A preamplifier with selectable equalisation

The SSM2275, a Butler front-end rail-to-rail low noise stereo amplifier integrated circuit, was used in the original design of this circuit but it has since become difficult to obtain. However, a good substitute is the NE5534AN which is made as an 8-pin dual in-line package costing about £3 from electronic component suppliers such as Maplin.
The phono output goes to the inverting input of the I.C. and the output is fed through a selectable feedback loop to the non-inverting input. The required waveform shaping is obtained from this negative feedback and is realised by careful choice of resistor and capacitor values. The "line" output is a few hundred millivolts, ideal for direct connection to a computer sound card.
One channel, for mono, or two, for stereo, can be assembled. One of the selectable feedback loops is adjusted for the standard RIAA curve of post-1954 long play vinyl discs, so a two-channel stereo pre-amplifier may be preferred if LP records are to be transferred digitally in addition to 78s - for example, if you have any vinyl transfers of old 78s! Some benefit in noise reduction may result if you record 78s in stereo before reprocessing and then converting to mono, as explained elsewhere on this site.
The circuit diagram for the pre-amplifier is shown in the diagram. Construction can be done on a strip-board of 0.1 inch pitch. Either a single or a double pole multi-way switch (for mono or stereo respectively) is needed for selecting the appropriate feedback components for different record types

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The 1k variable resistor reduces the output but should only be used if clipping occurs, which is unlikely except when recording 78s from the mid-1950s onwards. The required feedback loop is selected by the multi-way switch SW. The feedback loops shown below, designed from standard value components, provide some of the equalisation characteristics given in the table elsewhere on this website. Any or all of them may be included in the final circuit.

The current taken by the circuit is less than 5mA with a 9 volt dual supply. Batteries are therefore ideal as the power supply.

If you are intending to use an audio editing program to equalise your records, you may decide only to use the flat feedback loop and have no switching. The necessary equalisation can then be applied by the audio software. Dedicated software equalisers are also available for the purpose.