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cdp mods - Help!
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Old 3rd October 2002, 07:21 PM   #1
Sparhawk is offline Sparhawk
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I'm doing some mods on my Yamaha CDX-550 (as mentioned in a previous thread about op-amps). I've managed to copy parts of the schematic from the service manual, thanks to my local Yamaha dealer.

Maybe this is a stupid question, but there is something in the schematics that makes no sense to me.

The DAC is labelled YM7141, which I'm assuming is some proprietary Yamaha part. It has four outputs labelled AORL, AORH, AOLL, AOLH which go to the output op-amps. I'm assuming maybe these stand for "Analogue Out Right Low, Analogue Out Right High", since they seem to be differential outputs.

Here is what I don't understand: between these outputs on the DAC and inputs to the op-amps are a bunch of NAND gates. Each output from the DAC goes to an input on a NAND gate. The other inputs on the NAND gates seem to go somewhere in the clock circuit. The outputs of the NAND gates then go to the op-amps.

Is there some obvious explanation I'm missing? Is this some kind of 1-bit voodoo, or something?
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Old 3rd October 2002, 08:40 PM   #2
ultranalog is offline ultranalog  Poland
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Seems like the dac is not so much a dac as it is a digital filter. Since it's a 1 bit system, you really don't need much else anyway, there's only one bit weight. The filters outputs are nicely reclocked by the nands; must say I've never seen it yet although I do like it this way. The nand outputs could be simply put through a longtailed pair with a 2nd order lowpass filter (fc ~= 50 kHz) in front of it to yield the analog output.

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Old 3rd October 2002, 09:10 PM   #3
Sparhawk is offline Sparhawk
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What is a "longtailed pair"?

There is a low-pass filter after the first op-amp. It looks like a first order, but I could be wrong.
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