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Old 26th October 2012, 05:52 PM   #7491
bcarso is offline bcarso  United States
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Quote:
Originally Posted by dvv View Post
This is impossible to argue. I could say in all honesty that my Yamaha CDX 993 does very well indeed on its own, with three separate stages at the output - I/V converter, buffer and output amp proper. The I/V stage uses an op amp (AD826), the other two are fully discrete with dual trannies, current mirrors and the rest of the bells and whistles.

Someone might say that while it does fine, it could do better still in some other way.

I believe - right or wrong - that it's the I/V stage which is critical. Get that right, using a very fast op amp, and you should be home and dry. I find it symptomatic that standard op amps (as Yamaha original had, I pulled it out and stuck the AD 826 inside) seem to lose quite a bit of focus and detail. It appears to me a very fast op amp will by default improve the sound.
Opamps are not optimal for I/V conversion, but until someone designs an integrated optimized device they are about the best we have, if we are constrained by size and cost. Barrie Gilbert once remarked to the effect that an opamp was the worst thing one could use for this task!

The problem is that opamp input Z, open loop, is high, but for fast and accurate I/V conversion we want to start low and reduce further. Patrick has shown an approach using common-gate FETs which is a good start. I augmented it a bit by adding a loop around the input device to reduce the input impedance, and additional enhancements are desirable.

If so-called "current output" DACs had outputs, as the name suggests, that were high impedance, the problems would be alleviated to begin with. But "current output" is almost a misnomer. A better description is "output with a code-dependent output impedance and limited voltage swing capability, which has to be terminated in something that behaves like a much lower impedance."

Brad
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Old 26th October 2012, 05:59 PM   #7492
bcarso is offline bcarso  United States
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Originally Posted by nigel pearson View Post
I think this will be some fun for the winter months . Rotel were very kind to offer upgrading advice . Were it not for the RC reference in that old copy of Hi Fi choice it wouldn't have occurred to me . The good news with this one is I can only use my ears as I doubt my test gear will show any difference ?
Depends on your gear. Measuring jitter is now commonplace, but still requires specialized equipment.

DF96 is spot on. Think of a quartz crystal as a big flywheel with low-loss bearings. If frequency change over a wide fraction of center frequency with agility were a requirement, it's a poor choice --- that might well be an application where an RC oscillator would be effective. But as he points out, that's not what we need for digital audio.
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Old 26th October 2012, 06:06 PM   #7493
benb is offline benb  United States
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Among other things, an R/C oscillator is also much more sensitive to power supply variation than a quartz crystal oscillator, and that will add to jitter. I'd want to power it with one of those super duper active parallel regulator circuits I've seen around here to get extra low noise on the power supply line.
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Old 26th October 2012, 06:11 PM   #7494
bcarso is offline bcarso  United States
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Originally Posted by benb View Post
Among other things, an R/C oscillator is also much more sensitive to power supply variation than a quartz crystal oscillator, and that will add to jitter. I'd want to power it with one of those super duper active parallel regulator circuits I've seen around here to get extra low noise on the power supply line.
Yes, and the devices we would use to provide the gain to overcome the RC network loss to sustain oscillation have varying amounts of noise. CMOS in particular is famously noisy at low frequencies. Using lower impedances and hence more power will help, along with low voltage noise devices. But again, where RC oscillators have been appearing lately is in cost-challenged designs with modest requirements for accuracy, not for high stability and low jitter.

Quartz may be challenged by micromechanical resonators at some point.
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Old 26th October 2012, 10:52 PM   #7495
tvrgeek is offline tvrgeek  United States
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OK, but how many times has your music gone through op-amp based A2D, D2A conversions before you got it?

Want stability? An atomic clock is only about $6K. Of course, the cable from clock to transport will cause more jitter that the rubidium clock is gaining you anyway. An integrated crystal/oven module is in the hundreds so a more practical choice. Nano machines are a very cool technology to watch. Just not here yet.

I'd like to see more effort on the recording end.
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Old 26th October 2012, 11:24 PM   #7496
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Could someone say how jitter is caused . I read about it so many times . Not sure anyone has gone inside the problem . People speak of the oscillator working with high Q . I remember the in port receiving a distorted sine wave from the crystal when I had a look . I doubt the scope was corrupting it greatly . I will try to recreate that tomorrow if I can , my new scope is very high impedance and floating which must help . If of any value I will post it . Seems to me a sine wave is a good starting point . It passes to a Schmidt trigger next I guess . High Q or no I see no connection to a sine wave ? If the quartz is producing a square wave into Q Q dash port the then that to me seems different and yes Q matters . I suspect it isn't . No doubt it was a ghost of my measurements . I will try to find out . I did load the crystal with 1 M so that might be why I saw a sine wave ?
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Old 26th October 2012, 11:35 PM   #7497
fas42 is offline fas42  Australia
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I've got a gut feeling that down the track the obsession with low jitter will be seen to be off the track a bit: I've never specifically worried about reducing jitter to get good sound, it's always been other matters that were more important in making the sound more "analogue" - whatever that means, more real.

Of course, if one puts a great deal of energy in improving jitter specs, then what you're doing is frequently improving other aspects of component performance, which is what I feel is doing the real job ...

Frank
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Old 26th October 2012, 11:45 PM   #7498
fas42 is offline fas42  Australia
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Quote:
Originally Posted by nigel pearson View Post
Could someone say how jitter is caused . I read about it so many times . Not sure anyone has gone inside the problem
The trick is not getting jitter, it's how to stop it happening! Nature abhors a perfect beat, at least at our scale of size, just like you never see perfect straight lines in the great outdoors. The slightest imperfection, bit of randomness, anywhere, will cause the striking of the beat to be a touch off ...

Frank
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Old 26th October 2012, 11:50 PM   #7499
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Rubidum clocks may be very accurate but not necessarily low jitter - in fact, a lowly integrated oscillator has probably less jitter than a rubidum clock, they are optimised for different purposes. For A/D, D/A you don't require perfect accuracy but you would like zero jitter. Two different things.

Jitter comes for instrance from the uncertainty of a zero crossing of a square wave. At some point, the receiver has to decide whether a bit is a '1'or a '0', and that zero crossing wavers a bit with power supply, temperature, noise, what have you, so that then causes the zero crossing to waver to and fro and that's jitter.
Malcolm Hawksford showed that even the data pattern (the music) determines some of the jitter, so jitter depends on the actual series of ones and zero that make up the music.

jan
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Last edited by jan.didden; 26th October 2012 at 11:54 PM.
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Old 27th October 2012, 12:01 AM   #7500
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I do wonder if many cloaks that worked well just had better power supplies ? One thing I will try is improving the power supply all around the DAC . That is a spectrum analyzer job so can be measurement based . I remember doing this years ago and getting all sorts of nasty things . In the end I measured across the decoupling caps , I feel that is reliable .
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