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Old 16th May 2009, 04:39 PM   #1
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Default Crackling Noise in Tube Amp (only) & PC Sound Card

Hi, tube folks,

I have a strange problem that seem to be beyond my audio skills. I'm use a PC (Intel Quad-Core, Gigabyte P45DS-5 ultra-durable mainboard, ESI Juli semi-professional sound card, in short, all highest quality components), and a couple of amplifiers, solid-state Sansui G4700 and vacuum tube Pioneer SM-83 (fully restored).

Problem - very high level of audible wide band crackling noise (measured s/n ratio is about 62 - 65 dB) when PC is being connected to the tube amplifier.

However, when I connect to the Juli solid-state Sansui G4700 (with the same cables, speakers, even located in the same place) - almost dead silence.

If I re-connect Pioneer SM-83 from the PC/Juli to the iPod (with the same cable) - no noise, everything is OK (PC, monitor and table lamp were left turned on to check if the noise appear because of poor shielding problems within SM-83).

Unfortunately, I have no other amp right now to test with the PC/Juli. But I have 2 other sound cards - Intel integrated on mainboard and PCI SoundBlaster Live 5.1. Crackling noise is present with them, too, so it is not fault of ESI Juli.

Checked Sansui G5700 schematic (very similar to my 4700), no signs of
any filters which could suppress this wide-band crackling noise (I do not think it is possible at all to eliminate it using RC filters). Frequency response - 10 - 70,000 Hz.

Input impedance of Sansui is 47k, of Pioneer SM-83 470k. I have
shunted Pioneer's RCA input with 68k resistor to lower input impedance, nothing changes.

Connected Juli straight to the power amp, bypassing 12AX7 pre-amp -
the same noise.

Rebooted from Linux into the Windows on this PC - noise have not gone.

As the resume - only combination of PC/Juli (or any other sound card) and tube amplifier produces this crackling noise. When working apart, no problems at all.

Anyone can suggest where might be the problem?

Thanks in advance for any help.
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Old 16th May 2009, 07:52 PM   #2
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sounds like you might have a grounding problem.

Try just touching the ground (outside shield) on the input rca's to the tube amp. If you get the hum without the signal, then it's a ground issue. You could make sure the two devices are plugged into the same outlet. Also make sure that the tube amp is grounded properly. Perhaps the input gnds aren't grounded to the wall? It should be easy enough to test this by taking a wire and touching it to the outlet ground and the input jack - assuming this is the right topology for your amp of course...

If you don't get the buzz until you attach the signal - then I'm stumped.
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Old 16th May 2009, 08:01 PM   #3
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Bum soundcard? Dirty connections?

Maybe pull the soundcard, clean the connections and reinsert.

Cheers!
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Old 17th May 2009, 05:44 PM   #4
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In, short problem solved !

1) Cleaned twice with deoxid sockets, switches, tubes connectors - no change.

2) I've got rid of crackling noise by suggestion of 2 smart people from AK and DIYtube. Safety grounding tap of PC switching power supply need to be
disconnected in order to break grounding loop.

Thanks a lot for everyone who tried to help ;-)
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