Inductor for Power Supply Filtering

SEMI-RANT: I cannot tell you how many times I've seen it said in an effects forum that wall-wart adapters* are poorly filtered if at all. This is oft-repeated misinformation that can end up confusing a beginning builder. I've disassembled many of the little black dc power packs and every single one of them had 1000uF of capacitance as a filter! That is why I find it amusing when some well-meaning enthusiast (who has never disassembled one) recommends merely adding 100uF or 220uF to the power supply as a means of making it more quiet. That's not too effective by itself.

A much more effective modification to reduce hum from a power source is to use an inductor in series before the added capacitance. This is a common practice in tube amps or car audio.

This quick tip is about how to obtain cheap and effective power supply filter inductors. It's simple really; there are many inductors sold as noise filters for automobile sound systems. Even Radio Shack has a 4 amp inductor for $3.99 that can serve well in the power supply of your pedalboard (RS Part No. 270-030A) and MCM Electronics, Parts Express or other online suppliers have them as well.

RS 270-030 noise filter The inductor (or choke) looks like a small transformer and is used in series with a capacitor filter to remove ignition noise or alternator whine from car stereo systems. The wire on the Radio Shack part is quite heavy to carry the 4 amp load and can easily handle the power requirements of effects circuits.

I used the Radio Shake choke shown here to make a power supply filter for the wall wart connected to a small practice amp that I was building. The power input jack connects to the inductor and it is followed by a 220uF capacitor to ground at the pc board. The extra filtering keeps the circuit clean and hum-free.

The specifications of the choke (from the Radio Shack site) are:

Inductance: .................................. 1.90 mH Min @ 1KHz 1V 0mADC
                                             1.72 mH Min @ 1KHz 1V 400mADC

DC Resistance: ....................................... 0.0872 Ohms +/- 10%

Insulation Test: ................................. DC 100V 100 Meg Ohm Min.

Hi-POT Test: ............................................ AC 100V 1 Minute

Current: ................................................. 4.0 Amps (Max.)

CONSTRUCTION

Size/Core: .................................. EI-41/0.5/Silicon Steel Core

Impregnation: ........................................ Varnish Impregnated

Color Code: ....................................................... Yellow

Frame: .............................................................. With

Dimensions (Choke Coil)(WD): .............................. 59.5 x 35.0 mm

The inductance seems a little low and I measured a somewhat higher value but at any rate it makes a good addition to an effects power supply to provide some extra filtering for a smooth DC supply. For the price you can't go wrong - try one out!

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