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Old 14th April 2007, 04:54 AM   #1
maxpou is offline maxpou  Canada
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Question Need a soft start?

Hi,
i would like to know if i need a soft start for my power amp. I have 500VA toroidal transfo and 40000uF by each channel. Thank You! Maxpou
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Old 14th April 2007, 06:04 AM   #2
taj is offline taj
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Yes.
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Old 14th April 2007, 06:54 AM   #3
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I'll agree if the problem is blown fuses but it's also good to have a softstarter if you want better protection using smaller fuses.

Inrush Current killing fuses
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Old 14th April 2007, 05:07 PM   #4
maxpou is offline maxpou  Canada
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Thank you! Peranders.
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Old 15th April 2007, 08:57 PM   #5
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Default softstart for poweramp?

I have done some simulatins using Tina Spice and YES, you do need softstart.
Dependent whether you switch on during zerocrossing of the mains or at the peak, the inrush currents can momentarely go up from 100 to 400Amps.
A good starting point therefore is to make sure your softstart circuit switches mains on as close to the zerocrossing as possible.
The circuit I simulated used a 500VA torroid with 2x55V sec.
and 2x 10000u caps.
By switching mains on at zerocrossing and a series resistor of 22Ohms for 0.5seconds made sure that the mains inrushcurrent
was limited to 15A peak at the first 50Hz cycle decreasing to nominal load in 7 to 8 cycles.

Reason for these simulations is that I decided to build a softstart circuit using zerocrossing softstart switch-on.
I am using TRIACs to switch the 22Ohms resistor, that gets bypassed after 0.5seconds with a second TRIAC.
I also have an audio sense circuit foreseen, where the system switches ON and OFF after 5 minutes without audio.
In this way I can remotely switch on my active speaker systems, directly from my pre-amp.

The one thing I noticed is that at full load simulation (10A continuous load) there are momentary current peaks to charge the 10000uF caps.
These are peaks of 30A with for some 1-2mS.
Such short current peaks are no problem for most TRIACs but I was wondering if in such cases you have to use Power Factor Correction as the load current does not have a sinus shape.

I added the zerocrossing and peak switch on currentwaveforms FYI.
Attached Images
File Type: jpg inrushcurrent sine maxload no r.jpg (19.9 KB, 678 views)
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Old 15th April 2007, 11:12 PM   #6
john65b is offline john65b  United States
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Well, if you all don't mind, I have a soft start related question -

I have been building UCD/Tripath/gainclone amps without a soft start for a while - I figured as long as I don't dim the light or blow fuses I was fine. The amps all vary from 150VA 50VCT Gainclones to dual 400VA 80VCT UCD400. All without soft start. Even the Hypex website indicated they may not be necessary...

Anyway, I am building a new UCD400 and it will be 750VA 40VAC Dual secondaries and worry about hurting amps or speakers in an "event"- there are dual mains fuses to single monster toroid, and each +V/-V rails will have fuses also (6 fuses total).

Anyway, is there a external soft start I can buy to plug between the socket and the amp so I don't have to stuff an internal soft start into the amp chassis?

Is there such an animal? Would be perfect if there was a combo soft start/surge protector/line filter available that I could buy (cheaply)...
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Old 15th April 2007, 11:14 PM   #7
john65b is offline john65b  United States
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I think slower than I type...ebay ebay ebay

http://cgi.ebay.com/SOFT-START-SIX-A...QQcmdZViewItem
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Old 16th April 2007, 12:36 AM   #8
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Quote:
Originally posted by john65b
I think slower than I type...ebay ebay ebay

http://cgi.ebay.com/SOFT-START-SIX-A...QQcmdZViewItem

Nothing but a few NTC resistors and a few MOV's in that thing


100 bucks too
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Old 16th April 2007, 12:43 AM   #9
pooge is offline pooge  United States
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Probably not even a soft start. Just voltage surge protection.
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Old 16th April 2007, 01:23 AM   #10
john65b is offline john65b  United States
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Looks like the only option is internal - hows this for a Soft Start?

http://cgi.ebay.com/Power-Soft-Start...QQcmdZViewItem
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