Originally posted by

**Joe Gwinn**View PostFor all the experimenters using a CSE-187L or any current transformer, Joe Gwinn is correct about this. I can't emphasize this enough: a very low resistance connection in making the single turn string loop is very important for maximizing the output level and tonal balance with lower total string loop impedance favoring lower frequencies better.

I took many measurements with the Extech LCR meter and here are some numbers to help those without the Extech LCR meter play with this CSE-187L transformer and still obtain some good results.

1. If you bent the premounted single turn of AWG 12 wire on the CSE-187L primary into a loop, butt the ends of the wire together and the solder to make a shorted primary loop you will have an output impedance of between 78 and 80 ohms. The difference is based on the winding turns tolerance on the secondary which is specified at 500 turns plus or minus a few percent.

2. The length of the primary AWG 12 wire is 1.89" for a DC resistance of 250 micro ohms or 0.000250 times 250,000 (turns ratio squared) for a calculated output impedance of 62.5 ohms. Transformers are not perfect so in a real transformer there is leakage inductance that tends to look like a small inductance in series with the primary loop of 80 - 62.5 or about 17.5 ohms due to leakage inductance.

3. Using AWG 11 copper magnet wire is 105 micro phms per inch. Copper Magnet / Enameled & Bare Wires A string loop is typically adds about 5 to 6 inches of AWG 11 wire to the 1.89 inch long primary wire resistance. Each inch of AWG 11 wire adds 26.5 ohms to the output impedance measured at the output of the CSE-187L. A 5 inch wire added to the string loop to span the string width adds 26.5 times 5 or 132.5 ohms plus the 62.5 primary loop resistance plus 17.5 leakage inductance for a total of 212.5 ohms as the low impedance output impedance.

This is close enough to the nominal 150 ohms impedance typical for XLR low impedance microphone connections. The key to using this low impedance output is to ensure low noise.

4. Low noise using the CSE187L current transformer is simple. Use 2-conductor shielded microphone cable with XLR pins 2 and 3 going to the floating output pins on the current transformer. Attach the mic cable shield to pin 1 on the XLR and the metal frame of the CSE-187L. Also, ground one end of the low impedance string loop to the metal transformer frame. In some cases you may need to add a 2.5K ohms resistor from each transformer output pin to ground (the transformer metal frame) to ensure the lowest noise connection. Experimentation will tell you what sounds best.

5. If you locate AWG 6 square copper wire at the Surplus Sales of Nebraska web site, you can make a hairpin U-shaped loop that is about 35 microohms per inch so your output impedance will be 80 plus 8.75 ohms per inch (.000035 times 250,000). 8.75 times 5 inches adds 43.75 ohms to the 80 ohms on the primary string loop for a total output impedance of 123.75 ohms.

The lower the output impedance will indicate a lower primary string loop impedance. Here is a little truck that can help squeeze out a little more bass response out of a CSE-187L current transformer. Obtain some AWG 20 magnet wire. This AWG 20 wire will just fit between the CSE-187L transformer secondary and the laminated frame to add from one to four more single turn primary loops all connected in parallel with the installed primary AWG 12 primary wire. This will lower the total string loop impedance and distribute some current directly over the secondary winding to slightly reduce the leakage inductance.

If you have an acoustic guitar, this pickup is idea to experiment with because you can mount it directly behind the fingerboard. Make the transformer connection span the B-string with the transformer primary being vertical so the transformer connection is directly under the B-string to dip the current loop under this typically loud string to better control its output. Solder the 2-conductor shielded wire to an XLR connector and plug it directly into a mic mixer. You will have a very nice acoustic guitar pickup without the classic high impedance pickup resonant hump in the typical midrange where the ear is most sensitive. The creativity is making the mechanical size of the pickup fit your available mounting space.

If you want to try using higher turn current transformers or toroids you may need to use two or more turns on the string loop primary to keep the impedance in the nominal 150 ohm range (100 to 400 ohms). A 1000 turn current transformer multiplys the primary string loop impedance by 1,000,000 (1,000 squared) while a 500 turn current transformer increases the string loop impedance by 250,000 (500 squared).

This is just some stuff I have learned by playing with current transformers and guitar pickups for several years now. The CSE-187L is a good current transformer to begin your experimentation to obtain a good, low noise, higher fidelity output.

Remember, with current based guitar pickups, the low impedance of the primary string loop controls the current transformer output impedance and the tonal voicing.

I hope this helps.

Joseph Rogowski

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