Discrete analog stage for these S-D differential out DAC's

regal · 15th December 2007, 10:24 PM

A I/V stage that will handle the new differential ouput DAC's like the PCM179x is really needed in the DIY world. There are a few great projects out there using these chips (Rakk, EZDAC) that would be a lot better with a D1 or Jocko type I/V.

Cauhtemoc · 16th December 2007, 09:38 AM

Just use two single I/V converters.

oshifis · 16th December 2007, 10:40 AM

You can try this topology:

Laszlo's Valve Output Stage with Lundahl transformer

regal · 16th December 2007, 12:14 PM

Isn't the problem that the 179x have a DC offset ? Or would you recommend just using coup[ling capacitors and not worrying about the offset ?

oshifis · 16th December 2007, 03:17 PM

It is not a problem. I designed it for the TDA1541A which has 2 mA offset, while the PCM1798 has 3.5 mA offset. The offset current flows from the centre tap of the input transformer (ground) into the current outputs of the DAC. This current is in opposite direction at each half of the primary, so they cancel each other.

In the op-amp output stage version the current output of the DAC is at ground potential, and the offset current flows from the virtual ground into the DAC output (and the same current flows from the op-amp output to the virtual ground). The DAC pulls the necessary offset current from the ground always, regardless of wether it is hard tied to ground, connected to a virtual ground, or connected to a transformer that has a center tap on ground.

00940 · 16th December 2007, 09:51 PM

A nice dac to complement the Aleph-X.

regal · 16th December 2007, 10:29 PM

Quote:

Originally posted by 00940
A nice dac to complement the Aleph-X.

Unfortunately 7.8 mA p-p is to much for D1 and its variants.

15th December 2007, 10:24 PM	#1
regal diyAudio Member Join Date: Jan 2004 Location: MD	Discrete analog stage for these S-D differential out DAC's A I/V stage that will handle the new differential ouput DAC's like the PCM179x is really needed in the DIY world. There are a few great projects out there using these chips (Rakk, EZDAC) that would be a lot better with a D1 or Jocko type I/V.

16th December 2007, 09:51 PM	#6
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	A nice dac to complement the Aleph-X. __________________ Ben.

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16th December 2007, 09:38 AM	#2
Cauhtemoc diyAudio Member Join Date: Jun 2007	Just use two single I/V converters.

16th December 2007, 10:40 AM	#3
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	You can try this topology: Laszlo's Valve Output Stage with Lundahl transformer

16th December 2007, 12:14 PM	#4
regal diyAudio Member Join Date: Jan 2004 Location: MD	Isn't the problem that the 179x have a DC offset ? Or would you recommend just using coup[ling capacitors and not worrying about the offset ?

16th December 2007, 03:17 PM	#5
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	It is not a problem. I designed it for the TDA1541A which has 2 mA offset, while the PCM1798 has 3.5 mA offset. The offset current flows from the centre tap of the input transformer (ground) into the current outputs of the DAC. This current is in opposite direction at each half of the primary, so they cancel each other. In the op-amp output stage version the current output of the DAC is at ground potential, and the offset current flows from the virtual ground into the DAC output (and the same current flows from the op-amp output to the virtual ground). The DAC pulls the necessary offset current from the ground always, regardless of wether it is hard tied to ground, connected to a virtual ground, or connected to a transformer that has a center tap on ground.

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