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Single-transistor pre-amp for very-low-impedance Guitar Pickups

A number of my posts (see menu at foot of post) have covered the construction and amplification of home-made guitar pickups fabricated from hard-disk magnets. The amplification so far discussed has generally included the use of op-amps – particularly the Texas TL071/072/074 series. With this post I discuss an alternative – a single transistor amplifier, whose output can be fed straight into a ‘normal’ guitar input.

The circuit is simple, and no details are giving regarding construction. However, I incorporated a version of the pre-amp into a ‘look-alike’ guitar amplifier ‘front-end’, based (loosely) on the Fender Princeton 112 plus, both for test purposes, and as an example of what can be achieved. This design was further extended to cover Piezo pickups, and of course, the ‘normal’ guitar pickup also.

The basic amplifier schematic is given below:

Very-Low-Impedance Single-Transistor Guitar Pickup Preamplifier

Very-Low-Impedance Single-Transistor Guitar Pickup Preamplifier

The voltage gain of this type of circuit can be very high, and a generalised formula for this is: Vgain = gm.R20
where gm is the mutual conductance of the common-base amplifier, and is directly related to the hfe (the small-signal current gain) of the transistor used.
So it will be seen that the voltage gain is directly proportional to the value of R20 in the circuit above.

I’ve shown typical values for a 9volt supply, and these are usually chosen so that the collector voltage of the transistor is at half the supply voltage – in this case around 4.5volts. With the values shown, the voltage gain is around 100.

The circuit could easily be built on a small piece of perfboard, embellishments might include a small box and 9volt battery, with input and output jack sockets, and maybe a final volume pot at the output – value not critical, between 10K-100K.

Components should be chosen for stability and low-noise – with 1% metal-film resistors and good-quality electrolytics. For the transistor, I used a BC549C, but there are many good alternatives – look for a low-noise NPN transistor with an hfe of at least 500.

As I said earlier, I incorporated the above design into a guitar amplifier front-end, the schematic of which is shown below. I won’t discuss construction here, but refer any putative constructors to the relevant photo album. (http://cullercoats.joebrown.org.uk/#41) I’ve outlined the very-low-impedance pre-amp in blue for clarity. Apart from the high-impedance piezo input, the pre-amp closely emulates my Princeton transistor amp. Note I’ve changed a couple of resistor values in the transistor stage to take account of the higher supply voltage. (15volt) The output of the VLI preamp is auto-connected to the main ‘normal’ pickup input, when this jack socket is not in use.

Two versions of the test pre-amp were built, the 1st included an over-ambitious close-spacing between the 805-sized SMD components, and this, together with a flawed order-of-component install, resulted in a very much less-than-perfect end result, and the destruction of two capacitors and a resistor through repeated re-heating to obtain short-free assembly. The 2nd version has a more realistic spacing for the SMD, and also incorporates a relay, controlled by the earth switch on the ‘normal’ pickup socket to switch between magnetic and piezo. 600dpi exports of both versions of the Eagle boards are included on the photo album.

Guitar Amplifier front-end incorporating normal, very-low-impedance, and piezo preamps

Guitar Amplifier 'front-end' incorporating 'normal', very-low-impedance, and piezo preamps

Related posts include the following:

Downloads.

Eagle project files for both versions of my test amplifier are here:
The 1st version schematic: http://joebrown.org.uk/images/smd_pillbox/GuitarPreamp.sch
and the related board layout: http://joebrown.org.uk/images/smd_pillbox/GuitarPreamp.brd
The 2nd (preferred) version schematic: http://joebrown.org.uk/images/smd_pillbox/GuitarPreampV2.sch
and related board: http://joebrown.org.uk/images/smd_pillbox/GuitarPreampV2.brd

I made front panels in Front Designer, and the project files are here
Preamp panel with MAG/PIEZO switch: http://joebrown.org.uk/images/smd_pillbox/GuitarPreampFrontPanel_with_switch.FPL
Sans switch: http://joebrown.org.uk/images/smd_pillbox/GuitarPreampFrontPanel.FPL
Output and control socketry: http://joebrown.org.uk/images/smd_pillbox/GuitarPreampFrontPanel_output.FPL
High definition exports of the panels are in the photo album.

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