Low Impedance Pickups: What Am I Doing Wrong?!

[Joe Gwinn]

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. ...

Copper is more conductive than brass (ratio of 1/8 to 1/3, depending on alloy), but it's easy to compensate by making the cross-sectional area larger, to achieve the micro-ohm loop resistances that are needed. Or one can use copper, though thick brass is easier to come by in hardware stores. (McMaster can help here.) The point is to avoid the need to solder to aluminum. Although one will require a big soldering iron in either case, copper or aluminum. Brass is easier to solder.

The only connector systems that I know of that are plausible in this application are gold-plated multiwire cage connector systems like: integramate Hyperboloid Contact Systems. These have been around forever, and the patents have expired, so there are many sources. I recall using these in the mid-1970s in electrphysiology experiments.

These are the original, if memory serves: Hyperboloid Contact Design - Smiths Connectors.

[bbsailor]

Those who are interested in making string loop couplings to a CSE-187L current transformer with a .078" diameter primary may use copper tubing available from Round Copper Tubing - K & S Precision Metals: Full Line Metal Specialists. If you use the part number 8120 which is .125 OD with a .014" wall thickness just double the wall thickness and subtract from the OD to obtain the ID or inside diameter of .097". Cut the copper tubing about .375" long and install the string loop wire into the copper tube and solder. If the fit is little too tight using your chosen wire string loop gauge, spread it with a nail punch or use a larger copper tubing diameter. Since the string loop wire will be typically a little thicker than the CT primary turn, the best place to reduce the total string loop resistance is by adding supplementary wires in parallel with the existing primary loop on the CSE-187L. I have found that I can easily fit two additional pieces of AWG 20 magnet wire about 2" long mounted right next to the primary wire but only if you poke the glue out of the space between the wire and the lamination frame. Clean off about .375" of insulation from each end of the AWG 20 wire and poke it into the copper tubing when it bulges after you squeeze it on to the CT primary wire. Solder with silver solder and this will make the guitar pickup a little more efficient.

I have been tinkering with this type of pickup for about 10 years now and choose to share what I have learned. If the output level of this pickup is similar to a microphone, typically 150 to 300 ohms, then it should be good enough to make a quick, good sounding guitar pickup ready to directly feed a mic level XLR input without the classic resonant hump that traditional high impedance pickups have.

There are many ways to make these pickups with off the shelf current transformer components. I am just sharing what I have found that works well with inexpensive and easily available parts. If other MEF members find other off the shelf parts that work well, let us know.

Joseph J. Rogowski


PS: To make the best low resistance connections I cut a piece of 600 grit sand paper about .25 inches wide and about 1 inch long and roll it into a tight spiral, insert it into the copper coupling and clean it before mounting and soldering.

[Ripthorn]

So with the success of my prototype, I bought some copper plate and ceramic bar magnets. I machined a slot in the plate and plan to solder tabs on the side for attaching the transformer primary (as opposed to bending anything). Here is my question: is silver solder that critical, especially given that my loop is basically .125" thick and .1875" wide all the way around? I have lots of regular solder. If silver solder really will make a big difference, I'll go with it, but I would prefer to not use it just because. Thanks!

[Joe Gwinn]

Hard and Soft Soldering

So with the success of my prototype, I bought some copper plate and ceramic bar magnets. I machined a slot in the plate and plan to solder tabs on the side for attaching the transformer primary (as opposed to bending anything). Here is my question: is silver solder that critical, especially given that my loop is basically .125" thick and .1875" wide all the way around? I have lots of regular solder. If silver solder really will make a big difference, I'll go with it, but I would prefer to not use it just because. Thanks!

The term "silver solder" has two meanings, depending on melting temperature:

Hard solder, which melts at an orange heat, and uses a kind of brass that contains no tin or lead. Also known as silver brazing, using a brass alloy that contains silver. This makes the best joint, almost equal to welding, but requires a big torch and a muffle to get a big piece of copper up to brazing temperature.

Soft solder, which melts below red heat, and is largely a tin-lead solder alloy with about 2% silver. This is much easier to use, largely because it doesn't need to be nearly that hot. A big torch and a muffle is still useful, though, but soldering will be almost instantaneous, a good thing.

A muffle is an insulated box that confines the torch heat. I make muffles by stacking fire bricks and half-bricks to make a 5-sided cavity within which the work is placed. The torch flame is directed at the work inside the cavity. The muffle helps greatly with getting an even heat.

Anyway, I'd start with the soft soldering approach, and see if it's enough - I bet it is good enough.

[bbsailor]

So with the success of my prototype, I bought some copper plate and ceramic bar magnets. I machined a slot in the plate and plan to solder tabs on the side for attaching the transformer primary (as opposed to bending anything). Here is my question: is silver solder that critical, especially given that my loop is basically .125" thick and .1875" wide all the way around? I have lots of regular solder. If silver solder really will make a big difference, I'll go with it, but I would prefer to not use it just because. Thanks!

Please post a photo of your setup so we can recommend the best way to obtain a low resistance connection.

Thanks for the update.

Joseph J. Rogowski

[Ripthorn]

Here are the loops and magnets. On the open end of the loops, I intend to solder "tabs" at a 90 degree angle that have holes for the transformer primary. As for soldering with silver solder, I have fire bricks and a torch more than big enough, so that's not an issue. I regularly heat treat cutting irons for hand planes with this setup, up to 1/4" thick, 2" wide, and 5-6" long. I get that red hot in the matter of just a couple minutes. I also have a propane torch since I probably don't need that much heat. It's not a question of whether I can do it, it's just whether or not the silver solder that I linked to earlier is really necessary over regular solder.

[David King]

That looks great, I take it you didn't have much trouble cutting a slot?

[Ripthorn]

Not much trouble. I did break on carbide end mill because I took too much depth of cut. Copper is very gummy and will bog cutters down. Carbide doesn't like being bogged down and will snap easily.

[bbsailor]

Here are the loops and magnets. On the open end of the loops, I intend to solder "tabs" at a 90 degree angle that have holes for the transformer primary. As for soldering with silver solder, I have fire bricks and a torch more than big enough, so that's not an issue. I regularly heat treat cutting irons for hand planes with this setup, up to 1/4" thick, 2" wide, and 5-6" long. I get that red hot in the matter of just a couple minutes. I also have a propane torch since I probably don't need that much heat. It's not a question of whether I can do it, it's just whether or not the silver solder that I linked to earlier is really necessary over regular solder.

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Using the copper plate calculator at this web link Resistivity Calc I was able to calculate the following from the information you provided. If the total length of the .187" wide by .125" thick plate string loop is 6" long then the resistance would be 174 micro ohms. This equates to about 44 ohms added to a shorted CSE187L 500 turn current transformer (CT) of 80 ohms so the total is about 124 ohms with the CT directly connected to the copper plate. If you use copper mounting tabs, you will add a more resistance but you should be still in the near 200 ohms range and OK with a low resistance connection, for feeding a mic input circuit. However, if you use a 2000 turn current transformer the output impedance would be about 4 million times 174 micro ohms or about 700 ohms plus what ever resistance the primary transformer loop of the 2000 turn CT adds to the circuit. What is the brand and part number of your 2000 turn CT?

What are your mounting constraints that may dictate where you place the CT in relation to the string loop? Can you make a 90 degree bend down to directly mount the CT under the string loop or to the outside of the string loop by soldering directly to the copper plate by drilling two holes to match the CT primary wire spread and wire diameter of about .078 inches?

If you go the 500 turn CT way, you will be in the same output range as the Les Paul Recording guitar and can use that schematic to have on board controls with the transformer at the amp end of the cable or use a mic mixer input.

Thanks

Joseph J. Rogowski

[Ripthorn]

The 2000 turn transformer was an inexpensive toroidal from ebay. I think I posted the part number earlier. I could drill directly into the loop, but the tabs I was thinking of adding would be .125" thick copper plate, .1875" wide and about .25" long, soldered to the underside of the loop. Part of my issue is the width of the magnet. I'll see if I can get pictures showing what I mean. I am thinking of using the Xicon 42tl019-rc matching transformer, which is a 10k primary and 600 secondary. That should get me around 21 dB of gain and help going into a regular amp. If need be, I can use an active onboard preamp, but I would prefer to avoid it.

[Joe Gwinn]

So with the success of my prototype, I bought some copper plate and ceramic bar magnets. I machined a slot in the plate and plan to solder tabs on the side for attaching the transformer primary (as opposed to bending anything). Here is my question: is silver solder that critical, especially given that my loop is basically .125" thick and .1875" wide all the way around? I have lots of regular solder. If silver solder really will make a big difference, I'll go with it, but I would prefer to not use it just because. Thanks!

An afterthought: Is there any reason one cannot just bend the copper tails up by 90 degrees, and drill holes in the newly made tabs for transformer leads, versus trying to solder tabs to the U-shaped plate?

Or, bend the transformer leads by 90 degrees, so they can go through holes drilled in the flat U?

[Ripthorn]

An afterthought: Is there any reason one cannot just bend the copper tails up by 90 degrees, and drill holes in the newly made tabs for transformer leads, versus trying to solder tabs to the U-shaped plate?

Or, bend the transformer leads by 90 degrees, so they can go through holes drilled in the flat U?

I am trying to fit it under an aluminum top plate of predetermined size. In order to fit the magnet in and everything, along with the thickness of the copper, there is no way that I can get a perfect 90 degree bend in less space, so I thought of soldering the tabs. However, with my new idea, it should make the 90 degree tabs a moot point.

The transformer I am using has huge primary leads that won't bend.

[Ripthorn]

Just by way of update, I did get the pickups finished. They are lower output than what I would like, and it turns out they won't fit under the covers I have, so I will be doing something else. I am thinking I will do a set like firebird pickups, but slanted and with 7 string magnets so that I can have plenty of magnet under the E strings.

[bbsailor]

Just by way of update, I did get the pickups finished. They are lower output than what I would like, and it turns out they won't fit under the covers I have, so I will be doing something else. I am thinking I will do a set like firebird pickups, but slanted and with 7 string magnets so that I can have plenty of magnet under the E strings.

If you can measure the output impedance at the current transformer (CT) output with an Extech LCR meter you can deduce the total resistance of the string loop and the connection quality (resistance) to the CT primary. The CSE187L is a 500 turn CT so the output impedance reflects the total resistance of the CT primary and the string loop (plus the connection quality). A shorted CT is 80 ohms measured output. If your mounted CT (to the string loop) measured output is 200 ohms, then the string loop is 200 minus 80 or 120 ohms. Then, divide 120 by 500 squared to obtain the string loop resistance or 480 micro-ohms. If your measured output is much higher, that speaks to the string loop connection quality to the CT primary any may account for the lower output. The output should be in about the 3mv to 4mv range, similar to a low impedance microphone level in the low impedance mic range of about 250 ohms loaded by a bridging input impedance of a mic mixer of about 2.4K ohms.

I hope this helps.

Joseph J. Rogowski