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Old 29th January 2009, 04:20 PM   #1
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Default Need help with tubed output for PCM1793 DAC

I'd like to convert the output stage in a PCM1793 based DAC from its current OPA2134 to tube. The Dac uses a differential +- output stage and that's the part I can't find any tube info on. I know I've seen posts on how to replace the differential op-amp section with a tube circuit.

Can anyone help?
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Old 29th January 2009, 05:34 PM   #2
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converting BAL to UNBAL using tubes and without transformers

The good stuff begins around Post #81 with a post by Tom S.
I think all the kinks in my version were worked out by Post #90.

Truly a masterpiece of dumb luck. Fantastic PSRR and CMRR.
Seriously, I didn't discover till after the fact that I had pulled
a Broskie and got all the bad stuff to magically cancel out.
Couldn't have designed this good if I was actually trying.

Mind you the power supply is deliberately shown with
absurd amounts of ripple, just to illustrate the rejection.
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Old 29th January 2009, 06:11 PM   #3
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For those too lazy to dig through 5 pages...

Mind you, this sim was with 10VPP of power ripple at 120Hz.
And yet another 10VPP of common mode abuse at 450Hz.
Not seeing detectable levels of either mixed with the output.
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Old 29th January 2009, 11:09 PM   #4
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Hi Kenpeter - Thanks for the info. Have you built this stage? IF so whith what DAC? Thanks.
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Old 29th January 2009, 11:17 PM   #5
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Check with Sunrise in the other thread, he said he would build the circuit proposed in post#95. Kenpeter came with the original idea for that one in the beginning of the thread.

I very much doubt the other one will ever be built.
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Old 30th January 2009, 01:58 PM   #6
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Your post #95 seems to imply a misunderstanding the theory
of operation regarding my circuit. Specifically where you claim
that it is an SRPP.

It might be more accurately described as a cascoded plate
follower. Differentially bridged into a White cathode follower.
And both "stages" abusing the self same pair of triodes.

An SRPP wouldn't have the extra plate resistor up top of
the totem, nor use feedback from this point to correct the
cumulative error of both a cascode and a cathode follower.

I conclude it has nothing in common with SRPP, aside from
first glance appearances.

You had already shot down the differential cathode follower
plan as having too much THD, what made you go back to it?
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Old 30th January 2009, 02:11 PM   #7
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http://www.tubecad.com/may2000/page16.html
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Old 30th January 2009, 03:12 PM   #8
Koifarm is offline Koifarm  Netherlands
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Default Re: Need help with tubed output for PCM1793 DAC

Quote:
Originally posted by kkillebrew
I'd like to convert the output stage in a PCM1793 based DAC from its current OPA2134 to tube. The Dac uses a differential +- output stage and that's the part I can't find any tube info on. I know I've seen posts on how to replace the differential op-amp section with a tube circuit.

Can anyone help?
Do you like to keep balanced output or not?
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Old 30th January 2009, 03:27 PM   #9
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No I want an output that will feed a line input on my preamp.
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Old 30th January 2009, 03:49 PM   #10
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kenpeter,

Who cares what if the name is SRPP or not !

Quote:
You had already shot down the differential cathode follower plan as having too much THD, what made you go back to it?
It had to much THD in its initial form, just had to be corrected to sim fine. Showed this in post#43:

converting BAL to UNBAL using tubes and without transformers .

The only thing was loosing 6dB but its just fine as output was on the high side from the DAC.

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