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Old 20th March 2004, 05:25 PM   #1
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Question Tube preamp huge DC thump!

Hello,
I'm trying to fix a friend's tube preamp, a AMC model CVT1030a.
The problem is that, at turn on and off, if outputs a huge DC thump, you can see the woofer going full run on both sides (in and out).
I learnt how dangerous can it be testing it with a test speaker.. the woofer got fried after 2 times I switched it....
Since I'm new to tubes I have to ask your help...
I was able to find a schematic of the CVT1030, that matches quite well.
I suspect the capacitor C209, labelled as NP 22uF / 64V, that in this amp has a different value (220 uF / 16V) and is polarized.

Can anybody help me?


Cheers

Andrea
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Old 20th March 2004, 06:10 PM   #2
adamamp is offline adamamp  Canada
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Just guessing but is it possible that the regulated SS power supply induces this effect? I have added B+ regulated power supplies to some classic HiFi, HH Scott, Trio and Eico. In each case there was a need for a slow charge(adding low ohm resistance to AC primary, more on that later if required) to avoid disturbing turn on interaction. I love the sonic effect of regulating a tube amp but using tubes and SS in conjuction can have its problems mostly related to power up. There could be another prob with the amp that you have though. Simple fix, turn power amp on after preamp and turn it off before preamp, a good practice anyway. IMO, user results may vary.
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Old 20th March 2004, 06:30 PM   #3
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you can avoid this just by turn on/off sequence:

turn on: source -> preamp -> amp

turn off: amp -> preamp -> source
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Old 20th March 2004, 10:15 PM   #4
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well, besides that I don't see why the designer should use a 220uF cap here, or even a 22uF, where a 2.2uF would do very well, I suspect he saw the problem and tried to fix it with the 2 zener diodes used as a clamp.

First I would suggest is the switch on/off cycle described by Pedroskova, than toggle the switch from N to M or D, and replace the mentioned cap by a 2.2uF cap, preferable a film type rather then electrolytic. This last action will at least reduce the amount of the thump.

Dick.
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Old 20th March 2004, 11:24 PM   #5
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Hi,

Quote:
well, besides that I don't see why the designer should use a 220uF cap here, or even a 22uF, where a 2.2uF would do very well,
Seems to me they were offered every possible cap that had two 2s in its name at a bargain price...

2.2µF into 1M? They're nuts and it's bad design practice to boot.

Cheers,
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Old 21st March 2004, 06:23 AM   #6
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Not sure if i understand. There seems to be a muting function in order to prevent thumps. Is it still working? Is it manual or automatic?
The 16v rating appears to be incredibly low, even the original cap seems underrated. Is it possible the output caps have developed a leak? I'd immediately replace them with 2.2-4.7uF/250v films.

And i just noticed there is bipolar supply. Not sure what happens if one of the polarities ramps up first. A delayed mute seems a must.
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Old 21st March 2004, 08:29 AM   #7
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Quote:
Originally posted by fdegrove
Hi,



Seems to me they were offered every possible cap that had two 2s in its name at a bargain price...

2.2µF into 1M? They're nuts and it's bad design practice to boot.

Cheers,
Can you elaborate a bit.. not sure I understand


Quote:
Originally posted by analog_sa
Not sure if i understand. There seems to be a muting function in order to prevent thumps. Is it still working? Is it manual or automatic?
The 16v rating appears to be incredibly low, even the original cap seems underrated. Is it possible the output caps have developed a leak? I'd immediately replace them with 2.2-4.7uF/250v films.

And i just noticed there is bipolar supply. Not sure what happens if one of the polarities ramps up first. A delayed mute seems a must.
The only "muting" function available is to switch manually to the Phono function (the other 2 positions of this switch are "direct" and "normal", that enables control tones).. but I can't believe they can sell a preamp that can damage the speakers so easily.
The fact that this problem is getting worse with time seems to me a project's fault that slowly damages a component (maybe this cap?)

Cheers Andrea

PS I put on my net disk (www button) a scan of the service manual. Being in Tif format you need to download it to view.
If resolution is not enough let me know and I'll send you a higher-res pic.
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Old 21st March 2004, 10:34 AM   #8
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At the risk of sounding harsh why not just turn the darn power amp off first? No more thump and no more lost sleep.

Joe
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Old 21st March 2004, 10:54 AM   #9
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Whoops!

Sorry pedroskova didn't see your post. You are 100% correct.

Why is it that I suspect the usage of a solid state amp here?

Why not install a switch to short the output to ground until the amplifier is up and running? Maybe if you want to get fancier a timed relay circuit to mute the output for an acceptable time period upon turn on and an immediate drop out on stut down.

Joe
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Old 21st March 2004, 12:39 PM   #10
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Quote:
Originally posted by burnedfingers
Whoops!

Sorry pedroskova didn't see your post. You are 100% correct.

Why is it that I suspect the usage of a solid state amp here?

Why not install a switch to short the output to ground until the amplifier is up and running? Maybe if you want to get fancier a timed relay circuit to mute the output for an acceptable time period upon turn on and an immediate drop out on stut down.

Joe
The switch is already present, if I set it to "phono" the thump doesn't occour.
But since this problem is getting worse (and at the beginning it didn't even happen) I suspect something's going to fail soon, so it's not only a switching sequence problem but something different.

Cheers

Andrea
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