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PostPosted: 14 Dec 2008, 16:15 
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Some of you may not have been aware of this but I have been fighting with some humm/buzz in my amps for longer than any person should have to deal with. I dont like it one bit.
I bought a new DMM from RS a few days ago. It has a Hz function. Nice. Autoranging to :) Extra special. RS232 port. Great if you have one of those on your computer. Anyhow I heard a buzz and immediately blamed the amp. Then, remembering my buzz fiascos of late I blamed my potentiometer. It has been in all the fiascos so it must be the culprit. I paused the music and put my ear to the speaker. Dam n there it is. Buzz. So I put the DMM on it. .4V@200ma of DC. Hmph. Lets try this neat Hz function. 60Hz. Well, hello... So I took it to the output of the amp. The same. Then I tried the output from CDP. The Same. Then I unplugged the CDP from the amp and measured the output of the CDP. Wait I dont even have to measure, when I get the DMM over top of the CDP it measures 60Hz. Move it away and 0 Hz over top and 60Hz. I guess that explains it :)
Ri

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PostPosted: 14 Dec 2008, 19:17 
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Isn't the "buzz" so annoying! it's so hard to find out were it is coming from sometimes, like my viking amp has a small amount of buzz in the left channel, I think it has to do with the input to the power amp as the buzz does not get louder when I increase the volume control, but it's still really annoying! but because it's not very loud I decided to leave it alone for now.

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PostPosted: 14 Dec 2008, 20:07 
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Hey, great stuff, you found the hum problem! It is both sides of the CDP? Glad to hear you are happy with the MyRefC.
Cheers

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PostPosted: 14 Dec 2008, 20:18 
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So what do you mean by both sides of the CDP? Its at both RCA sockets which makes me think any pre I build should be balanced.
Thing is I dont get humm on my Harmon Kardon from the CDP. Maybe there is something in the HK that gets rid of it.
I have about 260mV of DC from the wall into the transformer and coming back out the other side. The speakers end up getting about 400mv at .2Amp of DC.
I cant imagine how great it would be to have a scope. Just this little DMM is making a few things clearer.
Serious about the balanced Pre. Have you built one Gio? I dont know much about them and just read a guy saying you need to build your amp for balanced. Is this a fact?
Ri

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PostPosted: 14 Dec 2008, 20:38 
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Hi Ri, yes, I meant both the left and right output of the CDP. My guess is that the output DC blocking caps are no good, but it seems odd that both of them are not working (perhaps someone removed them). It is very simple to find them. Open up the CDP and follow the trace back from the RCA output. It will be something like a 22uF cap. Your Harmon Kardon will have a DC blocking cap at the input which is why it does not have the problem.

What do you mean by balanced? When I hear that, I think recording equipment and XLR connections.

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PostPosted: 14 Dec 2008, 20:48 
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Well, I can only guess that its both sides because my DMM registers 60Hz before it even touches the CDP. :shock:
When I said balanced I think that also means XLR. So here is what I mean. Two signals going into the Pre. One for each channel. The pre compares them and removes anything that is exactly the same between the two channels.
Ri

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PostPosted: 14 Dec 2008, 22:11 
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Ok, I have never built anything that does that automatically. A couple of mono pots is probably as close as I will get to that. ;)

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PostPosted: 14 Dec 2008, 23:25 
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Well, maybe we just have to learn more about it :)
Ri

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PostPosted: 18 Dec 2008, 08:09 
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I believe NP has balanced Preamp on the Pass DIY site, possibly some efficacious reading there on the subject too...

There were some posts on using Two B-1's to form a balanced preamp set-up in the big thread on DIY Audio too.

John


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PostPosted: 18 Dec 2008, 11:45 
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I will have to read up on balancing the B1. I saw his balanced Bride of Zen or whatever it was called, but it looks huge. He is using huge transistors and I just figured its probably a classs A preamp which is really more than is needed.
Ri

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