Preamp Stage For Ceramic Phono Cartridge Or Violin Pickups

While we have published a number of variations on a standard RIAA preamplifier for magnetic phono cartridges, we have not published a preamp stage for ceramic phono cartridges. Typically, these were supplied as turnover cartridges in record changers but there were higher quality versions such as the Decca Deram. These phono cartridges are piezoelectric devices which require a very high input impedance. Similarly, violin pick-ups made by Fishman, Barcus Berry and others are piezo devices. These two circuits have been requested for a violin pickup but could equally well suit a ceramic or crystal pickup. The op amp circuit uses a TL071 connected as a voltage-follower. It can run from a battery supply of ±9V.

Circuit diagram:

The alternative transistor circuit uses a BC549 connected as an emitter-follower but with bootstrapping of the input bias network to provide a high input impedance. Both circuits have input coupling capacitors but since the transducers are capacitive (ie, piezo) they could possibly be omitted. Both circuits will probably need to be followed by further gain, depending on the output level. For a violin pickup, a parametric equaliser is also recommended, and for this we would suggest the 3-band parametric equaliser published in the July 1996 issue of SILICON CHIP. With a slight change to the feedback of the first op amp in this circuit, the extra gain required could also be provided.

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Simple Tachometer Circuit or Revolution Counter Circuit

The explained idea of a simple tachometer circuit can be used as a revolution counter tester for determining or setting the correct optimal engine speed which may ensure maximum engine efficiency from the vehicle. The pulses emanating from the contact breaker are shaped and limited by Fl2/R5/ D2. Transistor T2 conducts only when the contact breaker is open, because the full battery voltage is then present at the input (PL2) of the rev counter (see figure 48). Capacitor C2 is then discharged rapidly through R6 and T2. When T2 is cut off when the contact breaker closes, C2 charges only slowly via R4.

This ensures that the trailing edge caused by the closing of the contact breaker is applied to the next stage as a steep pulse, while the noise spikes are suppressed by the slow charging of C2. It follows that C2 must be carefully matched to the particular car engine. Diodes D4 and D5 protect the trigger input of IC1 against too high positive and negative voltages. The real heart of the circuit is C1: this versatile counter is connected as a monostable multivibrator (MMV). After every trigger pulse at pin 2 (negative voltage jump applied via C4) it generates an output pulse of constant period which may be preset with P1. Unfortunately, the counter auto-triggers if pin 2 is still logic low after the output pulse period has lapsed, and because of that R8 ensures that C4 can only pass very narrow trigger pulses.

The resistor and capacitor form a so called differentiating network (see figure 49).  It would now be possible to connect a suitable moving-coil meter to the output (pin 3) of IC1. Because of the inertia of this instrument, the individual output pulses (pulse width = O.7C7 (P1+R11) sec) are averaged (integrated). The meter reading corresponds exactly to the average value of the pulse height (see figure 50) and is therefore  provided Pl has been adjusted correctly proportional to the instantaneous engine speed.

The comparator, A3, following IC1 has no inertia and the output pulses from IC1 must therefore be smoothed’: this is effected by two integrating networks, R7/C5 and R9/C8 (see figure 51). Across C8 there exists therefore a reasonably constant direct voltage (the remaining ripple caused by the original pulses may be ignored) which is directly proportional to the engine speed (see figure 52). This direct voltage is set to 5 V at maximum engine speed during the calibration which follows later.

The following image shows the simple tachometer circuit design

The waveform images can be verified from the folowing images.

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Dark Activated Led or Lamp Flasher

This circuit adopts the rather unusual Bowes/White emitter coupled multivibrator circuit. The oscillation frequency is about 1Hz and is set by C1 value. The LED starts flashing when the photo resistor is scarcely illuminated. The onset of flashing can be set by trimming R2.

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