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Old 12th May 2013, 11:44 PM   #371
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I don't use the TDA1541 Dave, I have PCM1704 which are much lower in output 1.2mA, compared to the TDA5141's 4mA, this is why believe 2 stack 844's with the PCM1704 is the ideal where maybe 4 stack with the TDA1541 would be the ideal having more than double the output. If current starvation is an issue with either the signal or the glitches with these 844's. Or as I said before maybe I'm hearing the difference of 50ohm 1 x 844 loading of the PCM1704 compared to 25ohm 2 x 844's.

Cheers George
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Old 13th May 2013, 12:23 AM   #372
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Quote:
Originally Posted by georgehifi View Post
I don't use the TDA1541 Dave, I have PCM1704 which are much lower in output 1.2mA, compared to the TDA5141's 4mA, this is why believe 2 stack 844's with the PCM1704 is the ideal where maybe 4 stack with the TDA1541 would be the ideal having more than double the output. If current starvation is an issue with either the signal or the glitches with these 844's. Or as I said before maybe I'm hearing the difference of 50ohm 1 x 844 loading of the PCM1704 compared to 25ohm 2 x 844's.

Cheers George
George,

It is not quite so simple.

There are a few issues to consider that will cause the I-V to 'fall over'.

1) OP impedance of DAC 2) OP current swing

The low the (internal) OP Z of DAC, the less linear the I-V will be, comparatively speaking. Also a non linear or non resistive DAC OP Z will
cause additional distortion.

Other non linearities are non linear capacitance in I-V. These are voltage dependent capacitances of the semiconductors. This will manifest at
higher frequencies when you start stacking too many 844's there will be a
point that there are more losses than gains.

Another is thermal distortions. When current is modulating power through IP
grounded base stage of 844 the die is heating and cooling causing distortion.
This will manifest more with low OP Z DAC's. By stacking this is reduced.

More in this than you thought hey

With a discrete circuit -all- these distortion mechanisms can be reduced greatly - even without the use of feedback.

Terry
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Old 13th May 2013, 12:49 AM   #373
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Quote:
Originally Posted by zenelectro View Post
George,

With a discrete circuit -all- these distortion mechanisms can be reduced greatly - even without the use of feedback.

Terry
I also believe discrete would be better again Terry, but the simplicity and massive gains of the use of the 844 when subbing it for a normal 8 pin dil i/v opamps with very few mods is a winner, it can be done in a few minutes by those with very little knowledge.
Then if you go the discrete way this could take a lot longer than that if you could find a proven circuit, even Pedja Rogic said with his discrete clone of 844, it was hard to say if it sounded better than the AD844 itself. And he has not heard the advantage of stacking the 844. And you can still use the good internal buffer it has.

PS: Terry, as you suggested to try, (believe my ears not my eyes) I have taken off lp filtering on the 844 (cap across TZ resistor) and for good measure the LP filter on the input of the buffer I'm using, no noise heard, but I can see it on the scope, I think you were right it does fractionally sound better (more relaxed) with no filtering at all from dac output to rca's, still only 2mV of very high crap on the output rca's.



Cheers George.
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Last edited by georgehifi; 13th May 2013 at 01:01 AM.
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Old 15th May 2013, 02:09 AM   #374
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Quote:
Originally Posted by georgehifi View Post
...the simplicity and massive gains of the use of the 844 when subbing it for a normal 8 pin dil i/v opamps with very few mods is a winner, it can be done in a few minutes by those with very little knowledge.
Spot on! Sometimes the beauty of diy isn't implementing the perfected-this or the ultimate-that. It is rather the serendipitous and simple combining to achieve an enormous bang-for-buck!


Quote:
Originally Posted by georgehifi View Post
...even Pedja Rogic said with his discrete clone of 844, it was hard to say if it sounded better than the AD844 itself.
Well, he actually said it was close enough to not worry about it too much:

bear in mind that it will not be easy to beat the AD844. The layout and supply have to be done carefully and it worth to check if the output buffer will improve something or not. Ultimately this circuit has less grain and can achieve a better resolution but this does not mean that you will notice the grain or lack of the resolution listening to the common base AD844 I/V, you will not. Other than known shortcomings the monolithic solutions also have some advantages which are not only practical by nature.

from: Pedja Rogic, Discrete Diamond Non-Feedback I/V Stage for TDA1541(A) DAC

Quote:
Originally Posted by georgehifi View Post
And he has not heard the advantage of stacking the 844.
Well, he fully investigated the lack of heavy bias:

Interesting fact is that the AD844's internal circuit (as is) also could achieve superb distortion performance used as a common base stage, but it needs a higher bias current. Some other variation on this theme can perform well too, and some can perform even better. So, the bias used in the AD844 is below what we need here ...

from: Pedja Rogic, Discrete Diamond Non-Feedback I/V Stage for TDA1541(A) DAC


And his discrete circuit addressed it so in a sense he heard higher bias 844.

Last edited by AudioLapDance; 15th May 2013 at 02:17 AM.
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Old 15th May 2013, 08:16 PM   #375
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Thumbs up AD844

Hi Guys, Well I have the DDNF and I did use Pedja's 844 circuit from the Aya II dac. Yes.... I think there are some similarities. In my system they are really not close in sound quality terms. I found the discrete circuit to be very clean and dynamic. The 844 was nice however over time I realized it was just off the mark. The current starvation thing I am sure is the issue. When I get some time I'll need to try a buffer off the TZ point as George suggest. For the TDA1541(A) I'd think the DDNF might be a better match. Maybe adding another discrete current mirror section would be worth a shot. Wish I was up to the task to model that.... Don't have that software or experience. Stacking 2 844's seems worth while. Four is an issue I'd think. ! ? Dave
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Old 15th May 2013, 09:33 PM   #376
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Dave what buffer were you trying this time? Hope you weren't trying to drive from the TZ, as that is sad, it has an output impedance some say of 3 megohm, it really needs to see the direct input of a fet buffer or tube with no loading resistors, just like it see it's own internal buffer inside the AD844, and that adds to it as well, as it cannot be disconected.
Re stacking with the TDA1541 Supra (Mick) over on a nother AD844 forum that's going, tried 3 and thought it was great, went to 4 and is thinking it maybe a bit too tight & hifi sounding, but he said in some systems it could be just right.
It's a bit like loading down a moving coil cartridge, as I remember with a Supex 901, it got tighter and tighter and more dynamic sounding the closer it got down to 0ohms loading, and that was system dependant as well, I remember with 50kohms it was sloppy as?

Cheers George
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Old 15th May 2013, 09:43 PM   #377
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Quote:
Originally Posted by Torchwood421 View Post
Hi Guys, Well I have the DDNF and I did use Pedja's 844 circuit from the Aya II dac. Yes.... I think there are some similarities. In my system they are really not close in sound quality terms. I found the discrete circuit to be very clean and dynamic. The 844 was nice however over time I realized it was just off the mark. The current starvation thing I am sure is the issue. When I get some time I'll need to try a buffer off the TZ point as George suggest. For the TDA1541(A) I'd think the DDNF might be a better match. Maybe adding another discrete current mirror section would be worth a shot. Wish I was up to the task to model that.... Don't have that software or experience. Stacking 2 844's seems worth while. Four is an issue I'd think. ! ? Dave
I went to 4 844's on my 1541, it worked perfectly, but I preferred the sound of 3, 4 got a bit "hifi" sounding, it is probably technically better, but moved too far away from "1541" sound, but having said that, this is still the best I've ever heard the 1541 sound, and by a large margin. I have a stack of very expensive good "modern" dacs here and I'm enjoying the old 1541 very much.
I do think the Rogic 2SK170B buffer I'm using is contributing to sound, very good power supply, good layout etc.
I've been known to be very critical of the hype around the old 16 bit dac chips, and I must admit this has surprised me greatly. I tried a few 844 variations 10 years ago, wish I ad realised the advantages of stacking them then.
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Old 15th May 2013, 10:45 PM   #378
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Hi George, Yes... I know it has to be buffered from the TZ pin 5 connection. It's on my to do list. 3 Megohms doesn't leave many options. The discrete buffers in the thread or maybe the antiquated BUF03. I have a pair of LT1010's however I need to check the datasheet as to the input impedance. My comments are about Pedja's pin 6 normal output schematic on the Aya II dac. I need to order up a few 844's. I'm lucky that the board I built my circuit on still has enough room to add the buffer. Hi supra, Thanks for the suggestion on 3 844's. I do think there may be a limit on paralleling devices. I remember seeing TDA1543's stacked up in another thread. Like domino's. Wonder what that sounded like? !
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Old 16th May 2013, 02:25 AM   #379
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Quote:
Originally Posted by supra View Post
I went to 4 844's on my 1541, it worked perfectly, but I preferred the sound of 3, 4 got a bit "hifi" sounding, it is probably technically better, but moved too far away from "1541" sound,....
I wonder if your experience might be partly explained interms of the TDA1541's THD output characteristics with different I/V values.

Apparently the minimal THD occurs with an I/V resistance of <13ohms.

If the AD844 has an input impedance of 50ohms the THD of the TDA1541 can be expected to drop with stacking.

I wonder if the romantic coloration traditionally ascribed to the TDA1541 is not an intrinsic property of the chip but due to the induced THD with a too high I/V stage?
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Old 16th May 2013, 02:54 AM   #380
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Quote:
Originally Posted by Torchwood421 View Post
Hi George, Yes... I know it has to be buffered from the TZ pin 5 connection. It's on my to do list. 3 Megohms doesn't leave many options.
Now I'm lost - in my understanding you're using the AD844 open loop as a transimpedance amp where the gain is set by the resistor hung off pin5. So then the impedance at pin5 is just the impedance you hang off it, and the 3M is in parallel with that resistor.

Anyone tried a very low impedance shunt yet on their AD844s supply?
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