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Advice on Filter Capacitors for this LM4780 Board

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#1
Hi to all,
I am just looking for advice on the board shown.
It requires DC-voltage, but has already has 3x2200µF capacitors for each voltage rail mounted.
See schematics C20/C28/C29 // C2/C13/C23
So building an unregulated power supply with recitfication and smoothing,
I am wondering what smoothing capacitor value will match the 6600µF already given.
I will run it with a 250 to 300VA transformer, DC voltage will be about 32V.
All the best and thanks,
Herbert
 

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solderdude

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#2
Does it say it just needs rectified DC or if it should be regulated ?

The more smoothing caps you use the higher the start-up current will be and the higher the (pulse) current rating of the rectifier will have to be.
 
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#3
Hi,
thanks for the reply!
It does not say anything, bought it maybe 3 years ago, the designer´s website is offline.
But my choice would be rectified.
To my understanding this results that the caps on the board become part of the smoothing-
and that probably the value of the smoothing capacitor should be related to them,
so it should have the double, triple or quadrouple value...?
Should be 15000µF enough, summing up to roughly 22000µF?
 

solderdude

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#4
They do not have to be related to each other.
These capacitors are on the board to keep the path to the output devices as short as possible so short pulse currents are buffered by those caps.

Indeed for the rectified DC volltage this capacitance will add to the ones in the rectifier/smoothing caps.

The smoothing/reservoir caps for the rectifier thus could be anything between 1,000uF (where the bulk of the capacitance would come from the boards) to for instance 100,000 uF in which case the reservoir caps supply the bulk capacitance.

You can safely use somewhat longer wires from the power supply board to the rectifier board in this design as.

Should the amp not have had this much capacitance on board then the smoothing caps would have to be mounted closer to the amp and using thicker wires.
The bridge rectifier must be able to handle the short and very high peak currents when larger value smoothing/reservoir caps are used.
 
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#5
Good, thanks for he thorough explanation. 24VAC rectified to 32VDC,
Toroid rated at 300VA total, 6ohm speakers in reality, let´s assume 4ohm in theory.:
What capacitance would you advice?
 

solderdude

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#6
What room do you have in your enclosure ?
There probably are nice programs for calculating the minimum value given a certain amount of ripple voltage is allowed.

You could look into the specs of the amp chip used and search for the PSRR (or PSSR) to find out how many dB rejection there is up to 1kHz.
Then, when knowing the efficiency and listening distance of the speakers one could calculate the minimum ripple voltage on the smoothing caps which is allowed under full load.

Or you can go the practical route and just use caps between 4700uF and 47000uF and see if it works.
Going to big in capacitance can become problematic as well. values between the suggested ones are (most likely) fine.
 
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#7
Not much room- there is just space for a PCB with 2 large caps + rectifying bridge.
Specs do not give any frequency, 120/105dB are the typical values, 85 the limit.
I was wrong with the speakers, they have 8ohm and efficieny of 91dB.
Any help in calculating appreciated, I have no talent for mathematics.
Maybe calculating for 4ohm (if at all needed) woul make things safe for other speakers
PSSR.jpg
MidiMaster.jpg
 

solderdude

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#8
In that case I would just look which space you have available for caps (mimimum 50V rated) and see what capacitance you can fit in there.

50W per amp will be drawn and even with 100dB suppression quite some ripple voltage is allowed so any capacitance between 4700uF and 22000uF will be fine. fast transients will be drawn from the caps on the PCB.
 
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#9
Love this forum! (And of course it´s participants)
All necessary things answered within three hours and no audiophile bla-bla :)
 

solderdude

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#10
Of course the diodes must be a special type of high-speed super fast soft recovery Schottky of a specific brand and must be snubbered with 100nF to completely undo these characteristics. Otherwise the sound will be dull and lacking in stereo image.
The smoothing caps must be oil dipped paper capacitors as big as a house otherwise the sound won't be 'organic' and 'buttery smooth' with a choclaty flavour... I assumed that would be common knowledge...
Ohh forgot but the wiring must be OCC copper with aluminum cladding, silver coated and have some gold flashed and cryongenically treated.
The insulators MUST be cotton as any type of plastic simply won't do.
 

SIY

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#11
One more thing (not tongue in cheek): caps typically come in two different temperature ratings, 85°C and 105°C. The latter will cost a bit more, but will give higher reliability / longer life. Worth the slight extra expense.
 
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#12
Of course the diodes must be a special type of high-speed super fast soft recovery Schottky of a specific brand and must be snubbered with 100nF to completely undo these characteristics. Otherwise the sound will be dull and lacking in stereo image.
The smoothing caps must be oil dipped paper capacitors as big as a house otherwise the sound won't be 'organic' and 'buttery smooth' with a choclaty flavour... I assumed that would be common knowledge...
Ohh forgot but the wiring must be OCC copper with aluminum cladding, silver coated and have some gold flashed and cryongenically treated.
The insulators MUST be cotton as any type of plastic simply won't do.
I will get some virgins to braid the cables under full moon.
Furtunately no sex before marriage became fashionable again.

One more thing (not tongue in cheek): caps typically come in two different temperature ratings, 85°C and 105°C. The latter will cost a bit more, but will give higher reliability / longer life. Worth the slight extra expense.
Any advices? The 50V / 18.000µF Nichicon I would choose are 85°C. (Quick pick @ Digikey, choosing "Audio" as one filter, assuming
to get the the best ratings in the audible range though the caps are for smoothing)
Normally I would aim for 105° ratings but again, space is restricted, not more than 52mm height allowed.
And there is only space for two cans, not four or six.
All the best, Herbert
 

SIY

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#13
Any advices? (Quick pick @ Digikey, choosing "Audio" as one filter, assuming
to get the the best ratings in the audible range though the caps are for smoothing)
Yes, take "audio" out of the filter. Seriously. This is a power supply, and IME, any cap labelled "audio" is either a standard cap with a different label (and price!) or highly inferior in quality, but wrapped in a good story.
 

SIY

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#15
Within reason, not hugely. This is not a terribly critical spot.

edit: Ultra-low ESR increases ripple currents, so given that this isn't a switching supply, I'd avoid special low ESR caps. Ordinary filter caps with a high temp rating will overall work best.
 

SIY

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#17
That's a good name-brand cap. They'll work fine in this application. And a 10k log taper pot will be more than sufficient. You could substitute the pot for R11/R7 and R18/R20 in your schematic.
 
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#18
Thanks again! But this circuit, a headphone amp salvaged from a broken CD-Player, might
do the job of the pot as well? Because I would like to have have an extra headphone output.
I assume because of the high Impedance of IC404 (NJM4560) i could use the pot R532/R432)
(though being 20k) for driving both, the LM4780 and the Headphone circuit in parallel
I would use a breaker in the headphone jack to mute LM4780
via pins 14&20 when a headphone is inserted...
 

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#20
When I decide to use a 10k pot together with the headphone board pictured above, do I have to alter/double the values of R431/531?
I guess 20K would work as well, but as the channels of stereo pots are mismatched @lower volumes, I assume 10k would work better...
 
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