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Old 31st October 2004, 01:08 PM   #1
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Default Optimal supply design for UCD and Zappulse modules

Since there seems to be a lot of opinions on the best design for a UCD or Zappulse power supply, and the advice is rather scattered over a number of threads, I think it would be useful to document a decent basic design here. Hopefully others will also post tweaks as well.

In any design we need some parameters to optimise to, so I am going to loosely propose the following (which might hopefully be applicable to most builders)

- Need a good basic design. There is little point trying to add exotic parts for a 1% improvement if a better design exists that gives a much better improvement
- Stereo amps. Two modules in a box
- Modules placed physically apart (to reduce interference, and give more options to control heat)
- Case sizes need to be reasonable 80mm to 120mm maximum. For most purposes, this probably means that dual large torroids are not feasible, hence dual mono supplies are going to be difficult to implement. Note that LCAudio do have some very slim 200va torroids where you can pack 2 in a 1U case, and 4 in a 2U case!
- These digital modules appear not to need quite such high end supplies as a more traditional design. Therefore the design should be simplified appropriately.


Now, from reading here:
http://www.tnt-audio.com/clinica/ssps1_e.html

..and also from the notes on the UCD180/400 threads, and from comments from Lars and Jan-Peter, I believe the following design is suitable for a decent digital amp power supply:

[IMG]htp://www.wildgooses.com/downloads/ps.jpg[/IMG]

However, there are still some unknowns that I don't quite understand:

Firstly, the small caps across each diode were suggested in the UCD180 thread.. Are they still appropriate to this design and have I drawn them in the correct place? In the UCD 180 thread it was suggested that they only be place on the AC to DC negative diodes, is that advice good, and how would it relate to the design above? The LC Audio design does not include these

Secondly, LCAudio, and the link above recommend a small cap placed (100-330nF) between the two ac inputs to each rectifier. Should we add this to the design here?

Thirdly, does the use of double rectifiers make much useful difference? Would, say, two torroids and single rectifiers per channel be more useful for these modules in practice?

Finally, I drew in the two pairs of caps to be physically closer to the modules. Is this sensible/necessary?

Does anyone have any comments on this design? Can we improve it (usefully)? Any suggestions on minimum sizes for the various components (given a certain application)

In my case, I have a choice of a single large 1000Va torroid + 2 rectifiers to feed two UCD400s. Or I have a pair of slimline 200Va torroids that I could use with one rectifier per channel (dual mono though). Any thoughts on how these two options would stack up?

Thanks

P.S. Anyone know why my images aren't appearing inline?
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Old 31st October 2004, 09:53 PM   #2
indoubt is offline indoubt  Netherlands
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Interesting topic.

I consider buying the ultrafast diode bridges from the group buy elsewere in this forum for my zappulses (2.2 SE) or alike

Probably one toroid per zap, so I'll need four single bridges.

Whether the capacity will be a singe big one per rail or a configuration as per TNT suggestion I do not no yet.

The 200VA seem to small. It has been regularly stated that the Zap modules benefit from sufficiently sized transformers to really make them sing.

What is unclear to me is whether that only relates to VA rating or also to the secundary voltage.
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Old 31st October 2004, 10:09 PM   #3
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Quote:
Originally posted by indoubt

Whether the capacity will be a singe big one per rail or a configuration as per TNT suggestion I do not no yet.
Thanks for the thoughts.

Bruno (UCD designer) has stated in the UCD180 thread that quality of caps is more important than size. His own reference amp uses (I think?) 4,800uF caps (somewhere about post 258 in the UCD180 thread I think?)
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Old 31st October 2004, 10:29 PM   #4
indoubt is offline indoubt  Netherlands
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I am aware of that, I'm still indoubt as I have 8* 68.000Uf in stock. Unfortunate they are 50V. I'm still considering to go to a 33V tranny as I do not need the full power of the zapp but would like to have the current (2*800Va tranny or alike)

I have some time before building starts. Tomorrow I'll get the key of a new house so I will be building although the amp will have to wait. The zapps are on stock waiting to be used.
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Old 31st October 2004, 11:25 PM   #5
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I remember I read somewhere that ultrafast diode bridges is not as good as Fast Slow recovery bridges.....
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Old 2nd November 2004, 11:44 AM   #6
tiki is offline tiki  Germany
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Default Softstart circuit

Hi,

In the last days I experimented with a softstart circuit. The result is to be seen here. May be, this circuit was done before anywhere, but I did not find this version. The sum of all elements of the PSU's softstart part cost below 10 Euro. It is very simple too.

The SCR's gate is very sensitive, so high resistor values work well. It's extra supply with the two 1N4004 is not tested yet, I connected the circuit to the 10000F caps directly until now. This version's disadvantage was the long switch-off time (appr. 10 seconds, dependend on the on-state duration). With the extra supply this will be shortened to below a second, hopefully.

The diac is necessary to turn on the SCR safely (another pulse generator may do that too). The two resistors at the relays' contact hold the inrush current below 2A. It takes around 0,7 seconds to load the main caps up to appr. 5V below the nominal supply voltage. After that the relays switches on and the caps will be charged fully. The circuit works fine.

Any suggestions for advancements?

Best regards, Timo
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File Type: pdf psus_softstart.pdf (16.2 KB, 918 views)
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Old 2nd November 2004, 09:11 PM   #7
tiki is offline tiki  Germany
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Default Softstart addendum

Good Evening,

Some modifications of the circuit above:
C4: 100F -> 22F,
added diode (small signal) between C1+ (anode) and C4+ (cathode),
Resistor 15k between C4+ and GND.
And the diodes D5 and D6 were added, of course, while C4+ was disconnected from the positive rail.

This modification shortens the relay switch-off time to below 0.5s (after mains switch). This time is independent from the rail voltage now. That means, if the mains switch is off for a period longer than 0.5 seconds, the softstart is functioning, otherwise (well below 0.5s), the relay switches immediately on. That's not too bad, because the rail voltages are still near to their nominal value. So there cannot be an exceptional inrush current in this case.

The supply voltage of the relay has a ripple of about 3Vpp due to the small filter cap of 22F. This is ok, because there is no connection to the rest of the amplifier circuit except ground and secondary transformer AC. And the currents are very low, therefore I do not expect any disturbances.

The delay is in the range between 0.5 and 1 second with the given values (on my bench).

The relay is a 16A-type for 48V nominal supply, consuming less than 10mA.

I would be happy about some criticism. Maybe, this circuit could get better.

Best regards, Timo
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Old 3rd November 2004, 09:49 AM   #8
micheat is offline micheat  Canada
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What I am curious to know is, how important is the use of the ferrite beads? Also would using 2 10000uf capacitors per modules have any negative effect on the sound, over just using 1. Or how about 1 15000uf cap per module. If cost was absolutely no object, what would be the very best way to go for the absolute highest sound quality? Considering the efficency of the modules, and ability for them to operate with such a small power supply, compared to class a, or class a\b designs, cost wise a person can put alot better quality parts into the power supply, and still keep the project cost low. Hopefully achieving better over all sound quality then class a or a\b designs, costing the same price. At least thats what I'm hoping.
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Old 4th November 2004, 02:53 AM   #9
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How do you calculate the correct size for resisitors across the power supply caps? These will be for discharging the capacitors on power off. Will these affect the sonics of the UcD400's?
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Old 9th November 2004, 09:33 PM   #10
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"Bleeder" resistors, easy, use RC time constant to come up with a value that will discharge them in a reasonable amount of time, 5 to 10 seconds perhaps, make sure of the wattage rating as well.

Sound? Try it and found out, I doubt it though.
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