Low Impedance Pickups: What Am I Doing Wrong?!

[Ripthorn]

Great information, thanks. I am a physicist by profession and training, but electrodynamics is a little rusty (acoustics is my research specialization). I will play around with a few things and see what happens. Again, I'm thinking I will put the actual sensing coil under the aluminum and have the aluminum just be for looks. Going to drop by home depot on the way home for a foot of 10 AWG romex to pull the strands out of and see what happens.

[bbsailor]

Great information, thanks. I am a physicist by profession and training, but electrodynamics is a little rusty (acoustics is my research specialization). I will play around with a few things and see what happens. Again, I'm thinking I will put the actual sensing coil under the aluminum and have the aluminum just be for looks. Going to drop by home depot on the way home for a foot of 10 AWG romex to pull the strands out of and see what happens.

Use this web link to calculate the effective resistance of the string loop. Resistivity Calc Put the size of the aluminum loop wire in and calculate its effective resistance for the total length used. Use the wire table to see what resistance AWG 10 will be in microohms per inch. If you can purchase solid wire by the foot, then purchase AWG 6 or 8, bend into the final shape and carefully drill a hole to match the CT primary diameter about .125" deep in the flattened (filed) end of the wire to make a very snug connection. Then silver solder. AWG 10 might be a little to thin to drill out. If you can find some thin wall copper tubing or crimp connections to make a low resistance joint to connect the string loop to the CT primary.

Joseph J. Rogowski

[Ripthorn]

By silver solder, do you mean something like this?

[bbsailor]

By silver solder, do you mean something like this?

Yes

[Jason Rodgers]

Thanks for all the comments, guys. Here is my thinking: I have enough space that I could run a copper loop under my aluminum frame and use the aluminum just as decoration. All the other hardware on the guitar is made by me out of aluminum, so the aluminum look needs to stay, but functionally speaking, I can get a loop of 10 or 12 AWG wire under there. I could then install mic matching transformers for each pickup in the control cavity (I have plenty of space there). Does that sound like a good plan? Just so that you can see what I'm working on, this is the guitar, which is my own design I call the Photon. It is about 1 1/4" thick, weighs about 6 lbs or so without pickups, and plays very smoothly.

Coolness all over there! As an aside, watcha got going on with the string retainer at the head?

[Ripthorn]

Coolness all over there! As an aside, watcha got going on with the string retainer at the head?

The string retainer is a block of aluminum tilted back at 10 degrees to maintain string pull over the zero fret. Each string is secured with a cone point set screw. Four stainless screws hold it into the neck.

[Joe Gwinn]

Hi all, I wanted some low impedance pickups to go with the prototype guitar I recently finished, but I am having a heck of a time getting them to work. Here are some vital specs for what I am trying to accomplish:

- 1/8" thick aluminum frame
- CSE187 transformer (also have a cheapo 2000:1 transformer from the bay)
- Ceramic bar magnet pulled from cheapo strat pickup (just to try the idea out)
- 470k burden resistor from output signal to output ground to mimic a 500k volume pot

I have tried wiring the transformer up with the frame on either side of the transformer, with and without the load resistor. I have tried both transformers that I have, I have tried using neodymium disc magnets instead of the ceramic, orienting each of the four long faces of the magnet "up", all to no avail. Here is a picture of one of the arrangements I tried. I used brass strip to connect the transformer to the coil in an attempt to avoid the higher resistance of lead wire. To test, I have held the arrangement over the strings of one of my guitars and strummed with the pickup plugged in to a clean amp. I'm at my wits' end, as this doesn't seem like it should be too difficult. What am I missing?

As others have said, the bolted aluminum to brass junction is likely to cause trouble. You will need to solder the brass to the aluminum. One can get solder intended for aluminum. And use a very large iron. It helps to cover the junction area with flux and tin the aluminum by scrubbing the molten solder into the aluminum surface, which was wet sanded just before attempting to tin the surface. When the ends are tinned, was off using very hot water, then solder leads to the aluminum.

Or, make the big loop out of brass.

[Ripthorn]

I got some 6 AWG bare copper wire at Home Depot this afternoon. I will give that a shot tomorrow (I hope) and report back. I will also use a mic preamp before going to the guitar amp.

[bbsailor]

As others have said, the bolted aluminum to brass junction is likely to cause trouble. You will need to solder the brass to the aluminum. One can get solder intended for aluminum. And use a very large iron. It helps to cover the junction area with flux and tin the aluminum by scrubbing the molten solder into the aluminum surface, which was wet sanded just before attempting to tin the surface. When the ends are tinned, was off using very hot water, then solder leads to the aluminum.

Or, make the big loop out of brass.

Joe,

I have nothing but respect for your technical competence.

Brass, however is not a good metal to make a low resistance string loop from or even soldering links. See this web page for a metal resistance comparison. Resistance Values of Various Metals.

The key to obtaining a good low resistance output from a Triad CSE-187L current transformer (CT) is to make the string loop resistance near 800 micro-ohms total. The pre-installed U shaped primary is about 250 micro-ohms and when shorted out by bending inward and soldering together produces an output of 80 ohms as measured by the Extech LCR meter. This includes the leakage inductance of the single loop going though the CT primary of about 17 ohms over the primary DC resistance multiplied by the turns ratio (1 to 500) squared. I find that using AWG 11 copper wire at 105 micro-ohms per inch (for a total string loop length of about 6 inches) is the smallest practical wire gauge to use that will keep the output impedance near 235 ohms.

As the resistance of the string loop is reduced by using thicker wire, the output impedance of the CT goes down as measured by the Extech LCR, as expected. Since most microphone input impedance values represent a bridging impedance to a low impedance microphone in the 150 to about 300 ohm range, they are typically 2.4K ohms. That is why I chose the CSE-187L CT as it is cheap, easily available, has a metal frame and allows the transformer frame to be easily grounded (by soldering) along with the primary string loop to make a good ground connection to a two conductor shielded mic cable feeding an XLR connector with the CT output going to pins 2 and 3 and pin 1 going to ground.

One can even add two parallel AWG 20 copper magnet wires in parallel with the CT installed primary to reduce the impedance by about 10 ohms. This is good when making two string loops out of AWG 6 copper wire, one loop for 3 strings each and wiring them in series with a pot to control the level of the bass or treble strings. However in this case, drilling out the center of the wire to match the CT primary wire diameter is critical to making a good low resistance string loop. The output of the CT as measured by the Extech LCR is a very good indicator of the quality of the string loop joints.

I find that copper is the best string loop material when making soldering connections with home made low impedance pickups. Aluminum makes a good frame but only if uncut and stamped out like Lace does with the Alumitone. However, Lace uses C shaped laminations with the aluminum frame going through the center of the lamination to magnetically couple the current to the two coils under the frame with no need to mechanically reattach or rejoin the aluminium frame.

If you know of any other ways to get low impedance string loop matches, I am open to any recommendations.

Joseph J. Rogowski

[Ripthorn]

I did get a working prototype this evening. I used 6 AWG solid wire that I drilled out and soldered into I don't think my solder joint was very good, though. I had a heck of a time getting enough heat for the solder to flow very well. However, I got sound. I had to have it very close to the strings (almost touching) and a ton of gain from my mic preamp. By my rough calculations, it was between 50-60 dB of gain needed. However, here are things that were working against me:

- Poor solder joint, using regular 60/40 solder
- Hole sized slightly larger than primary wires
- Likely weak ceramic magnet
- Single ended signal into the mic preamp instead of differential

I tried it with the aluminum frame over the top and it seemed to boost output a little bit, but that could just be placebo effect. I have a couple 2000:1 CT's here, so I will try one of those to see what I get. The 6 AWG wire is a pain to work with. A question on lighter gauge stuff: can I hammer out the ends to drill and solder to, or will that add too much resistance due to reduced cross sectional area?

[bbsailor]

I did get a working prototype this evening. I used 6 AWG solid wire that I drilled out and soldered into I don't think my solder joint was very good, though. I had a heck of a time getting enough heat for the solder to flow very well. However, I got sound. I had to have it very close to the strings (almost touching) and a ton of gain from my mic preamp. By my rough calculations, it was between 50-60 dB of gain needed. However, here are things that were working against me:

- Poor solder joint, using regular 60/40 solder
- Hole sized slightly larger than primary wires
- Likely weak ceramic magnet
- Single ended signal into the mic preamp instead of differential

I tried it with the aluminum frame over the top and it seemed to boost output a little bit, but that could just be placebo effect. I have a couple 2000:1 CT's here, so I will try one of those to see what I get. The 6 AWG wire is a pain to work with. A question on lighter gauge stuff: can I hammer out the ends to drill and solder to, or will that add too much resistance due to reduced cross sectional area?

Use the XLR balanced mic input directly wired to the CT and bypass your volume and tone controls just to see if the CSE187L properly drives this input. I use this setup all the time with a Popsicle stick mounted pickup on an acoustic guitar and it works very well with the mic level much less than half way up with good gain and low noise. See post 24 and below for wiring details.

If you have a Harbor Freight near, look for their 5/16 inch diameter 1/8 inch thick neo magnet 10 pack.

If you use 8 inches of AWG 6 to make the string loop for the 2000 turn toroid CT you will be using about 264 micro-ohms on the primary string loop. Multiply this value by 4,000,000 or 2,000 squared and you will get 1056 ohms. Then add about 160 ohms for leakage inductance and the output impedance should be about 1216 ohms. This will drive a 10K line input very well and have about 4 times the voltage output of the 500 turn CT. The key to making a good low resistance joint is to cut the ends clean, sand off with very fine sand paper, obtain copper tubing to make a joiner compression fitting. If the copper tubing is a little to small in diameter use a tapered nail punch or tool from Harbor Freight to stretch the copper tubing to tightly join the AWG 6 wire together after you loop it through the 2000 turn CT. If you taper two outer ends of the copper joint, place it into a vice and squeeze the string loop together. The copper tubing will slightly stretch. Use a 100 watt soldering iron or a Weller high wattage soldering gun and solder the joint to keep it secure.

Bending AWG 6 can be a pain but you need a good vice and a good bending jig made from a thick pipe or drilled out metal handle near the diameter of the wire.

Just for experimenting, make a 10 foot test microphone cable attached to an XLR connector to match your mixer XLR input connection. Then place an alligator clip on the two signal wires pins 2 and 3 and an alligator clip on the cable shield pin 1 or ground. For the lowest noise, ground the string loop to the ground or shield connection.

I hope this gets you going in a better direction.

Keep us posted about your progress.

Joseph J. Rogowski

[David King]

Harbor Freight stopped carrying the little NDyms a couple of years ago sadly. It takes a lot of mass to heat and solder 6Awg. I'd go with a blow torch and heat the whole length of the copper until your solder melts. A good flux is nice too along with a lower temp eutectic, silver bearing solder.

[Ripthorn]

Now that I have proved out the concept, I have a couple future steps:

- use the 2000:1 transformer I have and make sure it works
- Machine the string loop from copper plate so that I get exactly the shape I want. This will help mounting to the aluminum, attaching the transformer primary, and just my sanity because my OCD won't force me to try and get a good surface finish on bare copper wire

[Mike Sulzer]

This will drive a 10K line input very well and have about 4 times the voltage output of the 500 turn CT.

But you would not want to use much less than 10,000 ohms if wide bandwidth is the goal. Using the inductance calculator from here: Inductance Calculations: Rectangular Loop, and the values N = 1, w = 6.e-2m, h = 8.e-3m, a = 2.05e-3m, and ur = 1 gives L about 3.275e-8 H. Multiplying by 4.e6 gives about .13 H. The L/R times constant with 10K is 1.3e-5, giving a cutoff frequency of about 12,420 Hz.

[David King]

I'll point out in case you don't know that machining copper can be a real drag. Try to get free-machining stuff but at the very least use lube of some kind and be aware that some lubes will discolor the copper. Lard/bacon fat works well according to lore, others swear by turpentine.

You could just anneal your wire, straighten it by rolling it between two boards, bend it and then flatten it nicely in your Kurt milling vise.