A New Design for Audio Clipping Pre-amplifiers Based on ... - IJME

capacitor is located in parallel with ‘R F ’, which decreases the effective ‘Z F ’ value. Asfrequency increases, the impedance of the capacitor⎛⎜ Xc =⎝12πfc⎟ ⎞⎠decreases, therebydecreasing the value of ‘Z F ’ and the resulting gain (A V ). The second capacitor is locatedin series with ‘R I ’, which increases the effective value of ‘Z I ’. As frequency decreases,the impedance of the capacitor increases, consequently increasing the value of ‘Z I ’ anddecreasing the gain. Due to the relationship between these two capacitors and the gain, afrequency bandwidth is created. Each capacitor’s sets the high and low frequency limits<strong>for</strong> this bandwidth.<strong>Clipping</strong> is a function of the negative feedback configuration. In this configuration, two1N914 signal-clipping diodes are designed to work in apposite directions parallel to ‘R F ’.These diodes have a threshold voltage at which they are <strong>for</strong>ward biased and act like ashort circuit. The effect of this threshold voltage is that as the amplified sinusoidal signalfrom the guitar ramps up to the 0.5V - 0.7V limit, it receives the full gain determined by‘A V ’. When the signal passes into the threshold range, the diodes begin to conductcausing ‘A V ’ to drop of towards unity, gradually clipping the peaks of positive andnegative alterations of the signal. Since the signal is already harshly clipped due to thedistortion from high gain op amp, diode’s gradual conduction smoothen corners of theclipped output.Tone and Volume ControlsThe output from clipping/gain stage is fed to an active tone filter, which utilizes thesecond op amp within the dual op-amp package (JRC4558D). Its role is to cut out anyharsh high frequencies, produced as a result of the distortion introduced by the clippingand gain. The cut-off frequency <strong>for</strong> this low pass active filter can be adjusted by apotentiometer wired across the negative and positive inputs of the op amp. Theadjustment of cut-off frequency is warranted when the proposed pre-amplifier is usedwith various electric guitar devices. The tuning will be accomplished during the initialstages of device set up.The volume control is another potentiometer device with the output from the toneconnected to one terminal, V IN-DC connected to the other, and the signal out connected tothe wiper. As the potentiometer is adjusted, various levels of signal are fed to the outputbuffer due to the voltage divider between V IN-DC and the wiper output. The adjustment todeliver substantial changes in the volume level, together with smooth audible tone due tohigh frequency cut off has been considered as notable feature of this pre-amplifier.PROPOSED ARCHITECTUREA complete basic schematic of the audio clipping preamplifier based on the SiliconControl Rectifier evolved from the previous discussion is as shown in figure 2. Therelevant improvements and the technical details of this circuit are as follows.International Journal of Modern Engineering Volume 8, Number 1 Fall 2007 114