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Improving passive I/V for Pi dac 8 x TDA1387
Improving passive I/V for Pi dac 8 x TDA1387
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Old 19th June 2017, 06:34 AM   #11
ChuckT is offline ChuckT
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Thanks. Great info.
I am using Moode on Pi3 with upsampled output from 88k to 192k, so I am wondering maybe the EQ filter is not needed. If I remove the passive filter, can I connect direct from dac I/V to RCA output with a simple filter (eg 10nf to ground)?
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Old 19th June 2017, 07:11 AM   #12
abraxalito is offline abraxalito  United Kingdom
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I'll have a look at increasing the damping of the filter - that way you'll get the benefit of the rejection of images at higher frequencies but not have the rising HF response needed for NOS eq. I think its going to be simply tweaking a resistor value....

Attached the revised schematic - I took it that you'd settle on 100R for the I/V resistor and adjusted the other values accordingly. It looks impressively flat now
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File Type: png 100RTDAfilter.png (6.4 KB, 134 views)
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Old 20th June 2017, 05:15 PM   #13
sumotan is offline sumotan  Indonesia
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Hi Guys
Is there another way around this ? Years ago I experimented with something similar
base on an article in Amateur Audio by Rubinson. No matter how the filter was tweaked,
it didn't sound good. If I recall correctly that filter was a butterworth network

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Old 21st June 2017, 12:56 AM   #14
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by sumotan View Post
Is there another way around this ?
'This' means the I/V stage or the filtering stage? Or both?

Improving on passive I/V isn't possible, the next step up is going to active I/V so as to present a lower impedance to the DAC's current source outputs. By 'active' though I don't mean opamps rather a discrete transistor circuit.
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Old 21st June 2017, 06:15 AM   #15
sumotan is offline sumotan  Indonesia
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Apologies Abrax I was referring to the filter network

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Old 21st June 2017, 06:16 AM   #16
abraxalito is offline abraxalito  United Kingdom
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So your question is 'is there another way to implement the filter?' ? Or 'do we really need this filter?' ?
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Old 21st June 2017, 06:23 AM   #17
sumotan is offline sumotan  Indonesia
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Yes both Abrax. My experience back then was with
a CS4328 & UltraAnalog dac. In the end I found that a direct connection sounded best . Was using a tube stage. Perhaps it was the tube stage that
managed to mask some of the ultrasonics
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Old 21st June 2017, 06:35 AM   #18
abraxalito is offline abraxalito  United Kingdom
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My experience with passive (LC) filters has been they've only helped or had unnoticeable effect, never reduced the SQ with any DAC I've built. But then I've only been using SS output stages and amps, no toobs.
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Old 21st June 2017, 06:41 AM   #19
sumotan is offline sumotan  Indonesia
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Well can't argue with you on this Abrax
Perhaps it too was my inexperience back
then.

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Old 21st June 2017, 06:52 AM   #20
abraxalito is offline abraxalito  United Kingdom
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Well with those two DACs you mentioned you'd not have much drive capability available. Passive (LC) filters tend to be low impedance so need to be driven from low impedance (or alternatively, matched impedance as is the case with passive I/V drive).
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