Cap recommendation for DAC Vref, Vcom pin - diyAudio
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Old 13th September 2009, 07:25 PM   #1
jkeny is offline jkeny  Ireland
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Default Cap recommendation for DAC Vref, Vcom pin

I'm using a Vout DAC chip, PCM1793 & about to get delivery of another Vout DAC, the new Sabre ES9022. I've noted that these Vout DAC have a Vcom, Vreg pin (named differently in the different datasheets) which seems to decouple the bias/Vref.

I have heard that this pin is sensitive to the decoupling cap used here & I noted some of this variability myself on the PCM1793 DAC. A thread on Diyhifi suggests that firstly the datasheet value for this cap is very undervalued at 1uF & suggests a 100uF, low esr type - an oscon or panny FC/FM. Then Jocko rows in with high leakage in these caps being very bad for noise issues. Then somebody else says that film types will have too high an inductance.

My own brief experiment with limited cap supplies lean me towards film type. It seems to be like signal passing cap; film type needed for high frequency clarity but higher capacitance needed for better bass response. I only had low value film polypropylene to try.

Before I go off wasting money on expensive high capacitance film caps - has anyone got any experience in this area they could share? I was recommended to look at Audyn Plus caps & reading about them they are built to minimise inductance, so a good start.
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Old 17th September 2009, 12:35 AM   #2
njepitt is offline njepitt  United Kingdom
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I'd be very interested in an answer to this also... Is it possible that this is a place where using both in parallel might be a good idea? I mean, a larger electrolytic (say 47 - 100 uF) and a higher-quality low-value film cap across it as a snubber...

Cheers

Nigel
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Old 17th September 2009, 12:41 AM   #3
jkeny is offline jkeny  Ireland
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Quote:
Originally Posted by njepitt View Post
I'd be very interested in an answer to this also... Is it possible that this is a place where using both in parallel might be a good idea? I mean, a larger electrolytic (say 47 - 100 uF) and a higher-quality low-value film cap across it as a snubber...
I can say that a low esr cap, the Sanyo Sepc sounded bad
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Old 5th August 2010, 11:55 AM   #4
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Default Hi,

Any conclusions on this? I have the same dilemma right now. There is a discussion about it on other forum, but no final conclusion.
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Old 6th August 2010, 09:59 PM   #5
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Jocko is right - leakage current usually does matter here, because impedance is fairly high and therefore small currents make for (relatively) large voltage noise.

1LSB on 16bit at 2Vout rms is only 32uV remember...

I like using film caps, 5mm pin spacing is mosty OK here IME. A better answer is usually to build an active buffer to drive the Vref pin with the expected voltage, because this can be a very low impedance source over a wider bandwidth when done right. Think 'filtered low noise reference plus suitable buffer'; detail left to your imagination.
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