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Old 10th January 2013, 01:38 AM   #2661
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I will do a check every six months or so for peace of mind.
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Old 10th January 2013, 02:00 AM   #2662
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Quote:
Originally Posted by Puffin View Post
UPDATE :

Got it down to 0 and attached speakers. Offset seems to drift, but nothing more than 50mv.

Is it a matter of continually adjusting or where is it best advised to leave it?
50mV is not a problem, I would set it so that it is closest to zero when fully warmed up.
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Old 10th January 2013, 05:33 AM   #2663
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Quote:
Originally Posted by chris ma View Post
I will do a check every six months or so for peace of mind.
For my peace of mind in my fully DC linked designs I have resorted to a DC sensing circuit that switches off the amp if the averaged DC at the o/p reaches about 1.4V which - even with 4 ohm speakers - will only generate about 1/2W worst case

Last edited by mikelm; 10th January 2013 at 05:53 AM.
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Old 10th January 2013, 07:32 AM   #2664
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Quote:
Originally Posted by mikelm View Post
50mV is not a problem, I would set it so that it is closest to zero when fully warmed up.
Thanks.
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Old 10th January 2013, 07:59 AM   #2665
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Quote:
Originally Posted by mikelm View Post
For my peace of mind in my fully DC linked designs I have resorted to a DC sensing circuit that switches off the amp if the averaged DC at the o/p reaches about 1.4V which - even with 4 ohm speakers - will only generate about 1/2W worst case
On Rod Elliots site you'll find a design that switches of the speakers in case of a DC offset. It prevents turn-on noises as well.

Such a thing protects your speakers when one of the power supply rails die or one of the output transistors or ...

Saved my speakers more than once.

MArco
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Old 10th January 2013, 12:07 PM   #2666
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Quote:
Originally Posted by deduikertjes View Post
On Rod Elliots site you'll find a design that switches of the speakers in case of a DC offset. It prevents turn-on noises as well.
Yeah - saw that one but in the end I designed one that didn't include switched outputs - because we expect / hope that this will be a very rare event that this circuit will actually switch I elected to simply have the amp turn off which means the relay can switch the mains - a less critical connection in terms of impacting audio quality.

Last edited by mikelm; 10th January 2013 at 12:11 PM.
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Old 10th January 2013, 02:55 PM   #2667
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I intend to fit a speaker protection/turn on delay circuit.
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Old 10th January 2013, 08:17 PM   #2668
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The trick is to find a relay which doesn't degrade the sound to much when it is connected into the signal path. I tried a Velleman kit with my JLH some years back but it added a hard, metallic quality to the sound. I eventually settled on a circuit which shuts the amp down by reducing the power supply rails to almost nothing via small relays connected in the voltage regulator circuit of the power supply. It works like a dream and doesn't degrade the sound.

Tim.
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Old 10th January 2013, 08:39 PM   #2669
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Well as a speaker protection i use a capacitor at the output A sandwich of capacitors actually. And use a soft start circuit for my transformer. No bump at all at turn on.
An amplifier i had before i could hear a squeeking sound for a short moment. Its was maybe a capacitors charging up
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Old 10th January 2013, 09:37 PM   #2670
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My JLH 96 has no capacitor, so I fitted fuses, assuming it would be a stop gap, and haven't gone back.
After a good year of occasional use, they haven't blown, and I doubt they are really impairing the sound (best amp I've owned!). When warm, the offset is typically around 20mV I guess (they've varied over time as I've tweaked the bias current).
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