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DAC connection problem

Discussion in 'Hi-Fi Stereo Systems & Separates' started by hilljd, Nov 19, 2002.

  1. hilljd

    hilljd
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    I have a couple of simple questions, but the background is a bit complex so bear with me.

    Current stereo components are TAG DVD32R/AV32R/Chord1200C. The system is used 2/3 for stereo and 1/3 HT. I’ve always assumed the weak point in this chain was the DACs in the AV32R, rightly so it seems. The long term plan is to upgrade the TAG to super SHARCs and the better DACs, but most importantly the analogue bypass. Then get a proper stereo DAC for the main channels, most likely the Chord DAC64. But all this will have to wait until next summer when the SHARC upgrades are available and I can live without the TAG preamp for a month or so.

    Meantime opportunity strikes and my dealer gets in a second hand dCS Delius DAC. This also works as a preamp so I took it home for the weekend to test. I’ll avoid the temptation to wax lyrical about this box, suffice to say it is definitely not going back (my girlfriend quickly dubbed it the dCS Delicious, quite apt).

    So now I’m stuck with two preamps, no bypass. The DVD32R should handle this as it has two RCA digital outs, just a new cable required. But the amp interconnects are a real drag as I have to swap between the two preamps dependant on which I want to use. Apart from anything else this isn’t good for the jacks on the boxes.

    So I had an idea. The Chord amp has dual inputs, RCA and XLR. The TAG uses the RCA, but the dCS includes XLR. I could connect both preamps at the same time if I got hold of a pair of XLR interconnects. So to my questions:

    1/ Does this work at all? Is there any chance of some kind of feedback loop between the preamps causing trouble?

    2/ At some point I’ll mess up and the inevitable will happen; both preamps will send a signal at the same time. How is the amp going to react to this?

    I know someone is going to suggest some sort of 2:1 junction on the interconnects, but this basically amounts to the same thing (doesn’t it?). It also has the major drawback of possibly negating the benefit of some expensive interconnects so I’m not keen.

    Any relevant experience or bright ideas? Thanks for any input.

    Jeremy

    PS: I suspect I’m displaying my ignorance of DSP technology here, but has anyone managed to get one of these dCS boxes to process a stereo signal from a (film) DVD?
     
  2. Reiner

    Reiner
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    PS: I suspect I’m displaying my ignorance of DSP technology here, but has anyone managed to get one of these dCS boxes to process a stereo signal from a (film) DVD?

    You have to select the PCM/Stereo Soundtrack. If you select DD or DTS it will be streamed as such to the DAC which lacks the necessary decoder. If there is no PCM/Stereo track on the DVD you have to use the internal decoder of the DVD player, i.e. use the analog outputs - where your DAC then would be bypassed/become redundant.

    As for the pre-amp/power amp question: I think it's not a good idea to connect both, unless you are very sure only one pre-amp is switched on at a time.
     
  3. hilljd

    hilljd
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    That was quick, another early bird!

    Thanks, what you've described is as I understood things. However this DAC is a little different... It seems will not recognize DVDs at all as they do not carry the appropriate 'audio flag' in the bitstream (I have tried). The manual says that you can disable this flag checking and so read any disc. But it warns you to be *very* careful doing this as the DAC will then attenpt to decode whatever it gets and who knows what might happen. I imagine it would have a fit with 5.1DD.

    So as far as I can see the only means of playing a DVD would be to switch off the flag checking, put the DAC into standby, cue up the DVD with a PCM soundtrack running, then switch the DAC on. And it's still all a bit risky if, for example, the disk stops and goes to a menu where a DD signal is sent. Hence the question, have any dCS users found such a procedure that works as I'm reluctant to experiment.

    Ditto with the parallel preamp connection. It would be a very easy experiment, but do I want to try? Noooooo! Someone out there must have more guts than me and actually tried it!

    Jeremy.
     
  4. Nic Rhodes

    Nic Rhodes
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    I have sympathy with you on this one as I have been down this route as well. The old Tag DACs are good. The new one are REALLY good. There is quite a difference here but the DCS is a serious bit of kit. I have it’s main competitor so I have been trying similar things but it should be remembered that these are £10k DACs competing with an AV processor.

    The DCS, I think, and just about every DAC only recognises PCM signals only. It tolerates DD but really hates DTS signals. This produces driver killing sounds so be aware. Please be very careful here. The set up you have is fine but it only needs some one else to make a simple mistake and bang, or in my case a little too much C2H5OH from Scotland.

    Re The inputs to these amps having both RCA and XLR. It is a bit more complicated than that unfortunately. The ‘switches’ FOR rca / xlr don’t operate as intelligently as we might think. For example, very few amplifiers are balanced internally. They have a little circuit in the input which changes the balanced signal to an unbalanced one for the amp (just like the RCA input!!). The switch doesn’t always switch from balanced to unbalanced but more often it take the little ‘extra’ circuit that does balanced to unbalanced out of the circuit path. This might be fine with an unbalanced input but with a balanced input both inputs are still ‘hot’. This then means you need to look at the output from the driving pre amp and what it does when not working or when in standby. Each pre amp is different. Look to how these outputs mute, open circuit etc. To be honest it is a real mindfield. Some work (by luck) and others do not (most quality kit). I found the best solution in the end was a HQ two way switch, which was made for me by my DAC supplier. It is undetectable in the circuit even with this level of kit.

    I did have one other solution. My DCS competing DAC has a matching ADC. I can feed the Tag stuff into this and run everything through the (DCS) DAC with a digital volume control. This is not as daft as it sounds. I can’t tell any difference with the extra AD operation. Mind you at Tag the other day I could spot no difference between the 5.1 input and the re digitised input. Does DCS do a linked ADC? ADC is much more stable than transport to DACs largely due to no SPDIF screwing things ups.

    With the DVD32R you also have the problem of digital outputs and sync linking. The AV32R needs the link for best performance but this will then not operate with the DCS!

    Does this help any?
     
  5. hilljd

    hilljd
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    Great, I was hoping you'd contribute, thanks.

    Yes, it certainly helps. Now I am pretty certain my ideas were all bad! No more messing with DVD bitstreams and no trying parallel connection; don't want any fried toys thank you. Oh well, I can live with swapping a few interconnects for a while.

    Glad you are impressed with the new TAG AV32R DACs, sounds like a very worthwhile upgrade. I can see a hefty bill from TAG looming next year!

    Very interested in the AD/DA comment. A brilliant solution since it then works for all inputs, film too. But I don't think dCS make a domestic AD converter, only pro. And that's probably horribly expensive too. I was thinking of trying connecting the DAC analogue outs into one of the AV inputs, just to see how it sounded, but I can't imagine it will produce anything like the DAC alone can, and probably little change at all.

    Thanks again.
     
  6. Nic Rhodes

    Nic Rhodes
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    It might pleasantly surprize you :) The bottle kneck in performance is the SPDIF not the ADC or DAC.
     

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