Installing a speaker proetection relay on Pioneer receiver

[neilb1906]

Hi all

I am almost complete in my resto of a cheap SX450. Gone really well and is sounded immense for a 15 w job. Beautiful deep sound since a total recap and reflow of joints and power transistors reseated.

Im wondering if anyone has installed a speaker protection mod to a Pioneer? Im concerned about the switch on thump i get through my expensive Monitor Audios.

The circuit itslef I dont forsee as a problem, simple DTDT relay and a transistor/capacitor set up for a delayed activation. The problem I see is placing this circuit in the speaker line before it hits the rear speaker posts.

Any comments/observations welcome.

 

[larryderouin]

It would be easier to connect the 450 to a power strip, then turn the VOLUME ALL THE WAY DOWN, and set the selector to SPKR OFF, then power down. When you turn it back on, hit the power strip, count slowly to 5 by 1000 counts (ie 1000 - 1, 1000 - 2, up to 5) then with the VOLUME ALL THE WAY DOWN, switch to SPKR A. By the 5 count the voltages should all be stabilized and you shouldn't get the "THUMP".

 

[tarior]

The fact that it is making a loud thump when powering on or off indicates that something isn't functioning properly.

 

[larryderouin]

Did you replace the Main Filter Caps yet. As the 450 is direct coupled there are no output coupling caps. I'd also look at C221 on the amp section itself.

 

[tarior]

The main filters would have to be seriously shot to cause thumps on power on/off.
I was thinking more along the lines of new a snubber array on the power switch, double check all regulated (and unregulated while we're at it) voltages, shitcan all known recurring failure semis like 2SA725 that these units are chock full of. Things like that.

 

[neilb1906]

Thanks for your comments so far, I will look at the snubber option on the mains switch. Yes, the main caps have been replaced.

 

[QSilver]

The thump is caused by a muting circuit IIRC. It works by delaying the power up of the initial stage of the power amp. After a time set by one of the capacitors, a transistor turns on which allows power to flow to the differential pair input to the power amp section. This can cause a thump but shouldn't if the timing is correct.... I think it is designed to match the rise of the positive and negative supply rails. If these charge at different rates (which they usually do) then you get a thump until they reach an equilibrium. Hope that makes sense...