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Old 18th August 2012, 11:26 PM   #1
quan is offline quan  Australia
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Default jims audio soft start module

Hi just finished building my first amp ever- telos clone. I am using 2 toroidal transformers 500VA, 50VACx2. It is working fine ,dc offset measures 5mV both channels. The only thing i noticed is each time i turn the amp on there is smoke coming from the resistor until the relays kicks in then it is ok. Is there anything i should worry about? Should i change to a higher power rating resistor?
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Old 19th August 2012, 12:03 AM   #2
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Quan,
Amateur here but I'll volunteer an answer. Put a meter across the resistor and turn on the amp. The inrush current will be high to begin with so note what is the highest voltage at turn on. Then figure your maximum amperage, voltage/resistance. Then multiply that amperage times the voltage supply to give you the maximum wattage at start up. I would then double that wattage for the new resistor.
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Old 19th August 2012, 12:32 AM   #3
Johno is offline Johno  Australia
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Sounds like you need higher powered resistors.

I use 4 x 68 ohm 10 W (jaycar wire wound) in parallel with lots of air around them feeding a pair of 300VA transformers. That gives me about 10A peak on the first half cycle (depending on where in the half cycle it is powered up) falling rapidly to the idle current. The relay switches after about 100mS. No noticeable resistor heat.

The soft start circuit and pcb are my own design and I am intending to use it unchanged for a 2 x 500VA Leach clone currently under construction.

Cheers
John
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Old 19th August 2012, 03:23 AM   #4
quan is offline quan  Australia
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Default jims audio soft start module

I have email the seller and just await for the responses. May be the standard module only meant for lower VA. The information came with the board stated -can handle up to 30A.
quan
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Old 19th August 2012, 04:34 AM   #5
Johno is offline Johno  Australia
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Had a look at Jim's website, circuit specifies 68 ohm at 7 W as the current limiter - should be OK unless it takes too long to switch?
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Old 19th August 2012, 04:39 AM   #6
AJT is offline AJT  Philippines
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i used Dale 10 ohm 50watt aluminium clad power resistor, after a delay of about 2~3secs, this resistor is then shorted out by the relay...something like this:


Click the image to open in full size.
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Old 19th August 2012, 06:14 AM   #7
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by quan View Post
The only thing i noticed is each time i turn the amp on there is smoke coming from the resistor until the relays kicks in then it is ok.

Is there anything i should worry about?
hey, you got good answers, so I guess its allowed to smile now

but without the smiling, in worst case it could cause a fire
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Old 19th August 2012, 07:32 AM   #8
quan is offline quan  Australia
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Originally Posted by tinitus View Post
hey, you got good answers, so I guess its allowed to smile now

but without the smiling, in worst case it could cause a fire
Yes ,definitely . i need to work out why it puffs a smoke when turns on otherwise definitely no sleep. I am not so sure now. I took another closer look . The smokes actually come from either the X2 capacitor which rates at 0.6uF 250VAC or the 3W resistor parralel to it( value of 470K/3W). I don't know what to do now. It is not coming from the load resistor which points out earlier as 10W/68R:scratch. I replace another unit which i bought from the same vendor , again the magic puffs of smoke appear again. I am getting nervous as this is my first ever amp build.
quan
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Old 19th August 2012, 10:20 AM   #9
quan is offline quan  Australia
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I am begin to believe it is the dodgy X2 class capacitor which lets out the smoke every time power on until the primary of the trannies are in direct connect. The first X2 capacitor was a Carli brand , the other one is a RIFA TYPE. I am not sure is it just faulty which is so usual for 2 separate types of X2 capacitor or the rating of the cap is exceed with the initial surge?
quan
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Old 19th August 2012, 10:43 AM   #10
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Can you provide the schematic? I suspect the design uses just a capacitor to drop the voltage for the relay coil. This is a bad design and will lead to a high surge current through the capcitor which is why it smokes.
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