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Old 18th September 2008, 07:32 PM   #1
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Question Need help troubleshooting preamp trouble

I bought a second hand CJ preamp "in perfect working order" and quess what? That's right, it has problems. Did you know that on ebay, perfect means fouled up?

Here's the deal. It sounds really sweet but when it is switched off a huge pop occurs that puts my Adcom GFA-555 into a tailspin (I think protection is the audio equivalent of a tailspin but I havn't checked wiki yet). This pop happens instantaneously with the power down.

This occurs whether the switch is used or if I just pull the power cord. I suppose a field is collapsing when it is powering down and a cap is not doing it's thing, but I need some schooling before I start troubleshooting. Perhaps a short list of the usual suspects.

I have the schematic and I can read and understand the main gist of it. I also have a cap tester that should arrive today or tomorrow. The schematic is here at DrTube if you care to see it: http://www.drtube.com/audioamp.htm#CJ

Thanks for any insight you can provide.
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Old 18th September 2008, 08:44 PM   #2
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Your CJ most likely does not have a problem. You can not turn off most tube pre amps with and amplifier still power up because you will get a huge thump. The thump is a huge voltage spike at the output of the pre amp. RULE OF THUMP Always turn on pre amp first, let it warm up for few minutes and then turn on amplfier. To shut things down first power down amp, let it discharge for few minutes and then shut pre amp down. If you follow the rule of thump you should have no problems.
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Old 18th September 2008, 08:54 PM   #3
pchw is offline pchw  United States
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You didn't say which model that you have. So, 2 lame answers:
1. Some models don't have the relay to short the output to ground. If that's what you have, it is not a problem of the preamp.
2. For those models that have relays, the relay is defective and sticky and not shorting the output signal to ground. In this case, you can replace it.

I think a better practice is to power on the preamp first and then the power amp, and power off is in reverse sequence - amp first and then preamp. If you do that, you can avoid the unpleasant thump. Notice that the Adcom has a big power supply, you probably want to wait a little longer before powering off the preamp.
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Old 18th September 2008, 10:18 PM   #4
kevinkr is offline kevinkr  United States
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Default Re: Need help troubleshooting preamp trouble

Quote:
Originally posted by Captn Dave
I bought a second hand CJ preamp "in perfect working order" and quess what? That's right, it has problems. Did you know that on ebay, perfect means fouled up?

Here's the deal. It sounds really sweet but when it is switched off a huge pop occurs that puts my Adcom GFA-555 into a tailspin (I think protection is the audio equivalent of a tailspin but I havn't checked wiki yet). This pop happens instantaneously with the power down.

This occurs whether the switch is used or if I just pull the power cord. I suppose a field is collapsing when it is powering down and a cap is not doing it's thing, but I need some schooling before I start troubleshooting. Perhaps a short list of the usual suspects.

I have the schematic and I can read and understand the main gist of it. I also have a cap tester that should arrive today or tomorrow. The schematic is here at DrTube if you care to see it: http://www.drtube.com/audioamp.htm#CJ

Thanks for any insight you can provide.

You didn't mention which model you actually purchased so it is impossible to make the determination as to whether the pre-amplifier you bought actually has a muting relay or not.

In general even with a muting relay the turn on/turn off sequence recommended in previous posts is the right thing to do to avoid this issue. And it is always a much bigger issue with a solid state amplifier as opposed to a tube amp driven by a tube pre-amplifier.

I agree there is probably nothing wrong with your CJ pre-amp.
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Old 18th September 2008, 10:35 PM   #5
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You're correct, I didn't. It's a PV10-AL.

I didn't know about the rule of thump. Now I do. I think I still have a problem, because as you mention, the Adcom has a very potent power section and takes about a minute to power down. It will be difficult to convince my lovely wife of the importance need to be so patient. Her patience is scaled in porportion to her height. I'm thinking about the relay idea.

Right now, they are all connected to this cool Digital Equipment Company power conditioner built for mainframes. It powers everyting off at once. Perhaps a 60 second timer to automate one bank of the outlets to keep the PV-10 powered up for an extra minute??? Humm, this could get pretty geeky.

Thanks to both posters for the good info. This is a great forum.
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Old 18th September 2008, 10:43 PM   #6
kevinkr is offline kevinkr  United States
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Mute relay on PV10 only really functions during power up. The timer chip will not close the mute relay contacts at power down until it is way too late.

I think you need a different power sequencer that turns off the power amplifier at least a minute or so before the rest of the system.

Tell your wife to be patient until you get the issue sorted out as both speaker and amplifier damage can occur under these conditions.
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Old 19th September 2008, 01:58 PM   #7
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Default OK, but why?

Yeah Kevin, I see. If the mute relay to work upon power-down, a timer would be needed to keep the power up for a few milliseconds until the relay had time to open.

Still, I remain curious as to what causes the thump. By that I mean, which component is storing the charge that lets go upon power down. I'd like to understand the mechanism.
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Old 19th September 2008, 02:18 PM   #8
kevinkr is offline kevinkr  United States
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Default Re: OK, but why?

Quote:
Originally posted by Captn Dave
Yeah Kevin, I see. If the mute relay to work upon power-down, a timer would be needed to keep the power up for a few milliseconds until the relay had time to open.

Still, I remain curious as to what causes the thump. By that I mean, which component is storing the charge that lets go upon power down. I'd like to understand the mechanism.
Actually it is the reverse, the relay needs to have the power removed immediately as it is an N.C. (normally closed) type. The relay does not close immediately when the power is removed. When energized it is open.

There are lots of things that store energy in the pre-amplifier and when you remove the power a large transient is generated as the field in the power transformer collapses, the output coupling caps are charged to whatever voltage was present at the CF outputs and as soon as you remove the power they start to discharge delivering current to the output circuitry. The supplies are collapsing, the operating points of all of the active stages are shifting as cathode emission falls, static operating conditions no longer apply. There are lots of other mechanisms, beyond these, you could spend months analyzing and measuring what is going on and not understand it all - and if you can do that then you can very easily modify it to fix the problem.

One thing you could do pretty easily would be to modify the timer circuitry and its power supply so that the coil is quickly de-energized at power off, you could for example use a small ac powered relay that opens immediately when the power goes off and put its contacts in series with the mute relay coil.. Not elegant but it might work if the mute relay closed before circuit disturbances generate the output transient you are hearing.. Worst case it should reduce it considerably if not eliminate it.
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