Low Impedance Pickup Research

[capehead]

[B]AND NOW FOR THE REST OF THE STORY

[/BQuestion for Capehead: Can you estimate the number of turns on each coil, the coil wire gauge, the measured wire resistance of each coil by itself and tell us if the two coils were wired in series or parallel? Use this wire calculator below and calculate the resistance of each alumium string loop in microohms. Carefully measure the alumium length, width and thickness and enter it into the calculator. If you have an Extech LCR, add the output readings of each coil at both 120Hz and 1KHz. Resistivity Calc Change the metal type on the bottom before you start.

I hope this makes current based pickups a little more understandable and even more fun to discover.

Joseph Rogowski

Hi Joseph, unfortunately those pics of the Alumitone were not ones I took myself; they were a totally random find from a Spanish language website. In fact when I found it, I wasn't even researching the Alumitone but I knew I had to download those pics because I knew they'd be useful when I decided to take the plunge and make the effort to experiment with this concept myself. By my logic, when I go about winding the secondary bobbins I'm likely gonna use the Seth Lover logic and basically wind them until they are full and see what happens

[fieldwrangler]

Capehead -

Indeed, I mentioned these reverb tank laminations in my first post on recent prototypes (around the middle of page 4 of this thread I think.) I used them on one of the earliest versions I tried after coating them with lacquer. Still have some around, but I'm now trying other materials; not sure where it will all end up.

Thanks, though, both for highlighting this resource and for your excellent illustrations!

While here, I'll add that I've been making transformer bobbins using rectangular model railroad tubing for the cores and nylon washers (slightly filed out) slipped over sections of the tubing for flanges.

By the way, does anyone know what these laminations are made from?

[fieldwrangler]

Also, I must admit that the 400hz high frequency spec on some of these off-the-shelf current transformers does worry me, although Joseph indicates that at low signal levels this might not be all that significant.

Has anyone found an off-the-shelf current transformer with a ratio of around 1:500 that's specified for something closer to full bandwidth audio?

[fieldwrangler]

By the way, does anyone know what these spring reverb driver and pickup laminations are made from?

Also, I must admit that the 400hz high frequency spec on some of these off-the-shelf current transformers does worry me, although Joseph indicates that at low signal levels this might not be all that significant.

Has anyone found an off-the-shelf current transformer with a ratio of around 1:500 that's specified for something closer to full bandwidth audio? I see a few toroids that come close. As far as the units I'm cobbling together around here I continue to use the hum-cancelling C core format; now I'm wondering if this is really as necessary as I thought. I mean, are all of those internal mic transformers hum-cancelling? Do they just shield them well with mumetal or something?

[bbsailor]

By the way, does anyone know what these spring reverb driver and pickup laminations are made from?

Also, I must admit that the 400hz high frequency spec on some of these off-the-shelf current transformers does worry me, although Joseph indicates that at low signal levels this might not be all that significant.

Has anyone found an off-the-shelf current transformer with a ratio of around 1:500 that's specified for something closer to full bandwidth audio? I see a few toroids that come close. As far as the units I'm cobbling together around here I continue to use the hum-cancelling C core format; now I'm wondering if this is really as necessary as I thought. I mean, are all of those internal mic transformers hum-cancelling? Do they just shield them well with mumetal or something?

The C core laminations is only necessary to couple the secondary coils to the alumium core of a pre shaped alumium or other metal frame that functions as the primary string loop. If you are using copper wire you can dispense with needing to use C shaped cores by directly fitting and soldering the current transformer primary to the string loop. In the case of the Prem Magnetics SPCT-251 with a higher permeability core, you can loop some square AWG 6 or 7 through the open primary by carefully bending this square wire into a long hairpin loop to include the space of the transformer about 1 inch on each side plus the bend radius and then add the string width of about 2 inches on each side then you need to join the open end of the hairpin turn together after it is mounted into the SPCT-251. I would add about .25 inches in each side, drill a small hole for a small machine screw through a piece of copper tubing about .375 inches long to match the spacing of the hairpin turn on the open end. Clean the insulation off the ends to allow a good contact of the copper tubing. Tighten the screw to make a good electrical connection. Solder if necessary to improve the copper tubing connection to the square copper wire. Another alternative is to make a 90 degree turn on each end and find a creative way to securely join the bitt ends of the square wire together by drilling a hole in each end of the wire to fit a short length of AWG12 copper wire into about a .125 inch deep hole on each end of this butt end connection that is a very tight fit. Add solder to keep the joint secure and makes a very low resistance joint. Measure the AC R output of the CT when doing this and stop when the Extech LCR meter has its lowest R reading and when the calculated total string loop resistance times the turns ratio squared is close to your measured value after adding about 25 percent for leakage inductance. Make sure that you let the joint cool before taking the final measurement to be accurate. You can try then loading this approximate 3K plus output with two 250 ohm .125 watt resistors from each output secondary connection to ground to have a balanced output and minimize noise. Experiment with different resistor values depending on the input impedance of your chosen preamp input stage.

Using square copper wire with a CSE-187L allows you to carefully drill a tight fitting hole in the wire to press fit the transformer primary into the string loop wire either at the end of the hairpin turn or directly under the B string to somewhat lessen its output. Remember the resistance of the string loop, including any prewired CT primary determines the output impedance and the tonal quality of the pickup.

Radio Shack has some miniature audio transformers that use interocking C core laminates that could be disassembled to serve your needs. Search on line about how to disassemble an audio transformer without destroying it beyond reuse.

I hope this helps.

Joseph Rogowski

[fieldwrangler]

This week's schedule pointed me towards trying one of the Vitec toroids (57P1820G 300:1.) Sound & output of this version are quite promising; gonna need to tame the string pull and try some things to reduce hum & buzz. Mumetal shielding & 300 ohm or so resistors to ground for the tranny plus experimenting with grounding the sense loop at one point will be on the docket for next week.

[bbsailor]

This week's schedule pointed me towards trying one of the Vitec toroids (57P1820G 300:1.) Sound & output of this version are quite promising; gonna need to tame the string pull and try some things to reduce hum & buzz. Mumetal shielding & 300 ohm or so resistors to ground for the tranny plus experimenting with grounding the sense loop at one point will be on the docket for next week.

Fieldwrangler,

Here is an experiment to try with these toroid CTs. Run a heavy round copper wire (0.188" max dia) through two CTs stacked together. Treat each CT as one half of a center tapped transformer. Join two CTs together in phase in series and use the series connection point as the common ground point for the coax shield connection point. Try loading each CT with various resistor loads (75 ohms up to about 300 ohms or a small variable pot) and listen to the sound tone with each loading value.

Here is another experiment that will be very educational and informative. Connect one CT to the audio output and then listen when the other CT is open. Then, short out the second CT and listen to the level and tonal change. This is what happens when the string loop resistance/reactance changes. Adding the second CT actually increases the reactance of the string loop much like using a thinner wire as the string loop. Then, when you short out the second CT you are effectivly making the string loop have a lower resistance/reactance.

This gives you more insight as to how to voice the combination of string loop resistance/reactance and the number of turns on the secondary much like Lace does on the Alumitones. Try wiring the CT outputs in both series and parallel and listen for the tonal shifts. Try different resistive loads (100 ohms up to about 500 ohms) on the parallel connection also.

If you like the sound of two CTs then you have the option of wiring the two CTs right next to each other or mount one CT on each end of the string loop to make it narrower to fit more confined spaces. You can even make two side by side string loops with its own set of CTs wired in a humbucker configuration with N pole magnets facing the strings on one string loop and S pole facing magnets on the other string loop and the output of the CTs wired in signal adding phase like in a humbucker.

Another experiment is to use multiple smaller insulated magnet wire (4 strands of AWG12 or 7 strands of AWG18) string loops individually joined together to minimise the skin effect at higher frequencies where the upper harmonics may be generating less current. To get a little more room for your wire to fit you can even file the center of the toroid opening with a file. Where you join the individual string loops together, use that point to add the string loop ground to connected to the coax shield point minimize the noise.

I hope this helps.

Joseph Rogowski

[capehead]

Anyone here speak/read French? Here's another conversation about the Alumitone/ultra low impedance pickup concept I found with some really good internal pics of the disassembled secondary section: LUTHERIE AMATEUR ? Afficher le sujet - Micros Alumitone, comment ça marche ... One question I have had since I found those other 2 pictures is about the number of wires coming out of each coil section; unless I'm missing something here, why would there be 4 wires coming out of each coil?

[MarkusBass]

Have you tried automatic translator:
www.lutherie-amateur.com/Forum/viewtopic.php?f=17&t=17277&start=120 - Translator

Mark

[capehead]

Have you tried automatic translator:
www.lutherie-amateur.com/Forum/viewtopic.php?f=17&t=17277&start=120 - Translator

Mark

Well, that certainly helps quite a bit, but I've always found that these auto-translators can make reading the results 'interesting' to say the least; one sort of has to mentally fill in the blanks in regards to the mistranslated words. Here's something else I found via a link through this forum of someone describing what they found upon opening up the cover over the secondary coils: chitarra.accordo.it/article.do?id=59694 - Translator Maybe it is the translation which isn't making it clear, but I'm still trying to figure out what all those other taps/wires coming off the coils are about...

[David King]

They are using lots of argot so I doubt AT will help you much. Looking at the last photo on the page you linked to (P7). It looks like they wound the coils right over the lead-out wires to anchor them. One end of each of the 4 goes to each end of the coils. You can see some of those connections in an earlier photo under the resin. The coil wires are wrapped around the lead-out's insulation and soldered at the stripped ends. The other ends of the lead-outs were connected together for the series link or connected to the pots etc. All ends accounted for.

[MisterBzr]

I've build several single loop pickups using the AS104 500:1 current transformer and I'm very pleased with the results.
For the sake of experimenting and comparison, I would like to make a single loop pickup but instead of using the CT I would like to make a simple on board preamp (using phantom power).
I've googled on mic preamps, so far I only find high impedance preamps for condenser mics.
Is there an easy but good circuit available?

Hans

[Mike Sulzer]

I've build several single loop pickups using the AS104 500:1 current transformer and I'm very pleased with the results.
For the sake of experimenting and comparison, I would like to make a single loop pickup but instead of using the CT I would like to make a simple on board preamp (using phantom power).
I've googled on mic preamps, so far I only find high impedance preamps for condenser mics.
Is there an easy but good circuit available?

Hans

No, there is no preamp that will give good signal to noise ratio at such a low voltage. It is remotely possible you could make one using many, many paralleled devices, but it would take amperes of current, and it would be hard to fit in the available space.

[MisterBzr]

Alright, that answers my question then: Thanks!

[bbsailor]

I've build several single loop pickups using the AS104 500:1 current transformer and I'm very pleased with the results.
For the sake of experimenting and comparison, I would like to make a single loop pickup but instead of using the CT I would like to make a simple on board preamp (using phantom power).
I've googled on mic preamps, so far I only find high impedance preamps for condenser mics.
Is there an easy but good circuit available?

Hans

Hans,

To better understand this low impedance string loop pickup and the transducer challenges of using very low impedance circuits, read up about "ribbon microphones". Although the ribbon mic operates on a different principal of a "moving conductor in a magnetic field", they are operating in the very low fraction of an ohm range (typically .1 to .3 ohms). This creates a challenge of obtaining a low noise signal at an impedance necessary to feed a mic input circuit. Typically, the ribbon mic output impedance is in the range of 150 to 250 ohms so a .2 ohm ribbon with a 1:35 turns ratio transformer puts out about 35 x 35 x .2 or about 245 ohms. Going to a higher turns ratio transformer provides a higher output but the output impedance raises too high above the standard impedance of mic input circuits of about 2.4K ohms where you want the bridging impedance to be about 10 times the source impedance to maximize the signal to noise ratio and not affect the high end frequency response too much.

Only very few high end and expensive ribbon mics use active electronics but these mics are in a much higher primary loop range than a guitar pickup string loop. The typical guitar pickup string loop upper limit is about 1000 micro-ohms or .001 ohms and is much lower than the ribbon mic loop impedance of up to about .3 ohms (including the transformer primary resistance).

When using current transformers to make guitar pickups, I have found that the 500 turn CT puts the output impedance in a good range for mic input circuits but only if you are using a thick enough primary string loop. If you make the total string loop resistance, including the resistance of any pre-installed CT primary (the CSE187L primary turn is rated a 250 micro-ohms), about 1000 micro-ohms, then output impedance will be near 250 ohms. If you use a 1000 turn CT you will get a higher output voltage but the you need to use a very thick string loop of about AWG 6 and expect the output impedance to be closer to about 500 ohms to be at the upper limit of a low impedance transducer impedance range. Transformers and transformer matching act like filters to change the tonal characteristics of the CT-based guitar pickup. The science tells us "the how" and "the why" but the ear tells us what sounds good and what we should pursue.

Keep on experimenting and thanks for sharing your experimental results.

Joseph J. Rogowski