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fret 3rd April 2012 08:02 AM

Looking for an inductor: 50mH @ 3A?
So far it's been hard to find. Element14 has a 2.3A inductor at that rating but I need 3A. The circuit I'm looking at building is this one.

Anyone know where I can get some at a reasonable price?

dchisholm 3rd April 2012 09:30 PM

That circuit is a generic test jig that emulates no particular speaker cabinet, so the actual inductance value isn't critical. (Although it should be measured and documented, so it could be repeated.) Anything from 40 mH to 60 mH, perhaps wider, will probably be acceptable.

Some ideas to consider:
  • Combine several inductors in series/parallel arrangements. If you can find, say, 27 mH with a 3 amp (or higher) current rating, put two of them in series. If you can find 100 mH with a 1.5 amp (or higher) current rating, connect two in parallel. If you can find 50 mH with 1.5 amp rating, put two in series (to make 100 mH with 1.5 A rating), then put two of those pairs in parallel to make 50 mH at 3 amps.
  • (When connecting inductors in series or parallel, pay attention to layout so their fields don't interact with each other to any appreciable degree.)
  • Think about creating your own inductor. I might start by looking at the large toroids offered by Amidon or Fair-Rite Products. Their catalogs and web pages contain the design information you need.
  • This thing doesn't need to behave like an inductor much above 100 Hz or so. Power supply filter chokes built with E-I laminations should be quite adequate. For example, the Hammond model 193T is 50 mH rated for 2 amps. The Hammond 195R10 is 50 mH rated for 10 amps; a pair of 195P5's in series gives you 60 mH at 5 amps.
  • I'd consider rewinding a filter choke salvaged from an old power supply, battery charger, or vacuum tube amp. Look for a choke weighing around 5 pounds (2.2 kg) or more. Strip off the old windings. Do a test winding of 10 or 20 turns; measure the resulting inductance; and use that to determine the number of turns you need. (Inductance is proportional to the square of the turns count.)
  • Commercial C-cores are also available in small quantities for winding your own chokes. (Use the "Search" on this forum to find vendors.) I know the manufacturers give design information for using them as transformers, but I don't know if the they supply design information for choke applications.
  • As a last resort, I'd wind an inductor on a salvaged power or vacuum tube output transformer core. Probably have to gap the center leg with a hacksaw, and hope there's enough varnish to keep the pieces from falling apart. Wind a test coil and procede as described above.

fret 3rd April 2012 10:30 PM

Hey great ideas!

I did find this guy:
25.5mH 4A

So 2 of those in series should do the trick. Price seems ok too. All the rest of the parts are available at element14 as well.

DigitalJunkie 4th April 2012 12:33 AM

You should be able to use lower current inductors,if you plan on using this at lower power levels.

sreten 4th April 2012 01:18 AM


Why not simply use a real speaker in a real box ?

rgds, sreten.

fret 4th April 2012 01:44 AM


Originally Posted by sreten (
Why not simply use a real speaker in a real box?

FYI Whats not in the circuit shown is the part that taps off the signal to be processed and output via an XLR. Basically a high resistance voltage divider going into a ACD and then a DSP chip running a IR convolution of some sort. Probably a speaker/mic IR like those from Redwires.

- Sometimes the volume is an issue (kids asleep in the same house, small clubs with low stage level).
- Repeatability with mic positioning is not the greatest. In all my years of mic'ing up a tube amp, it always sounds different.

Now maybe it would be nice to have say a single 12" guitar speaker/cab inline with this load/processor for stage monitoring, and the XLR output for the PA. I will look into that once I've got more of this up and running.

jamesdb 4th April 2012 03:19 AM

Signal Transformers has a choke CH-4 that is 70mH 4A, or CH-6 is 40mH 6A.
Not cheap, though.

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