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Preamp with humming noise!
Preamp with humming noise!
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Old 18th May 2011, 10:51 AM   #1
Matthewong is offline Matthewong  Malaysia
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Default Preamp with humming noise!

Hi, I have an old tube preamp which my friend sold to me. The problem with this preamp is whenever I plug in the CD player output to the preamp input, the humming sound start to scream. When disconnect the RCA from CD player, the humming sound will stop immediately, no matter whether the CD player is on or off. I did try other sources, like cassette player, DVD player... all also the same. What is the problem for the preamp and how to solve this problem????
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Old 18th May 2011, 11:07 AM   #2
rsumperl is offline rsumperl  United States
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Preamp with humming noise!
Most CD players have a high signal level output (~2v) that is probably overloading the pre-amp. Most pre-amps were used as additional amplification for magnetic phono cartridges. My CD player as well as my Sirius satellite radio have output levels of ~2v which drive Pete's red board to full volume.

A great pleasure in life is doing what people say can't be done.
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Old 18th May 2011, 12:33 PM   #3
Chris Daly is offline Chris Daly  Australia
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Use a L Pad divider ie 4 resistors - 2 for each channel to reduce the CD output to an acceptable level .

Cheers / Chris
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Old 18th May 2011, 01:38 PM   #4
chrish is offline chrish  Australia
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I don't think signal level is the problem. Matthewong stated that the humming is loud when the CD player is plugged in "if it is on or off". To me, this sounds like a ground loop problem. Does any other line-level signal source cause this same problem? Maybe you have an iPod or similar non-mains powered device you can plug in to help see if it is a ground loop issue.


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Old 18th May 2011, 02:10 PM   #5
Andy5112405 is offline Andy5112405  United Kingdom
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A lot of older equipment had the input RCA connectors connected to GND. This, as CHRIS says, can cause GROUND LOOP problems. You could try disconnecting the input sockets from the chassis to see what happens.
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Old 20th May 2011, 11:17 AM   #6
Matthewong is offline Matthewong  Malaysia
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I have checked all the RCA input and output, found one of the RCA touched the body, so changed to new RCA socket and the humming noise reduced a lot. Now the humming noise is low, but still audible when my ear near the speaker. When I turn my volume to max, then the humming noise is loud, would like to know how can I totally kill the humming noise which is quite anoying. One of my friend told me that tube preamp will surely have some humming noise especially when turning the volume past 12'o clock mark, is that true????
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