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-   -   Whic DAC chip to use with a PMD100 filter?? (http://www.diyaudio.com/forums/digital-source/2086-whic-dac-chip-use-pmd100-filter.html)

Brussel Sprout 25th January 2002 08:17 PM

Whic DAC chip to use with a PMD100 filter??
 
I have a Cambridge audio S700 DAC which had quite a nice tone, but not the best imaging or separation in the world. These things improve imensely after I did a bit of reclocking but now I want to change the DAC chip..

The S700 uses a CS8412 reviever, PMD100 filter and SAA7350 DAC. I like the PMD100, but the SAA7350 has seen better days, so what would be a good DAC to use in conjunction with the PMD100?? I'd like something which would require a pretty simple output stage, and sound better that the 7350 - Obviously :)

I was thinking of using something along the lines of a PCM1728, but another option would be for me to add a new DAC chip and use an LCaudio Zapfilter for the output stage.

Any Ideas??


Mark.

echo 25th January 2002 10:46 PM

Mark,

I think you might have a problem interfacing a delta-sigma dac like the PCM1728 dac with the PMD100 filter. All of the BB delta-sigma (bitstream-type) dacs, as well as those from Crystal incorporate digital filters internally (to the best of my knowledge...) I would suggest looking at a sign-magnitude (multibit-type) dac instead; BB has a few that would work great for your application. The PCM1704 is their latest and greatest, and will accept 20 or 24 bit data (the PMD100 outputs 20bit, right?) but is only available in a surface-mount package. For digital, this is clearly a benefit, but can be more difficult and tedious to work with. The PCM1702 is a very similar 20bit dac in a DIP package. I would probably choose the PCM63 20bit dac (also in a DIP package), if you can find a couple- it was just recently discontinued by BB. This chip was very popular in commercial dacs with the PMD100 filter, and is still regarded as one of the best chips made (it is only a bit 'outdated' because it will not accept 24bit data, which the PMD100 does not output anyway.)

Unfortunately, all of these dacs output only current, so the output stage may be a bit more complicated than with a dac which outputs voltage, but if you opt for the Zapfilter, that should not be an issue. Otherwise, all you need to add is a simple I/V stage- you can find examples on the Burr-Brown datasheets for the dac chips mentioned above.

Hope this helps, and good luck with your project!:D

-Mark

wildmonkeysects 26th January 2002 02:05 AM

More on PMD 100 interface
 
I second the emotion about BB 63, although now a classic, it is PFG.
Up the bypass caps to internal nodes, ref, offset, etc to much larger values.
As to I/V, I would strongly suggest going passive, or cascoded passive. My favorite is a "looped cascode" feeding a transformer, link below, which is quite a step above the standard opamp I/V shown in databooks.

http://www.AudioAsylum.com/audio/twe...ges/36482.html

One can use the above circuit with a bipolar iout DAC such as the BB 1700/1702/1704 or Signetics 1545, by adding a high impedance current source larger than the peak DAC output current to bias the cascode.

[The BB1700 is a dual 18 bit DAC, surprisingly good sounding.] Also, the PMD100 will put out 24 bits, but only with dither disabled.

Reclocking is a very good thing, with a few subtlties here: One cannot register/reclock the bitclock and data lines with a 256fs clock, so the BB 1700/1702/1704, which updates iout on a rising edge of bit_clock after a falling edge of latch_enable, preclude(s) true reclocking at the DAC unless one uses a 512fs clock and divides down to feed the transport.
One can reclock at the input of the PMD 100, where the data is still 1fs, with audible benefits. But is still best to reclock the relevant signal line at the DAC, so the BB63 takes the cake on that one: iout is updated on the negative edge of latch_enable allowing reclocking with a 256fs clock.

Also also, the 8412 loop filter is happier with component values other than the standard databook ones: try 0.22uf and 475 or 499 ohms with a 3300pF cap added from filt pin to ground.

Enjoy...

rfbrw 26th January 2002 08:04 AM

wildmonkeysects wrote
"One cannot register/reclock the bitclock and data lines with a 256fs clock"

And why not ?

rfbrw.

Carl Ekblom 26th January 2002 11:26 AM

The Wolfson WM8740 (www.wolfsonmicro.com) accept PMD100 filter by bypassing its own digital filters. It will then be running in 8fs mode and it has differential voltage outputs.

Regards

ftorres 26th January 2002 10:07 PM

Quote:

Originally posted by rfbrw
wildmonkeysects wrote
"One cannot register/reclock the bitclock and data lines with a 256fs clock"

And why not ?

rfbrw.

IMHO, it would be difficult to register/reclock the bitclock @ 256fs with a master clock @ 256fs, but maybe I'm wrong....

rfbrw 27th January 2002 12:06 AM

At 256 fs there are 4 Mclk cycles for every Sclk/Bclk cycle. So long as the Mclk edge occurs at the right moment in relation to the Sclk, data will be transferred from the D input to the Q output.

rfbrw

Dave 27th January 2002 04:32 AM

There is no reason why you can't reclock a bit clock running at 256fs with a low jitter 256fs Master clock. You just run the data, bit clock and word clock through some D flip flops which are clocked with the 256fs Master Clock.

The critical thing is to watch the timing at the D flip flops clock in relation to the data inputs.

To get around this I was thinking about using a small FIFO say 16 words by 4 bits in place of the D flip flop. See the "Jitter in DAC" thread for more info on this.

I would say that any of the PCM63/1702/1704 would be fine to use with the PMD100.

Although I have never heard the zap filter I think its a nice design and would go very well with any of these DAC chips. Or you could just build a similar output stage yourself - It will probably work out much cheaper.

What about going the fully balanced route?

Brussel Sprout 28th January 2002 03:28 PM

Hi everybody,

Thanks for all the ideas!! I think I'll go for either the pcm1704 or the pcm63 (most probably the '63) I had actually been considering something with a transformer output, just out of curiosity, so I might give it a go..

I've also been toying with the idea of building an integrated amplifier with a transformer attenuator - perhaps I could do away with the output transformer and end-up with an integrated DAC-Amp?? Hmm I must think about this. Anybody got any opinions??


Mark.

ftorres 28th January 2002 03:49 PM

I surely miss something ...
 
2 Attachment(s)
Quote:

Originally posted by Dave
There is no reason why you can't reclock a bit clock running at 256fs with a low jitter 256fs Master clock. You just run the data, bit clock and word clock through some D flip flops which are clocked with the 256fs Master Clock.


Excuse me Sirs, but there's something I don't understand - I surely miss something obvious. How can you sample a signal at the sampling frequency ?


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