Inductors for passive filters - diyAudio
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Old 28th November 2010, 01:46 AM   #1
miallen is offline miallen  United States
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Default Inductors for passive filters

Hello,

I'd like to create a series of RLC filters for a guitar preamp for a more sophisticated response than the usual Bassman tone stack. I'm still pondering RC circuits (twin-t notches can be pretty good actually) but I'd like to at least explore using inductors to get a complex frequency response with sharp notches and low / high cuts.

However I'm having trouble finding inductors. I suppose a Fasel inductor might be good for part of the circuit. I know some people are using small signal transformers. But it would be nice if I had a proper collection of inductors from 1 - 1000 mH to experiment with. Also it would be fabulous if they were toroidal and came with metal covers or whatever is necessary to include them in the preamp enclosure without having to worry too much about interference. Cost is not a large concern. This is a one-off project for myself.

I've looked through Mouser and the usual places but it's not clear that they have the parts I should be using.

Any pointers?

Mike
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Old 28th November 2010, 04:21 AM   #2
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Assuming your post is only about sourcing the parts then I'd ask why not wind your own? Easy enough to wind rudimentary coils for experimenting. Or do a tapped one so you can switch?
You could also get them custom wound. I believe Dave Slagle likes projects that are off the beaten track and supports them - also helping with design decisions. Maybe he'd do you super custom one of a kind switchable Fasels. . . . . (Ooh yeah)
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Old 28th November 2010, 05:52 AM   #3
benb is offline benb  United States
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These will be high-value inductors, as you indicated. One source might be guitar pickups themselves, though these also have substantial resistance and significant interwinding capacitance. Also they have high sensitivity to AC magnetic fields such as from power transformers.

Toroids have substantially better immunity to such fields, but putting the number of windings on to get the inductance you want could be problematic.

Active simulation became popular as opamps became more available and cheap, and also have the advantage of much better immunity to magnetic fields than real inductors. Look up gyrator as one such circuit.
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Old 28th November 2010, 03:01 PM   #4
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Go to a TV repair shop and get some old discarted VCRs (first generation, if possible), better yet the old RCA video disk players. You will find many coils of varying values in the video prrocessing circuitry. After all, this is DIY! E
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Old 29th November 2010, 05:47 PM   #5
miallen is offline miallen  United States
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Well that doesn't sound promising. Despite the aesthetic purity of winding my own 1H toroid inductor by hand I think I can say with certainty that that's no going to happen. I just picked up some UAF42 and Fasel inductors on Ebay. I should be able to get a mildly interesting response with those parts at high frequencies. But steep cuts at low frequencies seem like they could be a challenge.
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Old 29th November 2010, 06:23 PM   #6
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A source of small high value inductors would be a boon to making paasive line level LC XOs too...

Like the Marchand, or the attached one that appeared at ETF 2010.

dave
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Old 3rd December 2010, 03:36 AM   #7
miallen is offline miallen  United States
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Wow. Fasel inductors are fun for the whole family. I just spent all night noodling over different RLC circuits. Highpass, lowpass, notch and bandpass with various Q. I created a circuit tuned to a high E and it would really resonate on it, other E notes and somewhat on notes that are harmonics of E. If I played the low E open I could clearly hear a high harmonic. With a high Q bandpass it would really ring to the point where it sounded like reverb.

My only concern is that with some circuits there was a faint buzz in the background when not playing. I'm not sure at this point how much the inductors are responsible if at all. I was using a simple breadboard with florescent lights and power cables all about. I'll have to create a test filter circuit in an aluminum enclosure with external power. Hopefully I can isolate and eliminate any real noise.

I tried both Dunlop FL-01 which is the yellow cup core winding and the red FL-02 toroidal winding. I could not tell the difference. They performed identical near as I could tell. The toroidal was $14.95 and the cup core was $13.95.
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Old 3rd December 2010, 07:39 AM   #8
Enzo is online now Enzo  United States
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And please go over to Duncan's Amp Pages and download the free Tone Stack Calculator. It is a nifty little program that shows real time changes in freq response of a number of common tone circuits. You can vary the setting if the controls and also vary the part values. Watch the changes on the response curve next to it. Loads of fun and free. Compare Vox to Fender, amaze your friends.

There are other good things on that site too.

Here is the TSC page
TSC
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