IRF610 I/V convertion from Pass Labs D1 dac - single ended - diyAudio
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Old 14th May 2009, 05:03 PM   #1
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Default IRF610 I/V convertion from Pass Labs D1 dac - single ended

Help needed again from people who have built the SE version of the above.
After turn on, the source voltage needs about 1 minute to settle towards 0v. It increases gradualy from -3v. Even then it doesn't stay stable: Drifts 10-20 mV around zero.
Almost the same when turning off: slowly decreasing to -2v, then back to 0 again.

The dac chip, in my case pcm1704, is directly coupled to the source of the IRF. Isn't this dc offset at start up/shut down dangerous for it? or it souldn't behave so and there is something wrong with my circuit?
Supplies are regulated (lm317/337).

Does anybody measured distortion of the SE version?
In my case unacceptably high: -45db 2nd, -60 db 3d at 1,2Vrms output, driven from an op-amp through a 1Kohm resistor (0,8Vrms across it).

Generator+ opamp alone have less than -80db 2nd and 3d at this level of output.

Thanks in advance,
Greetings
Konstantinos
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Old 14th May 2009, 07:45 PM   #2
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I just read the last day that others (3+) reported the same happening. Check the threads over Pass forums.
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Old 15th May 2009, 05:54 AM   #3
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The Pass I/V needs about 1 hour to reach thermal stability in a draft free environment regardless whether it's balanced or SE. Until that point is reached the offset will drift, with the swings reducing as the stage warms up. Wayne Colburn recommends 2 hours warm up before serious listening.


It shouldn't hurt the PCM1704. The recommendation seems to be to let the I/V warm up for at least an hour on the test bench before connecting to your dac. Adjust the DC at the input to zero, then power down, hook up to your dac, leave to warm up, then re-adjust to zero.

Not sure how you can reasonably claim you are measuring the Pass I/V if you are using a I/V resistor before the input. The Pass I/V is designed to cascode the ouput of the DAC. Whatever you are measuring it definitely isn't the Pass I/V operating as designed!!!!
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Old 15th May 2009, 10:47 AM   #4
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Quote:
Originally posted by spzzzzkt

Not sure how you can reasonably claim you are measuring the Pass I/V if you are using a I/V resistor before the input. The Pass I/V is designed to cascode the ouput of the DAC. Whatever you are measuring it definitely isn't the Pass I/V operating as designed!!!!

I am not quite sure either, but according to the datasheet, the pcm1704 has an output resistance of 1 Kohm, so a voltage source with a resistance in series should simulate it.

Since the op-amp's output Z is negligible, i believe i do it the right way.
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Old 16th May 2009, 10:45 PM   #5
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In the co-linear DAC's the final internal component(s) are bit switch mosfets which have their drains connected to the iout bus. The first mosfet in the Pass I/V cascodes the bit switch mosfets.

You can't replicate this cascode arrangement with an opamp and resistor replacing the bit switch mosfets at the input to the I/V.
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Old 15th October 2009, 04:33 PM   #6
crt is offline crt  Indonesia
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sorry for my poor english
and i'm newbie



i have a plan to use D-1 I/V converter for PCM1704 (SE version), with this schematic

[IMG]Click the image to open in full size.[/IMG]

about caps value is 10nF with 1,5pF or 1,5nF ?
whose design this ? i forget where i get this schematic
or should i go with original version

is this D-1 really need output caps ?
and i have a plan to use this D-1 with B-1 buffer that already have input caps (for DC blocking).

Regards
La Ode
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