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Old 31st October 2006, 05:10 AM   #11
jwb is offline jwb  United States
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It is a kind of bizarre circuit. Were the designers trying to drive a mile of cable or what? Also, I seem to recognize this diagram. Isn't it a well-known commercial DAC?

Edit: yup, it's a Music Fidelity.
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Old 31st October 2006, 06:08 AM   #12
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Quote:
Originally posted by jcx



You might want to start with Class A biasing all 4 of the I/V op amp outputs with ~ 5 mA from the negative rail - very specific to NE5532/4 where the pull down is very much slower than the internal NPN pull up half of the output stage - I don't recomend this generally but it really is applicable to the NE5532 in a I/V stage

>Already done with 4x AD825's, running hot<


your Vos tweak tirm pot may be upsetting the diff to single end conversion - ideally R103/RR105 = R104/R108 and any Vos should be added elsewhere - the fact that you need Vos adj is further evidence against the discrete op amp

>The trimpot was adjusted to give 1-2mv stable dc offset, they ended up with a value within 2% of the original 2k2 resistor <

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Old 31st October 2006, 06:18 AM   #13
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Quote:
Originally posted by jwb
It is a kind of bizarre circuit. Were the designers trying to drive a mile of cable or what? Also, I seem to recognize this diagram. Isn't it a well-known commercial DAC?

Edit: yup, it's a Music Fidelity.

Very good jbw, it is the Musical Fidelity A3-24, and yes the output I believe is capable of outputting 6watts, maybe they wanted to drive a pair of horns or something direct, no pre, no poweramp, that got to be heaven.
This is one of the finest dac's that I've heard, very detailed, extended, yet sweet as a sugar, and incredibly powerful, great for driving my Lightspeed Attenuator LDR passive preamp which is a pig of a load.

Cheers George
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Old 31st October 2006, 03:49 PM   #14
jcx is online now jcx  United States
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that discrete amp is better sized to do a sub-par job of driving 6 Ohm loads

for measurably better performance you sould look at Walt Jung's "Op Amp Audio" section in his book for the line driver circuits - diamond buffer biased Class A with >100 MHz transistors or modern CFA/DSL driver op amp inside a high gain op amp feedback loop

the other purpose of the network on the discrete op amp input is to balance AC cmrr, by changing R104,R108 you give up some cmrr at higher frequencies
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Old 31st October 2006, 07:05 PM   #15
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Quote:
Originally posted by jcx
that discrete amp is better sized to do a sub-par job of driving 6 Ohm loads, the other purpose of the network on the discrete op amp input is to balance AC cmrr, by changing R104,R108 you give up some cmrr at higher frequencies
Thanks jcx, I realize that I can do better by changing the output, to a diamond buffer, but I just wanted to get the best out of this without completely changing it, and didn't I read somewhere that the Pass D1 has an output impedence of 1k? if it does that's too high for me anyway.
As for the change in R104-R108 it's only a few ohms to null out the dc offset, from what was about 10mv with the 2k2 to 1mv with a few ohms difference.

It already sounds a little better with the 47ohm output resistor shorted, I think now to give it a little more Class A with heatsinks on the Darlingtons, as RFBRW has suggested to further sweeten it up, and maybe enlargen the sound stage with more 3d, as has been my experience with increases of Class A with power amps.

Cheers George
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