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Old 13th December 2005, 10:23 AM   #1
guzzler is offline guzzler  Scotland
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Default Differential I/V stage? And RX input

I'm looking to build a DAC based around a SINGLE PCM179*, and I'm having trouble finding a decent I/V stage, as the the output is differential. I don't need or want balanced outputs, so the one of the datasheet isn't much use.

The only schematic I've seen is a 3rd order Bessel filter done by Aos

http://www.aoselectronics.com/gfx/ally/ppa_db_v1.1.gif

However, it looks a little too complicated for my taste, and I'd prefer a simpler discrete output stage so I can tweak the sound a little more than by rolling opamps...

Any suggestions MOST welcome

Also, regards input to a CS8416, I've been reading around on the forums about both transformer inputs, and differential inputs using logic buffers. What is the general consensus of opinion on that these days?
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Old 13th December 2005, 11:03 AM   #2
rbroer is offline rbroer  Netherlands
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There a several I/V circuits on this forum, you need two of the single ended I/V's.

To create a single ended buffered output signal, you can get some inspiration here:

http://ultranalog.com/cdenhancer/sacdenhancer/


I'm puzzled though why you need an I/V for your PCM1793

I would suggest sticking to opamp rolling
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Old 13th December 2005, 11:32 AM   #3
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Default Re: Differential I/V stage? And RX input

Quote:
Originally posted by guzzler
I'm looking to build a DAC based around a SINGLE PCM179*, and I'm having trouble finding a decent I/V stage, as the the output is differential. I don't need or want balanced outputs, so the one of the datasheet isn't much use.

The only schematic I've seen is a 3rd order Bessel filter done by Aos

http://www.aoselectronics.com/gfx/ally/ppa_db_v1.1.gif

However, it looks a little too complicated for my taste, and I'd prefer a simpler discrete output stage so I can tweak the sound a little more than by rolling opamps...

Any suggestions MOST welcome

Also, regards input to a CS8416, I've been reading around on the forums about both transformer inputs, and differential inputs using logic buffers. What is the general consensus of opinion on that these days?

Hi Guzzler,

Note that the 1793 has internal opamps and provides voltage outputs. There is no I to convert to a V. This is clearly shown in the data sheet.

If you want to convert the balanced output signal to single-ended, one option (passive) is a 1:1 transformer, with the primary across the outputs and the secondary grounded at one terminal and the other to the RCA center pin.

Jan Didden
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Old 13th December 2005, 02:00 PM   #4
guzzler is offline guzzler  Scotland
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Sorry, my fault not explaining clearly enough. I'm not looking at the PCM1793, rather the PCM1794/PCM1798. Thanks for the replies anyway
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Old 13th December 2005, 02:49 PM   #5
guzzler is offline guzzler  Scotland
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Iíve had a look a the RAKK DAC schematic, looks interesting. However, Iíve never been a huge fan of transformers in non-digital signal positions (personal preference, sound technical reasons, donít want to go into the argument about which sounds better etc).

Ideally, what Iíd like would be a solid-state discrete stage powered from a bi-polar supply to remove the output capacitor. Looking at Elso Kwaks schematic, heís got the o/p capacitor (which Iím not really adverse to, but would prefer to be without). For the input, would it be as simple as in the RAKK DAC with a resistor between I+ and I-?
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Old 14th December 2005, 09:01 AM   #6
guzzler is offline guzzler  Scotland
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anyone?
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Old 14th December 2005, 09:06 AM   #7
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Quote:
Originally posted by guzzler
anyone?

Well if you want to go from balanced to single ended you need either a passive devise (transformer) which is straightforward, or an active device and you can either brew something yourself or use any of the audio balanced receiver chips like those from TI (e.g. OPA1632 or the INA134/137 series). Alternatively, why not use just ONE output into your preamp/power amp and forget about the other? You may have some more 2nd harmonic distortion, but that may even be an advantage as it will help to mask 3rd hasrmonic etc.

Jan Didden
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Old 14th December 2005, 10:46 AM   #8
00940 is offline 00940  Belgium
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The easiest way, but not discrete, is to do it like Bel Canto did it for the DAC2. Opa1632 (or ths4130) for balanced I/V followed by a INA134 to convert to single ended.

Or if you can accept complicated and expensive, there was this : A nice dac to complement the Aleph-X.

But it gives balance output, so you will need to use an ina134 afterwards.
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Old 14th December 2005, 11:00 AM   #9
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Quote:
Originally posted by 00940
The easiest way, but not discrete, is to do it like Bel Canto did it for the DAC2. Opa1632 (or ths4130) for balanced I/V followed by a INA134 to convert to single ended.

Or if you can accept complicated and expensive, there was this : A nice dac to complement the Aleph-X.

But it gives balance output, so you will need to use an ina134 afterwards.
Well if they used THS4130 AND INA134 that's not very smart. You can just ground one output of the THS and run that SE.

Jan Didden
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Old 14th December 2005, 11:04 AM   #10
00940 is offline 00940  Belgium
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Don't you improve THD measurements by running it fully differential ?
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