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Resistor inside an inductor.

Martaine2005

May 12, 2015
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Hi all, I have an amplifier that I’m cleaning the dust from inside and noticed that a resistor is inside an inductor. It’s the same on both amp sides. I’ve never seen or noticed this before. What’s the purpose of it?. I can’t see if it’s in parallel or not without taking the board out.
TIA

Martin
 

Martaine2005

May 12, 2015
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Here’s a photo.
 

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Ylli

Jun 19, 2018
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Parasitic suppression. Any high frequency parasitic oscillations drop across the inductor and are dissipated by the resistor.
 

bertus

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Nov 8, 2019
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Hello,

You can also find coils wounded around a resistor.
These are sometimes used in RF circuits as chokes:
Coil_on_resistor.jpg
Here is a homemade version.

Bertus
 

danadak

Feb 19, 2021
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That coil is not wound on the resistor. Is it for dropping the coil Q ?

1673460153439.png


Regards, Dana.
 

Ylli

Jun 19, 2018
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It does, but perhaps a better way to look at it...
Since this network is in series with the outputs, at DC the coil is effectively a short circuit and the output amplifier just sees the impedance of the speaker and it's feed line. At ultrasonic frequencies, where parasitic oscillations are likely to occur, the coil has enough inductance that the network looks more like the value of the included resistor, such as 2.2 ohms. So the load on the amplifier becomes the speaker impedance plus that 2.2 ohms. It just helps to stablize the amplifier.
 

Harald Kapp

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Typical output audio amp output stage.
This example supports the pcb space theory.
I very much doubt that the inductance and the resistor have a mutual influence other than the one described by @Ylli .
 

Martaine2005

May 12, 2015
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Thanks for all the input.
This has made me read many sites about parasitic suppression.
From what I read and understand, it does a bit of all the above suggestions. Still, way over my head. But depending on L and R will vary the frequency at which oscillation suppression is neutralised.
Do I take it that this oscillation would cause feedback? Or is it more to do with stability of the amp?. Because if it’s only a fixed L-R combination then surely it can only have any affect on a certain frequency?. Or do I not understand?.
It is quite complicated even for grid resistors.

Martin
 

danadak

Feb 19, 2021
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Possibly a spice sim might help :


Regards, Dana.
 

Martaine2005

May 12, 2015
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I must admit that I haven’t been practicing with LTspice but will have a look over the weekend. Thanks.

Martin
 
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