bosoz keep playing music

i think this has been covered before, but i couldnt find it.

basically, when i turn off or unplug my bosoz, it keeps playing for up to 2 minutes or so. its slightly bothersome because it's like the power button is broken. is there a way to simply make it turn off when you cut power?

ive heard of people putting something across the + and - rails that bleeds them when it gets turned off or something? i guess i could also do some sort of relay at the output or whatever, but id rather not put anything in the signal path.
 
Blues,

thanks, but thats not how i make stuff :)

apassgear,

my PSU doesnt have bleeder resistors. its a by-the-book design of the original power supply. here is schematic and board:

http://web.vip.hr/pcb-design.vip/bls-pwr.html

ive never done a bleeder resistor before, but doesnt it go in between the + and - of the PSU caps? and if so, what value is good, and does it need to be higher powered? i have some 0.47ohm and 1ohm 5-watters laying around, would those work?
 
powered down, your circuit should be enough to bleed your ps caps if you want to work inside your chassis...as I've said in other threads I always keep my ps caps fully charged by cutting supply at the rails and just running my ps with LED as the standby load. Cutting off the rails at the circuit board will cut the music too if that's what you want...you're wasting energy in charging/discharging caps everytime you flip the power switch.
 
Blues,

the circuit bleeds down slowly unfortunately. it gradually gets quieter and quieter until it just fades out. its maybe a minute or so realistically.

i understand what you mean about having the power switch control the rails and wasting energy charging and discharging, but that isnt really an option right now.

plus, i would rather NOT have the PSU boards running 24/7. the preamp wont get turned on and off that often. i just want it to stop playing when its turned off.

MikeW,

getting a new power switch is NOT an option. that power switch thing took me 2 months to design and make. its not getting changed. it has more poles on the bottom that i could use, but i would rather not run AC and signal through the same switch.

and as far as the AC being close to the inputs, that was just a design flaw. it doesnt seem to hurt anything, i cant hear any noise on the output, it seems very quiet. i will either move that connecting board or build a steel shield around the AC inlet.

SO, back to the original problem at hand... there should be a very simple way to bleed off the PSU quickly when there isnt any power. what exactly will a bleed resistor accomplish? i would rather not rewire the whole damn thing with new switches, relays, etc...

can a bleeder resistor accomplish what i want, and if so, what value should i use?
 
I'm not a big fan of bleeder resistors for any number of reasons. The usual argument for bleeder resistors is safety. This will probably start another firestorm, but voltage is kinda like the "threat" that Iraq posed to the US pre-invasion...overblown. People are absurdly, irrationally scared of voltage. Below, say, 75 to 100VDC it doesn't even produce any sensation as long as you have dry hands. Does some current flow? Of course. But it's not a big deal. Above 100VDC (or 50VAC) I start taking things more seriously. Wet hands? I don't touch anything more than a 9V battery. (Sweaty car repair guys can get a tingle from a car battery, i.e. around 13V. Sweaty telephone repairmen cuss like the dickens when they touch 48V, but note that they're still alive, just tingly and mad.)
Okay, take the safety reasoning any way you like, it's no skin off my nose.
Onwards and upwards.
What? The preamp plays after you drop power and this is a problem? Gracious me, I'm offended by a piece of equipment that doesn't play after I turn it off. Tells me the power supply isn't big enough.
Okay, okay...if you want it to turn off, you want it to turn off.
(mumble, mumble...mutter, mutter...grumble...)
Possibilities:
--Use a relay at the signal output. This will have to have its own (pretty small) power supply. Less than a hundred uF, since you want to deplete the charge--via the relay coil--fairly quickly. The "it's in the signal path" objection doesn't have to be a problem. Don't switch the signal, switch ground. The signal goes straight through at all times--unswitched. When you want the output to mute, engage the relay to bring ground to the signal, not the other way around.
--Use a "bleeder" resistor to do something useful, like run the LED. Not enough current draw? Drop even more by having lotsa lights. Make it look like some of the old '70s gear just after LEDs came out and people couldn't get enough of them. Use small lightbulbs instead of LEDs, they use a lot more current.
--The brute force method--use a relay, not to ground the output, but to drop the rails. Use a 10 ohm 5, 10, or 20W resistor so as not to have a dead short to ground. When the power goes off, the relay connects the rail via the resistor to ground. A 10 ohm resistor will make short work of all but the most robust power supplies. Note that the wattage of the resistor is not really very critical. Sure you're starting out at 50V or whatever, but a tenth of a second later you're only dropping 20V, and a tenth after that it's only 10V, and a tenth after that...you get the point. You can put a surprising amount of power through a resistor as a decaying pulse and it'll barely get warm to the touch. Just don't sit there and turn the poor thing off and on a thousand times in a row.
--Use the psychological approach. Use a DPDT switch. Use one pole to turn off the power. Use the other to switch off the LED instantly. If the LED doesn't glow, you know it's off even if the music is still playing.
--Take out some of the capacitance in your power supply. Don't blame me if it doesn't sound as good afterwards.
(...grumble, grumble...who'd want a power supply that ran out instantly, anyway?....mutter, snarl...harrumph!...confounded stereo doesn't even sound decent until it'll play for half an hour after you kill the power... assorted curmudgeonly noises...grumble...)

Grey
 
well, thanks GRollins.

i know not everyone on this forum follows my projects, so i remind everyone that this preamp isnt for me, its for my dad. and, i am a perfectionist. i would lose sleep (literally, i wouldnt be able to sleep) knowing that i spend a year building something and the off switch doesnt actually turn it off. the thing has to be perfect. the input switch switches inputs, the volume controls volume, and off should mean off.

oh, and none of this is for safety at all. i have exposed transistor casings, live heatsinks, etc... like all my projects, it has a case :) the case isnt live, so all is good.

out of the possible options, i of course only like a few. i wont be adding any more lights or LED's. plus, i like how they are wired right now. they turn on and off just how i want them too. plus, there are only two at a time, so that wouldnt be enough current draw to kill the PSU's.

i really dont want to use a relay at the output. the amp is silent when kicking on and off (well, its not exactly silent when going off, but from dead off to on there are no pops) and switching inputs whilst music is playing is dead silent too. i would really hate to add relays that might potentially introduce popping and clicking. once again, im really anal.

the psychological approach is a good one, but i see it as a band-aid, and im not for cutting corners. once again, i would lose sleep at night knowing that the led was really just for show and didnt indicate power :) its just not a good design.

now you say you dont like bleeders for reasons of safety? well, in your "brute force" approach, couldnt i omit the relay all togehter and just use a bleeder at the caps (there are only 4 relatively small ones) to drain them? no relays needed. i have a little DC motor that i use as a drain for PSU's when im playing with them. and this motor takes about 1/10 of a second to drain the rails completely. just a tiny tap with the wires and they are down to 1-3v. so, wouldnt a resistor accomplish this?

OR, do i have a problem with my preamp? shouldnt it drain quicker? hmm.....
 
Blues said:
R=t/C; where: R=resistance in ohms, t=discharge time in seconds, C=capacitance in farad...increase resistor wattage if it smokes:)

oh ok. hum. so, if i have a 10,000uf cap, and i want to discharge it in 1 second, i need a 100ohm resistor. i wonder if that will smoke...

and i just connect it across the + and - terminals on the cap right?
 
A bleeder resistor robs current that could otherwise be used by the circuit. Unless you choose to switch it in and out with a relay or an unused pole on the power switch, it's going to be there all the time; a nasty little leech sucking power out of your circuit.
Keep in mind that the circuits you're comparing this to have pathetic, puny power supplies that collapse quickly because they were anemic to begin with. You're trying to provide your father with a perfect circuit...by making it less perfect. If you're going to saddle the poor power supply with a bleeder, a far more elegant solution would be to simply remove part of the capacitance that's in there. The power supply will collapse more quickly and you can put the caps on the shelf to use in another project later on. I'm not sure I see the logic in putting together a beefy, high end power supply, only to hobble it down to the level of a mid-fi unit. Most, if not all, high end circuits run on for varying periods of time after they're turned off. The ones that don't invariably use relays to drop the outputs. A good power supply is the mark of a good piece of gear.
A weak argument can be made that a bleeder resistor serves as a crude regulator of sorts, but there are far better ways to accomplish the same thing. Using one just to drop a power supply after the circuit it turned off is...oh well, it's your/your father's circuit. Do as you wish.
Sigh.
I'll head on to bed and leave you folks alone...g'night.

Grey
 
GRollins,

you just convinced me NOT to use bleeder resistors.

after thinking about it, i will do a small xformer with 12v secondaries that will power a couple relays. they will be connected in between the PSU and the preamp. when power is applied, the relays will latch and put power to the preamp. when power is turned off, they will unlatch and disconnect the preamp from power. BUT, the BOSOZ does have 1000uf caps at its power inputs, so hopefully that wont operate it for long. ill just have to see.

Blues,

when i build something, i dont like compromises. i want a complete anyone to be able to use it. i dont want someone to say "how come when i turn it off, it doesnt stop playing music". in my mind, thats just a poor design.

to all:

here is something that is bothering me though... is my preamp working adequately. im re-reading the spec sheets from passdiy.com and it appears i should be getting a 10W draw from each channel. so, 10W being delivered by 4000uf of capacitance (two 1000uf caps on PSU and two 1000uf caps on the preamp itself) . seems like its not drawing nearly enough current... i guess ill have to measure that myself. from the specs it looks like it draws 80mA for each FET, so 0.16A for each channel. wow, i guess an 800va transformer really is WAY overkill :) i didnt realize that.