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Old 16th December 2002, 04:53 PM   #1
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Default Ferrite Beads

Is anybody here using ferrite beads on internal wires in their amplifiers? A search looked like some people were, but not too much mention of them from what I saw.

As I was shopping around, figured I would see what people had had good and bad experiences with.

Also, my experience is more with generators operating at much higher frequencies, but I'm new to DIY audio, so maybe something like this isn't needed at all at these power levels and frequencies.

Anybody with any suggestions or experiences, please share them.

Thanks
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Old 30th December 2002, 09:52 PM   #2
dhaen is offline dhaen  Europe
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Default How many turns?

Even with 4 or 5 turns, the thing is only suitable as a UHF RF choke.
At very low impedance nodes, the reactance becomes significant at low-megahertz.
I can't think of any true audio applications.
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Old 30th December 2002, 10:31 PM   #3
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Default Re: How many turns?

Quote:
Originally posted by dhaen
Even with 4 or 5 turns, the thing is only suitable as a UHF RF choke.
At very low impedance nodes, the reactance becomes significant at low-megahertz.
I can't think of any true audio applications.
Ferrite beads don't have turns. They're simply cores of ferrite that a wire runs through. The purpose is to act as an EMI filter. This same idea would apply, but only if a filter like this was needed. I don't know if its needed or not.
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Old 30th December 2002, 10:48 PM   #4
dhaen is offline dhaen  Europe
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Quote:
Ferrite beads don't have turns
They can. They are used in UHF circuits with very thin wire as RF chokes.
I admit they are more usually used with a single (thick) wire running through them.
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Old 30th December 2002, 11:19 PM   #5
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That would be a ferrite core and is basically a toroid.

I like word games

Quote:
Originally posted by dhaen

They can. They are used in UHF circuits with very thin wire as RF chokes.
I admit they are more usually used with a single (thick) wire running through them.
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Old 30th December 2002, 11:47 PM   #6
dhaen is offline dhaen  Europe
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Quote:
That would be a ferrite core and is basically a toroid.
Absolutely.
But then again, a conventional "bead" is merely a torroid with half a turn

I like word games too

Basically what I was saying was that there isn't a node with low enough impedance for an impact within the audio spectrum.

As far as RFI is concerned, they're of little use at the input, because the impedance is high.
At the output it just might be another matter. They could be good at stopping >1 or 2GHz fom entering the feedback loop from your speaker wires , but as has been pointed out before, Zobel networks normally stop that.

Cheers,
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Old 31st December 2002, 12:02 AM   #7
ted is offline ted
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Actually the Heathkit AA-1640 used ferrite beads on the negative power rail.

http://www.continental.freeserve.co....wer_output.gif
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Old 31st December 2002, 12:18 AM   #8
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Quote:
Originally posted by dhaen

Absolutely.
But then again, a conventional "bead" is merely a torroid with half a turn

I like word games too

Basically what I was saying was that there isn't a node with low enough impedance for an impact within the audio spectrum.

As far as RFI is concerned, they're of little use at the input, because the impedance is high.
At the output it just might be another matter. They could be good at stopping >1 or 2GHz fom entering the feedback loop from your speaker wires , but as has been pointed out before, Zobel networks normally stop that.

Cheers,
Typically, I'm used to using them on wires between driver circuits or current sense circuits and PAs, on the high power wires. Depending on how an amp is built, this could be more than just the output wires. but I guess it would vary depending on the type of amp and construction.

Although you could throw them on the input to reject anything picked up on the interconnect coming from the source, I didn't think that noise would be a problem, like you said as well.

As far as I can tell, you would need a feedback loop without a filter for this already. With no feedback, or with a good filter on the feedback, it wouldn't do much. But that's what I don't now, since I'm new to frequencies this low.
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Old 31st December 2002, 01:21 AM   #9
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Default on ferrites

Ferrite beads or cores are lossy at high frequencies (hundreds
of khz or MHz), they are therefore very good isolation
devices to prevent rf from entering a circuit via power supply
lines, and to prevent rf from the circuit to pollute other circuits
connected to the same supply as the circuit itself.
The great thing about ferrites is that they are lossy, and
it absorbs rf energy and coverts it to heat. This way, none of
it gets anywhere. Inductors and capacitors merely move
rf energy around, either preventing them from entering
a particular circuit or bypassing it to ground. This is not
neccessarily the best solution for emi as this unwanted
rf energy still needs to be directed to a clean ground.

Ferrite beads are very useful in digital audio circuits, as they
operate with square waves, and hence, very large amplitude
harmonics at tens of MHz. If you put a scope probe at the
power supply of your cdp filter and dac chips, you may see
square waves riding on it. This can, and usually will,
contaminate the other power supply lines connected to it.
A ferrite removes all this cleanly.

Even in low frequency circuits such as amplifiers, rf energy
can enter the circuits via the power supply cord. And if these
are large enough, they would cause intermodulation distortion
in the amplifier by modulating the powe supplies. So ferrites
are also useful, but you need really high current ferrites for
these.

Hope this helps,
Yv
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Old 2nd January 2003, 11:36 PM   #10
Bobken is offline Bobken  United Kingdom
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Hi,

Beware of using ferrites on a power amp's output, as the well-respected UK audio designer John Linsley Hood has said that this can (will?) cause severe unwanted distortion in some cases.

From memory, I seem to recall that this is due to some hysteresis effects in the ferrite, itself.

I will try to find the reference to this caveat in my 'library' of books/articles, and will refer to the text in more detail, when I have come accross it.

Of course, using the much smaller values of inductance like 10uH made up of a few turns of wire (often wound around the body of a 10 Ohm, or so damping resistor), as is frequently used in power amps at their outputs, is a different matter.

This kind of wire-wound inductance seems to upset the HF square waves somewhat with some damped ringing, though, but does assist with stability if unusual speaker loads are contemplated.

Regards,
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Bob
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