Sound Quality Vs. Measurements

[DF96]

All oscillators have jitter, because all oscillators are at heart a very narrow filter fed with noise. The noise is unavoidable and essential - without it the oscillator will never start. The issue is to have as little as possible, given the technology being used, and with the least unhelpful statistics. Whole textbooks are written about the theory of low-noise oscillators.

Even if you had a perfect oscillator, then as soon as you do something with the signal you can introduce jitter because all circuits have noise.

[tvrgeek]

Even a piece of wire.

[benb]

Originally Posted by nigel pearson

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 .

Actually, that's almost there - now add a little noise to the sine wave, and you can see that the Schmidt trigger will trigger a little earlier or later on each cycle, depending on whether the instantaneous noise value adds to or subtracts from the sine wave near the trigger voltage. This variation in timing is, of course, jitter.

[dvv]

Originally Posted by bcarso

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

Fair enough, Brad, but if so, does it not make sense that for the op amp, which we know or assume to be the weakest link, we should pick one of the better ones?

Beside exchanging those two op amps, I didn't touch anything else in my player. And changing it was immediately heard as a positive difference - not radical, not in-yer-face, but easily heard, and in my view, very beneficial.

AS is out of the box, that player sounded not only unlike most other players, not only definitely analog with no trace of the usual digital shortcomings, in fact it sounded a little too warm, bordering on syrupy. While this can be enchanting for the first half hour, after a day or so, it begins to sound a little false. Much warmt, easy flowing sound, but I still had a feeling it was missing out somewhere.

The op amp change brought about a sastisfying effect - it lost none of it very analog sound, but gained in definition and focuis, a bit more detail and definitely more ambience. It shifted the tonal balance and brought in more detail.

Remember, the base unit price was €800, or about $1,100 or so, meaning this was a mid price device, far removed from any High End. Yet, it managed to box well above its price range. Modified, in my view, it became the one to beat at that price point.

And that was 10 years ago.

But the real big deal was the subsequently purchased real time DAC, that one kicked the whole game at least a notch, and I'd say even two notches.

[dvv]

Originally Posted by nigel pearson

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 .

I think this is what makes my player so good, as ever, it's the power supplies at work.

Most uncommonly for its price class, it has two separate and in looks hefty power transformers, one for the digital section and drive, the other for the analog section. This is followed by discrete full wave bridge rectifiers, each feeding two 6,800 uF capacitors by Matsushita. Thereafter, there is some local voltage regulation for individual circuits.

You don't see power supplies like that every day, and frankly, the Japanese industry has used half of that to peddle 2x50...60W stereo amps. The point being, somebody took their sweet time over this model. To the best of my knowledge, in this price class, there was nobody to even approach it for build quality and quantity.

[abraxalito]

Originally Posted by bcarso

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."

Looks like you're hanging out with the wrong chips. Try TDA1387, nice compliance range, decently high output impedance.

[nigel pearson]

My tests with crystals proved difficult . Here is a 3.2768 MHz taken down to 100 Hz for a strobe via 2 x 74HC4060 . There is an unfiltered 7805 regulator in addition . If ever a better example of how not to design an audio device this is it . Great for a strobe .

Thinking about what I saw in the past . It was my old oscilloscope doing Fourier . If it was 10 MHz and the device 3.2768 MHz it was working as at the very least a first order 10MHz filter . Hence a distorted sine-wave ( 1/3 F3 = 33 % distortion ) . Knowing how things more easily do square-waves than sine the crystal in such an oscillator is a square-wave . How nice I hope to find out soon .

Keeping that square wave clean seems critical long before wondering if it wobbles . The point about zero crossing voltage is excellent . How well the chip can recognize that point must be in question , and the nature of the signal etc being a source of unavoidable jitter . Knowing how sensitive even NE 555's are to instantaneous voltage fluctuations I have to think keeping voltage extremely stable and clean must be the major source of jitter avoided . DF 96 what you said seems the best first attack , trust what I already have . Making a PSU clean is nice gentle work . I said to my friend it will take a month , seem about right .

[bcarso]

Originally Posted by abraxalito

Looks like you're hanging out with the wrong chips. Try TDA1387, nice compliance range, decently high output impedance.

That is an unusually high compliance and output resistance, although many will sneer at its "mere" 16 bit resolution.

Try some of the ESS parts these days by comparison . Patrick's preferred old part is about 1k, and quite constrained on output voltage swing.

[bcarso]

Originally Posted by dvv

Fair enough, Brad, but if so, does it not make sense that for the op amp, which we know or assume to be the weakest link, we should pick one of the better ones?

Oh sure. And we need to be careful about what constitutes "better" in this application. One suspects that the recent material presented on differential nonlinearity of opamps will be applicable, for example.

My point is that most are blinkered about the approach to I-V conversion. Arbel had a similar problem trying to persuade people that an analog differentiator was best done by prefacing an opamp with a common-base stage, then applying feedback around the whole shebang.

I may use the development of optimized I-V converters as an excuse to buy an Oppo player Even since an ancient Sony died I've been without decent silver disk playback.

[nigel pearson]

Could we use a discreet I to V ? I wish Wave would say one using EF184 or something .