Making a great-sounding magnetic pickup out of scrap magnets
I recently found a big-bodied 12-string acoustic guitar which has a sufficiently-wide neck to accomodate my rather large fingers. I always add a peizo-pickup to my acoustic guitars, as I prefer to allow for some tailoring of the finished sound. In the case of this guitar, despite having a sweet tone, its acoustic output is lower than I would have liked, so I intended adding a pickup to this also. Before removing the strings I also toyed with the idea of adding a magnetic pickup, which would add a whole new dimension to the sounds that the instrument would produce, but browsing around on the Web brought the idea of purchasing a suitable magnetic pickup to an abrupt halt – a halt caused by the unashamed greed of pickup manufacturers in their ridiculous pricing of these units.
There’s almost as much rubbish talked about guitar pickups as there is about any other consumer electronics, usually in an effort to get the customer to spend more than he intended, and much more than the hyped-up goods are worth.
A magnetic guitar pickup is usually a magnet, or magnets, surrounded by typically 4-5 thousand turns of fine enamelled copper wire. This stuff is very much less stronger than a human hair, and therefore difficult, if not impossible, as well as being incredibly tedious, to wind yourself, to say nothing of the cost.
The traditional design also suffers from a couple of drawbacks. It’s relatively large inductance limits the capture of the higher frequencies produced on the guitar, as well as being a wonderful hum-finder. Although the latter can be dealt with by changing the coil design (hum-bucker), the former can be a serious problem, only partially overcome with treble-boost circuits and the like.
Build it yourself from junk
Mainly because of cost, I decided to build my own magnetic pickup. In any project I attempt, I try to play to my strengths, avoiding gaps in my knowledge and/or experience by sticking to what I know and do reasonably well.
Since fine copper wire is both expensive and difficult to use, the choice for the coil is much fatter wire. The downside is that there will be a large reduction in turns around the magnet(s) and consequently less output from the pickup. The upside will be that the coil will have very much less inductance than the standard pickup and therefore both it’s frequency response and resistance to hum-pickup will be increased.
|The photo opposite shows the prototype magnetic pickup mounted across the bridge end of the sound-hole. In the finished assembly, the guitar lead is routed inside the body to a socket replacing the strap-peg.|
With the previous in mind, and following my personal 1st rule that I always should try re-using/modifying would-be trash before buying anything, I looked amongst the workshop ‘junk’ for candidates for magnet and wire.
I dismantled an old (crashed) hard disk-drive, and removed the upper and lower carrier arms with the strong field magnets attached. The magnets were bonded to the non-magnetic arms using some sort of resin. They were easily parted by holding the arm in a vice, and putting a small screwdriver against one side of the joint and administering a sharp tap with a small hammer. These magnets were semi-circular in shape and magnetised through their depth, so it was possible to snap them together to form a larger semi-circle. (see photos below)
A word about the magnets. These, although reasonably thin, are incredibly powerful, and at one point I managed to painfully nip the skin on one of my fingers when they flipped together unexpectedly. I suspect they are of the rare earth type – probably samarium cobalt. Because they are so powerful, they tend to attract any small bits of metal swarf around, so keep your work area scrupulously clean, and inspect the magnets for detritus before assembly of the coil.
For the coil, I used a medium thickness enamelled copper wire retrieved from a defunct UPS transformer. I measured it’s diameter with a digital vernier, and this is 0.56mm, or 0.022 inches, which corresponds to 24swg (here in the UK) or 23awg in the USA.
For the coil former, I cut 2 pieces of Paxolin (Tufnol/SRBP) 40mm X 68mm (1.57inch X 2.7inch) and 1.37mm (0.05inch) thick.
So far, I hadn’t bought anything, as all of the above were items from my personal re-cycle stash. The idea is to simply build a pickup to suitable dimensions, using anything to hand, and then ‘make’ it work using hi-gain, low-cost electronics.
After carefully marking a 4mm hole to clear the magnet inner circumference on the top coil former, I made a sandwich of the magnet and 2 pieces of Paxolin together with a small quantity of epoxy resin glue. (Araldite here in the UK) Holding the pieces together while the resin set was a no-brainer. I simply sat the bottom coil-former on top of my steel vice, and placed another small piece of steel on the top of the sandwich – the magnets did the rest. I then left the sandwich a few hours for the resin to fully cure.
When the resin was fully-set I drilled the 4mm hole through the whole assembly for the central fixing bolt. I also drilled 4 small holes sufficient to accomodate the thickness of the wire in one corner of the bottom former. I threaded one end of my wire through two of the holes, locking it in position, then gently, but firmly wound the wire on until the former was full. I cut to length and then locked the finish by threading the wire through the remaining 2 holes. I had measured the length of wire before commencing, so I measured the remainder and calculated that I had wound on just less than 2 metres. (78 inches)
Testing the pickup
I scraped off sufficient of the enamel on each wire and soldered a single-core, screened audio cable to the pickup.
I measured the inductance (just out of interest) and this was just over 9μH, then connected the lead to my oscilloscope. I fixed two blobs of Blu-Tak putty on the underside of the pickup and fixed it firmly next to the bridge on one of my acoustic guitars.
I left sufficient clearance under the strings so that even when fretted up the neck, no string came closer than an 3-4mm (0.125 inch) to the pickup. I plucked each string in turn and noted it’s output on the oscilloscope. Each string delivered between 1 – 3 milli-volts – very low by guitar pickup standards. Encouragingly, each string trace showed a large number of ’sprogs’, as well as the string fundamental frequency, hinting at a very ‘bright’ harmonic-rich output.
The fairly-low output of the pickup needs to be amplified to be compatible with the standard input on an electric-guitar amplifier. Also it’s very low impedance need to be matched in such a way that losses are limited, and frequency-response retained. Good-quality microphone capsules have a similar low output, but tend to have a higher impedance, so a microphone transformer will not be suitable here – anyway such a component is likely to be costly, and as such is ruled out. A suitable transformer would be one with a very low impedance input and a medium (say 800 ohms – 2K ohms) output impedance. A very cheap and likely candidate is a low-power transformer intended for transistor audio amplifier output, but used ‘wrong-way-round’. This can be followed by a low-noise gain block implemented by ultra-cheap and readily available operational amplifiers.
For the transformer, my local electronics stockist (here in the UK) ESR has the LT700 (part number: 309-020), priced at £0.94, (plus the dreaded VAT) whose spec is as follows:
Secondary 2 x 3.2Ω
Frequency Range ?
Dimensions 16 x 20 x 16mm
ESR will supply goods to anywhere in the world, at competitive prices, and can suppy all of the components (except the hard-disk magnets!) used in this project. I am happy to recommend them.
For the record, the information on a label attached to the transformer is as follows:
PRI: 1.2k ohms
sec: email@example.com ohms
I measured the inductance of each winding, and the primary winding measured 150 milli-henries, each secondary measured 4μH.
With the experimental pickup connected to one of the 3 ohm windings, the turns ratio, hence the ’step-up’ in voltage is 1:20. (given turns ratio is square root of 1.2KΩ/3) In practise this was found to be optimistic, the voltage gain was nearer 1:12, still not enough for to match the guitar amp. More importantly though, the traces on the oscilloscope showed the ‘brightness’ of the signal to be preserved. This encouraged me to lash up a pre-amplifier from a couple of op-amps.
Experimental Breadboard Version
In the circuit below, the 1st op-amp’s input impedance is set to 47KΩ by R1, and it’s voltage gain by the combination of R2 with R5 and R6. This gain can be varied between 0 and about 36. The 2nd op-amp buffers the 1st stage and affords another voltage gain of about 4. The TL072 op-amps have JFET inputs which have a very large input resistance, allowing us to define the input impedance by choice of R1. I’ve chosen 47KΩ to accomodate a wide range of input impedances, as the pickup design may have more turns depending on wire used.
|A few words about the diagram opposite. First I couldn’t find a suitable symbol for the LT700 transformer in Eagle, (my PCB/Schematic layout editor) so I’ve shown this at the far left as two rows of pin headers. Secondly, there may be confusion over the value of R6 and R5. R6 is 1Kohms and R5 is 470 ohms. The two 100nF decoupling capacitors should be sited as close as possible to the op-amp supply pins V+ and V-. I tend to use dual-supplies of 15-0-15 volts with my op-amp projects, so if you wish to run the pre-amp on a 9v battery, then the 2 X 10K resistors, together with the 2 X 100uF capacitors should be included to provide a ‘GND’ point that is one-half of the supply voltage. A higher supply voltage could be used to a maximum of 30volts – make sure any electrolytic capacitors are rated accordingly. The final 2.2uF output capacitor should be of the reversible type when used with a dual-voltage supply, but can be of the ordinary polarised type if a single supply rail is used.|
|The photo shows the 8-pin dual op-amp TL072, with LT700 transformer on the right, together with the lead to the pickup. My ’scope leads are connected to the input and output of the pre-amp. A screened lead also connects the output to my bench power-amp. Note in my lash-up I didn’t include the 50K preset (R2 on the schematic), but simply substituted this for a 47K fixed-value resistor. I recommend use of the preset if you build the pre-amp, as this will allow for a wide choice of gain.|
|Use of these plastic bread-boards enable a very quick assembly to test a circuit and/or ‘tweak’ component values.|
After tweaking the resistor values (to arrive at those in my schematic) it was time for a test. Now my bench amplifier is a rough and ready 30watt design built around a single TDA2050 IC, and could be classed as lo-fi, but in no way represents a typical guitar amplifier as regards performance. The speaker is a cheap 8 inch chassis, and this setup is normally just used for testing audio stuff actually works. The frequency response of the speaker is not very good, but plugging in and playing the guitar I was blown away with the cracklingly crisp highs generated by the prototype pickup. Each string sounded bell-like, and output was more or less consistent across each string. I’ve a whole range of guitars including both good-quality Fender and Gibsons, and I’ve never heard a pickup sound so bright. (see downloads section for mp3 of my audio tests)
Building Mark I Pre-amp
Entering a schematic in Eagle usually means that a printed circuit board layout can be generated reasonably quickly, but at this stage I decided to leave some scope for further experimentation and build the 1st version on punched stripboard. In the UK this goes under the title of Vero Board – what it is called in your country could be something else. Anyway it’s an SRBP board punched with a matrix of holes spaced at 0.1 inches with strips of copper running in one direction across the holes. I’m not going to detail the building of this version on the stripboard, but it should present you with no problems if you haven’t the facility to make printed circuits.
|The pre-amp occupies the upper half of the board, with input transformer and dual op-amp (in a 8-pin DIL socket) clearly visible. The section on the lower-left is the 2 filter capacitors and supply connector to +15, 0 and -15 volts. The section on the right is a tone control built around a single op-amp, and you can just make out that there are two sets of pin-headers where I’m trying various resistor and capacitor values, without having to solder and unsolder these to the stripboard itself. The reason for the cut-out will be apparent in the following photos.|
Parts List For Magnetic Pre-Amp
All resistors 1/4 watt
R1 : 47K ohms
R2 : 50K ohms log taper pot (or preset)
R3 : 4K7 ohms
R4 : 22K ohms
R5 : 470 ohms
R6 : 1K ohms
R7 : 470 ohms
R13, R14 : 10K ohms
C1 : 470nF polyester
C2 : 2.2μF 63v electrolytic reversible (non-polarised)
C8, C9 : 100nF ceramic 63v
C10, C11 : 100μF 25v electrolytic
Op-Amp : Texas instruments TL072 8-pin DIL package
1 X 8-pin DIL Socket
J1 : 2-pin 0.1inch pin-header
X1 : 2-pin 0.1inch pin-header
X3 : 3-pin 0.1inch pin-header
Transformer : LT700 (see text above)
A Loud/Soft Footswitch
A habit I’ve gotten into recently when building my own guitar FX pedals is to provide a footswitch for loud/soft. I use a DPDT (Double-pole,double-throw) footswitch using one pole to connect the output of two pots, which can be used to set the soft/loud levels output. The other pole is used to select an LED indicating which pot is currently active. Since a picture is worth a thousand words, I’ve included this arrangement below, to use or discard as is your preference.
Parts List For Loud/Soft Switch
LED1, LED2 : 3mm LED (any colour)
R1, R2 : 47K log taper (suggested)
S1 : DPDT Footswitch
R3 : 680 ohms
Note that I’ve specified the value of 47K (log taper) for the 2 pots. These should ideally be chosen to suit your guitar amp input, but anything up to 250KΩ should be OK.
|The rough-and-ready prototype box top panel showing the footswitch and controls. The two 3mm LEDs mentioned in the text are visible to the left of the two volume controls.
A front panel cover has been designed in ‘Front Designer’ and is discussed elsewhere in the text.
|The ‘temporary’ reminders of which socket is which give lie to my protestations of no loss of mental acuity despite my advancing age.|
I generated a top panel for the prototype in ‘Front Designer’, and I’ve included a picture of this below. Note that the finished panel has the design dimensions 90mm X 117. The Front Designer project is given in the Downloads section at the end.
Fitting The Pickup
The magnetic pickup could be firmly mounted semi-permanently near the bridge with sticky-back tape or similar, but since the pickup output is so bright to start with, I opted for the non-destructive option of mounting across the sound-hole using a simple clamp. A small length of 9.4mm wide (0.375inch) X 1.8mm (0.07) thick steel was cut to size, bent slightly to form a shallow bow, then drilled and tapped to take an M4 screw (4mm). A piece of Paxolin was cut to the same size and drilled to clear an M4 screw, then the pickup and clamp pieces were mounted on the sound-hole of my 12-string guitar, tightened using the centre screw just sufficiently that it won’t easily move. (see 1st photo on this page.)
The strap-peg of the guitar was removed and set aside, and a hole 8.5mm (0.3 inches) was drilled where the strap-peg screw had been, and the small-diameter audio cable from the pickup was routed through this hole ready to be soldered to a socket.
Note that at this point, I had decided to also fit a peizo pickup, and this meant that the fitting of a simple 1/4 inch jack socket wasn’t an option, so I intended using a 4-pin mini-DIN socket.
I made a strap-peg carrier out of 1.76mm (0.07 inches) thick extruded Aluminium, which I buffed to a fine satin finish, to take both the mini-DIN chassis socket, and the existing strap-peg, and fed the audio cables through an aperture cut in the carrier, then soldered both the magnetic and peizo pickup leads to the mini-DIN socket. IMPORTANT If you are fitting both types of pickup, don’t be tempted to use a common braid/screen for these. Each cable is destined to be fed to a different amplifier, and you will invite all sorts of earth ground-loop problems if you disregard this warning. I’ll say it again – each pickup signal and earth should be insulated from the other.
I fastened the carrier and socket to the guitar using smallish brass screws after slightly counter-sinking the holes in the mini-DIN socket.
The photos below show both the pickup clamp and output socket arrangement I used, plus an upside-down reflection of the now-naked trees in front of my house, and the junk on the workshop window-sill.
|Note how the bottom clamp piece has aligned itself to the magnetic field of the pickup.|
If you wish to add a tone-control similar to my prototype above, the following schematic shows the method I used.
|The circuit performs a treble-boost, with it’s performance primarily set with the components R9, R10, R11 and C4. Input from the pre-amp (taken directly from Pin 7 of the TL072) is applied to Pin 3 of the TL071. The base frequency at which boost starts to occur is set by R11 and C4, with overall gain determined by R9 and R10. If you want more overall gain, increase the value of the fixed resistor R10, whilst increasing the value of C4 will lower the frequency at which boost occurs. (In other words you can fiddle with these values to achieve the result you desire) The filter capacitors C10/C11 and GND voltage point divider resistors R13/R14 are not required if you already have these in the pre-amp, or are supplying the tone-control with +15, 0 and -15 volts. The two decoupling capacitors C1 and C2 (100nf each) should always be fitted, and as close to the supply pins on the op-amp chip as possible.|
Parts List For Tone Control
All resistors 1/4 watt
R9 : 25K ohms lin pot
R10 : 22K ohms
R11 : 1K5 ohms
R12 : 470 ohms
R13,R14 : 10K ohms
C1,C2 : 100nF ceramic 63v
C4 : 10nF polyester 63v
C5 : 2.2uF 63v electrolytic reversible (non-polarised)
C10,C11 : 100uF 25v electrolytic
Op-Amp : Texas Instruments TL071 8-pin DIL
1 X 8-pin DIL socket
Schematics & Printed Circuit Boards
I’ve included The Eagle project with schematics and suggested board layouts for both magnetic and peizo pickups below, together with the tone control facility mentioned above.
Downloads: Project Files
Downloads: Mp3 Test Tracks
Update 10th August 2010
I’ve heard reports of the zipped (complete collection) MP3 files below not downloading correctly, and ascertained that this is so. I’ve removed the ‘MP3′ from the filename, re-zipped the files, and the problem (I hope) has gone away. Note the MSE (Microsoft Security Essentials) can be guilty of stopping some zip files reaching your computer intact.
In the following, recording was done using on an old(ish) Yamaha tape deck, and an (even older) Shure SM58 mike 1ft away from the front of my Peavey 30-watt valve (tube) amp which was connected to the Magnetic Guitar pre-amp, and a PA speaker connected to home-built power-amp connected to the peizo pre-amp. The output of the tape was then captured using Sound Recorder in Roxio Media Creator, and the final wav files derived using the Roxio wave editor. Finally, these were converted to mp3 – again using Roxio MP3 music disk creator.
Note that there are no effects used – the phasing sound is simply the 12-string guitar itself. All of the tracks are recorded with the tone control described above set half-way.
Where the word ‘Both’ appears in the title, both types of pickup were switched on.
No plectrum is used, all tracks are picked with either thumb only (melodies), or claw-hammer style. (thumb and 3 fingers)
I’m still getting used to playing 12-string, so forgive fluffed notes and rattles!
Oh Yes! I nearly forgot! The guitar is a (reasonably) cheap ‘Freshman’ 12-string, which cost me ~ £150.00 from ‘GuitarGuitar’ here in the UK.
|Bonny Swans||Clean Amp setting|
|Celtic Rock||Slightly dirty Amp setting|
|Dancing At Whitsun (Both)||Both pickups on, slightly dirty, with some overloading of mike (apologies)|
|Dancing At Whitsun (Mag)||Mag pickup only, with clean amp|
|Dum De Dum||Slightly dirty amp|
|Lark In The Morning||Dirty amp|
|Little Tin Soldier||Clean amp|
|St. James Infirmary||Clean amp|
|Take Your Time||Clean amp|
|Walk In The Room (Both)||Clean amps|
|Walk In The Room (Mag)||Slightly dirty amp|
|All Tracks – zipped with playlist||(GuitarPickupTests.zip)|
And now for something completely different..