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Old 28th December 2012, 04:19 AM   #31
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IME (not directly with PCM1704, but other multibits) its not so much feedback vs non-feedback rather HF linearity which matters most. A passive LPF is a worthwhile introduction which proved to me that, prior to including it, my I/V and post-amp performance was still limited by HF non-linearities. Yes, a discrete design is certainly one way to get the necessarily linearity, but its not the only way.
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Old 28th December 2012, 07:26 AM   #32
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If you are familiar with the rationale behind zero-feedback amp designs, you'll have an easier time getting what I think about this. With, e.g. typical active iv converters, you have very high level feedback around an opamp(chip or discrete matters not). Because there is a small but finite time required for the signal to go through the opamp, what is fed back to input is a delayed signal, which works just fine for sinewaves & other non-repetitive waveforms, but fails to do as intended for non-repetitive waves like music, and rather creates new distortion components(generally at a much lower level than the distortion it's intended to cancel in a well designed feedback amp stage, but real). Now think about how significant that time delay becomes to the gltch pulses & other digital artifacts coming out of the dac chip. The most telling thing about this to me, in my experiments with removing feedback completely from everything after the dac chip, is that the sonic effects of jitter become very, very, very nearly inaudible on a ladder dac when feedback is eliminated, and are hugely reduced on bitstream dacs. Even comparing a top shelf transport, e.g., and upgraded Museatex C-Lock CDD, to a very crappy dvd player as transport(e.g., Pioneer DV505), even my young, female, sound engineer assistant had a very, very hard time picking out any differences between transports, in the first trials using a Pioneer DV-S9(in dac mode, whether set to internal or external clock) rebuilt with my no-feedback, high current drive output stages after it's PCM1702's. My other DV-S9, upgraded identically, but with it's conventional(but also upgraded) iv & output stages, made it quite easy & obvious to hear which was the better transport. With ladder dacs, the only potential damage, as I can see it, that jitter can manifest in the output signal is in the nature of the glitch/noise spectra, exactly where feedback will have the most negative effect. Further, having zero feedback, combined with way overkill output current drive ability, has revealed to me nuances of tonal layering & sense of open space(and I don't mean just top end) that I had never heard before. Any other no feedback and/or passive iv designs that I have yet seen fail to address how mucn current it actually takes to drive a line, and a preamp/amp input, extremely well, which I think accounts for why such designs have not captured much attention in the market.
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Old 28th December 2012, 07:35 AM   #33
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I get the independence of transport with my multibit DACs too. The transport I used did at first make a difference but that was caused by CM noise over the S/PDIF cable, not jitter. With the optimum routing of that input's grounding, transport dependencies have evaporated. For transport I'm using about the cheapest DVD player I could buy. I'm still using a feedback circuit after passive I/V. I don't buy the delay argument myself but I don't nowadays use normal opamps and certainly not any as active I/V. I get what you're saying about 'open space' - that came to my sound when I installed the passive LPF.

If you're experiencing what at first blush seem to be line driving issues, check that you're not getting CM noise back-injected into your DAC from the subsequent kit. Putting a CM choke in line might allow you to check to see if this is upsetting your SQ.
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Old 28th December 2012, 09:33 PM   #34
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I'm still using a feedback circuit after passive I/V. I don't buy the delay argument myself but I don't nowadays use normal opamps and certainly not any as active I/V .[/QUOTE]

or some reason passive I/V (with a cap) followed by an opamp gain (either discrete or IC+ filter + buffer)) sounds better to me than letting an opamp try to do the I/V conversion. Typically measures as good. I call it the passive I/V + phonostage. The cap + resistor prior to NFB Jefet gain just makes it work.Jim Hagerman I think was the first to come up with the idea


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Originally Posted by abraxalito View Post
If you're experiencing what at first blush seem to be line driving issues, check that you're not getting CM noise back-injected into your DAC from the subsequent kit. Putting a CM choke in line might allow you to check to see if this is upsetting your SQ.
Find a transport designed by an RF engineer and 99% of transport "issues" go away. Can't help but think that noise on spdif is a bigger issue than jitter.
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Old 29th December 2012, 01:02 AM   #35
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Quote:
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or some reason passive I/V (with a cap) followed by an opamp gain (either discrete or IC+ filter + buffer)) sounds better to me than letting an opamp try to do the I/V conversion. Typically measures as good.
I concur - and I think I know why. In making the measurement, I presume that's a THD+N measurement. But real music has lots of 'tones' - dense spectrum, with each individual freq at a much lower level. So IMD performance at much reduced signal level (say in the range -40 to -60dB) is the real issue for SQ, not THD at high level. Thus THD measures linearity in the wrong place on the transfer function. I reckon an opamp being used to do I/V isn't linear enough in that low signal range and we hear that non-linearity as 'greyness' and loss of soundstage depth. The best opamps for I/V are CFB topology and so I conclude the poor linearity is the LTP input stage primarily.

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Find a transport designed by an RF engineer and 99% of transport "issues" go away. Can't help but think that noise on spdif is a bigger issue than jitter.
Agreed.
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Old 8th January 2013, 10:51 AM   #36
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Hello Gabriel,

Did you manage to get any progress with RD30 dac ?
I'm looking forward to hear your impressions about it after some listening period.

Regards,
Tibi
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Old 11th January 2013, 10:23 PM   #37
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Hi Tibi
As soon i will finish the Aikido Unbalancer as I/V stage i will post here my personal impressions.
Best regards
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Old 13th February 2013, 05:14 PM   #38
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Hi
Today m manage to finish the Unbalancer, I made some measurements but the signal is to low about 362mV RMS so tomorrow I will made some modification, I will eliminate the negative feedback to see how high the signal will be, I want to have at the output 2V RMS. If I will not be satisfied I think I will order the I/V made especially for this DAC
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Old 25th March 2013, 06:50 PM   #39
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Hi
I manage to finish the Unbalancer, now I have at output 2Vrms. At this point I made an A/B test between the DAC and my Dune HD MAX with has a AK4422 convertor
The differences are visible the sound coming from the RD30 is more precise more fluent more musical. Now I work at the case.
My dream comes true.
To have a 4xPCM1704 playing HD files is unbelievable. Most of the DAC made with this IC are made especially for 44.1 kHz sample rate and maximum 96 kHz but this RD30 can play 24/192 and 24/176 kHz without problems.
This DAC satisfy me. I will be glad to share with any one my experience with this DAC and the Unbalancer as I/V.
Maybe In the future I will order the I/V made for this DAC but because I use a tube amplifier I prefer to have also tube on I/V
Best regards
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Old 22nd March 2014, 03:33 AM   #40
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Quote:
Originally Posted by stephensank View Post
IMO, you are absolutely wasting your PCM1704's if you use i/v, lpf or output stages that have loop feedback. I've done my Pioneer DV-AX10(differential 1704's, SM5847 DF) with passive resistor i/v, followed by discrete jfet voltage gain and current amp stages(ample current to directly drive inefficient Yamaha orthodynamic headphones beautifully) of my own design, and it is absolutely spectacular.
Funny, according to Pioneer, the DV-AX10 uses Analog Devices multi-bit 192/24 converters (almost certainly sigma delta) not 1704's.

DV-AX10 - | Pioneer Electronics USA

I'm having very hard time finding out which converters used 1704's. Most I find mentioned actually don't. According to Stereophile the Mark Levinson 360 and 360S do:

Mark Levinson No.360 D/A converter | Stereophile.com

Also, the Wadia 861

http://www.stereophile.com/content/w...specifications

Last edited by charlesp210; 22nd March 2014 at 03:38 AM. Reason: added wadia
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