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

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Z = Z || R(12)INITBWhere: ‘Z IT ’ represents the base impedance.By applying appropriate values, a high level ‘Z IN ’ can be obtained. This high level of‘Z IN ’ is adequate to overcome any signal loss due to the high impedance of the guitarpickups. Increasing the value of ‘R B ’ would subsequently increase the value of ‘Z IN ’. Forexample, a case with a value of 1MΩ <strong>for</strong> R B yielded a Z IN = 605KΩ. <strong>Based</strong> on thecalculations <strong>for</strong> ‘A V ’ and ‘Z IN ’, the current gain ‘A I ’ can be obtained as follows:AI = AV(ZIN/ RL)(13)An increased ‘A I ’ can be achieved by increasing the value of ‘R B ’ . For the case of R B =1MΩ, ‘A I ’ was found to be 118.58.<strong>Clipping</strong> and GainThe discussions outlined in the previous section reveal three important aspects within thepre amp’s functionality that could be improved upon. These improvements greatlyenhance the ability of pre-amp and are the main contribution of this research ef<strong>for</strong>t. Thedesign details of these changes and how that enhances the capability of the amplifier isdiscussed next.Gain Improvement: Firstly, the gain of the non-inverting negative feedback amplifierusing the pre-existing configuration was increased. This would extend the range ofclipping produced by the output of op amp. In order to achieve this increased gain, asummation amplifier setup is proposed which will double the gain of the previous setup.Functionality of this proposal can be derived as follows:A = [(R + R) / R ] V + [(R + R) / R ] V(14)VFAINAFBINBIt has been observed that the summation gain approximately doubles the ‘A V ’ level,allowing a larger range of clipping. If a larger level of gain was needed, more ‘V IN ’ legscould be summed together. The details of this modification can be seen from figure 3.Improvements in <strong>Clipping</strong>: This is an area in which many boutique pre amp builders haveattempted to alter by implementing a variable clipping system to replace the stock diodes(1N914) within the audio clipping pre-amplifier. Due to the deviation found in variousdiodes voltage conduction ranges, a multi-position selector switch connecting varioustypes of diodes in parallel to ‘R F ’ can produce a crude variable clipping system. Theproblem with this system is that it is limited to the number of types of diodes that areimplemented, and the range of clipping is not continuous but rather stepped. In order tocorrect this, a continuous variable clipping system using a Silicon Controlled Rectifier(SCR) and RC network is proposed. The SCR is a three terminal device composed of anInternational Journal of Modern Engineering Volume 8, Number 1 Fall 2007 116
anode, cathode, and gate. It conducts from the anode to the cathode when there is anadequate gate signal level (‘I GT ’ and ‘V GT ’).Figure 3: Proposed clipping and gain circuitFigure 4: Typical SCR Characteristics [ 9 ]Figure 4 shows the <strong>for</strong>ward and reverse characteristics of an SCR. By replacing diodewith the SCR, and by feeding the gate from the output of the op amp through a variableRC phase shifting circuit, the SCR can be turned on and off at various points of the signalInternational Journal of Modern Engineering Volume 8, Number 1 Fall 2007 117
Page 1 and 2: Nr. 2/2004/ Inform
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Page 8 and 9: waveform. This cau
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Page 12 and 13: Figure 9: 1N914 Diode “smoothed c
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A New Design for Audio Clipping Pre-amplifiers Based on ... - IJME

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