How to convert P3A DAC to balanced output? - diyAudio
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Old 2nd January 2003, 11:58 AM   #1
protos is offline protos  Greece
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Default How to convert P3A DAC to balanced output?

Having recently much appreciated the benefits of balanced connections between my Pass BLS pre and Aleph power amps I would like to also have a balanced connection from my source , the perpetual technologies P3A dac which only happens to have single ended outputs. This DAC has four B-B 627 opamps driving the output so I guess in theory it could be possible. However I can't seem to find any schematics and I would need some good advice because the board has a lot of SMD components.
Help please?
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Old 2nd January 2003, 07:46 PM   #2
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The Crystal 4397 is a balanced out DAC (see Crystal website for 4397 info). You can simply use two of the opamps for each channel (one would amplify one phase and the other the other phase). You could simply use a one pole filter on each and run the outputs of the op amps an XLR. This way it would not only be balanced but you would have only one op amp in series with the signal per phase and only one pole of filter. Less is better....usually, in this case, for sure.

Alternatively, you could hook up a high quality transformer, with no filtering directly to the pins of the DAC and the output to the jacks. See Jensen transformer/DIO mod sites for schematics.

Thirdly, you could use a zero feedback buffer per phase. Or a differential Mos-fet stage ala Pass.

Ric Schultz
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Old 3rd January 2003, 01:59 PM   #3
protos is offline protos  Greece
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Ric ,
Thanks for your reply. However being quite new to electronic DIYing I am not sure what a one pole filter is.
I think your suggestion of using one op-amp per phase and taking the output from there is probably easier to implement.Forgive me for asking beginner's questions but the way I understand it is that the Crystal DAC sends a balanced signal to the two opamps for amplification. Apparently the designer then decided to have only single ended outputs so he probably has shorted the output of one opamp to ground. This means that if can "unshort" one of the opamps I could use it's output as one of the phases.The problem is doing all that without schematics.
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Old 3rd January 2003, 05:17 PM   #4
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Actually, both phases of the DAC are either summed by the first or second op amp. So, all the op amps in the P3A are being used, but in a way that is not very transparent. As far as a single pole filter, this is simply a resistor in series with cap to ground or a cap across the feedback resistor. You might ask in one of the other forums for specifics.

Ric Schultz
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Old 20th December 2003, 08:05 AM   #5
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Default same question

Hi,

yesterday I tried to draw the output schematics of my P-3A cause I wanted to know a bit better what I was modifying and would like to have a balanced output.
Attached is the drawing of one channel of the output stage.
The first part is clear to me being a two pole low pass filter turning the balanced output of the DAC into a single ended signal.
Ive got a few problems with the second opamp. This seems to be inside the feedback loop of the first and is biased into class-a via a 10k resistor to the negative power supply.

Also the 20k resistor to the output relais (when muted) is not clear to me.

Can someone shed a bit more light on this circuit?

william
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Old 22nd December 2003, 09:51 AM   #6
protos is offline protos  Greece
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You seem more adventurous and experienced than me.I still have not attempted trying to read the multi-layer board.Recently I experimented trying to power the unit up with batteries and I failed miserably.I just could not understand how the power supply works.From a first guess it looked like 9v ac is rectified to 12v and then made to +- 12v.So I said well why not just supply 12v dc on the output of the rectifiers with a battery and it should work but of course it didn't.There are some other ic down the line that are not powering up like this.Any ideas?
By the way, what were your findings on the modified vs the stock unit? I also have the P1a but I amnot using it.
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Old 22nd December 2003, 10:23 AM   #7
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Hi Protos,

the power supply uses some kind of voltage multiplier. At the two big 35V/1000 muF caps I measure around 25,.. volts (+ and -).
The analogue output is fed with +-16V regulated by two shunt regulators with Tip31 and tip....

The modified unit sounds quite a lot better than the standard one. Just a bit less like a CD. Bass is more powerfull, treble is sweeter and theres a lot more air around the instruments / singers. I can listen a lot longer than with the standard unit before I switch to my turntable........

Still no one answered my questions about the output stage

william
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Old 31st March 2004, 09:30 AM   #8
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Is it possible that it's configured as some kind of CCS? I have not tried modeling it yet - but I'm sure that would reveal all!

I have just bought a P-1A, and now I'm hunting for a P-3A on the auctions. I got the P-1A for $515 inc. shipping to UK. They're difficult to get hold of here!

Is it possible to use the P-1A on its own? How does the room / speaker correction work? Is it DIY or a pay-for service?

Thanks,
Gaz
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Old 31st March 2004, 10:34 AM   #9
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Hi Rarkov,
The room speaker correction option which attracted a lot of buyers seems to be a huge consumer rip-off by PT and should be condemned outright.
They have been promising availability of this option for about 3 years now and nothing has come out .If you ask them as I did you get a"we are still working on it" type of BS.
3 years? I mean come on we are not talking about manned exploration of Mars here.My top of the range Pioneer car head unit has this option in-built with mic included at no extra cost and PT is suggesting something like 600USD for every correction.So I guess if you have different speakers and change rooms you can end up paying a few thousand dollars.
I am not as enthusiastic of the P1a as of the P3a.In fact I simply don't use it anymore because I think the modded P3a on it's own is great.The P1 smooths some things over in my opinion which may sound better in overly bright systems but it also casts a bit of a veil and is slightly less dynamic and lively with slightly softer bass.Everybody was raving about it but now I can't really see why especially considering it's price.
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Old 9th November 2004, 12:50 PM   #10
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Further listening with an upgraded system suggests that the P1A is indeed a worthwhile upgrade over the modded P3A.
I think I was fooled originally by the increased output of the P3a of 6db.However if you adjust volume levels correctly the P1A is much more dynamic in the analogue sense of the word with better separation.With digital it sometimes easy to be fooled until your ear can compare with a better source or vinyl.In this sense the P1A comes much closer to good vinyl replay than the P3A by itself.The P3A sounds more compressed in the way that modern pop recordings are made to sound louder but lose the nuance and microdynamics.These are restored with the P1A.
I also tried the P1A with battery power and it was a big step up.
Now to battery power the P3A with +-24V.I think this combination should be a stunner.
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