Microphones may be classified into two groups: high-impedance (≈ 200 kΩ) with high-voltage output and low-impedance (≈ 200 Ω) with low-voltage output. The output from a high-impedance microphone can be amplified simply and effectively with a standard inverting or noninverting operational amplifier configuration. However, high-impedance microphones are more susceptible to stray RF and 6O-Hz noise. They have a fairly flat frequency response but are usually restricted to short cable lengths (10 feet or less). Long cables result in a high frequency roll-off characteristic caused by the cable capacitance.
Low-impedance microphones also have a flat frequency response but their low output levels impose rather stringent noise requirements on the preamp. The preamp shown in Figure 3-26 operates from a low-impedance, unbalanced, two-wire microphone where one of the wires is ground. The circuit consists of the LM318 preamp and the tone control circuitry.
The LM318 op amp is operated as a standard noninverting amplifier. Resistor Rl (47 kΩ) provides an input path to ground for the bias current of the noninverting input. The combination of R2 (560 Ω) and C2 (10 µF) provide a frequency roll-off below 30 Hz. At 30 Hz and above the gain is relatively flat at about 50 dB, set by the ratio R3/R2. R3 (220 kΩ) furnishes negative feedback from the output to the inverting input of the op amp. C3 (1.0 µF electrolytic) ac couples the preamp to the tone control section.
The top half of the tone control section is the bass control. The bottom half controls the treble frequency response. These tone controls (R5 and R8) require audio taper (logarithmic) potentiometers. The 50-kΩ potentiometer on the output can be used to set the output or gain of the preamp. Figure 3-27 shows the bass and treble responses of the circuit.
A preamplifier is needed to amplify the signal generated by a tape head or phonograph cartridge. It is also common to include, with the preamplifier, a means of altering the bass and treble frequency response. The “purist” may want the amplifier to be “flat”, which means no change from the input’s frequency response. This condition should occur with both bass and treble controls at midposition. Sometimes it may be necessary to compensate for the effects of room acoustics, speaker response, etc. Also there is simply a matter
of personal taste; one person may prefer music with heavier bass; another may prefer stronger treble.
Active tone control circuits offer some advantages: they are inherently symmetrical in boost and cut operation and have very low total harmonic distortion (THD) because they are incorporated in the negative feedback loop.
The circuit shown in Figure 3-28, is a form of the socalled “Americanized” version of the Baxandall negative-feedback tone control. At very low frequencies the reactance
of the capacitors is large enough that they may be considered open circuits, and the gain is controlled by the bass potentiometer. At low to middle frequencies the reactance of the 0.03µF capacitors decreases at the rate of 6 dB/octave, and is in parallel with the 100 kΩ bass potentiometer; so the effective impedance is reduced correspondingly, thereby reducing the gain. This process continues until the 10-kΩ resistors, which are in series with the bass pot become dominant and the gain levels off at unity. The action of the treble circuit is similar and becomes effective when the reactance of the 0.003-p.F capacitors becomes minimal. This complete tone control is in the negative feedback loop of the second TL080. Figure 3-29 shows the bass and treble tone control response. The response curves were run with 1.0 V equal to “0” dB as the “flat” response line.
The first TL080 is a preamp with a 100Ω adjustable gain change pot. This gives a gain adjustment of about 6 dB for matching to the output of a particular pick-up or tape head. The negative feedback loop of this TL080 contains the gain setting and frequency compensation components.