/01 (4)
- Re: FRENCH ART - more on tweaks, STEVE and STEVEN - Steven S. 16:14:56 10/14/01 (2)
- Re: FRENCH ART - STEVEN - Claude G 08:43:33 10/15/01 (1)
- Re: FRENCH ART - STEVEN - Steven S. 13:33:37 10/15/01 (0)
- Re: FRENCH ART - more on tweaks, STEVE and STEVEN - steve jones 14:40:55 10/14/01 (0)
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