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Old 7th October 2012, 04:02 AM   #21
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Hey Ian, was that post to me?
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Old 7th October 2012, 05:48 AM   #22
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No, sorry if I gave that impression, lanchile...I did use your post as a reference though, as I happen to agree - from past experience at least.
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Old 7th October 2012, 06:35 AM   #23
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You can try using a bleeder resistor across the +/-17 volts capacitors. This will force the capacitors to discharge faster making the thump shorter. I will try a 2K resistor across the +/- 17 volts to see if it will dampening the "thump". This may help when the amplifier it is powered off.
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Old 7th October 2012, 07:02 AM   #24
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Originally Posted by Ian Finch View Post
This is the basic problem with any mass produced solid state amp. in the silicon era before the the general use of relays around 1980. Even capacitor coupled output designs could deliver a strong thump.

Some high quality units seemed to have had care taken to reduce DC offsets and the imbalance in the charging currents to the preamp and ancillary circuits (this usually causes most of the noise) but all amplifiers had some degree of "thump/crack" and it was once part of everyone's audio experience. I notice that a lot of people who weren't around then, fret about damage to their speakers or just the surprise of it as they aren't accustomed to this in modern audio systems which have muting/protection circuits.

There is an important issue with re-capping that DIYs often ignore by fitting low ESR caps everywhere. Be careful buying caps on a popularity basis - you don't want low ESR cap grades for filtering DC (the big reservoir caps) and they will probably make the noise pulses shorter and louder. You'll have to replace caps anyway, but just use regular types, a little larger in capacitance (<= 100%) may actually be helpful.

As in previous posts, this muting is in the form of a speaker protection relay disconnecting the speakers until a few seconds have elapsed from turn-on and dropping out immediately the power is turned off. You can purchase pre-built universal modules of these (a PCB with ready built circuit) on E-pay for peanuts or you could DIY completely from schematics shown regularly on the forum, if you wish.

Unfortunately, short of just reducing the sharpness or "crack" of the sound , you can't make the effect disappear with a simple tweak or part replacement. I think though, that unnerving as the effect may be, any speaker that can't handle the typically brief burst of energy at moderate level is sure not going to handle music too well either. The rest is down to you doing the proper mod. or simply getting used to retro audio.
Thanks for the comment and insight Ian....
This receiver was a fixer upper from a friend who wasn't expecting much other than a possible repair of a bad channel, which I have accomplished.
I am certainly guilty of "mass cap replacement syndrome", which was my bad.
It would have been better to replace the actual bad components (which I did anyway) and left the rest of it alone....LOL
I am going to put the original PS filter caps back in and see if those were actually better with this noise pop/thump issue.
My pop/thump fix for this unit may be just adding a speaker switch on the back to provide some simple manual protection.
It's intended use by my friend (if it was salvaged) was pretty minimal anyway.
So I'm really just saving an old 1963 SS receiver from the dump.
It was a fun little project though...the pop/thump was just very surprising to me and I do remember that being a problem with older designs. I'm just not interested in damaging my speakers in the process!
I really should be using some test speakers!
I will try a few more things to see if I can minimize it, put it back together and let it go at that. My friend can and will easily accept a little switch action to avoid this quirk.
Thanks again for everybody's help, advice and knowledge...this is a great place to get info on this sort of stuff.
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Old 7th October 2012, 10:59 AM   #25
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Originally Posted by nigelwright7557 View Post
I would still try the cap across the switch.
I had a disco with dual record decks and every time I turned on a deck i would get a huge crack through the speakers. I put a cap across the switches and that brought the crack down to an acceptable level.
Same with my guitar amp. Acceptable now.
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Old 7th October 2012, 12:45 PM   #26
balerit is offline balerit  South Africa
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The only real cure to this problem is to have a slow turn on of the power supply.
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