Hi-Fi add-on to reduce surface noise from records

Although records are capable of very high quality reproduction, with use there tends to be a gradual decline in fidelity due to record wear and an accumulation of minute dust particles in the grooves. This results in a loss of treble signals, plus the generation of a substantial amount of high frequency noise (mainly the all too familiar 'clicks' and 'pops').

When playing records that are severely affected in this way it is usually beneficial to use a scratch filter. This attenuates the highest treble frequencies and therefore gives a reduction in the noise. It also cuts treble in the programme material, but this disadvantage is usually more than outweighed by the reduction in noise and subjectively there is an overall improvement in quality.

For the best results it is necessary for a filter of this type to give little attenuation below the cut-off frequency, and a very rapid rolloff above this frequency. This necessitates the use of an active filter such as the one featured here. Its response starts to roll off at about 4.5kHz, reaching the -6dB point at 6kHz and the attenuation is nearly -40dB at 20kHz.

The Circuit

The circuit, which appears in Figure 1, uses a conventional arrangement. TR1 is a field effect transistor used in the source follower made and gives approximately unity voltage gain. It is biased by R1 and R2 via R3-R6, and R7 is the source load resistor.

The filtering is provided by R3 to R5, and C2 to C6. R3, R4, C3 and R5, R6, C5 form straightforward RC low pass filter. At low and middle frequencies C2 and C4 effectively have an infinite impedance, and no significant effect on the signal. This is simply because any change in voltage at the input to TR1 is matched by a similar change in voltage at the output. As C2 and C4 are connected between these two parts of the circuit, there is a constant voltage across them so they are ineffective. This technique is known as 'bootstrapping.'

At high frequencies the two filter sections produce losses that result in a lower signal level at the output than at the opposite ends of C2 and C4. In consequence there are further losses due to R3, C2 and R5, C4 forming additional filter sections. This gives a steep slope (about 20dB per octave) above the cut-off frequency.

S1b provides on/off switching and is ganged with S1a & c which bypass the unit when it is switched off. The current consumption of the circuit is only about 1mA.

Construction

Figure 2 shows the Veroboard layout and wiring of the Scratch Filter. There are three breaks to be made in the copper strips. Of course, two boards are required for a stereo unit, one for each channel. It is advisable to use a metal case for the unit so that the circuitry is screened from sources of electrical noise, including mains hum.

Ideally the Scratch Filter should be connected between the pre-and power-amp stages of the amplifier or receiver, and a tape monitor socket can be used if available. This allows the unit to be switched in and out from the front panel, and so the bypass switching of the filter may be omitted.