Preamp Equalization Network

With turntable and record player ceramic cartridges, the amount of built-in RIAA compensation can vary and therefore it is difficult to have a network that provides perfect equalization. Generally, these types of cartridges have a lacking in the low frequency response, hence, that is where one ought to improve the response. After much experimentation, I came up with three possible network configurations for use with my ceramic cartridge preamp circuit.

For the resistors, in all cases, you should use high-precision ones when possible. For the capacitors, C2 is 1200 pF with 2 % tolerance and 250 V rating. It looks a bit like a block of fudge and made of polystyrene. These are supposed to be very high-end and loved by audiophiles due to their superior specifications. Since the first stage is extremely critical, you should use the best component available. C1 is 1500 pF with 10 % tolerance and 100 V rating. It is a green coloured mylar capacitor. If you can find polystyrene ones with better tolerance figures, then that is even better. If you had a rotary selector switch, you could incorporate one or two of the following networks to make a truly versatile preamplifier to suite all audio tastes.

For Good Bass Response

My favourite network is this one because the audio has a fuller bass which sounds extremely good when using those cheap ceramic cartridges. There is no loss in treble, and you can further boost it through the amplifiers treble control. In this network, R1 and R2 are 20 MΩ metal film resistors with 1 % tolerance. If you use this circuit, make sure your turntable and amp are producing a clean signal without any mains hum or rumble from the platter. If you power the preamplifier with a 9 V battery, then you will be able to hear how clean the initial signal is. Use this network with high-quality turntables and power supplies that produce a totally clean audio signal. Since most people do not have high-end equipment, manufacturers use a cut-off frequency of around 100 Hz. However I always say that instead of reducing the quality of the preamp, one should improve the quality of the turntable!

For a strong RIAA correction, when the cartridge does not have any built-in compensation, R1 is 10 MΩ, and R2 is 120 kΩ.

Variable Equalization

In this configuration, R2 is 120 kΩ, in series with a variable resistor VR. The value of VR can be 1 MΩ or more. As the resistance increases, the bass response improves. This would be useful if your cartridge lacks low frequency response, and you need to improve just that.

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