Low Impedance Pickup Research

[MisterBzr]

Alright, that answers my question then: Thanks!

[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]

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

[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.

[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...

[MarkusBass]

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

Mark

[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?

[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

[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]

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]

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?

[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]

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?

[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

[bbsailor]

I have been reading and doing my best to mentally absorb as much of this 'alien pickup technology' as I can. I'm getting close to being able to wrap my head around it so for that, much thanks to you far more knowledgeable guys in this area (especially bbsailor) for enlightening the rest of us. Anyways the point of me responding to this thread is that I wanted to tell you all that I might have found an easily available way to get those types of laminations used in the Alumitone p/ups; from the transducers of reverb tanks! I was going through my stuff looking for something and an old reverb tank fell out of the box I had it in and landed top side down. Upon retrieving it from the floor I looked at the transducers and realized they were at the least similar as all the pics/drawings I've seen from the underside of Alumitones and their current transformer secondary. Do you think they would work with this type of thing? [ATTACH=CONFIG]32575[/ATTACH][ATTACH=CONFIG]32576[/ATTACH][ATTACH=CONFIG]32577[/ATTACH]

AND NOW FOR THE REST OF THE STORY

Best way to proceed with this current-based pickup idea is to make low resistance string loops that conducts well enough to generate enough current to be transformed. It is transformed by the coil turns, minus any losses from leakage inductance, string loop resistance, reflected impedance and the effect of the impedance transformation by the square of the turns ratio. In this case for our MEF experiments, I chose to go the XLR mic impedance route because everyone has access to this type of device or an inexpensive Shure (or similar) mic matching transformer. Thick copper wire is easily available. Bending wire and soldering are relatively simple tasks for people who build and tinker with guitar electronics. The higher impedance Alumitone design route uses a lightweight alumium shell that matches the shape and mounting style of the standard pickup footprints (single coil and Humbucker, P90, etc) by stamping the string loop out of an alumium plate about .125" thick. The string loops are stamped out and the frame is bent to allow a coil to be added. OOPS!! how do I add the coil with the frame already stamped out? Ahhh!!! Magnetic induction!!!. Detect the currents flowing in the metal alumium frame and find a way to arrange two loops to function like a humbucking pickup but focusing on detecting the best common current path, through where the center metal frame joins the outer loop. In this location the induced currents from the two string loops merge and also cancel out noise externally induced noise picked up by the string loops of the alumium pickup frame. This is the classic humbucker pickup design adapted for current based pickups, easy mass production and easy quality control to produce a reliable product.

Alumitone tunes their pickups by knowing how manipulating the turns ratio relative the the string loop resistance and how to shift the tonal output in various directions and then see what classic pickups these alterations sound like and they make a model that sounds like famous guitar stars pickups.

The only way to extract the currents flowing in the alumium frame is to use C-shaped interlocking transformer laminations of sufficient permeability to effeciently transform the small currents in the primary string loop to a voltage necessary to drive your amp input. In the high Z mode an output between 75mv (millivolts) to 250mv is typical. Scaled back by a 10x ratio an output in the 7.5mv to 20 mv. Afterall, you want the primary string loop to stay with a very low resistance and with a stamped frame there is no joint to worry about, just the resistance of the alumium frame loop that gets transformerd by the amount of turns on the secondary coil(s).

How do I replicate, with easy to obtain parts, the class of current-based pickup that is also the basis of the Alumitone pickups so pickup experimenters could successfully dabble, discover and understand what they are hearing?

Then I thought...use copper, make good low resistance joints and listen to how changing various variables such as string loop resistance in microohms and current transformer turns ratios and output loading affects the sound and tone of the pickup response. Using a single CT or dual CTs, one on each end of a string loop and how you wire the dual CTs, in series or parallel, load one CT with a resistance or capacitance and listen to how the pickup sounds.

Here are some interesting thoughts. The primary (non harmonic) frequencies range on a guitar is 82Hz to about 1350 Hz on a 24 fret guitar. Most of the tonal coloring done by High-Z pickups is done with very fine wire, winding styles, magnetic field, shape and strength. Most of the combined inductance, capacitance and resistance loading tonal effects are in the harmonic range. Guess what, playing with the resistance and eddy current factors in the string loop and how these factors reflect into the output coils by the amount of secondary coil turns affects the output impedance and the overall tonal shift to accomodate listening preferences, not to make any statements like this is better, because better is in the ear of the beholder but understand why it is better or not and more importantly understand how it can be replicated if desired.

Lower noise is better so if you can find ways to minimize the picked up noise you do not need as high an output to be usable. CSE-187L is a very inexpensive 1 to 500 turns ratio current transformer rated for up to to 400 Hz ( but it works up higher also) that has a primary transformer loop made of AWG12 copper wire that is about 1.85" long and 250 microohms. That means that unless the primary transformer turn is supplemented with additional wires or replaced with a larger wire, wire bundle, even litz wire, tonal changes can be replicated by using tranformer theory that can help predict the effect of various variable matches on tonal shift and ultimately the sound we hear. A CSE-187L with the installed primary bent into a tight loop is about 80 ohms on the secondary output and this includes about 17 ohms for lakage inductance. Adding a 5" of AWG 11 to make the hairpin string loop at 105 microohms per inch adds 525 microohms and this adds about 131 ohms to the existing 80 ohms for a total output of about 211 ohms, pretty near the middle of the low impedance microphone range of devices. That is why the 500 turn CT is just about right for these experiments using common XLR inputs but the choices are endless for discovering some interesting combinations.

This use of transformer technology now adds other possibilities to make passive pickups with expanded tonal ranges and operate with lower noise.

The CSE-187L CT has a metal frame that if all labels are removed and cleaned, the shielded mic wire can be soldered to the CT metal frame to help keep the noise low. Grounding the string loop and any metal coated magnet will also help. In some extreme noise cases, adding a 1000 ohm resistor down to a 200 ohm resistor form each end of the CT secondary to ground will help with a noisy input and adjust the loading effect on tone. Reducing the load on high turns ratio CTs, 2000 turns and higher, can help tilt down the high frequency response as the output is now loaded by an output equal to the primary string loop resistance times the turns ratio squared. A 700 micro ohm string loop with a 2000 turn CT (Prem Magnetics SPCT-251) reflects an output of 2,800 ohms plus about a 25 percent leakage inductance effect increasing the output of about 3500 ohms. This is much too high for typical XLR loading requirements where the load should be 10X higher than the source. If the source and load were to match there would be a 6db reduction in voltage due to the loading effect. This loading effect is not flat across the hearing band as it will tilt the pickup output range to suite your ears needs and desires or improve pereived deficiencies.

This is tone shaping using the passive properties of transformer technology in this new and unique way of making guitar pickups. Off-the-shelf current transformers, copper tubing, various types and sizes of copper wire, winding techniques can produce passive pickups with some interesting and low noise effects.

I am assuming if we can keep the upper frequency limit controllable up to about 10KHz, we might not be missing much beyond that.

Question 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