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Old 8th June 2003, 05:49 PM   #1
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Default More on Clock Power Supply

I have been reducing the power supply noise to a Tent XO module feeding a PCM1704 DAC.

With a TL431/BC156 series pass ps as posted widely, the measured noise level was quite high, at around 60 uV wideband.
The sound improved over a resonant crystal arrangement, but not that dramatic.

Reducing noise gradually by using OSCONs at input, across the TL431 and at the output, I managed to get noise down to about 15 uV and here the sound improved significantly, with clearer highs and better bass, and a much more relaxed presentation.

Connecting a TL1763 LDO, however, I managed to get noise down to 8-10 uV, with reduced hf components but some 50 Hz evident in the trace. The sound has now become even better, with clear and smooth presentation. There was no need to use inductors at all.

Getting to this stage wasn't easy, and many insitu measurements were made relative to changes to the ps. However, it has proved worthwhile in that the sound is now much better. May be I should try a battery next? Throw away your 7805 and LT1086s, and try something better. The LT1763 has 20 uV specified noise, but a 150 uf OSCON at the output did the business.

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Old 8th June 2003, 10:59 PM   #2
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Hi Fmak,
So, do you have a schematic of you final design?
Curious to see it.

Randy
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Old 9th June 2003, 06:35 AM   #3
pedro is offline pedro  Spain
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Me too! :-)
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Old 9th June 2003, 07:10 AM   #4
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[QUOTE]Originally posted by randytsuch
[B]Hi Fmak,
So, do you have a schematic of you final design?
Curious to see it.

-------------------------------------------

Just the LT1763 SMT 8 (not available in anything else) with 220uf feeding input and 150 uf 6.3V OSCON SG at the output. The chip sits on a DIL adaptor and a 6x3 cm ground plane, with fly lead connection to the pins. The whole thing was quite laborious to assemble. The larger LTs have higher noise but are easier to mount.
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Old 9th June 2003, 08:47 AM   #5
pedro is offline pedro  Spain
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How do you connect "sense" and "byp" pins in the LT1763 to the "out" pin?
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Old 9th June 2003, 09:37 AM   #6
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What type of XO do you have?

After reading your post I've been thinking about stacking some 9V batteries as a PSU for my XO3.
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Old 9th June 2003, 09:47 AM   #7
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Quote:
Originally posted by Tazzy
What type of XO do you have?

After reading your post I've been thinking about stacking some 9V batteries as a PSU for my XO3.
Hi all,

I am happy to see that the hunt for low noise is paying off....

8-10uV equals 8 to 10 nV/Sqrrt Hz, when 1 MHz BW is still used (Fred ?).

Not bad for a commercially available LDO !

Using batteries, or a seperate transformer finally reduces the last trace of noise from a player, not to mention the last bits of potential ground loop problems

succes to you all

Guido
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Old 9th June 2003, 10:57 AM   #8
ALW is offline ALW  United Kingdom
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Default Cascaded reg's

The ground loop / 50(60) Hz stuff can be the hardest to get rid of, but with care it's not difficult.

Try cascaded reg's, connected as tracking pre-regualtors and always take the outputs from the o/p and ref terminals of the reg.

The latter bit is critical - if you take the o/p of most 3-terminal adjustables from anywhere other than the adj terminal sense point (i.e. where the R / C connect) you'll get signficant measurable and audible noise increase, which is a killer to a clock supply.

For digital (NOT analogue) supplies an AD8065 in a Jung reg gives VERY low noise that will be hard to maintain by other means.

This solution sounds better than a quieter AD797-based supply in my CDP, I suspect this is the better HF dynamic noise characteristics where the ear may be more sensitive to phase noise.

The noise on the PSU can be viewed as a modulation of the clock, the spectra is pretty important in my experience.

Andy.
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Old 9th June 2003, 11:10 AM   #9
ALW is offline ALW  United Kingdom
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Just for info - the plots below are the same reg, in the same CDP, measured in different places: -

Bad...

Click the image to open in full size.

Better...

Click the image to open in full size.

Unless you connect the circuit to the right place, all your hard work goes down the pan
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Old 9th June 2003, 11:15 AM   #10
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Quote:
Originally posted by ALW
Just for info - the plots below are the same reg, in the same CDP, measured in different places: -

Bad...

Click the image to open in full size.

Better...

Click the image to open in full size.

Unless you connect the circuit to the right place, all your hard work goes down the pan
Andy

What is de unit of measurement ?

regards
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