More circuits for the practical musician
For guitar practice a small self-contained amplifier plus a pair of headphones is often more convenient than a high power combo. This simple headphone amplifier design will operate well with most high, low and medium impedance headphones giving good volume and low noise/distortion levels. It also includes Bass and Treble tone controls.
IC1 is a preamplifier stage which gives the circuit an input impedance of 100 kilohms and provides 20dB of voltage gain. This gives the circuit adequate gain for use with most guitar pickup and headphone combinations, but if necessary R4 can be raised in value somewhat in order to give a higher level of gain (increases in the value of R4 give a proportional boost in gain). A volume control could be included at the input of the unit if desired, but as virtually all guitars incorporate a volume control this is not essential.
IC2 is used as the output stage, and this provides an adequate output current to drive any normal type of headphones. The tone control networks are included in the feedback circuit of IC2, and this stage has a nominal voltage gain of unity. However, the tone control networks can be adjusted to boost or cut the gain over their respective operating frequency ranges, and about 12dB of boost and cut is available at 100Hz and 10kHz. VR1 is the Bass control and VR2 is the Treble control.
The circuit has a quiescent current consumption of about 2 milliamps, and this only rises to around 5 milliamps at high volume levels. A small (PP3 size) 9 volt battery will therefore give many hours of operation. When using the unit with high and medium impedance headphones results are likely to be best with the phones wired in series, but for high impedance types results will probably be best using parallel connection.
This circuit is a form of auto 'ducking' or 'voice-over' unit, and it is based on a CMOS 4016BE quad analogue switch. The music signal is fed to Input 1 and the voice signal is applied to Input 2. Under quiescent conditions the control input of IC2a is taken high so that this switch is turned on and couples the music signal at input 1 through to the output. Tr2 acts as an inverter so that IC2b is turned off and the voice signal is cut off from the output. However, if Tr1 is biased into conduction the two control voltages swop states so that the music signal is switched off and the voice signal passes through to the output. In order to produce the 'voice-over' effect it is merely necessary to supply a suitable bias to Tr1 whenever a voice signal is present at Input 2.
This bias is supplied by IC1, which amplifies the voice signal, and the amplified signal is then rectified and smoothed by D1, D2 and C3. R4 enables the voltage gain of IC1 to be varied from zero up to a maximum of 22 times, and this is set for the lowest gain that gives reliable operation of the unit. With a higher level of gain the unit will work, but there is a danger of background sounds producing spurious operations. The circuit has a fast attack time so that the voice signal is switched through to the output almost instantly as it commences. The circuit also has a fairly rapid decay time so that the music signal is switched through to the output again soon after the voice signal ceases, but the decay time is sufficiently long to avoid switching during brief pauses in the voice signal.
As the circuit stands it will only accept a mono music signal, but by duplicating the switching circuit using the remaining two sections of IC2 it would be possible to control a stereo music signal and mix the voice signal into both channels. The current consumption of the circuit is only about 2 milliamps and a supply voltage of around 9 to 18 volts is suitable.
Feature by Robert Penfold
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