My amp plays a long time unplugged...

eiraved

Plane Whisperer
Subscriber
I'm troubleshooting some issues with a McIntosh MC2500 while using headphones. Since it has four 38,000uF caps, I'm kinda' careful to discharge them prior to sticking my hands back in there. So, after testing with my iPod as a source and using headphones, I decide to discharge the caps by unplugging the amp from the outlet, then turning the amp on with the iPod playing music and waiting for the music to stop playing through my headphones. I was quite surprised to find that it took well over 100 seconds for the music to go silent through the headphones! That's a lot of stored up charge in those caps! Has anyone else attempted this? Let us know your findings.
 
I haven't with that one. I've had a few amps that would play through speakers for 15 seconds. Speakers will definitely discharge faster than headphones.
 
Yeah, makes sense. I would expect that my speakers should drain the caps faster than some cheap headphones.
 
I completely recapped this amp except for the main filter caps due to cost. This test convinced me that they may still be ok, LOL!
 
The GFA-555 will run the reQuests for over 30 seconds at moderate volume. I wonder how long it'd go with headphones... (Not that that's really an option, but still.)
 
Yeah, 30 seconds with reQuests is quite a long time! You may be able to eclipse 100 seconds on headsets.
 
Most amplifiers would run for many seconds if the those designers eschewed AC detection power off circuitry. Fortunately, such circuitry has been present in most quality HiFi gear since the 1970s.

Long run time on power off is not a sign of anything good or superior. It merely highlights the lack of adequately complete protection circuitry. Some amplifiers with non-symmetrical current draw from the PSU rails will exhibit considerable offset fluctuations as the unpowered rails collapse differentially in such a situation. Competent design includes either front end muting, or speaker disconnection upon AC cycle loss. This is normally monitored direct, from a half wave rectification of a secondary tapping.

In the case of the MC2500, they likely figured relay disconnection was unnecessary due to the output transformers and instead of muting the input stage on AC dropout, they saved a few cents and simply didn't bother with any power-off muting at all. According to the schematic, they spent a whole lot of time and effort on their turn-on delay and power-guard circuitry and simply ignored turn-off muting.

It also appears that they omitted the virtually bog-standard bleeder resistors over the main filter capacitors, installed to safely and predictably discharge the otherwise high voltages present after turn off.

Very slack engineering on a TOTL 500w/ch monster amplifier IMO.
 
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Long run time on power off is not a sign of anything good or superior.
Thanks for your comments as they are relative, but not the reason why I created the post. I wasn't bragging, but was (simply) just amazed about how much juice was held by the caps and wondered if anyone else had this experience and wanted to talk about it.

As far as McIntosh engineering is concerned, they had their reasons to do what they did and that's a whole other post in itself! If you don't care for McIntosh, that's fine also.

You are aware that every manufacturer has disclaimers to prevent the "amateur" (note the quotations) from opening their equipment to perform repairs, notably to prevent serious injury or death. If you and I decided to work on the amp (experienced restorers), we would know to take the necessary precautions to prevent an accident from occurring. Could McIntosh have introduced additional features to make the equipment safer for the technicians? Of course they could have, but consumer electronics was made with the consumer and not the technician in mind. Anyway, the McIntosh authorized repair facilities know how to safely repair their equipment so these added features (and added cost) were probably not considered necessary in their eyes as most consumers could care less.
 
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It has occurred to me that running the speakers on power down was probably not a good idea. Now that that's confirmed, won't be doing it any more. Don't want to hurt my speakers or my amp.
 
It's not a good idea to do so, and I don't run my speakers during power down either. The OP was about troubleshooting an amp using headphones, and those headphones I'm using are cheap beater headphones I use specifically for this purpose. No great loss if they get smoked...:no:
 
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