Introduction: This design example provides two simple, inexpensive drivers for headphones and audio lines that are designed for electric guitars and violins, but are also suitable for many other applications.
The popular 555 timer can be used as a PWM/D class amplifier for musical instruments or other applications. It can operate from a supply voltage range of 4.5V to 16V and can output a drive current of 200mA. The audio signal is sent to the CV (Control Voltage) pin of the 555 timer.
This design example provides two simple, inexpensive drivers for headphones and audio lines, as shown in Figure 1 and Figure 2, respectively. These two drives are designed for electric guitars and violins, but are also suitable for many other applications. For such simple applications, noise and total harmonic distortion (THD) are not important considerations, so these two values ​​are not measured.
Figure 1: Headphone and audio line drivers with op amps and NE555 timers. It is also possible to use a CMOS version (such as LMC555) but with a lower output current. The advantage is that the operating frequency is higher.
The following are some design considerations:
◠The input resistance of the CV pin is approximately 3kΩ. In most audio applications, some kind of audio preamplifier/buffer is required.
â— CV requires a very large input audio signal. The required amplitude depends on the power of the 555 timer and the required output audio power.
â— The 555 timer is used as an oscillator and is modulated by a lower frequency audio signal applied to the CV. The oscillating frequency should preferably be at least 10 times the maximum desired audio frequency. For audio applications, the frequency should be between 60kHz and 200kHz. This simplifies the filtering of high frequency noise generated by the 555 timer and maintains high switching efficiency.
â— Pay attention to RF emissions. A first-order low-pass filter should be placed at least between the output of the 555 timer and the speaker or headphone. If the cable is long, the cable parasitic capacitance (preferably twisted pair) should be considered.
With the formula Av1 = 1 + R6 / R12, the first stage gain is set to about 11 by R6 and R12.
The frequency of the timer when there is no analog signal input on the CV depends on the values ​​of R7, R8 and C5. The standard calculation formula is as follows:
f=1.44/((R7+2R8)&TImes;C5)(Hz)
The output signal of the NE555 is transmitted to the connectors OUT1, OUT2 and OUT3. R9, C7 and load can be used as low-pass filters to filter out the high-frequency components produced by the timer. If unfiltered, these components can cause radiation, causing problems with sensitive electronic devices around the amplifier. The cutoff frequency of the filter should be minimized and headphones with higher resistance should be selected.
Junction Field Effect Transistor (JFET) version
The circuit can also use field effect transistors or bipolar transistors to achieve high input impedance and amplify the audio signal before the NE555.
The input stage of the circuit is built around JFET T1. Resistors R4* and R5* should have a minimum applicable value. They should produce a certain gain and have a low output impedance to drive the 555 timer. In the absence of an input signal, the DC voltage between point A and point B is approximately VEE/3. This circuit is simpler than the circuit shown in Figure 1, but it may be necessary to adjust R4* and R5* depending on the transistor T1 selected and the voltage gain required for the first stage. The problem here is that the difference in JFET parameters for a given type may exceed 4:1. When switch S1 is closed, the gain of T1 is set to the maximum value.
Figure 2: Headphone/line driver with JFET input and NE555.
The bipolar transistor T2 can increase the driving capability of the JFET. In addition, it also supports the use of higher R4* values, which increases the voltage gain of T1.
The circuit shown in Figure 3 uses two 555 timers operating at different frequencies to achieve different sound effects.
in conclusion
This type of circuit can operate over the entire voltage range of the 555 timer (4.5V to 16V), but higher +VEE (such as 12V to 16V) is preferred. This will produce more output power, and most op amps and JFETs will perform better in this voltage range.
This type of circuit can drive high-impedance speakers and headphones – more than 24Ω is preferred. In either case, it is better to keep the peak output current of the 555 timer below 150mA. This will keep the chip's power consumption within an acceptable level. If the output current is much higher than 100mA, the voltage drop of the 555 timer output transistor will increase rapidly.
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