Resistor inside an inductor.

[Martaine2005]

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]

Here’s a photo.

 

[Ylli]

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

 

[Harald Kapp]

A matter of available pcb real estate?

 

[Ylli]

Typical output audio amp output stage.

 

[bertus]

Hello,

You can also find coils wounded around a resistor.
These are sometimes used in RF circuits as chokes:

Here is a homemade version.

Bertus

 

[Ylli]

Hello,

You can also find coils wounded around a resistor.
These are sometimes used in RF circuits as chokes:

Yes, have done it myself may times.

 

[danadak]

That coil is not wound on the resistor. Is it for dropping the coil Q ?




Regards, Dana.

 

[Ylli]

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]

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]

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]

Possibly a spice sim might help :

Spice Modeling Acoustic Properties of Speakers

Did you know that you can model electro-acoustic properties of loudspeakers? I use LTSpice for my simulations. I create a circuit based on the electro-mechanical properties of a woofer, and an enclosure. It takes the form of a high pass filter of sorts. Using Thiele-Small parameters, you can...

www.diyaudio.com

Regards, Dana.

 

[Martaine2005]

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

Martin