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Old 24th August 2012, 04:44 PM   #591
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Originally Posted by jackinnj View Post
Hmmmm, I don't know if I did extensive testing when selecting the choke -- 12mH was just something I pulled out. i have one of these Murata DC-DC converters in the preamp stage of a Pioneer SX-838 which I rehabilitated.
12mH is a lot! I think that lowest DCR and lowest inductance that does the job is all that's required. I don't know how to figure that out though.
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Old 24th August 2012, 04:49 PM   #592
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What is the typical current draw of this device when powered from USB bus?
Can't measure myself, away from home.
Meanwhile I'll look for DC choke.

Last edited by LinuksGuru; 24th August 2012 at 05:01 PM.
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Old 24th August 2012, 05:43 PM   #593
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DCR shouldn't be too much a concern -- it's a ferrite -- and the DC-DC converter is regulated switch mode. Any sag and the pwm is adjusted to bring the output to the stated voltage level.
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Old 24th August 2012, 06:08 PM   #594
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So, basically, are you saying that any choke that fits in the holes is good?
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Old 25th August 2012, 01:23 PM   #595
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You could probably take something out of a XT or AT power supply and it would work.
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Old 25th August 2012, 01:28 PM   #596
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I'd like Pete's opinion before I go modifying his circuit. (as if I had one of those power supplies lying around anyway... )
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Old 28th August 2012, 08:53 AM   #597
AndrewL is offline AndrewL  United Kingdom
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Originally Posted by dirkwright View Post
I'd like Pete's opinion before I go modifying his circuit. (as if I had one of those power supplies lying around anyway... )
Me too, Pete? Do you think this is the source of my noise?

much appreciated,

Andrew
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Old 28th August 2012, 11:47 AM   #598
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This device draws up to 500mA from 5V, choke must be rated 500mA DC. Its quite big, much larger then fuse. I think it might be better to remove protection diode.
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Old 30th August 2012, 07:50 PM   #599
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After all, I killed all gremlins which haunted my box.

As I wrote before in one of posts, I had oscillation (or injected noise, hard to state clearly), strange tiny peaks in frequency response curve, and absolutely unsatisfactory THD+N even in loopback mode (input and output connected together on 33K resistor terminals).

I don't know which part of the tweaks actually solved problem(s), but they are dirty cheap anyway.
1) I occasionally burned U1 DRV134PA, and replaced it with THAT 1646P08-U.
2) Added the following ceramic capacitors in order to suppress noise:
1uF - 5V supply (soldered straight on USB socket PCB terminals);
0.1uF - parallel to C22, C7, C8 (near Murata switch mode DC-DC converter);
1uF - parallel to C13, C18, C11.

0.1uF - high quality ceramic NP0 type
1uF - ceramic Z5U type

Choke would be the last option if anything above doesn't work, DC 500mA 10-12mH is quite big, it doesn't fit nicely.
4 terminal chokes (from many switch mode PS) are common mode types, they are not suitable for this application.

Now in loopback mode I've got the following (1.9V from ESI Juli, balanced connections, 192000 mode, 191072 FFT):
1 KHz: THD - 0.0075%, THD+N - 0.082%
20 Hz: THD - 0.041%, THD+N - 0.19%
20 KHz: THD - 0.048%, THD+N - 0.094%
SMPTE IMD 250Hz/8KHz - 0.035%, MD2 - 0.034%, MD3 - 0.0038%
50 Hz noise: -104dB
100 Hz noise: -120dB
150 Hz noise: -120dB

Next step will be building foil shield inside plastic box in order to get rid of annoying 50/100 Hz noise. Its not that bad, but could be better.
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Old 31st August 2012, 07:16 AM   #600
AndrewL is offline AndrewL  United Kingdom
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Hi Linksguru

Could you post an picture of the inside of your box?

thanks,

Andrew
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