Ground Bounce/ Oscillations in SPDIF - diyAudio
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Old 3rd March 2004, 10:20 PM   #1
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Default Ground Bounce/ Oscillations in SPDIF

By chance I looked at the spdif output of an Assembalge D2D twin PLL dejittered output and found quite a lot of oscillations along the top hat, at around 360 MHz on a Tek 24565 scope. The eye of the pattern is very sharp and seems to justify the low jitter claim (2 ps?) but the trest of the pattern is polluted.

I have never likedc the sound of this unit (aggressive highs) and I wonder if this is due to the oscillations. If so, what is the cure? A filter?
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Old 3rd March 2004, 10:47 PM   #2
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fmak,

That is the best thing you can do for a DAC is multilayered grounds

I just finished working on the same thing but in a PS audio transport. The solution was to use a Jocko Homo clock circuit and some more stuff.

1) I pulled out the unitís Hartley oscillator, it was connected to a 74ACT04, using all the gates, which fan out and drove the output clock and a Phillips controller. This got rid of all the ringing on the master clock for the SPDIF. Jocko and driver built in.

2) I replaced a bunch of the power supply caps with OSCON low ESR caps and added more bypass caps.

3) PS Audio used a 74ATC74 to drive the SPDIF transformer and the AESEBU circuits. They were ringing like mad. I replaced them with 74ALS74. I put them on sockets just incase I want to change them.

4) I removed the .1uf AC ground cap that connects the secondary of the transport's pulse transformer.

These changes made a world of difference in the noise transmitted to the DAC.
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Old 4th March 2004, 06:43 AM   #3
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I just finished working on the same thing but in a PS audio transport. The solution was to use a Jocko Homo clock circuit and some more stuff.

I pulled out the unitís Hartley oscillator, it was connected to a 74ACT04, using all the gates, which fan out and drove the output clock and a Phillips controller. This got rid of all the ringing on the master clock for the SPDIF. Jocko and driver built in.

These changes made a world of difference in the noise transmitted to the DAC. [/B][/QUOTE]
--------------------------------------------------------------

Thanks, two clarifications:

You have just finished working on the Assemblage D2D, or the PS Audio transport? If it's the former, can you please email the details?
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Old 4th March 2004, 02:18 PM   #4
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FMARK,

I was working on a CD Transport and separate DAC. If your Assemblage D2D uses any of the logical families replace them. HC, HCT ACT, VHCT with ALS. Next you should check that all the logic chips are decoupled with a .1 or a .01uf ceramic cap. You should also consider changing the electrolytic caps to a OSCON versions.

I have worked on some of the older Assemblage DAC and there where quite noisy. With out having the schematics I can only make a few recommendations

http://www.jeffchan.com/audio/d2d.html
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Old 4th March 2004, 03:36 PM   #5
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If your Assemblage D2D uses any of the logical families replace them. HC, HCT ACT, VHCT with ALS.
-----------------------------------------------------------------------------
Why ALS; can you please explain?

Next you should check that all the logic chips are decoupled with a .1 or a .01uf ceramic cap. You should also consider changing the electrolytic caps to a OSCON versions.
-------------------------------------------------------------------------------
Have done.

I have worked on some of the older Assemblage DAC and there where quite noisy.
------------------------------------------------------------------------------
I put in a 1:1 toroid transformer from the 75R output which is the only one w/o output transformer and this got rid of much of the 360MHz ringing!

The AES/EBU output which is transformer couples. doesn't seem to have a problem.





Let me explain how I got to this, I spent some months using the D2D and was never totally happy. I currently have a Sony SCD777ES which I find to be a very good transport but there is some visible blurring (I think it's jitter) in the eye pattern of some 1.5 nS. Using the D2D , there is considerable ringing at around 360 MHz in the upper trace although the eye itself is tight.

Substituting the D2D with an older SF Ultrajitterbug, I how have improved sound both in the bass and treble, and in imaging. Blurring of the eye is reduced to 0.3 nS buried in noise.

All this was done on a TEk2465B 400MHz+ scope which seems to lock onto evereything and enables 10x zooming. Best I have got unfortunately.

It just shows that it is worthwhile looking at equipment performance.
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Old 4th March 2004, 06:44 PM   #6
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fmak

Well the Ultra Analog Receiver is a better part than the 8414 is. It is lower jitter part and has better internal isolation. Of course, it will only work to 48 KHz.

I ready do not know how the Sony interface is designed, however as stated you have loads ringing on the clock, that is bad new for the DAC.

I would start, if you have not already, so by looking at the power supplies on the DAC with the Sony SCD777ES disconnected. Not that the supplies are bad, it just easy to see what going on at the power supplies. Then Connect the Sony SCD777ES and see if the noise levels change. If the noise changes then there could be several different types of problems, circulating ground currents, ac coupling cap to ground at the SPDIF interface, and more.

Do you have schematics for the Sony SCD777ES, get one f you don't.
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Old 4th March 2004, 07:39 PM   #7
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Question Logic Family Choice

Quote:
Originally posted by jewilson
FMARK,

I was working on a CD Transport and separate DAC. If your Assemblage D2D uses any of the logical families replace them. HC, HCT ACT, VHCT with ALS.

Hi Jim,
Traditionally HC logic is used for digital audio. I once tried VHC logic and found it better sounding than HC or AC logic.
I have no experience with ALS but I can of course give it a try.
The more it experiment the more I am convinced good digital sound stands or falls with the clock in the transport.
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Old 4th March 2004, 08:38 PM   #8
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Elso,

When your switching at high clock speeds, High Speed CMOS dumps more current to ground the Advanced Low Power Schottky.

I was looking for my older data books that show the differences output designs between the two, however I could not find the applications and schematic data for the High Speed CMOS. I have the stuff for ALS in some of my old TI data books. If I find the more I will post it.

Also, as the clock speed increases for High Speed CMOS the it will use more power than ALS at high speeds.
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Old 4th March 2004, 08:40 PM   #9
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Question Logic

Quote:
Originally posted by jewilson
Elso,

When your switching at high clock speeds, High Speed CMOS dumps more currect to ground the Advanced Low Power Schottky.

I was looking for my older data books that show the differences output designs between the two, however I could not find the application and schematic data on the High Speed CMOS. I have the stuff for ALS in some of my old TI data books. If I find the more I will post it.
Jim,
Please read the more I experiment the more etc. in my post.
Seems like someone changed position of the keys on my keyboard. Is this some new kind of virus?
ALS is bipolar logic. HC and VHC is CMOS logic I learn from Fairchilds logic selection guide.
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Old 4th March 2004, 10:28 PM   #10
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Default Re: Logic Family Choice

Quote:
Originally posted by Elso Kwak


Hi Jim,
Traditionally HC logic is used for digital audio. I once tried VHC logic and found it better sounding than HC or AC logic.
I have no experience with ALS but I can of course give it a try.
The more it experiment the more I am convinced good digital sound stands or falls with the clock in the transport.
You mean clock next to DAC chips, wherever they may
be.

T
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