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amp whistles when I move speaker cable

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4/24/2003 5:01 AM
JR	amp whistles when I move speaker cable I have my amp running into a THD Hot Plate attenuator; when I move the speaker cable (or squeeze the insulation) going from the amp to the Hot Plate, I get a loud whistling through the amp. Why??!! JR

4/24/2003 8:53 PM
MBSetzer	maybe you are using a guitar cord instead of a speaker cord Mike

4/24/2003 10:18 PM
JR	Nope, it's a nice big honkin 12 guage speaker cable!

4/25/2003 9:00 AM
Dave H	Is one side of the OT secondary grounded? You can sometimes get oscillation when the winding is floating. Dave

4/25/2003 3:41 PM
JR	Should be but I'll check - everything is star grounded to the chasis but the chasis is aluminum so maybe there isn't such a great connection? Thanks for the suggestion! JR

4/25/2003 10:42 PM

JR,
What tune does it whistle ? This may be a clue as to what is wrong. (harrr! sorry, couldn't resist).

One thing that comes to mind is if you are holding the cable in your hand when the amp whistles, you are changing the characteristics (ie inductance) of the speaker cable while holding it. This means you are changing the dynamic loading of the amp, perhaps affecting the feedback loop (assuming there is one). If this is the case, try figure out why. Just a guess on my part, but it seems the amp should be more stable than that. Are you using too much negative feedback ? Anything weird about the speaker load ?

Have you tried other cables?

HTH,
Jack

4/25/2003 11:03 PM
JR	Hmmm.. well it's the cable that goes from the amp to the Hot Plate - the cable from the Hot Plate to the speaker is fine so maybe the attenuator has something to do with it?? The feedback loop is pretty normal, a 47k tied to the 4 ohm jack. I'll try another cable too. Thanks! JR

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