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Old 19th February 2014, 01:37 PM   #731
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ADI's AD603 and AD605 work well as voltage controlled attenuators I've found. Marvels of engineering by the designer of AD844, Barrie Gilbert.
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Old 1st April 2014, 08:42 PM   #732
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Thumbs up 844 VS Pedja discrete circuit with BUF03

Hi George, Well I am nothing if not slow...... I finally updated my second 1541A S1 crown dac to Cinemag based output filter (Mark II). That puts the 2 dacs in a position for comparison. So with a triple stack of AD844 with BUF03 off the TZ versus discrete Pedja I/V buffered by BUF03 I find they are really quite similar. Over all tonally about the same. I lean slightly toward the Pedja discrete circuit. Without the BUF03 I found the discrete circuit just slightly reticent. Very detailed with a laid back character. I will need more listening time with the Mark II to make any further comments. For anyone wishing to build the discrete version and buffering the output with some sort of buffer just resize the coupling cap and series resistor. One issue I have is that I have + and - 19 VDC for the Pedja however that is way to high for the BUF03. For now I have another power supply powering the BUF03 stage. I will try regulating the + and - 19 VDC down to 12 or 15 VDC rails and see if I really need to add another transformer and regulator board. My system is Magnepan MG 2.5R with custom crossovers. Amp is a "Class D Audio" SDS 470 300 WPC. Preamp is the Nelson Pass BA-3.
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Old 1st April 2014, 09:46 PM   #733
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When you say coupling cap, I have managed dc coupling from dac to 844 i/v to buf03 to output rca's, with only 1-2mV of dc offset. But that is using the PCM1704, maybe this is the reason.


Cheers George
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Old 2nd April 2014, 03:56 AM   #734
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Default Coupling cap

Hi George, The cap is in the discrete Pedja circuit with the bipolar transistors. I have a pervasive 11 to 12 mV offset that the 2SK170 nulling circuit can't correct. I may try nulling at the BUF03. My concern is that it might not be stable over time. Guess that is worth checking. This is only valid with the TDA1541A. In the triple stack 844 + BUF03 I am direct coupled with 0.1 mV offset. I am quite happy with that. On the discrete I/V + BUF03, I still need to do more listening to really decide which is the winner. They sound more alike then different.....
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Old 15th April 2014, 07:20 AM   #735
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so after some goose chase i am going back to a second build of ad844 setup. i've tried couple passive>buffer settings and lastly susy opamp>1:1 trafo filter (they were all setup with differential tda1543 setup) but have not been able to recreate the balls to the walls dynamic sound of stacked ad844 setup. oh, also tried sen i/v- truly a remarkable step up but this 844 has that 'something' in the sound that makes you want to listen.

my question is- last time i used both pin 2,3 from +,- signal of dual 1543 to null the dc offset, then eventually ccs from tz pin for class a bias. with class a bias the sound became much more enjoyable, and now am wondering if that setup more or less achieved the crippled 844 state that pedja originally intended.
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Old 15th April 2014, 07:37 AM   #736
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I believe Pedja only used the 1541 with the 844 I maybe wrong, he will answer if he sees this. And yes I don't think he got into stacking the 844's as it is a big plus.

Cheers George
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Old 15th April 2014, 12:09 PM   #737
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I have never seen why there should be a need to stack 844s if you do it right - make the current go direct to ground at around 3R and the 50R input will see so little current that stacking is not needed. It will be like stacking 15 x 844s. The level will be lower, just increase gain of 844 on Pin 5, it is low noise, so gain can be increased substantially.

Cheers, Joe

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Old 15th April 2014, 04:07 PM   #738
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Default Triple stack VS Discrete Pedja diamond I/V

Hi all, I finally finished the second dac. I spent the last week going back and forth listening. The digital side is for both is.... DIR9001, NPC SM5814 (digital filter), TDA1541A S1 Crown. Both have Cinemag based passive filters. Both are buffered with the AD BUF03. So the difference is down to I/V's. Triple stack: Super powerful. Dynamic. Vocals are clear with a you are there sound. Slightly forward presentation an attribute common with the 1541A. Good details. Sound field comes from a black velvety background. The bass is mighty. Pedja's discrete diamond circuit: Open sound. Sound stage is pushed back a bit. Vocals are clear and articulate. Everything in the mix is easily heard. Sound comes out of nothing. I don't hear the velvety black background at all. Frequency response sounds flat from bass to as high as I can hear. It is a joy to listen to. I can't say it is as seductive as the triple stack. I miss some of the dynamic attack. No clear winner. A matter of taste I'd say. I like them both..... So the question might be which one is high end sounding. Based on what I hear I'd say the discrete circuit has a slight edge. I like how I can hear into the mix. I hear things more clearly. My system for reference is the Magnepan MG2.5R's, Nelson Pass BA-3 pre, Class D Audio SDS470 300 WPC amp. A recording I liked for this test. Roger Waters "Amused to Death" for its holographic sound field and low level sound effects like radio and tv sounds. I also used remastered Beatles, Pink Floyd. Jazz and Blues recordings. So I am expressing an opinion only. Your results could be different.
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Old 15th April 2014, 06:57 PM   #739
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Interesting. Which transistors did you use in the Pedja discrete circuit?
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Old 15th April 2014, 08:41 PM   #740
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Quote:
Originally Posted by Joe Rasmussen View Post
I have never seen why there should be a need to stack 844s if you do it right - make the current go direct to ground at around 3R and the 50R input will see so little current that stacking is not needed. It will be like stacking 15 x 844s. The level will be lower, just increase gain of 844 on Pin 5, it is low noise, so gain can be increased substantially.

Cheers, Joe

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Joe, go back and read a few more pages around 20 pages ago, it's not just about input impedance being halved with stacking, which was a plus, it also eliminates the 844 going into current saturation on loud passages and transients.

Cheers George
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