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Valve DAC from Linear Audio volume 13
Valve DAC from Linear Audio volume 13
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Old 5th October 2020, 09:49 AM   #971
merlin el mago is offline merlin el mago  Europe
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Ray what kind of LPF do you use?
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Old 5th October 2020, 11:36 AM   #972
nautibuoy is offline nautibuoy  United Kingdom
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Check out post #1 Felipe, specifically the Reconstruction Filter section - you'll find lots of useful info there on the filter options.

Ray
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Old 6th October 2020, 11:16 AM   #973
merlin el mago is offline merlin el mago  Europe
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Is this one?
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Old 6th October 2020, 03:33 PM   #974
MarcelvdG is offline MarcelvdG  Netherlands
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No, it's a variant of this one: https://www.diyaudio.com/forums/atta...b3_lundahl-png
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Old 6th October 2020, 04:44 PM   #975
diyAchim is offline diyAchim
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Probably a daft question - but why wind your own inductor?

Would a standard component between 1.5mH and 2mH not do largely the same?

Here's a list of components that where specified at 25kHz or 100kHz:
Inductors
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Old 6th October 2020, 06:15 PM   #976
merlin el mago is offline merlin el mago  Europe
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Quote:
Originally Posted by MarcelvdG View Post
It's an LC filter with a característic impedance of 402 ohms to match the output impedance of the DAC? What's the córner frequency & the power loss?

TIA
Felipe
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Old 6th October 2020, 06:59 PM   #977
MarcelvdG is offline MarcelvdG  Netherlands
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The output impedance of the DAC is about 11.2 kohm (differential, 5.6 kohm per side), as set by the anode resistors, so it's an LC filter with very different termination impedances. It is designed to have a third-order Butterworth roll-off at 42.9 kHz, but I don't know to what extent the winding capacitance of the transformer and C9 disturb that. The insertion loss is dominated by the transformer.

The thing is that I originally designed the filter for Jensen transformers, see post #182, Valve DAC from Linear Audio volume 13 Ray preferred Lundahl, so I changed it into the circuit of post #215: Valve DAC from Linear Audio volume 13 However, Jensen claims that the standard application circuit for their transformer has a flat input impedance well above the audio band, but Lundahl doesn't specify at all what happens with the input impedance of their standard application circuit at ultrasonic frequencies.

I later tried to guesstimate the parasitics of the Lundahl transformer from the rather sparse information in the datasheet and to come up with a filter that is both compensated for and as insensitive as possible to transformer parasitics, see post #837, Valve DAC from Linear Audio volume 13 . No-one has tried those filters yet, though, while Ray has listened to his version of the filter of post #215 and is very happy with it.
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Old 6th October 2020, 07:06 PM   #978
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by diyAchim View Post
Probably a daft question - but why wind your own inductor?

Would a standard component between 1.5mH and 2mH not do largely the same?

Here's a list of components that where specified at 25kHz or 100kHz:
Inductors
The reason I chose to wind my own inductors is that Siemens/Epcos/TDK provide enough information to calculate the distortion of a homemade inductor on a potcore, while standard inductors generally have no clear distortion specs. It would certainly be interesting to see how well or how poorly standard inductors work, though - when they have an air gap and are designed to saturate at currents far above the few milliamps of signal current that come out of the DAC, distortion could very well be a non-issue.
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Old 6th October 2020, 07:08 PM   #979
agent.5 is offline agent.5  United States
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Well, Jensen and off-the-Digikey-webpage inductors are easier for us over here across the pond.
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Old 6th October 2020, 07:57 PM   #980
MarcelvdG is offline MarcelvdG  Netherlands
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Digikey also has many potcores and RM cores, including models that are either identical or very similar to what Ray used, but sure, off-the-shelf is easier.
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