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Old 20th June 2012, 12:06 PM   #1
Cassiel is offline Cassiel  Libya
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Default Troubleshooting BIG hum problem. CJ CAV-50

Hi folks, the problem is simple: big hum when the pre out is connected to another amp. This is what I have checked so far:

1. The preamp circuit. Everything reads OK.
2. The tube. Changed, same noise.
3. Plugged in my headphones. No noise at all.

So it looks to be a grounding problem, right? The wires carrying the signal to the RCAs are not shielded but the ground wire is loosely twisted and only soldered to one side. You can see that in the picture - the red and black wires coming from the center of the PCB. If any of you have any idea of what I might still check please say, otherwise I really don't know what to do next. I'm no tech just a tinkerer, you know.
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Old 20th June 2012, 12:30 PM   #2
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Hi!

The most likely reason is a ground loop. If both the preamp and poweramp have a connection between signal ground and safety earth, you have a loop: signal ground is connected between preamp and poweramp via two paths: through the interconnect cable and through safety ground. Lift the connection between safety earth and ground in one of them, then the problem should be gone. If you want to operate the one in which you lift the ground in another system, you might need to reconnect.

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Old 20th June 2012, 08:38 PM   #3
Cassiel is offline Cassiel  Libya
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Thanks for the input Vinylsavor. Hmmm...I think I'm not going to be able to fix it.
First of all, to work with this amp is a PITA, the chassis is connected to ground in three places (huh?), maybe more, but three places so far. Then all the inputs and outputs are grounded in the same PCB, from there the ground goes to the volume pot and from there, to circuit ground. I have disconnected safety ground and the hum is as loud as before. Another thing, this amp has an EPL out which doesn't hum. It shares the same ground as the pre out which hums a LOT. Bah, I'm clueless. One thing I can tell though, the ground arrangement in this amp sucks.
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Old 20th June 2012, 09:10 PM   #4
DF96 is offline DF96  England
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PSU problem? The thing which is different between Pre Out and the other outputs is that this one actually goes through the valve so HT ripple appears at the output too.
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Old 20th June 2012, 09:35 PM   #5
Cassiel is offline Cassiel  Libya
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Yes, DF96, you can hear the hum with very sensitive speakers. This amp has a noisy PS, maybe because they went the audiophile way and avoided the use of electrolytic caps in the power supply. As a result it has very little capacitance, but it has a regulator. Anyways, I can't hear any hum with the headphones connected to the pre out so it must be a ground loop. I can't think of any other reason. Since I'm not going to fix the ground layout of this amp (too much work) it looks like the pre out is going to stay as an useless feature.

Still, I can't believe this problem is 'normal', I have connected it to several amps in different places and always...terrible, terrible noise. CJs specs say: Noise (PRE OUT): 94 dB below 2.5V Ha!
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Old 20th June 2012, 09:56 PM   #6
DF96 is offline DF96  England
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Puzzling! A grounding problem would be expected to affect all outputs. A PSU ripple problem would be expected to do the same thing for headphones or an external amp. However, headphones can only be grounded via the cable, whereas an external amp can find its own ground via some other route. Maybe it is the combination of poor grounding practice and poor PSU design - then maybe a ground connection has come loose somewhere.
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Old 21st June 2012, 01:13 AM   #7
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When you say that you've plugged in your headphones, what exactly does this mean? Who knows? might matter.

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Old 21st June 2012, 01:38 AM   #8
Cassiel is offline Cassiel  Libya
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Quote:
Puzzling!
Yes, that's why I want to find out. These strange problems are addictive.

Quote:
When you say that you've plugged in your headphones, what exactly does this mean?
It means I plugged my headphones with an adapter into the RCA pre out. Unfortunately, no noise. Makes troubleshooting much harder. I'm not annoyed yet but soon will be if I continue thinking about this. Not worth it really.
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Old 21st June 2012, 01:46 AM   #9
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Quote:
Originally Posted by Cassiel View Post
It means I plugged my headphones with an adapter into the RCA pre out. Unfortunately, no noise. Makes troubleshooting much harder. I'm not annoyed yet but soon will be if I continue thinking about this. Not worth it really.
Cool. Headphones vary in sensitivity all over the map, and some like my 600 Ohm Beyer's, or Sennheiser HD414's, etc. are insensitive enough to fool me all the time.
Without knowing exactly how loud yours play with an ordinary signal and hooked up the same way, it's hard to comment fersure, fersure (as the Valley Girl said about her contraceptives). Might be a red herring.

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Old 21st June 2012, 02:08 AM   #10
Cassiel is offline Cassiel  Libya
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Just to clear up doubts, my headphones sensitivity is 98 dB SPL/mW, impedance is 600 Ohms. And the hum I hear when the pre out goes to the amp is not small in nature. Even if the mismatch in impedance eats up some noise, I should still be able to hear some of it. Case closed really. Tomorrow I will wire it to triode, which is what I intended to do in the first place, and that's it. Thanks to those who tried to help.
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