Doodling down an idea for a simple RL hi/low/band pass filter. I’ve read that using inductors can give a steeper roll off and I want to use the band pass to create sort of pinch or calm space the sweep falls into.
Not used inductors much (caps being so much cheaper) so does anyone have any good RL filter schematics or can recommend a good range of inductor values to experiment with?
Has anyone tried making their own inductors? The closest I’ve come is making pickups.
I’ve made my own transformers and RF coils, but not inductors for AF purposes. It’s a tough process without the right equipment.
On the subject, I thought I’d share this circuit. I have no idea if it’s been done before, it’s just something I came up with. R2 is a 1M ohm pot for frequency control and R9 is the same for Q control. C1 can be used to tune the frequency too. You’ll need a big inductor, such as the primary of a mains transformer with the secondary left open. If you want a bandpass output, you can use the secondary side. Loading it will reduce the Q. It’s never going to be an ideal filter, but it sounds good.
Many thanks. This is interesting.
I know approximately nothing, except that the old Moog fixed filter banks used passive RLC filters:
Up to the sixties it wasn’t that unusual for people to wind their own AM antenna coils around a ferrite rod. Some people still wind custom secondaries for transformers. I don’t think it can be that hard. In principle at least it’s easy enough to measure the inductance of a coil by putting it into a tuned LC circuit with a tuning capacitor and measuring the resonant frequency.
Of course this is all for RF coils. For audio I don’t really know, but obviously loudspeaker coils are built to handle audio frequencies and they seem to be quite compact.
I can’t find much on CineMag’s website but here is one drawing apparently for audio inductors which are 25.5 mm diameter cylinders, 16.6 mm high.
Thanks. If you look at the filter diagram in Figure 20 which you posted above, the coils have inductances in the range 75mH to 3H. That multitap coil on the Cinemag website has a maximum inductance of 150mH, on the low end. On YouTube there are videos showing how to build a winding jig for coils. It would be a fun build to create a former out of greyboard or some 3D printing material and experiment with winding specific inductances in that range.
(Not volunteering. A hand wound coil with an estimated delivery date of 2025 is no use to anybody.)
Another point about inductors is that, as long as they’re not coupled (ie: not so close together that their magnetic fields can induce a forwards or backwards EMF in each other) you can exploit the series property: for two inductors in series L1 and L2, the resulting inductance is L1+L2. Just don’t put them close together.
Question : what would be the effect of the EMF caused by proximity on the performance of the circuit? Could you, for example, use a nearby coil to vary a property in another coil and change the performance of a filter circuit? A bit like a variable cap in an RC filter.
I imagine so, subject to the inverse square law. But more practically there are variable inductances that rely on adjusting a ferrite core.
I haven’t looked inside a radio in decades, but a typical multiple transistor super heterodyne radio of the sixties would have several such trimmers to control the RF and IF oscillators. They looked like oblong metal cans with a round hole at the top for a screw to trim the inductance. The closer you bring the coil and the ferrite core together, the higher the inductance.
Here’s a picture of a home made superhet radio complete with trimmers.
An example of modern variable inductors, of a similar design:
Thanks for the info. I’ve not been inside a radio since my childhood and CB days. I was just mulling the idea of controlling an inductor with another signal like cv etc.
Time for some experiments. Yay!
I know im going to have to relearn and research more. If anyone can recommended schema, good ranges of inductors to play with or books I’d be grateful.
I wonder if this concept might interest you. A DC voltage can be used to saturate the core of a coupled winding, substantially reducing the inductance of the AC secondary. Presumably this would alter the resonant frequency of an LC circuit incorporating the inductor, giving you a nice sine wave output of variable frequency.
The term “DC” here is relative, naturally. A CV signal controlling an audio frequency signal should work quite well with this technology, I expect.
Well that’s my week of research shortened considerably. Thank you @Bitnik this is great.
I made a filter calculator, which includes LC filter (not RL though), if that can help you.
If you want to wind up your own, it fairly easy. There is a lot of online calculators. You “just” need a waveform generator and a scope to measure them, or an LCR meter: https://www.wikihow.com/Measure-Inductance
You may want to use an air-core (relative permeability of 1) for your inductor, as it will not saturate, but saturation can bring fun effects on filters you can try various metal (iron, steel, cast-iron, ferrite, …), and various shapes (rod, torus, C). But it starts to get complicated…
So from looking at the online coil calculator, it seems that the inductance is proportional to the square of the number of turns. Using thin wire has a much weaker effect on inductance though it does enable you to pick many turns into the space.
This is great. It means if you use thin wire and lots of turns you can get some pretty hefty inductances. You can use an electric drill or some other rotary motor (a Dremel running as slowly as possible?) and a suitable jig.
I just saw a small mistake: d is not the wire diameter in the formula, but the inductor length.
That makes more sense. So short fat coils with lots of turns have higher inductance.
