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Old 10th January 2004, 01:44 PM   #21
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But I'm not using any cap in there.
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Old 10th January 2004, 01:50 PM   #22
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Quote:
Originally posted by fmak


I find OSCON audio type useful but have not much clear idea why.
Did you compare them soundwise to other types?

In my experience, each cap has certain sonic signature. It doesn't matter where you place it, this signature always comes out.
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Old 11th January 2004, 01:46 PM   #23
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I Agree with Peter, you can always hear the Sonic effects of Caps – great care has to be taken to reduce their effect in the circuit.

The Vref pin on an ADC/DAC is one of the most critical nodes in a circuit, and is often over looked. Internally, this pin is normally used to supply charge to a Switch capacitor network (in a 1 Bit or Low Bit DAC). The charge is drawn at the internal switching rate – but with an Audio modulation component. Therefore, this Node needs to be well de-coupled at both the Audio Band and “RF” frequencies.

If you where to perform an Impendence plot of any capacitor, you will see a wildly varying curve, with the De-Coupling properties of the capacitor rapidly reducing at LF. This increasing impendence at LF has a large impact on Bass quality – resulting in “1 Bit or Low Bit” DAC’s sounding “light” in the Bass.

Most Vref pins are at half VCC typically 2.5V. The internal Bias generator can only supply a limited current, so there’s no point in hanging large De-coupling caps on them – it will take forever to charge these up – and you will still be left with the Caps varying impendence which will impact its “foot print” on the sound.

The best approach is to add external Very Low Noise and ultra low impedance Vref Generator. Below is a circuit I often use. Pls. note, it’s for designs where Vref is 2.5V. Due to the “High” impedance of the internal Vref (VBias) Gen, they don’t normally mind the addition of the external Gen. Nevertheless; it’s prudent to match the output voltage of the External Generator, as close to the output voltage of the internal Gen as possible.

Due to the Feedback of the OP-AMP, the impedance of the Generator appears almost “flat” within the Audio band (in fact, the output impedance is much LOWER at LF) – thereby limiting the sonic “footprint” of the VRef Capacitor used. Its important to realise that RF decoupling is still required, and the OP-Amp will offer very little correction - due to its limited gain Bandwidth at RF. The RF quality of the 100nF ceramic is critical, try placing it directly across the DAC Vref Pin and Ground – and the electrolytic C5 should be a Low ESR such as an OS-Con or Tant. The quality will now be less critical, due the active correction of the OP-AMP.

Note, the feedback is taken FROM the DAC Vref pin; this removes the effect of PCB wiring and any noise. The limitation at LF is now due to the DAC’s package bonding impendence.

The FB connection should be as short as possible back to the Op-Amp.
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File Type: jpg dac vref gen.jpg (18.4 KB, 2640 views)
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Old 11th January 2004, 07:19 PM   #24
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Did you compare them soundwise to other types?

In my experience, each cap has certain sonic signature. It doesn't matter where you place it, this signature always comes out. [/B][/QUOTE]
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This I agree with in analogue systems. I have not the same amount of experience in digital circuits. Where I have used OSCONs (mainly SGs) they have been beneficial. Certainly in clcok circuits, they have a big effect on power supply noise. I have even tried 680 uF/6.3V! No lytic I have tried have the same effect.
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Old 11th January 2004, 08:36 PM   #25
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Default Leakage

Leakage currents for filter cap for the reference voltage in combination with 100K filter resistor can cause significant noise at the point where you are trying to filter it. Some of the low ESR caps have fairly high leakage currents Picking a low leakage cap and a smaller resistor is probably worth investigation. One had better be sure that the reference voltage pin of the IC you are driving will work be driven by a low impedance source who's voltage at the reference pin could appear before the ICs power supply votage comes up (depending on supply filtering for the IC and reference circuit. This approach will probably work in many cases but read the ap notes and data sheet thoroughly to avoid possible "got ya"s.

http://www.analog.com/UploadedFiles/...ower_sect2.pdf
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Old 11th January 2004, 10:14 PM   #26
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Hi Fred,

Thanks for your comments. I don't spec a Low ESR cap for C1 as it would be a waste of money in this postion. Dispite D1's fast "pull-up" action, its almost certain that the external Vref circuit will be slower to reach the correct voltage, then the internal bias Gen - thereby preventing any latch-up conditions - but a good point to be aware of.

I also checked the noise performance on an FFT, and any noise above 100Hz is below the Zoom FFT noise floor of -170dB ref 2.5V. A good result for such a cheap and simple circuit.
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Old 12th January 2004, 07:14 AM   #27
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I also checked the noise performance on an FFT, and any noise above 100Hz is below the Zoom FFT noise floor of -170dB ref 2.5V. A good result for such a cheap and simple circuit. [/B][/QUOTE]
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Thanks John, I shall try the circuit. Can you pl describe the subjective sonic benefits of it?

I have been looking at power supply interactions with XOs and as an aside did some FFT analysis of XO outputs. They vary enormously and some 'audiophile 'designs have very high odd harmonic contents (-8dB or so third) whilst others have much better suppression (-15dB third and few others). They also vary enormously in wave shapes, some with extremely distorted and asymmetric outputs.

A lot has been made of jitter, never this topic. I wonder how the XO output and waveshape affects the output of the digital streams in the processors.
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Old 12th January 2004, 09:15 AM   #28
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Default Re: Leakage

http://www.analog.com/UploadedFiles/...ower_sect2.pdf [/B][/QUOTE]
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Very good url; thanks.

John W AD suggests using the AD780 at Vref; claims to improve resolution of ADC by 1 bit. May be worth trying.

FredM
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Old 12th January 2004, 11:44 AM   #29
JohnW is offline JohnW  Hong Kong
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As has been stated, you always hear the sound of a Capacitor, without the external Gen Circuit – they all had their sonic “foot-print”, good in some areas – poor in others.

At the end of the day, I could never find a Cap that “sounded” neutral. With the external VGen circuit, Bass becomes very tight – controlled with no “Bloom” and the sound stage opens up! – best of all cap choice is not such a “No Win” situation.

Its strange how improving the Bass always seems to improve the sound stage???

Below is a Zoom FFT of the output of the circuit 0dB @ 2.5V, note that the Mains harmonics (50Hz / 100Hz etc) are mostly from the R&S UPD05 analyser – its worst in this respect then the AP.

Adding a lower noise Vref is not going to help. The ZMR25H is a noisy 2.5 volt regulator – but its noise is completely filtered by the circuit. The noise towards DC is again due to the UPD05. I don’t believe much could be done to improve the circuits noise performance.

Changing the 5534 to the AD797 might help, but I’ve always had problems getting the 797 stable in unity-gain situations – with mV RF oscillation at about 500MHz (very hard to see on a ordinary Scope). Adding the small value resistor in series with the FB loop didn’t help much - and worsen the noise floor. Add before anybody asks – Yes, the 797 was well de-coupled with ceramic SMD caps across its PSU pins and to ground.
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Old 12th January 2004, 03:16 PM   #30
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Rather than construct a circuit for this, is it possible to use a dedicated highly accurate vref chip for an improvement? Or would this be subject to the same problems?

like this perhaps?
http://www.linear.com/pdf/146010fs.pdf
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