"Piggybacked" PCM1704 - diyAudio
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Old 31st March 2010, 09:25 PM   #1
Salar is offline Salar  Germany
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Default "Piggybacked" PCM1704

Hi to all,
I have a Parasound D/AC-1500, running four PCM63K, the second pair is only used for XLR output. In short, two complete Stereo Dacs in one cabinet.
Using only RCA outputs, I would like to mod the second DAC with pcm1704.
The Parasound has been already upgraded with a DF1704, since yesterday
a DF1706, providing 24 bit output and running in parallel with the DF1704 has been already installed.
Now my question:
Can I pggyback two pcm1704 for gaining the same current output as the pcm63? IŽll have to use adaptor boards anyway, so I could also parallel them using the sourroundin caps/electronics needed for each PCM1704.
Is paralleling/piggybacking a wise step or useless, as long as you do not invert the second DAC and sum the currents using an opamp?

All the best, salar
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Old 1st April 2010, 01:26 AM   #2
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Piggyback will (theoretically) improve by 3dB the signal/noise ratio. But it is better to do the summation after the OpApm filters, so that OpAmp noise will too be cut by 3dB (that of course means doubling the parts).
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Old 1st April 2010, 02:01 PM   #3
Salar is offline Salar  Germany
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So, no problems when soldering the two current outputs together?
The output current is 1.2mA and the output impedance is 1kohm, will the output impedance stay the same, but
the current double?
BTW, output current of the PCM63 2 mA, output impedance is 670 ohm. The opamp connected to the PCM63 output is an AD841.

All the best, Salar

Last edited by Salar; 1st April 2010 at 02:18 PM.
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Old 3rd April 2010, 11:25 AM   #4
Salar is offline Salar  Germany
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O.k, it can be done. Stumbled over the PCM1702 evaluation board which allows up to 4 PCM1702 per channel.
But now I need help from you guys in calculating the feedback resistor and capacitor in I/V stage.
A formula is given for calculating the I/V conversion for paralleled DACS in the PCM1702 Evaluation boardŽs Datasheet,
but i was always v e r y bad in mathematics. BTW, output current and impedance of the PCM1702 and pcm1704 are the same.

So here is the math:

DAC current output of the original PCM 63 is +-2mA
Output impedance is 670 ohm
The PCM63 has already a built in 1.5kohms feedback resistor for I/V conversion. Parasound chose this resistor for the circuitry and added a 1000pF capacitor.
The following opamp is AD841.

Now I have TWO PCM1704 replacing ONE PCM63.

The PCM1704 has an output current of +-1.2mA, output impedance 1kOhm.

What resistor value and capacitor value do I need to get the same voltage output of AD841 for those two 2 PCM1704 replacing one PCM63?

Would also be intersting for listening purposes to match them to get the same output voltage. As I wrote,
there are two complete Stereo DAC boards in the Parasound D/AC 1500,
the second additionally used for XLR output. (Phase Inversion is done digitally, IC can be easily removed and bypassed) So my mod will result in two complete PCM63 and PCM1704 Dacs...

Thanks in advance,
all the best, Salar (soldering now for two days)

Last edited by Salar; 3rd April 2010 at 11:33 AM.
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Old 3rd April 2010, 12:35 PM   #5
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There are 2 ways:
1. tie toghether all the resistor terminals of the DAC's and to one OpAmp. You need to choose an OpAmp capable of driving the total ammount of current (from all DAC's). Or add a buffer after the OpAmp output (like BUF). For example LM4562 can drive max +/-23mA. OP275 can do 38mA.
2. use one OpAmp per DAC as per datashet and mix (add) all the voltage outputs with one output OpAmp buffer.

PS: When you parallel two sources, the resulting current doubles, while the internal voltage remains the same - therefore the equivalent output impedance halves (ohms law).

Last edited by SoNic_real_one; 3rd April 2010 at 12:44 PM.
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Old 3rd April 2010, 01:05 PM   #6
Salar is offline Salar  Germany
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Hello SoNic_real_one,
thanks for the hints, but I do not want to change the existing opamps.The PCM1704 has no resistor teminals (whereas the pcm63 has resistor terminals.)
In the PCM17032 evaluation board, the current output of the dacs is mixed and fed to one opamp.
So, I need to calculate the resistor value for +-2.4mA (PCM1704) in relation to the original +-2ma of the pcm63?
So the resistor for two PCM1704 will need 3.2kohm, or am I wrong here?
Impedance will be 500 ohm?
All the best,
Salar
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Old 3rd April 2010, 02:58 PM   #7
Salar is offline Salar  Germany
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1250 ohm, if I am not wrong.
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Old 3rd April 2010, 09:03 PM   #8
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For two, the impedance will be 500 ohm (+/-2.4mA).
The application from the catalog shows a 2.5kohm conversion resistor in the I/V stage - to make 2.4mA x 2.5k ohm = 6Vpp = 4.24Vrms. The filter stage I think leaves 2.12Vrms at the output.
For your two parallel DAC's you will have to divide the 2.5kohm by 2 = 1250 ohm.
Mind that the OpAmp output current will double too (+/-5mA).
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Old 4th April 2010, 09:34 AM   #9
Salar is offline Salar  Germany
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Hello SoNic_real_one,
thanks!
The orignal PCM63 had +-2ma, so the output current of the opamp was 4mA?
I am too dumb to draw conclusions from the AD841 darasheet, but as I understand, the maximum output current allowed is 50mA (nothing written about the max. input current anywhere...)
All the best,
Salar
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Old 5th April 2010, 02:09 AM   #10
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The output current of the OpAmp is what I am talking about. That's what drives curents into the virtual ground, not the input.
AD841 has 50mA but... is good for video apps, not for audio.
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