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Old 21st June 2007, 04:28 PM   #1
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Default Ferrite bed od the wires who goes to output transistors ?

Hello

Sometime we don't have choice to have a bit of long wires
going to the output transistors.

Most of the time having a bit to long wires going to the output transistors made the amp oscillate. But adding more capacitors can lower sound quality.

How about puting small ferrite bed on the wires who goes to output transistors to stop the oscillations ?

Thank

Gaetan
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Old 21st June 2007, 04:54 PM   #2
anatech is offline anatech  Canada
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Hi Gaetan,
Since you are already doing something wrong with long wires to the transistors, you then need it to work any way you can. Ferrite beads on the wires would only make things worse. If it wasn't oscillating before, it would after you stuck the beads on.

It's the inductance of the wire that causes the oscillation to begin with. The impedance of your supplies and base circuit go up at RF frequencies where you still have gain. This turns into positive feedback and the transistor "takes off" and oscillates. That is why capacitors are used right at the transistor leads.

The only way to avoid using these capacitors is to keep your wiring (or traces) short. Decoupling to ground may still be required.

-Chris
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Old 21st June 2007, 05:48 PM   #3
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Hi Gaetan

The only thing I have done with long wires is to "plat" them. This seems to give at least a good a result as twisting and holds them in place better. The idea is to bring the wires as close as possible so that the overall currents (and magnetic fields) cancel.

This will minimise the inductance of the wires.

cheers
John
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Old 21st June 2007, 06:04 PM   #4
anatech is offline anatech  Canada
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Hi John,
Quote:
The only thing I have done with long wires is to "plat" them.
I haven't heard that term before. Could you please explain what you mean to me?

Thanks,
-Chris
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Old 21st June 2007, 06:16 PM   #5
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hi Anatech

nothing special - same as plait or braid

Actually the result looks like a 3-phase supply...

cheers
John
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Old 21st June 2007, 06:50 PM   #6
anatech is offline anatech  Canada
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Hi John,
Thanks, that now makes sense. I was just unfamiliar with the term you used.

-Chris
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