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Old 1st October 2012, 08:25 PM   #11
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So what are we saying here. The resistance if the DMM in this case gives me a false reading. There is no DC voltage in reality?
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Old 1st October 2012, 08:31 PM   #12
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There can only stand DC over the coupling cap when the coupling cap leaks (what might be the case) or, when the connected apparatus has DC on its input connection.
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Old 1st October 2012, 08:35 PM   #13
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Quote:
Originally Posted by chokesrule View Post
So what are we saying here. The resistance if the DMM in this case gives me a false reading. There is no DC voltage in reality?
No, your reading is correct. See the post by 'disco'. Unless the 'low' side of the capacitor is referenced to ground in any way, you would see a DC voltage on it.

Putting your meter probe on the plate, you connect it to ground via the high internal resistance of the meter, then the voltage slowly disappears, depending on the resistance of the meter.

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Old 1st October 2012, 08:36 PM   #14
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Can you explain this then. I see what you are saying about the current being blocked. But the first time I ever tried valve rectification. I accidently wired the first 1uf capacitor in series . So when I switched on with the cd player running I would hear sound which gradually faded . Surely when there was sound, there was current flowing through a capacitor .Or was the sound I heard there infact powered by the "following" capacitors which had charged just enough at switch on in the instant it took the series cap to charge?
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Old 1st October 2012, 08:36 PM   #15
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Quote:
Originally Posted by disco View Post
or, when the connected apparatus has DC on its input connection.
If I understood the original post right, there was nothing connected except a voltage probe.
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Old 1st October 2012, 08:45 PM   #16
disco is offline disco  Netherlands
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Quote:
Originally Posted by Rundmaus View Post
If I understood the original post right, there was nothing connected except a voltage probe.
Theoretically speaking a connected apparatus could have caused DC, but we can rule that one out then.

TS: Only AC can be passed by a capacitor. DC is passed through a leaky capacitor, which should be replaced immediately.
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Old 1st October 2012, 08:46 PM   #17
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Originally Posted by chokesrule View Post
But the first time I ever tried valve rectification. I accidently wired the first 1uf capacitor in series [...] Surely when there was sound, there was current flowing through a capacitor. Or was the sound I heard there infact powered by the "following" capacitors which had charged just enough at switch on in the instant it took the series cap to charge?
AC current can flow 'through' a capacitor. In the simplest case of half-wave rectification, the voltage behind the rectifier consists of a series of pulses with the same polarity.

You can describe that as a 'mixture' of a DC part (the average/mean value of the voltage) and a superimposed AC voltage. This is why you're applying filtering, to remove the AC part and have pure DC for your circuit.

What you observed could be the current pulse from the charging of the series cap as you said. Also, the AC part that passed the series cap might 'power' your amplifier, although this should not make any sound besides 100Hz noise.
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Old 1st October 2012, 09:05 PM   #18
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cheers all
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Old 1st October 2012, 09:14 PM   #19
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Originally Posted by chokesrule View Post
cheers all
... and time to go to sleep, its nearly midnight here...

Floating circuit elements (like the open end of your capacitor) are always a pain in the ***, it is usually difficult to predict at what potential they will end up. There may be leak currents, small but present, especially if we talk about electrolytic caps.

There is more strange stuff like dielectric absorption causing a discharged electrolytic to 'recharge' if it is left open after discharging...

Usually, all parts of a circuit should have a defined DC operating point, if the output of the lampizator is designed as you described, it relies on the equipment following in the signal path to set DC conditions of its output.

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