Inductor wrapped around resistor - what for?

[uzernaam]

I've seen it a few times now... on the circuit board of an amplifier, a resistor with an inductor wound around a resistor. No idea why...

Is it a clever use of pcb real-estate? Or is there a reason to do this? In other words, does the inductor change how the resistor operates or vice versa?

[metalsculptor]

This device is a de Q'd inductor, RF parasitic suppression is what it is usually used for. Especially common in series with the anode caps of horizontal deflection amplifier valves. Often referred to as an RF choke.

The inductor dominates at low frequency giving the device a low impedance, the resistor dominates at high frequency.

[infinia]

Hi
Sometimes ferrite beads are used as input RF rejection filters on pro audio gear. Combined with series a resistor for lossy (non peaking) lowpass audio filter w/ stopband just beyond audio extending into the RF range.

[Samuel Groner]

He's not asking why there's an inductor and resistor, he's asking why the inductor is wound around the resistor.

As you said this saves some board space; also it is simply quite convenient to manufacture an air-cored inductor this way. At audio frequencies there should be little if any interaction of the two components.

Samuel

[AndrewT]

Hi,
I would not wind the inductor around the resistor. Keep them separate.
Secondly, I would not place the inductor (with it's resistor) near the amplifier PCB.
I would put it in the wiring route from PCB to chassis speaker terminals and away from the chassis walls.

This distancing of the inductor is to reduce the effect the chassis has on the linearity of the inductor and to reduce the effect the inductor field has on the Amp's circuits.

[DF96]

The usual reason an inductor is wound around a resistor is to deliberately make a poor (low-Q) inductor. At audio frequencies it acts as a piece of wire, as it has very low inductance. At medium radio frequencies the resistor loads the inductor. At high radio frequencies the presence of the resistive material within the inductor core ensures that it is lossy and so low Q. The net result is that it attenuates RF, but without having any serious resonances of its own. It stops parasitic oscillation. Quite often used in RF circuits, but less common in audio. Sometimes, to be a good audio designer, you first have to be a good RF designer!

[johansen]

The only place i've seen such is in older fm radios and oscilloscopes.

You're looking at .1 to 2 uH, for a 1/2th watt resistor depending on turns count so even at 1 mhz that's only single digit ohms, at audio frequency the resistance of the copper would be higher than Rxl, not including the resistor leads.

[macboy]

Wow, people keep talking about input filters and tubes and horizontal deflections,... I find it hard to believe nobody has seen the RL network that the OP is talking about.

I commonly see a paralleled inductor and resistor at the output of amps. It protects the amp from the potentially capacitive load of the speaker cables and blocks RF from being injected into the amp output to its feedback loop. Usually the inductor is air core and around .7 uH (a dozen or so turns of wire) and the resistor is around 4 to 10 ohm or so. You will see this in most commercial amps and, for example, in application circuits for National's chip amps.

To answer the OP's question directly, yes I believe there is no reason to wind the inductor around the R except to save space, and it makes a convenient form to wind around.

[DF96]

I think most of us have seen an output inductor! The wire is usually quite thick, in order to keep the resistance low, and it is rarely wound around a resistor. Winding around a resistor is likely to be more expensive than a separate inductor and resistor, as it may have to be done at least partly by hand, so it will only be done when there is a good reason. Saving space is unlikely to be a good reason. Stopping oscillation is a good reason.

[macboy]

Where else would you likely see an inductor in a solid state amp other than at the output? That is the reason I couldn't believe all those shots in the dark about the function of the RL network.

I agree that saving space isn't a good reason but since people around here tend to build things by hand (ever seen a DIY'er send their design to a proto fab?) and they like to do esoteric things too, it wouldn't be too unusual to see the output inductor wrapped around the resistor in pictures of people's DIY amps. It looks unique and custom made which gives us all that warm fuzzy feeling when we admire our own work.