Moving Coil Pickups for the Technically Curious

[Rick Turner]

I think the only valid claim might be the R/C network on the "large coil".

BTW, this does bring up one of the interesting issues with dummy coil hum canceling, and that is the role of the core of the coil both in terms of the inductive effect as well as the potential for capacitive coupling of induced electrostatic hum if the inner end of the coil is "hot". This was not a major issue for the Alembic system as we used ceramic magnet cores for the pickups and Plexiglas cores for the dummy coils...which were, by the way, rarely of never the same size as the pickup coils once we went with the active hum canceling system. So the whole idea of using different sized coils for pickup and hum canceling really goes back to 1972 or so. Prior to that, Alembic used a passive dummy coil per pickup with those coils being stashed in the electronics cavity. There are probably around 25 instruments that we made that way, including "the Pretzel guitar" that I made in 1969 which will be on exhibit at the Museum of Art and Design in New York in the fall of 2011.

As an aside, I see myself quoted elsewhere on the forum here re. potting coils. I've gone conventional now...vacuum wax potting. It works.

One other thing...I've just wound some pickups in both 40 and 44 gauge Teflon coated wire. Sounds good... No big deal to strip with some 220 sandpaper for soldering. Glad I bought it surplus by the pound, though; I checked prices...whew! Might have to save the stuff for exotic pickup winds.

[David Schwab]

I think the only valid claim might be the R/C network on the "large coil".

BTW, this does bring up one of the interesting issues with dummy coil hum canceling, and that is the role of the core of the coil both in terms of the inductive effect as well as the potential for capacitive coupling of induced electrostatic hum if the inner end of the coil is "hot". This was not a major issue for the Alembic system as we used ceramic magnet cores for the pickups and Plexiglas cores for the dummy coils...which were, by the way, rarely of never the same size as the pickup coils once we went with the active hum canceling system. So the whole idea of using different sized coils for pickup and hum canceling really goes back to 1972 or so. Prior to that, Alembic used a passive dummy coil per pickup with those coils being stashed in the electronics cavity. There are probably around 25 instruments that we made that way, including "the Pretzel guitar" that I made in 1969 which will be on exhibit at the Museum of Art and Design in New York in the fall of 2011.

I'll have to go check that exhibit out.

I loved the trapezoidal pickups you used to make. Cool idea.

Here's a cool shot of the dummy coils.

As an aside, I see myself quoted elsewhere on the forum here re. potting coils. I've gone conventional now...vacuum wax potting. It works.

Yeah, that was me. I was using varnish or polyurethane for a while, but I went to wax now. They took too log to dry.

One other thing...I've just wound some pickups in both 40 and 44 gauge Teflon coated wire. Sounds good... No big deal to strip with some 220 sandpaper for soldering. Glad I bought it surplus by the pound, though; I checked prices...whew! Might have to save the stuff for exotic pickup winds.

Yeah, that's expensive stuff! I've been using 40 gauge for some bass pickups lately.. it's has a cool tone. I've been trying to find some 41, but I never see any.

[earthtonesaudio]

I am almost finished building a non-conductive tailpiece to go with the Graph Tech-style bridge saddles in my guitar. I have some aluminum tape for the string ground return (will probably run it under the tuning pegs for electrical contact), and a Lexan plate for the tailpiece with some extra space for mounting a breadboard.

I plan on using a single opamp with a bipolar supply at first. It will be wired as a current summing amplifier with the strings being the inputs and the string ground providing the noninverting bias voltage and hopefully some common mode rejection at the same time. Next I will experiment with separate op-amps for each string and messing with the gain and frequency response for each string separately. My biggest concern is noise. I have some TL074 to try as well as some LM3900... I'm interested to see if the Norton amp performs better in this setup.

[Razar]

Hello all!

I recently made a video with my bass guitar that works on the principle that you discuss here. I posted the link in talkbass forums and one guy sent me a link here. So may be it would be interesting for you to check out the realization of the ideas you discuss here.

[David Schwab]

Hello all!

I recently made a video with my bass guitar that works on the principle that you discuss here. I posted the link in talkbass forums and one guy sent me a link here. So may be it would be interesting for you to check out the realization of the ideas you discuss here.

Welcome to the Pickup Makers forum!

Your bass sounds very good!

[bbsailor]

Hello all!

I recently made a video with my bass guitar that works on the principle that you discuss here. I posted the link in talkbass forums and one guy sent me a link here. So may be it would be interesting for you to check out the realization of the ideas you discuss here.

Razar,

You did a nice implementation of the moving coil pickup for your bass.

David,

Thanks for your interest in this topic.

Since you are a bass maker here is a quick way to try this on a 4-string bass without needing a common ground return.

1. Short string pairs together at the nut so that the E-A pair and D-G pair are in series. This allows the CT string-pair connections to be made behind the bridge. This can be easily done with a strip of copper foil in the string nut.
2. Make sure the bridge electrically isolates each string using non-metallic bridge inserts. Make a tail piece that also keeps each string electrically isolated such as using Lexan or Plexiglass with copper rivets that the strings can pass through.
3. Obtain a few CST-1005 (1000 turn secondary) current transformers (CT) and solder a heavy jumper wire from the E-string rivet to the A-string rivet and pass that jumper through the CT center opening. Do the same thing with the D-G string pair.
4. Run each string pair CT to an XLR connector into a low impedance microphone input. If hum is a problem, reverse the phase of one CT.
5. Ground one pair of strings to the XLR pin 1 (ground connection).

You will notice a much faster string attack and wider dynamic range.

If you want independent control of each string, use the truss rod as a common ground return and use one CT for each string looped through the CT and back to the ground return. The mechanics of how you accomplish this is where the creative aspects of making this pickup come into play. Here are some possible ways.
1. Use a brass nut
2. Make a collar (like a large keyhole shaped soldering lug) that fits under the truss rod adjustment screw and is connected to the brass nut.
3. Find a creative way to bring the end of the truss rod out of the neck heel for a low impedance connection back to the bridge to finish the ground return.
4. Design and fabricate a bridge where each bass string goes through the center of a separate CT.
5. Passively mix the 4 CTs with a volume control to adjust the level of each string.

With the nut end of the string being the common ground, there is no problem with the strings touching the frets.

You can do a quick experiment with passing one string through a single CT with a heavy alligator clip jumper wire making a ground return just to do a comparison of the string attack and dynamic range compared to traditional high impedance pickups. If you try this let us know what you think?

Do a web search on "Stringamp" to see how this idea is implemented on a violin or cello.
Very little technical information is available at these web sites. Magnets are not visible on the instruments but may be hidden under the end of the fingerboard.

Joseph Rogowski

[Man Of Steel]

I recently made a video with my bass guitar that works on the principle that you discuss here.

Cool beans, Razar!

And, as David Schwab has already said, it sounds great! Your video is nicely done, as well.

What transformers are you using, and how have you wired them?

[MrCandy]

Transformer in system like that is the most important part, so that it would be nice to learn more about it in detail, its AFC in particular.
If we consider industrial current transformer, then its AFC will be essentially linear
(the same goes for mic transformers) unlike hi-impedance resonant pickups.
Thats why transformers like that will work nice in acoustic instruments.
If we use thansformers in electro guitars/bass(for rock, pop, modern music), then they must have characteristics like hi-impedance resonant pickups.
In general, this is a whole topic needed to be studied deeply

[Razar]

David Schwab, bbsailor, Man Of Steel and MrCandy thank you all for your interest!

Here is a schematic that I actually use on this bass - it's the same as in video. There are also two 1 uF caps and two 10 Ohm resistors wired in parallel with the output of each string couple - they work like filters for the frequencies above the audible range.

When I connect this bass to mic inputs of my sound card (echo audiofire4) I have about 50 dB signal to noise ratio - I think it's about the same as for usual humbuckers, and it's better than single coils. And it sounds OK in mix with the other instruments, especially with EQ. Actually you never hear 'hum', like 50-cycle and harmonics, it's more like white noise 'hiss', and it depends on the mic preamp used. That's because mic preamp has to work on high gain, so its own noises may become audible on high volumes, but the bass actually lowers them with its low impedance.

String couples have reversed phase, bbsailor also suggested it. It has some 'humbucking' effect that you can hear on high volumes - for two couples the hiss is lower. And it helps when some strong sources of electromagnetic noises (like light dimmers) are near.

I think that in any way, high quality (or even not expensive) mic pre brings a lot for moving coil systems. It's better to buy something like that than to try using some crappy opamps in onboard preamps - they won't work better.

About the transformers. I haven't tried some cool stuff with permalloy shielding, something specially designed for ribbon mics - that might work better. The small transformers that I tried didn't bring any sense in construction, they only catch a little more hum.

Connecting the nut to the ground... I think it's hard to realize and practically it's not necessary. Like in microphones - the ground wire is not connected to th signal line. The signal goes through balanced cable all the time, double mic cable is used. I played this bass on stage connected through the multicore cable to the FOH mixer several times and it had no problems.

About the bridge construction: I purchased a Kahler bridge assembly and used only saddles from it. I mounted them with bolt screws directly to the wood, and soldered the cables to the screw nuts on back side of guitar. I think it's rigid.

And the last thing - about the frets/fret noise. In my case with separate preamps for each couple, there are no problems with them - no extra sounds, clicks or whatever.

[MisterBzr]

I've made a test setup by connecting a agw 14 wire with my brass nut and my bridge. I led the wire through a AS-104 1:500 current transformer. When experimentated with different magnets I got a pretty good signal, but I was also picking up some noise.

Maybe I want to make use of this concept. I want to make a guitar with 2 outputs, one for the bass side and one for the treble side.
From what I've read I can make a pickup by having one (aluminium) bridge for the bass side and one for the other. The bridge will be connected to the trussrod, the trussrod with the aluminium nut. Both the loops are led through a 1:500 current transformer.
I'm I right so far?
I'll need a long but rather weak magnet under the strings, would this be something or is it too weak?

Can this setup be made humbucking?

Hans

[bbsailor]

I've made a test setup by connecting a agw 14 wire with my brass nut and my bridge. I led the wire through a AS-104 1:500 current transformer. When experimentated with different magnets I got a pretty good signal, but I was also picking up some noise.

Maybe I want to make use of this concept. I want to make a guitar with 2 outputs, one for the bass side and one for the treble side.
From what I've read I can make a pickup by having one (aluminium) bridge for the bass side and one for the other. The bridge will be connected to the trussrod, the trussrod with the aluminium nut. Both the loops are led through a 1:500 current transformer.
I'm I right so far?
I'll need a long but rather weak magnet under the strings, would this be something or is it too weak?

Can this setup be made humbucking?

Hans

Hans,

Here is where experimentation and listening comes into play with what sounds best. Try to obtain some current transformers (CT) with 1000 up to 2000 turns. Then connect a wire of about awg 24 from the end of the string through the CT one, two, three up to a few times and back to the ground return and listen to what number of turns sounds best for each string. You can combine all six strings by wrapping the connections from all the strings through one CT and mix the outputs in the CT. Guitar strings typically have a fraction of an ohm up to about 2 ohms impedance due to the resistance of the string wire. Try obtaining some small 3.2 up to 8 ohm primary transformers with a 10K ohm up to 50K ohm secondary. Place the low Z side of the transformer across the string. Feed the high Z side of the transformer into the amp and listen to the sound change as you place a hand held magnet near the string and move it from the neck to the bridge.

Look up the word "stringamp" and see a European violin pickup maker who uses this technique to amplify his violins. He does not technically talk about how he does it but after these experiments you will know.

Joseph Rogowski

P.S. To make it humbucking just reverse the wind direction of adjacent strings.

[MisterBzr]

I've googled on stringamp and found someone who is testing this right now: Electrodynamic pickup
The stringamp website indeed didn't offer that much information.
I'm planning to make a test setup with a couple of strings, when I have some results I will post them.
Hans

[bbsailor]

I've googled on stringamp and found someone who is testing this right now: Electrodynamic pickup
The stringamp website indeed didn't offer that much information.
I'm planning to make a test setup with a couple of strings, when I have some results I will post them.
Hans

Hans,

Study up on transformer theory. Remember that the impedance ratio being transformed is based on the square of the turns ratio. A current transformer such as a Triad CSE187L uses a E-I laminated frame with a single AWG12 U-shaped primary of about 200 microohms. If the primary is bent and soldered together to form a shorted loop it has 79 ohms of AC resistance at 1Khz, 8.4MH and a Q of .6692. Now, if I add the U shaped primary wire removed from another CSE187L transformer to make a longer primary loop extending about 1 inch from the plastic mounting pad, it has 130 ohms AC resistance at 1Khz, 13.7mH and a Q of .6637. What this tell me is that the extended loop respresents 51 ohms so the same loop in the CT should also respresent 51 ohms. But it is 28 ohms higher and this additional resistance represents the inductance value (AL) of the laminated core material. All of this contributes to the output impedance that will feed either you mic input impedance typically 1.5K to about 2.5K ohms or the microphone transformer. What you want to do is design backwards from what you already know about the input impedance you are working with and controlling the matching to that impedance with the string sensing loop wire gauge as using thicker string sensing loop gauges favors the lower frequencies and ultimately controls the voicing of your pickups where you ear is the final judge. With a 200 microohms fixed primary turn plus say another 300 microohms to make a 2 inch long string sensing loop you would have 500 microohms times 250,000 for about 125 ohms output plus about 28 microohms for the AL value times 250,000 for a total of about 132 ohms, plus about another 100 ohms for leakage inductance for less than perfect magnetic field transformation for a measured value of about 230 ohms.

If you are using the moving strings as primary string loops you are in the .5 ohms to 2 ohm range of impedances, Look at using transformers with a 3.2 onm 4 ohm 8 ohm or 16 ohms primary. If the secondary has 500 ohms it will work well into a mic mixer. If you want to go directly into a high Z amp you need a secondary in the 20K to 200K range. You vibrating string will typically have between 2 to 5 millivolts P-P generated in it that needs to be effectivly converted to a more useable impedance and output level. Here is where your knowledge of transformer theory comes into play. Go to the Jensen Transformer web site to see very detailed technical information about audio transformers.

Joseph Rogowski

[MisterBzr]

Hans,
...
Go to the Jensen Transformer web site to see very detailed technical information about audio transformers.

Joseph Rogowski

Joseph,
Thank you for your reply, I've found the chapters / articles on the Jensen website.
To be continued...!
Hans

[MisterBzr]

Joseph,

I've read the chapter on transformer theory on the Jensen website and searched this forum for information.
I've noticed a couple of things:
1. (As you said) there is a wealth of information on the Low Z / moving coil pick up design on this forum, but the information is scattered;
2. (and because of this?) The same questions are asked several times (and patiently answered).

Would it be an idea to create a subforum on low z / moving coil pick up design, where all the threads on this topic can be stored and with a sticky thread which contains a primer / f.a.q. on the subject which can be used as a source of reference?

I'm very thankful for all the useful information!

Hans