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Old 2nd May 2013, 08:10 PM   #341
supra is offline supra  Australia
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I've also tried the stacked 844's and can hear why George was so impressed-I've never heard a TDA1541 sound like this, and I've owned and built more than a few. I used 3 stacked 844's but ordered more as probably need to put another one on to suit the 1541.
As George says it is very dynamic, very defined, none of the "euphonic" sound commonly found with 1541's. I'm using Pedja Rojics 2SK170 buffer, but I suspect the 844 will work with any buffer. It certainly brings out the very best from a 1541 and should make a significant difference to all R2R dac chips.
I'm not really a 16 bit dac guy but I'm really enjoying this dac- its like having an old girlfriend turn into a super model!
Nice work George.
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Old 3rd May 2013, 10:03 PM   #342
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Thanks Supra, glad to see someone has given it a go and found the magic that happens when stacking.
I believe that even with the PCM1704's 1.2mA output a single AD844 maybe current starved, maybe not by the signal transients but by the very hf glitches these have and these could be causing the problem of current starvation when a single is used. Or it's the halving of the input impedance when stacking.

Oh I also have another 2 must do's, I think maybe the TZ output does not like to see capacitive loading, all I have now is a TZ resistor of 4.7kohm between it and the buffer fets, even the input of whatever buffer you are using remove any filter caps to ground, so TZ doesn't see it. I measured the noise on the output, and has gone up slightly to 2mV but in the 10's of megahertz region, definitely can't hear it, and all dc offsets are still .1mV stable, after initial switch on and off spikes, I really have to get around to muting that as it will when I'm not careful one day take out my amps.
Also if the buffer you are using can handle it make the input impedance at least more than 1meg ohm, as I have done some tests on the bench and the TZ output drops by around 10% when it see's a 100kohm load, so now I have nothing between the TZ and fet inputs of the buffer except for the TZ resistor itself, and all is stable and wickedly fast.

Cheers George
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Old 3rd May 2013, 10:25 PM   #343
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I don't really care if it "sounds" good to anyone, but paralleling opamps is not easy, and stacking them is definitely not recommended, according to TI:
Paralleling Op Amps?is it possible? - The Signal - Blogs - TI E2E Community
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Old 4th May 2013, 12:41 AM   #344
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TI only say this when there is global feedback around it, I could imagine there is maybe a problem this way, they don't say anything running it with no feedback as we are. As well we are only stacking the input i/v and not the buffer section. You need to hear the difference to make up your mind with an absolute to if it works or not.

Cheers George
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Old 9th May 2013, 02:47 PM   #345
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Thumbs up AD844

Hi, I have tried the AD844. Liked it for a while. That was using Pedja's original Aya II schematic. I have had better luck with the Discrete DDNF circuit that Pedja posted some years ago. It sounds just as has been described here. There is a lot more current through the mirrors on that one. It sounds like 4 stacked 844's is the optimum for 1541's? I may have to try reworking my 844 board to use the TZ output and a buffer. That was something I never tried. Would be interesting to compare those. Based on a comment seen here I tried the OPA627 as an I/V. In my DAC that was real nice. I have simple I/V (no Sallen Key filter crap), GIC Bessel 7th order filter and AD8065 buffer. Awesome bass, clean with great drive.
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Old 9th May 2013, 09:49 PM   #346
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If you liked the OPA627 as an I/V, your in for a shock (dynamically) with the stacked AD844's and the signal taken from TZ to a good dc coupled solid state buffer. Just make sure the buffer is at least 1megohm input impedance, as I found that the TZ output droped by 10% when loaded down with 100kohm.
Also try to remove any capacitance that the TZ point may see, so long as you do not end up getting any audible noise in the end, as I have a suspicion that the TZ output prefers to see nothing on it's output, just the TZ resistor and the input of the fet buffer.

Cheers George
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Old 10th May 2013, 02:56 PM   #347
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Thumbs up 844's

Hi George, Sounds like a BUF03 might be good as a buffer for the TZ output pin. FET input very high input impedance. I have several of those. Never cared for the sound of the BUF04 or AD811's. Have you tried Pedja's discrete current mirrors? I see the PDF was posted earlier in this thread. I may try the OPA637 for I/V, if they are affordable. I think these maybe very expensive now. If you know of a better buffer please point me to it. Dave
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Old 11th May 2013, 12:19 AM   #348
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Very happy with the buffer I'm using at the moment http://www.diyaudio.com/forums/attac...set-adjust.jpg

But I have found this ready made one on ebay, do you buffer gurus think it's any better than the one I'm using? As the one I'm using I have measured at 100ohms output impedance, I would like to get lower if possible if this is or it's a better circuit?

Cheers George
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Old 11th May 2013, 01:47 AM   #349
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Hey George, sorry I dropped off the face of the earth but I see you've been going great guns! You are a wild man: no fear, I love it!

Looking into the output of your buffer, the lowest resistance is the pairs of 22 ohms on the top and bottom. So that means you'd see four 22 ohms in parallel or 5.5ohms.

Layout will be important, care to share the address or is it hush-hush ...

The output is doubled and the other trans must be doing some sort of cascode-thingy that I'll let the smart people talk about.

It's hard to find the BUF03 chip buffer but the BUF634 and LT1010 are plentiful.

Torchwood, would you mind sharing details of your 1541 offset adjust circuit and what the process is for connecting it the first time (paranoid about hurting my 1541s). Thanks.

Cheers,
Jeff

Last edited by AudioLapDance; 11th May 2013 at 01:52 AM.
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Old 11th May 2013, 01:55 AM   #350
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George - I can't remember if this has been discussed before but what are you using to power your AD844s? Hopefully not zeners like in that schematic - low impedance supplies are a must I've discovered. I have some simple shunts on my blog which you could try - they sound even better when lots of uF is added to them. Also what supplies are feeding your PCM1704s? My TDA1387s took a step up when I lowered their supply impedance.
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