These days, music is a major hobby for the young and not-so-young. Lots of people enjoy making music, and more and more dream of showing off their talents on stage. But one of the major problems often encountered is the cost of musical equipment. How many amateur music groups sing through an amp borrowed from a guitarist or bass player?
This is where the technical problems arise not in terms of the .25” (6.3 mm) jack, but in terms of the sound quality (the words are barely understandable) and volume (the amp seems to produce fewer decibels than for a guitar). What’s more, unpredictable feedback may cause damage to the speakers and is very unpleasant on the ear. This cheap little easy-to-build project can help solve these technical problems.
A guitar (or bass guitar) amplifier is designed first and foremost to reproduce the sound of the guitar or bass as faithfully as possible. The frequency response of the amp doesn’t need to be as wide or as flat as in hi-fi (particularly at the high end), and so this sort of amplifier won’t permit faithful reproduction of the voice. If you build an adaptor to compensate for the amp’s limited frequency response by amplifying in advance the frequencies that are then attenuated by the amp, it’s possible to improve the quality of the vocal sound. That’s just what this circuit attempts to do.
The adaptor is built around the TL072CN low-noise dual FET op-amp, which offers good value for money. The NE5532 can be used with almost the same sound quality, but at (slightly) higher cost. The circuit breaks down into two stages. The first stage is used to match the input impedance and amplify the microphone signal. For a small 15 W guitar or bass amplifier, the achievable gain is about 100 (gain = P1/R1). For more
powerful amplifiers, the gain can be reduced to around 50 by adjusting P1. The second stage amplifies the band of frequencies (adjustable using P2 and P3) that are attenuated by the guitar amp, so as to be able to reproduce the (lead) singer ’s voice as clearly, distinctly, and accurately as possible. To refine the adaptor and tailor it to your amplifier and speaker, don’t be afraid to experiment with the component values and the type of capacitors.
The circuit can readily be powered using a 9 V battery, thanks to the voltage divider R4/R5 which converts it into a symmetrical ±4.5 V supply.
Author :Jérémie Hinterreiter
