loud thump when powering up my amp

denimvenom · 29th October 2006, 07:15 AM

i know many older solid state amps do this, but for the sake of being specific, it is a 70's 'polytone custom bass amp 100' combo that i am having this problem with. i would like to stop this loud thumping sound when i turn on the amp, at least for the sake of protecting the speaker. it also thumps when i turn the amp off, though not quite as severely. ive already tried putting a .01 and then a .1 poly cap across the on/off switch, but neither helped at all. like i said, i know this is normal for some amps but with this amp it is a particularly loud thump - and i want it gone or lessened. i would really appreciate any suggestions or solid advice.

pinkmouse · 29th October 2006, 08:36 AM

With an old amp it is likely that some of the electrolytics in the biasing or feedback have dried out. A blanket replacement of all electros is probably a good idea. Pop the lid, and give us a couple of pics.

denimvenom · 29th October 2006, 07:53 PM

hi - ive already done that. i thought it would take care of some of that thumping, but its exactly the same as with the original electrolytics.
btw, i dont want to have to build some kind of elaborate speaker protection delay circuit. ild prefer a simple solution, if there is one. i just want to enjoy my amp. i just finished 'restoring' it. i may want to tweak one of the channels later to give it a little mid boost, but otherwise its done.
thanks for the response!

pinkmouse · 29th October 2006, 10:39 PM

OK, it's probably a "design feature".

Two options, a switch in the speaker output to turn the drivers on and off, (only do this when not playing anything through the amp or the switch will rapidly arc out), or a muting circuit. They are not really that complicated, have a look at Rod Elliot's site for some ideas.

Leolabs · 30th October 2006, 12:54 AM

Is it a single supply powered amp???

FastEddy · 30th October 2006, 01:00 AM

Single supply = yes.

denimvenom: You are probably in love with the looks and the "tube like" sound. You might consider replacing some of the parts with more modern technology.

bigpanda · 30th October 2006, 01:34 AM

It isn't that complicated as you might be thinking of, or is it?

FastEddy · 30th October 2006, 02:06 AM

According to this: " ... 90 watts, solid state, closed back, 2 channels each with two 1/4" inputs volume, bass and treble controls ..." ( from http://www.elderly.com/vintage/items/130U-7722.htm ) ... it would not be too hard to do a total make over, a DIY Upgrade ... throwing in a couple of decent 90 to 150 watt amp modules of modern technology and adding a simple, dual channel pre-amp with tone controls ... The power supply transformer is of an older type, but the power rails (+ / - DC voltage) would lend itself to a nice upgrade. (Of interest: http://aussieamplifiers.com/nx150.htm ... works on rails from +/- 40 VDC through +/- 65 VDC ... but there are many, many more available.)

(The really great thing about DIY modding up an older amp for the stage ... you would amaze your compadras with a really great, clean, clear sound ... and no one will ever steal it as it looks so old and funky.)

bigpanda · 30th October 2006, 02:18 AM

sorry, I mean the anti thump circuit.

FastEddy · 30th October 2006, 02:54 AM

I do too

... my solution may sound like a sledge hammer for a tack hammer job, but as long as your adding circuits and parts, you might as will fix all the problems ... (Im a little more into total DIY than simple slap dab, make due repairs, sorry.)

29th October 2006, 07:15 AM	#1
denimvenom diyAudio Member Join Date: Oct 2006	loud thump when powering up my amp i know many older solid state amps do this, but for the sake of being specific, it is a 70's 'polytone custom bass amp 100' combo that i am having this problem with. i would like to stop this loud thumping sound when i turn on the amp, at least for the sake of protecting the speaker. it also thumps when i turn the amp off, though not quite as severely. ive already tried putting a .01 and then a .1 poly cap across the on/off switch, but neither helped at all. like i said, i know this is normal for some amps but with this amp it is a particularly loud thump - and i want it gone or lessened. i would really appreciate any suggestions or solid advice.

29th October 2006, 08:36 AM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	With an old amp it is likely that some of the electrolytics in the biasing or feedback have dried out. A blanket replacement of all electros is probably a good idea. Pop the lid, and give us a couple of pics. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

29th October 2006, 10:39 PM	#4
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	OK, it's probably a "design feature". Two options, a switch in the speaker output to turn the drivers on and off, (only do this when not playing anything through the amp or the switch will rapidly arc out), or a muting circuit. They are not really that complicated, have a look at Rod Elliot's site for some ideas. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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29th October 2006, 07:53 PM	#3
denimvenom diyAudio Member Join Date: Oct 2006	hi - ive already done that. i thought it would take care of some of that thumping, but its exactly the same as with the original electrolytics. btw, i dont want to have to build some kind of elaborate speaker protection delay circuit. ild prefer a simple solution, if there is one. i just want to enjoy my amp. i just finished 'restoring' it. i may want to tweak one of the channels later to give it a little mid boost, but otherwise its done. thanks for the response!

30th October 2006, 12:54 AM	#5
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Is it a single supply powered amp???

30th October 2006, 01:00 AM	#6
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	Single supply = yes. denimvenom: You are probably in love with the looks and the "tube like" sound. You might consider replacing some of the parts with more modern technology.

30th October 2006, 01:34 AM	#7
bigpanda diyAudio Member Join Date: Jun 2004 Location: HKSAR	It isn't that complicated as you might be thinking of, or is it?

30th October 2006, 02:06 AM	#8
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	According to this: " ... 90 watts, solid state, closed back, 2 channels each with two 1/4" inputs volume, bass and treble controls ..." ( from http://www.elderly.com/vintage/items/130U-7722.htm ) ... it would not be too hard to do a total make over, a DIY Upgrade ... throwing in a couple of decent 90 to 150 watt amp modules of modern technology and adding a simple, dual channel pre-amp with tone controls ... The power supply transformer is of an older type, but the power rails (+ / - DC voltage) would lend itself to a nice upgrade. (Of interest: http://aussieamplifiers.com/nx150.htm ... works on rails from +/- 40 VDC through +/- 65 VDC ... but there are many, many more available.) (The really great thing about DIY modding up an older amp for the stage ... you would amaze your compadras with a really great, clean, clear sound ... and no one will ever steal it as it looks so old and funky.)

30th October 2006, 02:18 AM	#9
bigpanda diyAudio Member Join Date: Jun 2004 Location: HKSAR	sorry, I mean the anti thump circuit.

30th October 2006, 02:54 AM	#10
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	I do too ... my solution may sound like a sledge hammer for a tack hammer job, but as long as your adding circuits and parts, you might as will fix all the problems ... (Im a little more into total DIY than simple slap dab, make due repairs, sorry.)

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