How Many Microfarads before Soft Start is Necessary? - diyAudio
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Old 27th March 2005, 08:35 PM   #1
lgreen is offline lgreen  United States
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Default How Many Microfarads before Soft Start is Necessary?

Well the title says it all, when should one begin to worry that he/she has too many uFD's of capacitance to just power the amp on without accounting for a soft start? Assume a 500-800 VA toroid.

For example, I don't have a soft start on my Gainclone (has a 500 VA toroidal transformer and about 12,800 uF total between all channels). Seems to not be an issue, but I'm going to add more with some snubbing and was wondering when the inrush would just be too much so that I'd need to add some soft starting.

Heck, my BOSOZ preamp has 12,000 uF total but there I've got two power tranformers. Oh wait, I've got 3 and a few more power supplies in there. Forgot about that, need to add another 4,000 uF.
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Old 27th March 2005, 08:51 PM   #2
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Apms with toroids over 500W and e.g. 2x20.000uF or more should have soft start
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Old 28th March 2005, 05:39 PM   #3
lgreen is offline lgreen  United States
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Default What about the Brian GT Subberized GC?

What about the Brian GT kit, 4 x 10,000 uF caps?

Schematic of 1 Channel of Power Supply
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Old 28th March 2005, 06:31 PM   #4
SY is offline SY  United States
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Soft start adds about $5 and three cubic inches to a project. It seems foolish economy to not use it.
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Old 28th March 2005, 06:43 PM   #5
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Quote:
Originally posted by SY
Soft start adds about $5 and three cubic inches to a project. It seems foolish economy to not use it.
I love to know how far you get for $5.. I did mine follow Rod.E's project just the small cheapie transformer for the delay/relay costed $10..

The Bucther
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Old 28th March 2005, 06:44 PM   #6
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Quote:
Originally posted by SY
Soft start adds about $5 and three cubic inches to a project. It seems foolish economy to not use it.

Would you share what softstart circuit I could use that only costs $5? I've been using ESP's soft start and it costs about $40.

Thanks, Terry
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Old 28th March 2005, 06:58 PM   #7
K-amps is offline K-amps  United States
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Quote:
Originally posted by SY
Soft start adds about $5 and three cubic inches to a project. It seems foolish economy to not use it.
ok SY now you have to share it with us...
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Old 28th March 2005, 07:04 PM   #8
K-amps is offline K-amps  United States
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With shipping from Digikey to my place.... its already over $5 for me
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Old 28th March 2005, 07:04 PM   #9
lgreen is offline lgreen  United States
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Default softstart

I have no objection to the discussion regarding cost, but how about answering the question as well? Soft start at ????uF?

I suppose a single thermister in-line with one primary winding would be a cheapie solution. I'm making the elliot circ, and the relay itself cost over $10.00, the mounting is another 3, and the xformer is at least 5, I think its going to be much higher than $40.
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Old 28th March 2005, 07:26 PM   #10
SY is offline SY  United States
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Digikey? My goodness, don't you people know how to scrounge a relay? You're making a one-off, not going into production.

In any case, the relay and an antikickback diode, a resistor and a zener to tap power (may not even be necessary if your rail voltage is somewhere south of 30 volts), a small resistor and cap to set a time constant, a 100 ohm 2 watt resistor (value may vary depending on your transformer and caps), and a small transistor to trigger the relay. And you're all done.

Looking in my solid-state woofer amp, the relay cost a buck (two bucks at Radio Shack), the transistors, resistors and cap came out of one of my coffee can storage bins (ok, ok, if you don't have a coffee can, figure another buck or two)...
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