Moving Coil Pickups for the Technically Curious

[Corvus]

I have some basic conceptual questions for Joseph or anyone else who's interested.

I'm extremely interested in trying out the multi-channel aspect of this pickup type.

My main design requirement is a separate signal for each string, so that eliminates the option for humbucking using string pairs.

I also need a balanced signal from each string for connecting to my mic preamps in my audio interface.


So one question is: can I achieve a balanced output design for each string using a pair of current transformers wired out of phase?


And my other question: can I create a single string humbucking design using current transformers?


Please excuse my lack of understanding of transformers and electronics in general. I have already successfully prototyped a single string traditional humbucker design which sounds great and gives me a separate balanced output for each string. However the signal bleed between strings is a problem with high gain and the string separation advantage of this moving coil concept is really appealing to me.

Any helpful advice would be greatly appreciated.

Leo

[bbsailor]

I have some basic conceptual questions for Joseph or anyone else who's interested.

I'm extremely interested in trying out the multi-channel aspect of this pickup type.

My main design requirement is a separate signal for each string, so that eliminates the option for humbucking using string pairs.


I also need a balanced signal from each string for connecting to my mic preamps in my audio interface.


So one question is: can I achieve a balanced output design for each string using a pair of current transformers wired out of phase?


And my other question: can I create a single string humbucking design using current transformers?


Please excuse my lack of understanding of transformers and electronics in general. I have already successfully prototyped a single string traditional humbucker design which sounds great and gives me a separate balanced output for each string. However the signal bleed between strings is a problem with high gain and the string separation advantage of this moving coil concept is really appealing to me.

Any helpful advice would be greatly appreciated.

Leo

Leo,

What is the input level required by your balanced input mixer?

You will be required to arrange a creative way to obtain a common ground return from the nut end of the neck back down to the body of the guitar near the bridge that is a low resistance to minimize resistive losses. The lowest resistance string is about .5 ohms based on the core diameter of the string. You want the common ground return the be less than .05 ohms. Use either a brass nut or wire each machine head to the common ground return. Using the truss rod as a ground return is a good way to make the modification. Make a metal collar that looks like a washer with a long finger to attach to the brass nut to make a low resistance electrical connection from the nut to the truss rod. Then find a creative way to bring the tail end of the truss rod electrically into the body of the guitar near the bridge. Then use between a 4 ohm transformer primary (input side) transformer, however it may be labeled as the secondary. Use a transformer with a 500 ohm secondary although it may be labeled as the primary. The turns ratio will be 500 divided by 4 or 125. Thake the square root of 125 to be 11.18. This means that a 2mv signal induced directly on the string will be about 20mv out of the transformer going into the mic mixer. The you need a 13 pin (minimum) connector set to take the signal out of your guitar; 2 pins per transformer for each string plus one common ground. Make a breakout box on the mixer side. Ground the common ground return and the frame of each metal transformer to the common ground pin of the 13 pin connector to minimize noise.

Although current transformers will work, the 4 ohm primary to 500 ohm secondary will work better in this application. Try to obtain from Radio Shack the 5 pack of magnets .75" wide by 1" long by .187 thick with a .187" hole in the center. Mount these on a rubber mat and you can adjust the height of each magnet by compressing magnet against the rubber underneath. Mount them with the 1" length running in the string direction; with 3 magnets across. Try laying the magnetic field from the neck to the bridge with the magnets getting closer to the strings as they approach the bridge. The longer the magnetic field is under the strings the higher will be the output.

This will produce a pickup with total isolation between strings. Look for miniature audio transformers that are about .75" across in size of the metal laminated transformer frame. You will need to make a bridge with non-metal string intonation adjustments inserts. These are commercially available as "string savers".

Post photos of your progress so we can all learn more.

I hope this helps?

Joseph Rogowski

[bbsailor]

I have some basic conceptual questions for Joseph or anyone else who's interested.

I'm extremely interested in trying out the multi-channel aspect of this pickup type.

My main design requirement is a separate signal for each string, so that eliminates the option for humbucking using string pairs.

I also need a balanced signal from each string for connecting to my mic preamps in my audio interface.


So one question is: can I achieve a balanced output design for each string using a pair of current transformers wired out of phase?


And my other question: can I create a single string humbucking design using current transformers?


Please excuse my lack of understanding of transformers and electronics in general. I have already successfully prototyped a single string traditional humbucker design which sounds great and gives me a separate balanced output for each string. However the signal bleed between strings is a problem with high gain and the string separation advantage of this moving coil concept is really appealing to me.

Any helpful advice would be greatly appreciated.

Leo

Leo,

In reviewing my answer I see that I have not fully answered your humbucking question.

So here goes!

The six long metal string suspended in open air can be subject to some hum if you are very near a source. Take a soldering gun and hold it a few inches above the strings and activate the gun and listen for the hum. When you mix all the strings together you can change the input phase of adjacent strings so that the external hum field is neutralized as is done on humbucker pickups. This only fully works when the strings are mixed together for a single output or the mixer level is the same for each string. When the strings are used independently they are fully isolated from each other both in output phase and sensitivity to any external field and the humbucking effect will not work fully as effective as on a humbucker because the levels of each string can be independently varied and unless all strings are near the same level the phase reversal will be less effective but still can help minimize the noise.

Wire up two strings and test the transformer phase with the soldering gun generating a noise field to see how this works helping to reduce hum but works best when the strings are near the same mixer level. Here is where trying to get a mechanical balance using variable height magnets can get a better physical balance so the outputs of the mixer can be at or near the same level so the external noise reduction effect can more effectivly take place.

If you are using separate outputs, you need to keep the ground return resistance low as well as make sure that the ground return and the metal transformer frames are grounded. It also helps if you solder the mounting tabs of the six metal transformer frames together and shield all six transformers to minimize external field pickup. In my independent output moving coil designs I did not notice any hum problems that I could not solve by moving away from the source of the hum like the amplifier's power transformer and using the tips listed above.

I hope this helps?

Joseph Rogowski

[MisterBzr]

It's been a while, but tonight I've been experimentating with a 1:1000 current transformer and I'm pretty satisfied with the results.
I've placed the ct behind the (non-conductive) bridge and led a string through it, then I made a loop of two strings. I moved around one or two magnets at different positions lengthwise. I also changed the distance between the string and the magnets.

I'm planning to build a new 7 string headless guitar where the trussrod will be connected to a conductive nut. I want to make a string retainer after the non-conductive bridge (because the tuners will be mounted in the guitar body) and connect the retainer with the trussrod, thus making a loop. My plan is to put the ct's between the bridge and the string retainer, one per string to get a polyphonic pick up. I want to make a slot for the ct's in the guitar body and solder the ct's on a experiment pcb board.
Someone suggested that I could use two cat5 cables to get the 7 balanced signals to a breakout box, and from there on I can connect it to a soundcard / mixer or what so ever.
These are just my rough thoughts, and I would love to get some input.

[MisterBzr]

It's been a while, but tonight I've been experimentating with a 1:1000 current transformer and I'm pretty satisfied with the results.
I've placed the ct behind the (non-conductive) bridge and led a string through it, then I made a loop of two strings. I moved around one or two magnets at different positions lengthwise. I also changed the distance between the string and the magnets.

I'm planning to build a new 7 string headless guitar where the trussrod will be connected to a conductive nut. I want to make a string retainer after the non-conductive bridge (because the tuners will be mounted in the guitar body) and connect the retainer with the trussrod, thus making a loop. My plan is to put the ct's between the bridge and the string retainer, one per string to get a polyphonic pick up. I want to make a slot for the ct's in the guitar body and solder the ct's on a experiment pcb board.
Someone suggested that I could use two cat5 cables to get the 7 balanced signals to a breakout box, and from there on I can connect it to a soundcard / mixer or what so ever.
These are just my rough thoughts, and I would love to get some input.
One of the things is the lack of shielding of the ct (as mentioned in the last post). I've made several single loop pickups using a ct without shielding and I hadn't had any hum problems. If I need to shield the ct's, then I have to mount them inside the guitar body, in stead of on the body.

[bbsailor]

It's been a while, but tonight I've been experimentating with a 1:1000 current transformer and I'm pretty satisfied with the results.
I've placed the ct behind the (non-conductive) bridge and led a string through it, then I made a loop of two strings. I moved around one or two magnets at different positions lengthwise. I I'm planning to build a new 7 string headless guitar where the trussrod will be connected to a conductive nut. I want to make a string retainer after the non-conductive bridge (because the tuners will be mounted in the guitar body) and connect the retainer with the trussrod, thus making a loop. My plan is to put the ct's between the bridge and the string retainer, one per also changed the distance between the string and the magnets.

string to get a polyphonic pick up. I want to make a slot for the ct's in the guitar body and solder the ct's on a experiment pcb board.
Someone suggested that I could use two cat5 cables to get the 7 balanced signals to a breakout box, and from there on I can connect it to a soundcard / mixer or what so ever.
These are just my rough thoughts, and I would love to get some input.
One of the things is the lack of shielding of the ct (as mentioned in the last post). I've made several single loop pickups using a ct without shielding and I hadn't had any hum problems. If I need to shield the ct's, then I have to mount them inside the guitar body, in stead of on the body.

Hans,

Before just passing a single string through the CT try this to obtain a little better coupling between the string and the CT. Wind about 6 to 10 turns of AWG 18 copper wire through the CT and connect one end to the string ball and the other end to the ground return. This will improve the coupling of the string impedance to the CT. Try using a different number of turns of AWG 18 wire through the CT to obtain a better balance between the six strings that vary in resistance between about .6 ohms to about 2 ohms. Because each string only passes about a 2 octave frequency range doing this will get all the strings to produce about the same output. Try winding 6 turns then make a tap, then wind two more turns and make a tap and then make two more turns and make a tap. Then, switch between each tap and use your ear to listen and maybe an oscilloscope to measure the balance between each string. This is very primary experimentation that has not been documented before (except by me on this forum) but is applicable to what you are trying to do! Do a web search on the word "stingamp" to see a commercial implementation of this applied to the violin. Remember, each string is functioning like the ribbon in a ribbon microphone so this is rather old technology and physics that few have connected the dots to before as is applied to the guitar and midi.

Post your results so we can all learn from this very new form of applying the physics of a "moving wire in a magnetic field" (web search this phrase) to the design of guitar pickups for midi.

It is good to see someone pushing the technology envelope and experience the possible future of guitar technology.

Joseph Rogowski

[MisterBzr]

The last couple of days I had some difficulty to post a reply, my browser kept on loading the page.
But now it seems to work.

Joseph thank you for reply.
I will try what you suggested. The ballends of the strings are at the neck, but I will connect the tuner (which are mounted on the body) to the AWG 18 wire.
My idea is to place the ct's on a experiment pcb and shield them as you said earlier.
I've mentioned using 2 cat5 cables. How do you think of that? Each cable has 4 pairs of wire and a shield.
What kind of cable do you use?
Do you have some pictures of your ct's mounted in your guitar?

Hans

[bbsailor]

The last couple of days I had some difficulty to post a reply, my browser kept on loading the page.
But now it seems to work.

Joseph thank you for reply.
I will try what you suggested. The ballends of the strings are at the neck, but I will connect the tuner (which are mounted on the body) to the AWG 18 wire.
My idea is to place the ct's on a experiment pcb and shield them as you said earlier.
I've mentioned using 2 cat5 cables. How do you think of that? Each cable has 4 pairs of wire and a shield.
What kind of cable do you use?
Do you have some pictures of your ct's mounted in your guitar?

Hans

Hans,

Think of the DC resistance of the string as it's approximate impedance. You will need to try each string with from about 6 to 10 turns around the CT to obtain the best volume (peak to peak output) for each string as well as the most desireable tone. The best power transfer occurs when the source impedance and desitination impedance are the same. In the moving string pickup the impedance of the primary winding should approximately equal the string impedance as a starting point for planning, then listen to various primary winding taps to select the best output level and tone. What you are doing is trying to obtain a good enough signal to noise ratio of the vibrating string so you can easily transmit the signal through a twisted pair balanced cable to a destination impedance to process the signal. History reflects that transmission impedance range of from 150 ohms to 600 ohms are standard for long range transmission. Shorter range transmissions can be done with impedance up into a few thousand ohms but with greater impact by cable capacitance as the impedance goes higher.

Make the ground retrurn have a very low resistance to minimize losses. Then, you can place the individual string CT between the string machine head and the ground return.

On my initial attempt to do this I uses 6 miniature output transformers 8 ohms to 20K used backwards with the string going across the 8 ohms side. I mounted this bundle of transformers on the heel of the neck block inside the guitar. The I used an 8-pin microphone jack and plug set to have 6 hot pins, one for each string, then two pins were used for the common ground. I used 6-conductor shielded cable and made a breakout set of six .25" standard plugs on the end to allow each sting to be plugged into a six channel mic mixer for individual string control.

Keep up the good work.

Joseph Rogowski

[MisterBzr]

Experiments

Finally I got around to do some experiments.

My goal is to make a polyphonic pickup using the strings as a moving coil, using commonly available and preferredly low-cost products.

The guitar I used has a conductive tailpiece, but both the bridge and the nut are non-conductive (bone).
It's a 7 string guitar tuned in fifths (Low C -one semitone above low B of a 5 string bassguitar- to f# - 2 semitones above high e-string of a guitar),
string gauges are 0.145w, 0.090w, 0.056w, 0,034w, 0.022, 0.013, 0.009.

I made a ground return from the tailpiece, which I connected with a awg 18 wire. The wire was led through a current transformer (ct) and
attached to a machine head tuner. I also grounded the primary loop.
I used a breadboard to mount the ct and connected the ct with a mic-preamp.
This is what I used:
- several ct's: 1:500, 1:1000, 1:2000 and 1:2500.
- two button neodymium magnets with a 10mm diameter


Unfortunately I only had a simple multimeter which wasn't able to record any (low) AC voltage or very low resistance.
I used my ears and the led indicators of the preamp instead.

First: I switched between the different ct's and found that the 1:2000 CT works the best for me for 4 reasons:
- It gives a very reasonable output (compared with the 1:500 and 1:1000 ct's);
- The hole is big enough to led the wire through about 14 times (compared with the 1:500 and 1:1000 ct's);
- It's a lot cheaper than the 1:2500 ct (appr. 2.5 usd and 8 usd);
- It's a bit smaller than the 1:2500 ct, which will be an advantage when mounting 7 ct's in my guitar body.

Second: I wrapped the awg 18 wire several times around the ct and through the hole to obtain a good coupling.
Depending on the string I got the best results (highest output) between 6 and 12 loops.

Third: I moved the magnets around lengthwise, where I found an optimum by placing 1 magnet as near the end of the fretboard as possible
and 1 magnet just besides the bridge. I've altered the height of the magnets by turning the screws. I've tried to put the magnets as high as possible.
(I realise that the magnets of the regular pickups have there (small) share also).

Fourth: I switched between all 7 strings and tried to obtain a reasonable output / string. This was the case for the four highest strings.
I'm having difficulty to get a fair output for the bass strings, maybe because of the lower resistance of the strings (compared to the resistance of the ground return).

One of the very cool features of this design is that there is no cross-talk at all.

For now, there are a few things which I have to deal with:
1. The low output of the three lowest strings
2. The string is picking up some noise, which is acceptable for the highest strings, because of the greater output, but is way to much for the lower strings
3. There's a little bit of hum noticable when I touch the string
4. The ct needs to get shielded, so I have to figure out a compact way to store 7 ct's and 1 or 2 output jacks in one cavity
5. How do I transport 7 balanced signals the easiest way?

I would gladly receive some feedback.

Hans

[bbsailor]

Finally I got around to do some experiments.

My goal is to make a polyphonic pickup using the strings as a moving coil, using commonly available and preferredly low-cost products.

The guitar I used has a conductive tailpiece, but both the bridge and the nut are non-conductive (bone).
It's a 7 string guitar tuned in fifths (Low C -one semitone above low B of a 5 string bassguitar- to f# - 2 semitones above high e-string of a guitar),
string gauges are 0.145w, 0.090w, 0.056w, 0,034w, 0.022, 0.013, 0.009.

I made a ground return from the tailpiece, which I connected with a awg 18 wire. The wire was led through a current transformer (ct) and
attached to a machine head tuner. I also grounded the primary loop.
I used a breadboard to mount the ct and connected the ct with a mic-preamp.
This is what I used:
- several ct's: 1:500, 1:1000, 1:2000 and 1:2500.
- two button neodymium magnets with a 10mm diameter


Unfortunately I only had a simple multimeter which wasn't able to record any (low) AC voltage or very low resistance.
I used my ears and the led indicators of the preamp instead.

First: I switched between the different ct's and found that the 1:2000 CT works the best for me for 4 reasons:
- It gives a very reasonable output (compared with the 1:500 and 1:1000 ct's);
- The hole is big enough to led the wire through about 14 times (compared with the 1:500 and 1:1000 ct's);
- It's a lot cheaper than the 1:2500 ct (appr. 2.5 usd and 8 usd);
- It's a bit smaller than the 1:2500 ct, which will be an advantage when mounting 7 ct's in my guitar body.

Second: I wrapped the awg 18 wire several times around the ct and through the hole to obtain a good coupling.
Depending on the string I got the best results (highest output) between 6 and 12 loops.

Third: I moved the magnets around lengthwise, where I found an optimum by placing 1 magnet as near the end of the fretboard as possible
and 1 magnet just besides the bridge. I've altered the height of the magnets by turning the screws. I've tried to put the magnets as high as possible.
(I realise that the magnets of the regular pickups have there (small) share also).

Fourth: I switched between all 7 strings and tried to obtain a reasonable output / string. This was the case for the four highest strings.
I'm having difficulty to get a fair output for the bass strings, maybe because of the lower resistance of the strings (compared to the resistance of the ground return).

One of the very cool features of this design is that there is no cross-talk at all.

For now, there are a few things which I have to deal with:
1. The low output of the three lowest strings
2. The string is picking up some noise, which is acceptable for the highest strings, because of the greater output, but is way to much for the lower strings
3. There's a little bit of hum noticable when I touch the string
4. The ct needs to get shielded, so I have to figure out a compact way to store 7 ct's and 1 or 2 output jacks in one cavity
5. How do I transport 7 balanced signals the easiest way?

I would gladly receive some feedback.

Hans

Hans,

What is the input impedance of the amplifier or mixer? The reason why this is important is that CTs with a high turns ratio transform the primary string impedance circuit (including the turns through the CT) by the square of the turns ratio. The turns ratio is calculated by dividing the number of turns in the primary into the number of turns in the secondary.
I suspect that your CT has an AC resistance of between 5K ohms to 20K ohms based on the primary string circuit resistance and the number of primary CT turns. Make sure you use the appropriate input on your mixer to accomodate this source impedance without loading it too much.

With moving coil/string pickups, the length of the magnetic field under the string determines it's output. On the lower output strings try adding more magnets in line with the strings. Look to obtain some .25 inch X .25 inch X 1 inch long or longer ceramic magnets to lay under the weaker strings.

The Extech 380193 LCR meter would be a valuable tool for your experiments.

Make sure that the resistance connection from the actual string through the tuner is a good low resistance connection. Try it direct on the lower output strings to see if there is a loss in the tuner/CT connection.

Make sure that the common ground return is connected to the main ground going to the mixer/amp input to minimize hum.

I hope this helps.

It is good to see your progress.

Joseph Rogowski

[MisterBzr]

I think the impedance of the mixer is 600 Ohm (I found the tech specs of the Alesis IO2 and I'm using the IO4 but I presume the mic preamps are the same).

Yes, I've grounded the ground return with the main ground.
I will add some extra magnets for the thicker strings.
I hope to do some experiments with them next week.

[MisterBzr]

I've did some measurements with a LCR meter and calculated the turnratio using: Zp/Zs=(Np/Ns)^2 to match the 600 ohm impedance of the mic preamp.
This is what I found:

String	Gauge	Zp	Turnratio
C	0.145	0.366	1:40
G	0.09	0.528	1:33
D	0.055	0.643	1:31
A	0.036	1.094	1:23
E	0.026	1.36	1:21
B	0.013	1.853	1:18
F#	0.009	3.883	1:12

Am I right about the turnratios?

I also did some experiments with a 1:500 ct. I've measured a better output with my basstrings when I wrapped the wire 14x around the ct, (which is the maximum amount of times that the wire will fit in the hole of the ct) and using an extra magnet.

@Joseph: Why do you suggest to use ct's with 1:500 and higher turnratio's instead of transformers with smaller turnratio's, is it to get a better coupling and /or higher voltage output?
Can't I use transformers with a smaller turnratio and a thicker wire to lead through the transformer?

From what I hear without any eq and reverb, I like the sound of the tone it produces.

[bbsailor]

I've did some measurements with a LCR meter and calculated the turnratio using: Zp/Zs=(Np/Ns)^2 to match the 600 ohm impedance of the mic preamp.
This is what I found:

String	Gauge	Zp	Turnratio
C	0.145	0.366	1:40
G	0.09	0.528	1:33
D	0.055	0.643	1:31
A	0.036	1.094	1:23
E	0.026	1.36	1:21
B	0.013	1.853	1:18
F#	0.009	3.883	1:12

Am I right about the turnratios?

I also did some experiments with a 1:500 ct. I've measured a better output with my basstrings when I wrapped the wire 14x around the ct, (which is the maximum amount of times that the wire will fit in the hole of the ct) and using an extra magnet.

@Joseph: Why do you suggest to use ct's with 1:500 and higher turnratio's instead of transformers with smaller turnratio's, is it to get a better coupling and /or higher voltage output?
Can't I use transformers with a smaller turnratio and a thicker wire to lead through the transformer?

From what I hear without any eq and reverb, I like the sound of the tone it produces.

Hans,

The turns ratio is calculated by dividing the number of primary turns into the number of secondary turns. The reason why 500 turn CTs are good is because with only a single turn of wire, about 500 microohms under six guitar strings in a 2.25 inch wide string loop (with a magnet in the center) going through the CT primary you can adjust the output of the secondary to be within the low impedance microphone range of from 150 to 300 ohms by adjudting the thickness of the wire or winding additional turns on the primary through the toroid coil. However, since the string and the ground return forms the string loop source string loop impedance, you want to match this higher impedance (a few ohms to a fraction of an ohm) to the CT better than with just a single turn. Your 600 ohm input impedance is pretty low to be a bridging impedance. Typically the mic input impedance, while rated for low imepdance microphones, is actually about 10 times higher than the real microphone impedance.

If the CT has an output impedance of 600 ohms going into a 600 ohm load, you loose half your signal in this matching situation. Measure the actual impedance of the mixer input and strive for a loaded CT output impedance (loaded with the string input load) to be in the range of one fifth (minimal) to one tenth (optimal) the real input impedance. This is called "bridging impedance" and is very important for properly loading transformers located inside microphones.

Read up on transformer theory at the Jensen Transformer web site. Also look up "ribbon microphone matching transformers" on the web to see how these low imepdance ribbons are matched to thansformers to match these transformers built right into the body of the microphone and feeding balanced line cable in the 150 to 300 ohm impedance range. They keep the turns ratios between 1 to 37 to about 1 to 50 to control the output impedance to match the mixer's real input impedance which is typically 1.5K ohms to about 2K ohms. When the turns ratio gets much above these ratios, the impedance rises by the square of the turns ratio times the input impedance of the ribbon or in this case each individual string. I have a Prem Magnetics SPCT-251 CT that has 2000 turns on the secondary and I added square copper wire .160" X.160" on a single shorted loop as small as I could make it. The CT had an output impedance about 800 ohms. Divide 800 ohms by 4,000,000 (2,000 squared) or 200 microohms to see what I mean. This is much too high for a mic input but when I make the loop long enough to span six strings, I end up with an output impedance of about 3K ohms or 750 microohms. The output of current transformers is based on how much current is developed in the primary.

Read up on leakage inductance as it appears as an impedance in series with the primary and has the effect of limiting the amout of current that can be generated in the primary and ultimately transformerd to the secondary. A single loop has a higher turns ratio but with impedances in your string ranges, you need better coupling to the primary circuit with more turns. However, more turns lowers the rurns ratio. Theory can only take you so far, then it is build it, measure it, alter it, measure it again and finally listen to it.

Look at the very low resistance of the metal frames of the Lace Alumitone pickups to see what I mean. On the side you will notice interlocking "C-shaped" transformer laminations going through the alumium frame forming a current transformer. The location of this transformer lamination is in the place where the series connection of both single loops join and form a humbucking primary. By using stamped or cut alumium frames, they maintain a very low resistance of the primary string loop and just add the transformer laminations and dual coils placed on these laminations under the alumium frame to complete the CT circuit.

Since we are repurposing commercial off the shelf CTs for this unique guitar pickup application, we are working in new technological territory and must use known transformer theory, innovative thinking, and listening to find the best sound balance. Different transformer cores couple better at higher or lower frequencies and have different amounts of leakage inductance which affects the output level, output impedance and tonal spectrum.

Another type of transformer to try is a commercial miniature metal laminated E-I frame audio matching output transformer 8 ohms center tapped to 500 ohms up to about 1.2K ohms. Go to the Mouser Electronics web site. http://www.mouser.com/ds/2/449/XC-600127-202003.pdf Use the 8 ohm center tapped side as the primary and try the full winding and the center tap to find the best match. When you use a primary string impedance range from 0.3 ohms to about 3 ohms the transformer output will be in the 200 to 400 ohms range. Try to keep the transformer output impedance between one fifth to one tenth of the real input impedance of your mixer. Try to use the line input impedance if your transformer impedances are too high for the mic input. My experience is that the voltage generated on the string after the first matching transformer is well above the output range of a microphone but well within the input range of a mixer.

The optimum match is a balancing act between converting the actual voltage generated on the unloaded string of about 3 to 5 millivolts to a mic impedance from 150 to 300 ohms and a level that will work well within the range of the mixer input with a good signal to noise ratio. Read the voltage generated on the string unloaded, then attach the transformer and read it again. If the level drops to one half the unloaded voltage, the transformer load equals the string circuit impedance. You can use a length of magnets from the rear of the neck up to the bridge to obtain what I call the initial generated output balance. Then, by using various turns ratios on the current transformer or miniature audio transformer you can adjust for tone as each string only has about a 2 octave primary frequency range. When you are using one or two turns on the primary of a current transformer you are still in the current transformer domain and primary current primarely determines the output. Lower impedance (below a milliohm) equals more output. But when you start using multiple primary turns and impedances in the 0.5 to 2 ohms range you are starting to get into the more traditional voltage transformer range but not quite.

Keep good notes about individual string output, turns ratios, input impedances, output impedances and subjective tonal evaluations. We are collectively plowing new ground and sharing our collective experience and wisdom gained through experimentation. There is no reason why this new form of guitar pickup cannot be optimized and become more common as the guitar becomes a more universal input device and MIDI input for computer generated sounds.

Thanks for sharing your results.

Joseph Rogowski

[thethreemusketeers]

Joseph,
We are students of mr Bezemer and are currently working on a test model.
We are working with a seven string instrument, all strings have their own loop and coil.
They all have their own individual xlr cable output.
The 8th output is meant for the pickup, this is for testing purposes.
The idea is to use a mixing unit to be able to adjust each string individually.
Eventually we want to replace this for a d'sub multi plug xlr for convenience

We would like know what you think of it and if you have any advice or have any questions about it.
Potential questions from our side will follow later.
Willem, Jessie and Davey

[bbsailor]

Hello Musketeers,

Please identify the model of the toroid current transformer that you are using?

What is the actual input impedance of the mixer that you are using?

It seems that your goal is to obtain a good level and low noise output from each of the seven strings to allow individual fine tuning of the volume at the mixer?

Please identify your plan to test the output of your results?

What test equipment do you have access to?

It is good to see you tinkering in areas where few have gone before!

Joseph J. Rogowski