Safety timer/relay in JLTi/VBIGC not working as hoped - diyAudio
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Old 20th August 2009, 09:39 AM   #1
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Default Safety timer/relay in JLTi/VBIGC not working as hoped

I've got a bit of a problem I'm hoping someone might be able to help me out with.

I recently built a JLTi, or tube-buffered GainClone, according to this design here
Click the image to open in full size.

I then added a relay with a timer circuit, as per Nuuk's instructions, to short the output of the valve while it's warming up so that I don't damage my speakers.

As instructed by a thread I read here somewhere, I wired my relay to short from right after the coupling capacitor (the 3u3 in the diagram above) to the signal ground (I wired everything up according to this diagram here).

Unfortunately though, rather than completely shorting out the DC offset, while the valve's output is shorted as above, both channels of my amp output around -200mV DC, which I figure isn't too good. When running normally, the amp's DC offset is around -60 to -80mV.

Worse still, when I switch the power to my amplifier off, even with the valve's output correctly shorted by the relay, which switches as soon as it loses power, the amp's DC output reaches about 3V as it cools down (could it be that the chips' power capacitors are draining unevenly?).

Can anyone spot what I'm doing wrong? Am I shorting to the wrong place (currently just after the 3u3 capacitor in the diagram above)?

Also, I noticed after I'd built my amp according to the JLTi design above, that the VBIGC design on Nuuk's website looks significantly different:
Click the image to open in full size.
Did I pick the wrong design? What are the relative merits of these two designs?

Thanks heaps!
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Old 20th August 2009, 09:48 AM   #2
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1 M ohmn as feedback is unsuitable. Joes values are better and also to use the voltage divider for the feedback.

Where do you short the tube?
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Old 20th August 2009, 10:06 AM   #3
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Cool, I guess I picked the right design then.

I shorted just after (to the right of) the 3u3 coupling capacitor - shorted to the signal ground.
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Old 20th August 2009, 11:13 AM   #4
Ted205 is offline Ted205  United Kingdom
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why not short the output of the amp ?

i.e after the lm3886

or better yet use the mute pin on the chip
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Old 20th August 2009, 11:17 AM   #5
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It's an LM3875, so no mute pin unfortunately. Is it ok to short the output? Will that not damage anything?

(EDIT: Sorry, for confusion, it is a 3875, not a 1875 as I previously said here. Either way, no mute pin.)

Last edited by gaplessophile; 20th August 2009 at 09:01 PM.
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Old 23rd August 2009, 04:10 AM   #6
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I'm afraid I'm still completely stumped by this.

How are other people going about protecting their speakers in a tube-buffered gainclone?

Is my issue with the 3V DC offset during power-down (even with the tube effectively out of the circuit) unusual?

Thanks heaps for all your help!
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