VRDS-8 Analog Section Upgrade - diyAudio
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Old 20th September 2004, 05:45 PM   #1
GerryM is offline GerryM  Norway
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Default VRDS-8 Analog Section Upgrade

I have been working on upgrading my TEAC VRDS-8, trying to come closer to the presence and ambience of my analog system.

So far I have installed a clock upgrade, changed I/V and filter/output opamps to OPA627s, and changed all the decoupling/filter caps to BlacK gates.

The VRDS-8 uses dual PCM1702 DAC chips per channel. From the schematic of one channel (should be attached) it looks like I could get a balanced signal output at the output of the I/V stages (two per channel).

I like the idea of keeping things simple, less components in the signal path, and I know how good transformers can sound from my S&B TVC, so I was thinking of some kind of transformer connected right at the I/V opamp outputs (is the opa627 the best in this application), driving a balanced output.. this would also provide filtering...
Or maybe I could go for a simple circuit with discreet components...
Or.. any other simple suggestions? I know there have been many threads around this topic but they seem to deviate off topic and they probably go way back in time.
So what is the current thinking / experience?
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Old 20th September 2004, 08:53 PM   #2
zinsula is offline zinsula  Switzerland
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If you have balanced dac outputs, look at the Pass D1-DAC output stage: http://www.passlabs.com/downloads/ol...t/d1_servm.pdf

Several guys here stated that Op-Amps are not the ideal choice as DAC output stages http://www.diyaudio.com/forums/showt...844#post396844 and based on their explanations it makes sense, although I didn't have had the possibility to hear such a gear...
Not yet - I'll go and build one, but I have to sort out some other things before.

Another possibility is to build this: Easy-to-build I/V stage
But with diff outputs maybe you have to change quite a bit...

Cheers, Tino
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Old 21st September 2004, 01:07 AM   #3
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I would not assume that because there are two dacs it automatically follows that they are in a balanced configuration. The labelling R and SHIFT R suggests that they are in a staggered configuration.
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Old 21st September 2004, 07:29 AM   #4
GerryM is offline GerryM  Norway
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Since the outputs of both I/V opamps feed straight to the + and - inputs of the next opamp, I would have thought they could just as well drive a transformer and give a balanced output... or am I missing something?
Hasn't anyone done something like this?
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Old 21st September 2004, 12:49 PM   #5
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Quote:
Originally posted by GerryM
Since the outputs of both I/V opamps feed straight to the + and - inputs of the next opamp, I would have thought they could just as well drive a transformer and give a balanced output... or am I missing something?
<snip>
Yes, if the output to the dacs is staggered. You would have to remove the stagger and invert the input to one dac.
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Old 21st September 2004, 09:11 PM   #6
GerryM is offline GerryM  Norway
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Have you looked at the schematic?
I know its sideways (to fit the size limits) but if one DAC output was not inverted then nothing much would come out of the next opamp after the I/V stages since they seem to preserve polarity.
As far as 'staggered' goes, I fail to see the significance (a nice way of saying I dont understand but I dont believe it anyway until proved otherwise).
Could you explain what you mean?

Also for anyone else:
Now I have narrowed the transformer search to either a Sowter 9545 or Lundahl LL1694 transformer.
Has anyone got experience of these or other transformers used with DAC outputs?
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Old 21st September 2004, 10:42 PM   #7
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Quote:
Originally posted by GerryM
Have you looked at the schematic?
I know its sideways (to fit the size limits) but if one DAC output was not inverted then nothing much would come out of the next opamp after the I/V stages since they seem to preserve polarity.
As far as 'staggered' goes, I fail to see the significance (a nice way of saying I dont understand but I dont believe it anyway until proved otherwise).
Could you explain what you mean?
<snip>
It is not about the input to the I/V stage. It is about the input to the dacs. That part of the schematic is not there so one has to speculate. Based on other VRDS schematics and reviews a staggered configuration seems more likely to me.
Re staggered configuration
Paralleling up DAC chips
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Old 22nd September 2004, 08:18 AM   #8
GerryM is offline GerryM  Norway
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You are most likely right that the DAC output is staggered. The input circuit to the DACs (too big to attach as one legible file on the BB) has an AD1893 sample rate converter, followed by an SM5843 digital filter, plus lots of inverters, serial to parallel shift registers, analog multiplexers...
There is a control on the CD player for 'DS point' which I think changes the effective oversampling rate.
Tomorrow I'll stick a dual trace scope on the outputs of the I/V converters and see what it looks like.

However, back at the schematic - even in the case that the DAC outputs are 'staggered' or whatever, since they go to the + and - inputs of one op amp after I/V conversion, couldnt I still just wire in a transformer instead?
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Old 22nd September 2004, 09:57 AM   #9
pro is offline pro  Italy
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Hi GerryM.
On my Teac VRDS8 I built the one attached, and it works very well.
Ciao.
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Old 22nd September 2004, 07:39 PM   #10
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Quote:
Originally posted by GerryM
You are most likely right that the DAC output is staggered. The input circuit to the DACs (too big to attach as one legible file on the BB) has an AD1893 sample rate converter, followed by an SM5843 digital filter, plus lots of inverters, serial to parallel shift registers, analog multiplexers...
There is a control on the CD player for 'DS point' which I think changes the effective oversampling rate.
Tomorrow I'll stick a dual trace scope on the outputs of the I/V converters and see what it looks like.
<snip>
Much like another of the VRDS range. You could look at the LE input of the '1702 and check the frequency of LE. You could also see if frequency of LE changes with the DS point setting. If the LE signal is not common to both dacs check their positions relative to each other.
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