Cap recommendation for DAC Vref, Vcom pin

jkeny · 13th September 2009, 07:25 PM

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.

njepitt · 17th September 2009, 12:35 AM

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

jkeny · 17th September 2009, 12:41 AM

Quote:

Originally Posted by njepitt

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

barmanekm · 5th August 2010, 11:55 AM

Any conclusions on this? I have the same dilemma right now. There is a discussion about it on other forum, but no final conclusion.

martin clark · 6th August 2010, 09:59 PM

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.

13th September 2009, 07:25 PM	#1
jkeny Banned Join Date: Feb 2007 Location: Dublin	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.

5th August 2010, 11:55 AM	#4
barmanekm diyAudio Member Join Date: Nov 2009 Location: Kraków	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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17th September 2009, 12:35 AM	#2
njepitt diyAudio Member Join Date: Aug 2008 Location: Brasilia	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

6th August 2010, 09:59 PM	#5
martin clark diyAudio Member Join Date: Jul 2003 Location: Bath, UK	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.