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Re: FRENCH ART - more on tweaks

Posted by Claude G (A) on October 12, 2001 at 16:57:30

In Reply to: FRENCH ART - more on tweaks posted by Claude G on October 10, 2001 at 18:24:10:

Thanks to all for the encouraging posts.

As stated, I do have incomplete schemes (very bad copies). The top is missing, so that I do not have the power supplies, one quarter of U1, half of U2, and nearly all tracks are unreadable. Anyway, the French importeur promised me a better copy by post very soon.

However, I am just interested in electricity because I am a music and hifi fan : that’s the only reason why I look back in my engineer’s course! So I am not a skilled man for that, just a bit experienced, and, to be honest, I wrote so many posts because I wish to carry on the mods ASAP in order to listen to music, rather then soldering for ages! Anyway, your replies are really great and I do not regret having subscribed to this chat.

To be more specific…

Steve,
Thanks for your advice, great idea!! I hope place allows an adapter, but it is a good tip.
However, even if I can’t follow al the paths, my idea was even to take out U1 and U2 in order to find some easy connections to get to the tube and back using the existing tracks and do some connections only between the 2 remaining supports if possible. Would have been fabulous, but if I do not know yet what U2 does, or if some of the op-amps need to be active when the tube is running when doing this mod, I am afraid there is another problem with it that stops us right from the beginning (help welcome!).
From what I see, ie only the middle of the plans (!), the output stage U1 acts as a summer, a filter, and an output buffer. It sums the negative and positive output from the DAC for each channel (better for the sound), and our triode is, I believe, not capable of that, is it? That would mean an op amp to sum up both signals into one output, and then a valve output stage. But hold on, this means op amp AND valve? I prefer it direct, as you know, ie op amp OR preferably valve, but one stage only. Means surely an op-amp at last… help! Anybody to shake me and prove I am wrong?!!
I read the AD825 was very tubish from their best side, that might make me smile again?

Salut Michael!
Ton Français est excellent, il doit t’aider à comprendre mon “Franglais”! I am very pleased that you’re happy with the results. In fact, I now get used to the high frequencies, and enjoy this mod more and more without any problem with DC current, or plops. However, it has now become very transparent, and though, I miss a bit weight and density (was not there before anyway). It is minor, but the bass has not a state of the art impact and could go further down , the highs could me more subtle and yet also climb higher with a better energy distribution, everything could be more mature and musical (the old tweaked output stage of my CD sounds a lot better, unfortunately it is not fed by the same DAC!) .
The ART is surely capable of more (quickness, expression, refinement). It has of course nothing to do with the cap mod, as its only fault was to put the debate a level higher and to take off the curtain between the op amp and me! I think that’s the right mod to do, because nothing could have brought us back what we were losing there, and it prepares the field for a better output stage (I think all I am missing is still there!).
From the beginning, my feeling was that this outstanding unit had very obvious and big limitations due to the PS and the output stage. We are all lucky these are easy to overcome. It is sad that no one inside Paris wants to take the risk to solder a 14 pin support and 5 diodes on a double faced circuit !! I wish I could have the equipment for it. Anyway, I will keep you informed with the next mod (diodes) as soon as possible.

Hi Steven!
At last some welcome help on the op-amp subject, and an answer on the modwright mods. Thanks Steven! Do you know more on this mod, or other ones?
I am really not an expert. As stated, I only found LT1362 and LT1356, but I am still waiting for LT’s official advice.
LT1365 is a race unit with outstanding figures (1000V/us, maybe a bit to fast?). It requires however more current to be driven (4 times more then the TL074) : as I am not an expert, I do not know if everything is needed, but I looked therefore at the LT1362 (also very good apparently, it’s little brother in the same family with lower requirements). There is also this worrying “50mA Minimum Output Current”, do you know what it means? Another issue might be its bipolar technology (no FETs) : anyway, we got to try and listen to make a real judgment, I guess that’s the fun and also the risk. Surely not a bad choice and much better then the current amp!
Do you know if this mod is just “replace plug and play”, or if other adaptations are required (load, stability, resistances around?)

When choosing the amps, I looked at the general values, design, uses, the current consumption (how far can we go in that matter with our regulators?), and the slew rate. My experience is that any serious audio amp should have more then 80V/us (sorry for the OPA 627, maybe its outstanding but rather comfortable sound is linked to the fact it is a bit slow?). I had good experience with 300 V/us (AD827, outstandingly neutral and fine, yet not the most forgiving or involving, maybe to accurate and to fast so that swing is light?). However, around 150V/us should put us on the safe side on this subject, and, of course, numbers are not an issue, music matters only. Experience is welcome!
From what I read, and from the figures I have seen, a montage around the AD825 might be the choice, having the best of both worlds (very musical yet very accurate and transparent). It uses FET technology, supposed to be better sounding (well, my power amp does not, but in this case we don’t need to supply a lot of current, we need musicality at low power, so FETs should be right). Of course, we need 2 of them (or 4 if ADC use, but do we have enough current to feed them?) and a special montage, so a bit of DIY. Is it worth a try, or what do YOU think? What is your experience?

Read you all very soon,

Claude

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