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Yet Another Soft Start Circuit
Yet Another Soft Start Circuit
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Old 12th August 2019, 04:32 PM   #21
johnhenryharris is offline johnhenryharris  United States
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Not Bad. Simple yet effective. Just need to find the right relay with the pull-in voltage that works. You still need a power switch that can handle the turn on surge even with the NTC limiting the current, so about 12 amps, not too bad.
With my soft start the on/off switch handles 40 ma @ 12 v and the relay that bypasses the NTC pulls in after the surge has passed, yes the circuit has a bit more parts then this one.
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Old 3rd September 2019, 07:48 PM   #22
johnhenryharris is offline johnhenryharris  United States
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I was asked why such a big toroid transformer? The answer, lots of current capability and lower AC impedance/DC resistance. You want to have a capable power supply that will not voltage droop when the signal demands more current. A voltage droop will modulate the signal adding distortion. Lots of storage capacity in the capacitor bank also helps, bypass with film caps for better high frequency performance. And the lower impedance of the power supply seems to have a positive audible effect on the sound.
With lots of current you can have lots of output devices paralleled which gives you a lower output impedance which in turn has a better dampening factor for control of the speakers. Lots of output devices requires lots of current for idle bias to run class A and to lower higher order harmonic distortion products.
All of which makes for a heavy amp which most definitely will need a soft start circuit.
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Old 3rd September 2019, 08:47 PM   #23
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by johnhenryharris View Post
A voltage droop will modulate the signal adding distortion.
Not really. It will limit the max output level but that is seldom the issue. It does not lead to distortion unless the supply really collapses.

If you really want a stiff power supply that doesn't droop, put in a regulator. A good regulator help also to stop all that mains junk that often comes in through a toroid.

Jan
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Old 3rd September 2019, 09:11 PM   #24
tauro0221 is offline tauro0221  United States
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Hi,
Sorry to discourage you but the best way to power up/down an amplifier in the same line with nigelwright7557 it is like me used the zero crossing to ramp the AC up when turning ON the amplifier and ramp the AC down when turning the amplifier off. This method will allow to slowly charge the capacitors and at the same time preventing or minimizing the inrush current that happen on power ON. I have been using this method for 4 year without any problem. Use an Arduino to do ramping up/down at zero crossing. Simple.
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Old 4th September 2019, 09:00 PM   #25
spookydd is offline spookydd  France
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That's my take as well. I think as long as there is zero crossing detection and it's used properly to turn on at the right time, there is no need for the two-stage with bypass method.

Can you share something about what you've done?

Quote:
Originally Posted by nigelwright7557 View Post
I did a soft start circuit based on an 8 pin PIC micro, an opto-coupler and a triac.
Just dropped the PIC power supply off a resistor off the mains.
Worked a treat and no relays etc to burn out.
This sounds like an interesting approach with a PIC.

Do you have something to share that uses this method?
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Old 4th September 2019, 11:38 PM   #26
tauro0221 is offline tauro0221  United States
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Hi,
For those interesting in building the AC control attached it is the schematic on how to built the AC power control by ramping the AC on turning the amp ON/OFF. Also the Arduino sketch using for the control of the triac and one photo of my original prototype with a home made SSR relay. l
Attached Images
File Type: jpg Ac power control.jpg (263.8 KB, 225 views)
File Type: jpg prototype_ac_control.jpg (759.1 KB, 222 views)
Attached Files
File Type: doc ac control.doc (36.0 KB, 16 views)
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Old 5th September 2019, 05:27 PM   #27
johnhenryharris is offline johnhenryharris  United States
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So you need a PIC to do the work, and you have to program it, besides the other circuitry around it. I will take the simplicity of linear parts anytime I can and digital logic after that.
Hitting a zero crossing is nice but I tried that in the first version of my circuit and it made no difference to the total amount of current at startup. And I don't see a need to ramp down for turn off. Just disconnecting the mains works fine, the bias to the output devices drains the storage capacitors quickly and without thumps or noises, at least in all my amps.
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Old 5th September 2019, 05:32 PM   #28
nigelwright7557 is offline nigelwright7557  United Kingdom
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If an amp is thumping on power up or power down it has a problem.
Class A amps have an output capacitor so they are a special case.

The worst case I have ever seen was a IRS2092 class d amp.
On power down it would make a siren type noise for a couple of seconds then give an almighty bang through speaker. I had a chat with IR about it and they said the power supply was discharging too slowly. That was my fault as I had used very large smoothing caps.
So I came up with a PIC circuit that monitored VCC and if it fell below about 13 volts it reset the 2092 via an opto coupler. Worked a treat.
I also added a 4 second power up delay.
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Old 5th September 2019, 05:33 PM   #29
johnhenryharris is offline johnhenryharris  United States
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So is there a tern, other than distortion, where the rail voltages ripple/drop slightly with large lower frequency current draw?
I imagine you could add a regulator to the power supply but with up to 40 output devices current draw would mean a pretty hefty regulator. Maybe just regulate the front end and leave the output on a very stiff supply.
Thanks Jan for the input.
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Old 5th September 2019, 05:44 PM   #30
johnhenryharris is offline johnhenryharris  United States
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Hi Nigel,
That sounds like an interesting amp, with those noises you would definitely want some control on it.
My amps are Class A up to about 160 watts, the ones I alluded to with the 40 output devices, has an idle bias of 8 amps and even with 760,000uf in the power supply it drains them very quickly and quietly so I haven't seen a need to add your type of circuitry.
So different amps, different requirements.
Have a good one.
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