A Low Cost Audio Building Block
Late 1975 saw the release of PAIA's audio amp - the Pygmy. The Pygmy has a gain of around 50, and high (line) level signals can easily drive the Pygmy into natural distortion. Doing more work with the Pygmy, we found that some guitars and other signal sources had low enough output to prohibit driving the Pygmy fully into distortion. Many of the guitars whose pickups use reverse winding techniques to achieve humbucking action will have this problem.
To overcome low input levels, PAIA has released the #1710 preamp kit. The circuit, as shown in figure a, is a single transistor amp with a gain of 5. This circuit was designed to mount inside the Pygmy and use its supply, but the preamp is much more versatile when considered as a basic audio building block.
As musicians look for more ways to express themselves, they eventually try some signal processing devices such as distortion boxes, wa-wa's, and so on. If a musician accumulates a large assortment of "black boxes", he is likely to run into signal losses, higher noise, etc. One way to solve a lot of these problems is a preamp. That's where the 1710 shows its versatility.
The preamp is constructed on a small (1.5" X 2.5") circuit board which can easily be mounted in most any existing piece of equipment — inside your amp, a foot pedal, or even inside your guitar. Although the schematic (fig. a) lists a power supply of 12 volts, any supply from 9 to 18 volts can be used without affecting operation. This makes a 9 volt transistor battery ideal, especially since most foot units also use this type of power. Also, the gain of the preamp can be changed by varying the value of R4. If you need more gain, decrease R4 to 330 ohms. This will give a gain of 10.
The AC input impedance of the preamp is around 7K ohms. The output is approximately 500 ohms. No output capacitor is used on the 1710 since the Pygmy amp input is AC coupled. For experimentation in other applications, an output capacitance of at least .1 mfd. should be used.
If this unit looks interesting to you as an experimental building block, or as a functional item to be added to your present equipment, the complete kit including circuit board, parts and instructions is available from PAIA by ordering kit #1710 for $1.50 postpaid.
Feature by Marvin Jones
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