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Pre-Amplifier For Balanced Magnetic Guitar Pickup

This post is part 3 of the article ‘High Quality Guitar Pickups On A Shoe-string’, and discusses the fitting of an experimental balanced magnetic pickup to an acoustic guitar, together with a suitable pre-amplifier.

Although delighted with the success of my crude but effective magnetic pickup, the pre-amplifier required purchasing a small transformer, and I decided to see if this component could be eliminated. Given that this provided a voltage gain of around 12, removing it meant increasing the gain of the op-amp circuitry, so I decided to build a balanced-input circuit, in an effort to keep down hum and noise, and double-wind the coil around the magnet.

Another set of magnets was obtained from a crashed hard-drive, and using the same wire, the former was filled again, this time with the wire bifilar-wound. (2 wires used in the wind) A centre-tap should be made by joining the start of one wire with the end of the other, and this centre tap is then connected to the braid (screen) of a twin-cored audio cable, which becomes the GND connection.

Magnets on Spruce Former

Magnets on Spruce Former

The pickup former and magnets before glueing. Note there is no gap between the magnets, this is a trick of the light. The magnets and former should be glued together with epoxy resin (refer to Part 1) before winding the coil.
The bread-boarded prototype from which the schematic below was derived. Note the stereo socket connecting the pickup on the bottom left, and also that I’ve used a fixed resistor for R2. The +ve, (Red) -ve, (Black) and GND power-supply connections are visible on the right, while both the ’scope probe and mini-clip to the power-amplifier can be seen top-left of the board.
Bread-boarded prototype pre-amp

Bread-boarded prototype pre-amp

The experimental pickup Blu-tacked to one of my guitars

The experimental pickup Blu-tacked to one of my guitars

I covered the pickup wire with a thin strip of adhesive tape. Note the twin-core audio cable connecting the pickup to the pre-amp.
The pickup DC resistance was measured and found to total 2.75 ohms.


Schematic For Balanced Magnetic Pickup Pre-amplifier

Pre-Amp for Balanced Magnetic Pickup

Pre-Amp for Balanced Magnetic Pickup



Balanced Magnetic Pickup Pre-amplifier PCB and Component Layout

Below is the suggested layout for a PCB. The Eagle files are available here: Downloads: Project Files

PCB

PCB

PCB Components Layout

PCB Components Layout


Parts List for the Balanced Magnetic Pickup Pre-amp

All resistors 1/4 watt
R1,R3,R4,R5,R8 : 4K7 ohms
R2 : 4K7 lin pot/preset
R7 : 470 ohms
R9 : 1K ohms
R10,R11 : 10K ohms
R13,R14 : 10K ohms

C2 : 2u2 reversible (non-polarised) electrolytic 63v
C8,C9 : 100nF ceramic 63v
C10,C11 : 100u electrolytic 35v

Op-Amp : Texas instruments TL074 14-pin DIL package
1 X 14-pin DIL Socket
J2 : 2-pin 0.1inch pin-header
X1 : 3-pin 0.1inch pin-header
X3 : 3-pin 0.1inch pin-header

Texas Instruments Op-Amp Data Sheet for TL071-74: Op-Amp Datasheet

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1 comment to Pre-Amplifier For Balanced Magnetic Guitar Pickup

  • admin

    I received the following email from Rob of London asking to clarify a couple of points. I’ve included this here as a comment (with his permission) together with my reply.

    Hi Joe,

    A great webpage – tks for sharing. A few questions if I may……

    Re your pickup – the initial wind (before you added a secound outer coil), you never said how many windings you made – any idea, (or do you have the DC resistance of the coil?)

    re your second pickup , you decided to go balanced, so you simply wound two wires onto your bobbin at the same time – I’m after a very small humbucking low impedance pickup (which is how I found you webpages) …but I don’t want to wind two bobbins (ie the traditional way, becuase space is tight for the intended use), so I’m hoping your way cancels hum? Have I got the details of the winding correct……

    wire 1 start…………………………..(both wire wound the same direction adjacent to each other) ……………………………………..wire 1 end
    wire 2 start…………………………..(both wire wound the same direction adjacent to each other) ……………………………………..wire 2 end

    ….then, after finishing the wind, I then solder ‘wire 1 end’ to ‘wire 2 start’ – that ‘joint connection’ becomes the centre tap & the other remaining two wires are the +ve & negative?

    Many thanks once again – a brilliant attitude.

    rgds,
    Rob (in London)

    ———————————————————————————————————
    My reply:

    Hi Rob,
    Thanks for your email and the compliments.

    When I made the first pickup, my criteria was to produce a working device using scrap materials, but ensuring that what resulted was reasonable in size, and sound. Because scrap hard disk magnets come in all shapes and sizes, I had only a rough idea what sort of inductance would result. I therefore measured the whole length of wire I had (around 5 metres) and after completing the winding measured what was left. I discovered that I’d used almost 2 metres exactly. (purely by chance) I measured the inductance of the resulting coil and this was 9uH – very much less than ‘normal’ guitar pickups. The DC resistance wasn’t measured, but would be very low – I would need to look check this. (I did, the total resistance was 2.75 ohms).

    Because of the very low impedance, hum pickup is almost non-existent, but I wanted to see if I could get rid of the transformer, by dramatically increasing the gain of the amplifier, so decided to try the balanced version.

    Winding 2 coils together (a Bifilar winding) has significant advantages – especially for home constructors. The 1st one being that because you are winding 2 wires at the same time, you are assured that the coil will be balanced, and can therefore ignore counting turns. The 2nd is that because each turn is closely coupled physically, the signal generated in each coil will be exactly the same – which would not be true of a 2-layered arrangement., as a 2-layered coil will have significant inductance and DC resistance differences due to the physical geometry and length of each winding.

    Your discussion regarding the connections of such a Bifilar winding are entirely correct, and I couldn’t have put it better myself. The result is a coil that has twice as many turns as one single winding, and is centre tapped. When the centre-tap is grounded, signals originating from the coil itself are then anti-phase at each end. We use a balanced amplifier to correctly amplify the signal. Noise and hum not originating from the coil itself will be ‘in phase’ at the connection to the amplifier, and consequently will be cancelled out in the balanced amplifier.

    BTW I was so impressed with the results using the scrap magnets, I purchased a dozen small neodymium rod magnets that will fit neatly into plastic electric sewing machine spools. With judicious reduction in diameter of the cheeks of these cheap spools, six can be accommodated in the same space as a ‘normal’ guitar pickup. These (and associated circuitry) will form the basis of another article, but I’ll take a few photos of what I’ve used, and done tomorrow (I’m writing this in bed!) and forward these to you.

    Good luck with your project, further stuff will follow tomorrow.

    Regards,
    Joe Brown.

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