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Old 16th February 2006, 01:38 PM   #1
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Default Soft-start integrated with DC-protection?

Hi all
Do you think it would be wise to drive amplifier's mains relay so that it is 'open' when charging supply caps or there is DC on amp's output?
Seems to me that relay by-pass resistor should be of very high value, therefore time constant of soft starting must be more than a second for sure.
A possible issue is energy in reservoir caps as well as energy charging at 'failure' state must be low enough to really protect the coil from burning, a possible benefit is no relay in audio path.
What do you think?
Anyone tried something like this?

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Old 16th February 2006, 11:16 PM   #2
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HELLO
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Old 17th February 2006, 01:12 AM   #3
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Surely normal solidstate amps will settle without a load to quiescent about as quickly as with.

Obviously single rail amps with output capacitor like the old Quad 303 are a special case.
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Old 17th February 2006, 02:23 AM   #4
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See my own approach of this problem at: Soft Start and Standby Functions for your Aleph Amplifier
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Old 17th February 2006, 07:17 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi dark,
have you used PSUD2 to investigate the startup characteristics of your proposed PSU?

You will find that the smoothing caps are substantially charged in less than 400mS and fully charged in well under a second.

So, to answer your question. If the supply is large then there is some benefit in delaying the cap charging.

If you want the extra complexity you could consider a time delay on the soft start to get the transformer running, then a longer relay time delay to permit a slow charge to the caps. Finally the longest time delay to the speaker relay.

For 300VA and less then soft start relays are not needed on toroids with less than +-20mF.

400VA to 750VA and <+-75mF soft start is usually good enough.

If you go above 750VA and above +-100mF then slow charge may be helpfull.

The problem you are trying to overcome is voltage drop on your mains wiring at start up and fuse blowing.

You can fit enormous T rated fuses that allow repeated start up, but they offer little protection in the event of a serious overcurrent fault eg. my 625VA +-40mF needs T8 fuses on straight mains connection. T6A blew after a few dozen starts over a few weeks of use. But T3A now starts it with only a soft start (resistor + relay)

Finally, I would keep all the mains related components together on the one assembly. Keep the DC block with the soft start, spike absorbtion and bypassing.
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Old 18th February 2006, 11:07 PM   #6
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Big thank you guys

best regards

p.s.
Quote:
PSUD2
What's this?
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Old 18th February 2006, 11:17 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
has your search button stopped working?http://www.duncanamps.com/psud2/index.html
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Old 18th February 2006, 11:33 PM   #8
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Thanks
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