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Old 11th September 2011, 10:23 AM   #1
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Default Custom built amp, loud sound when switched off.

Hi all,
I finally finished my custom build amp. It's a simple chip-based amp, with a pre-amp and separate transformers for the amp and pre-amp.
I have a power switch that turns the amp on and off; however, when I switch the amp off, a loud cracking/pop/crackle sound goes through the speakers as the amp loses power.
Is there anything I can do to stop this? can I simple wire a small capacitor inline with each speaker to hold some power when the amp goes off?
Thanks for any suggestions
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Old 11th September 2011, 11:02 AM   #2
Mooly is offline Mooly  United Kingdom
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Many circuits give unpredictable behaviour as the rails collapse and it's impossible to give a definitive reason for your exact amplifier.

It could be the power amp that does this, or it could be the preamp generating noise and there is still enough power left in the rails for the chip amp to pass this to the speakers.

The rails to the preamp may be collapsing too quickly or not quickly enough in relation to the power amp.

The absolute fix (and to me essential as all audio circuits should be totally silent on power up/down) is to incorporate a relay delay to the speakers.

If you didn't want to do this then you have to first isolate the power amp inputs and see whether the problem exists just for the power amp section and then look for and try and devise a fix. Or it could be the preamp as mentioned and some form of muting is needed at power down on the preamp output.

The speaker relay covers all bases though....
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Old 11th September 2011, 12:38 PM   #3
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hi,
Thanks for your reply
How exactly do I go about implementing relays; what do I need to buy, and basically how do I set it up?
Basic circuit:
audio signal goes to pre-amp
Pre-amp output goes to power amp
power amp output goes to speakers
Power circuit:
mains power comes into both transformers
power from pre-amp transformer goes directly to pre-amp (the pre-amp is always on)
a switch is wired in series with the amps transformer, to switch the amp on and off
Thanks!
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Old 11th September 2011, 12:42 PM   #4
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Ashley there are a few schmeatics on here for wireing up a 555 timer IC, but that mainly solves power on thumps, if your rails are in the 24 to 60V region, you could use upc1237, the datasheet is pretty self explanatory.

If it is only a power-off thump you could use a simple mains powered AC relay on the outputs, which would disconnect the speakers the moment power is cut, and before the rails on the DC side of he rectifier starts to drop.

I suspect part of your problem is the the preamp side's PSU start dropping out before the amp side...
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Old 11th September 2011, 02:05 PM   #5
Mooly is offline Mooly  United Kingdom
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Although a delay is is easy to implement I found this which at the price may be hard to beat. This gives a delay at switch on and at switch off it relies on its own rails collapsing quickly dropping the relays out. I would guess the drop out time is around 200ms and determined by the value of the rail caps on the module and the current drawn by the relays.

This module also offers DC offset protection which is good thing and could save your speakers in the event a fault developed in the amplifier.

VELLEMAN KIT|K4700|K4700 2 CHANNEL LOUDSPEAKER PROTECTOR | CPC

Details and circuit,
http://www.velleman.eu/downloads/0/i...k4700_rev1.pdf

If you wanted to totally DIY it then it's a case of how simple or complex do you want ? Can you make your own PCB's etc.
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Old 11th September 2011, 08:23 PM   #6
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Quote:
Originally Posted by Mooly View Post
Although a delay is is easy to implement I found this which at the price may be hard to beat.
+1
Similar modules available on eBay - both 'kit' and assembled. Just search on 'speaker protection'....
I've used them a few times in chip amp builds and they seem to work fine.You do have to check the power ratings.
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Old 12th September 2011, 12:10 PM   #7
marce is offline marce  United Kingdom
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Cap across the switch, a capacitor turn of snubber...
I ahve same problem, so I am going to build a relay board, but with the addition of various switched mains outlets so that my equipement can be powered up and down with one button in the correct sequence.
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Old 12th September 2011, 10:16 PM   #8
Minion is offline Minion  Canada
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Why can"t you put the speaker output on the same switch as the power switch (on a different pole of course) ??

Then when you turn the power on you connect the speaker and when you turn the power off the you disconnect the speaker .....
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