DC offset on turnoff

dm2a · 13th September 2007, 08:46 PM

hello everybody,

When I turn off my Luxman L-3, after 5 seconds the woofers get " sucked in ".

DC offset is 0mV on both channels but on turnoff it goes up to 12Volts on both channels. I 've been searching about the subject but the only solution I can find is placing a relay or putting a 1µF cap on the powerswitch. Shouldn't it be possible to solve this whitout changing the amp's design ? I can't imagine the Luxman was meant to suck the woofers in on turnoff.

I've been working on this amp(new transistors and a few caps) so maybe I did something wrong.
I saw the power supply caps discharge at different speed, one in 5 seconds the other in 2 minutes. Is this normal ?

thanks in advance

richie00boy · 13th September 2007, 08:49 PM

A cap on the power switch will do nothing for this problem. It is inherent in the amp design and is due to what happens when the rail voltage falls below that required to maintain the current sources inside the amp.

A relay is the only way you can avoid it.

Netlist · 13th September 2007, 09:10 PM

I currently have an L-3 on the bench who behaves exactly as you describe except for the PSU caps.
Check the bleeder resistors across both caps.
Apart from that, 12V is present on both outputs after turn off (5sec.) and slowly decreases to 0 within about 15 seconds.

/Hugo

dm2a · 13th September 2007, 09:28 PM

Good to hear other L-3's are doing the same thing. Does it to the same thing to the woofers also ?
I'm using it with cheap speakers now but it doesn't look healthy, I'm afraid to use the amp on good speakers. Can 12Volts do any damage ? If it does I think I'll go looking for a relay.

dm2a · 13th September 2007, 09:28 PM

Forgot to say I measured the bleeder resistors they're OK.

Netlist · 13th September 2007, 09:33 PM

Woofers are moving backwards too but only very little. This depends on the type of woofer.
I wouldn’t consider it as dangerous but if you want to be sure, a relay is, as has been said, the only solution.
As for the caps, it wouldn't hurt the amp to replace them.

/Hugo

dm2a · 13th September 2007, 09:38 PM

I'll do that. Thanks for the advice.

Goodevening.

13th September 2007, 08:49 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	A cap on the power switch will do nothing for this problem. It is inherent in the amp design and is due to what happens when the rail voltage falls below that required to maintain the current sources inside the amp. A relay is the only way you can avoid it. __________________ And yes, there are capacitors in the circuit. One is even employed to form the dreaded bootstrap on the voltage gain stage. Get over it. -Burning Amplifier #2, Nelson Pass www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

13th September 2007, 09:10 PM	#3
Netlist diyAudio Moderator Join Date: Jan 2003 Location: Belgium	I currently have an L-3 on the bench who behaves exactly as you describe except for the PSU caps. Check the bleeder resistors across both caps. Apart from that, 12V is present on both outputs after turn off (5sec.) and slowly decreases to 0 within about 15 seconds. /Hugo __________________ Toothpaste is bad for the teeth; it’s too sweet.

13th September 2007, 09:33 PM	#6
Netlist diyAudio Moderator Join Date: Jan 2003 Location: Belgium	Woofers are moving backwards too but only very little. This depends on the type of woofer. I wouldn’t consider it as dangerous but if you want to be sure, a relay is, as has been said, the only solution. As for the caps, it wouldn't hurt the amp to replace them. /Hugo __________________ Toothpaste is bad for the teeth; it’s too sweet.

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13th September 2007, 09:28 PM	#4
dm2a diyAudio Member Join Date: May 2007	Good to hear other L-3's are doing the same thing. Does it to the same thing to the woofers also ? I'm using it with cheap speakers now but it doesn't look healthy, I'm afraid to use the amp on good speakers. Can 12Volts do any damage ? If it does I think I'll go looking for a relay.

13th September 2007, 09:38 PM	#7
dm2a diyAudio Member Join Date: May 2007	I'll do that. Thanks for the advice. Goodevening.

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