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How do I remove the thump when I turn an amplifier on?

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Is there a way to add a simple circuit to stop the thump when you turn an amplifier on?

Other than having the volume turned down there must be a way?

I have just purchased a Bell Quadmatrix decoder (looking forward to using it to decode some old SQ vinyl, nice wooden case which cleaned up really nice), and was wondering if there is a simple circuit or component I can add in to suppress the thump when i turn it on. The VU meters pulse at switch on and there is that thump that is annoying.

 

(Few other little modes to do Like add banana posts in leau of the 2 pin din sockets, and get those lights going again in due time of course).

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Owen will know, I think its something to do with capacitance across the power in - then maybe not..

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Cheers papa,

 

Owen you out there?

 

I expect I will have to recap the power supply also whilst there - it is after all 35 years old?.

Weird the way they did it - 2 capacitors upside down with soldered links to the PCB. Caps are held by restraints - jubilee clips. Yup should also recap the power supply me thinks.

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should be able to find anti thump circuit of the net. Built one years ago on a Dave Reid saken based amp which had a bad thump on power on. Was a pretty simple circuit with relay with about a 5 second delay that went across the output stage of the amp. I'll see if i have any info, seem to remember it was an eti mag circuit

 

 

 

edit

 

This is similar to what I built. The site also has another way of doing this with a mosfet circuit rather than using relays . http://sound.westhost.com/project33.htm

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indeed, the simplest method is a time delay relay on the output ( only need to switch one, not both + and - ).

 

one of the prime reasons you get a turn on "thump" is that the power transformer is cycling at 50hz - and therefore it charges one side of the voltage rails before applying power to the opposite side. Remember that the rectifier is a + 0 - device, as is the amplifier itself ( it swings voltage from above zero to below zero ). Other reasons are that caps can charge at slightly differing rates, and again, this will show on the + 0 - rails. There are also more reasons, but these are the ones that I have found most often make things go thump. Interestingly enough, I watched the outputs of an amplifier which did this on a scope once, and the thump was fairly random as to being positive or negative voltage, but it almost always peaked at 30v - thats a lot of power in context, and if you are using the amp to drive tweeters with no crossover, it will possibly damage them over time.

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Here are some pics of the internals and what i have to work with.

I know that from Jaycar the replacement capacitors will be heck of a lot smaller. The 2x 1000uF are 35volt. The other one one the power supply not sure but 35v. The two large ones are 2200uF with terminals marked A and B, I'm assuming that the A is the positive as it has red wire?

Lots of open space. One of the photo shows the CBS full logic PCB.

Attached files 158088=7317-Bell_01.jpg 158088=7318-Bell_02.jpg 158088=7315-Bell_03.jpg 158088=7316-Bell_04.jpg

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Replaced the 3 (the other one is on the rectifier circuit) 1000uF 35 Volt capacitors, bough from Jaycar. Thing didn't go bang so must have them polarised right. Which now leaves me with the 2 chassis caps of 2200uF (do they have to be that big in physical size? - anyway Jaycar didn't have a suitable replacement that size or value) and remove the thump.

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Jaycar kit KC5450 $41.90 does the trick. A universal speaker protection kit.

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Gawd looking inside that old quad decoder amp gave me some hideous flashbacks.

The awful thing is that amp thumps by design - it has a single rail power supply and when you turn it on those two big 1000uf 35 volt output coupling caps on the amp board are going to charge up to half the rail voltage THROUGH THE SPEAKER, giving rise to serious thumpage. The ONLY way you are going to eliminate this is to fit some sort of turn on delay in line with the speakers (relay + timer) and provide an alternative path for the charging current during the time before the relays close (resistor instead of the speaker). These things were designed by engineers then the designs were "refined" by accountants - just look at that magnificent choke between the reservoir and filter caps (the two big grey clamp mounted caps). I'm rather glad Bell didn't make parachutes.

M

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LOL thanks Moby. I have a Jaycar speaker protection kit - have to assemble and find a place to put it. Measured 20 volts at the output phono plugs for the front channel. Never measured the earth lug of the phono, so not sure if it to is 20 volts - which would mean 0 volts?

Going to shift the input PCB to closer to the input sockets to give me room for the protection PCB where the input PCB was. Measured around 47 volts at both the amp power inputs, 25 volts going into the decoder, 6 volts from a tap from the transformer goes to the lighting. Going to put a 20 volt zener across the decoder power inputs. Measured 20 volts going to the input PCB - why I do not know. Going to change the din sockets for two dual banana terminals.

Future will see a jaycar 50 watt stereo amp the SC-480 go in - expect I will have to rethink the power supply and maybe a transformer - why all this because i can and I want to.

Why spend the money - just want to have an old 70's kit that will not destroy modern kit and the joy of using the SQ decoder to decode SQ vinyl I have.

And Xmas looms for time to do it.

Cheers

Best regards

bill

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LOL thanks Moby.

I have a Jaycar speaker protection kit - have to assemble and find a place to put it. Measured 20 volts at the output phono plugs for the front channel. Never measured the earth lug of the phono, so not sure if it to is 20 volts - which would mean 0 volts?

Going to shift the input PCB to closer to the input sockets to give me room for the protection PCB where the input PCB was. Measured around 47 volts at both the amp power inputs, 25 volts going into the decoder, 6 volts from a tap from the transformer goes to the lighting. Going to put a 20 volt zener across the decoder power inputs. Measured 20 volts going to the input PCB - why I do not know. Going to change the din sockets for two dual banana terminals.

Future will see a jaycar 50 watt stereo amp the SC-480 go in - expect I will have to rethink the power supply and maybe a transformer - why all this because i can and I want to.

Why spend the money - just want to have an old 70's kit that will not destroy modern kit and the joy of using the SQ decoder to decode SQ vinyl I have.

And Xmas looms for time to do it.

Cheers

Best regards

bill

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