Flat response preamplifier


Short of using a specialist "flat" phono preamplifier (like the one illustrated above), there are several ways of amplifying the signal from the phonograph cartridge (pickup) to a sufficient level to digitise it.

You can:

The first two options are illustrated in the picture below.

The microphone input will work for moving-coil cartridges which have a great deal in common with dynamic microphones (moving-coils and ribbons), albeit with a signal about 4 times lower than a typical moving-coil microphone.* Acceptable results may be obtained if the microphone preamp' is well designed and the cartridge leads can be fed balanced to the microphone inputs with the shield derived from the turntable chassis and arm tubing. Even single-ended will work if care is taken to keep leads short, well made and away from sources of interference.

*And remember, just like dynamic microphones, phantom power should never be selected.

Some primitive ways to connect a record-deck to a computer sound-card/ interface: see text

Moving magnet cartridges are especially problematic. If the interface has instrument inputs (as does the Focusrite unit illustrated above), then the moving-magnet cartridge may be fed directly to these inputs for which the sensitivity is probably suitable (lower diagram). Noise may however NOT be optimal. Instrument inputs are usually on ¼" jack sockets so it's tempting to apply the phono leads direct to the interface with some RCA phono to jack converters. Whilst this is great for an initial test and to check the concept, it should be remembered that instrument inputs are very high impedance (or - at least - they should be) and this affects the response of moving magnet cartridges. Once again, this is the reason for the loading capacitor and resistor across the pickup. If done carefully, these components may be secreted within the jack plug.

*Moving magnet cartridges produce signals developed across a largely inductive impedance so the terminating impedance is important is obtaining a correct frequency response.

Roll your own flat phono preamplifier

Frankly, neither of the above arrangements is ideal. A much better solution is to modify an existing phono preamplifier unit so that it has a flat response and then apply the output of this to the computer audio interface/ sound-card at line-level.

A modified or specialist preamplifier may be used to boost the pickup signal level to a sufficient level that it may be applied to most audio internal or external sound cards

A commercial phono stage is a good starting point for modification because the designer will have taken the trouble to provide high-quality amplification, low-noise circuitry, decent power supplies and the correct cartridge loading.

Strap the big capacitor; remove the little capacitor.....is the golden rule!

RIAA correction may be implemented in various different ways. In a passive circuit: or in an active circuit, in which the frequency-dependent impedance is part of a feedback network. It may be implemented with discrete transistors, or valves (tubes), or with operational amplifiers. But in all these various incarnations, the modifications may usually be accomplished with a little detective-work and care.

The figure above illustrates a typical passive configuration of an RIAA equaliser. The necessary modifications are marked in red. As you can see, the golden rule is, strap (bridge) the higher-value capacitor and remove the smaller-value capacitor. By these simple means, the RIAA correction is modified to give a gain a few dB from the 1kHz reference point so that neither noise, nor overload margin of the preamplifier is compromised. The only practical thing to watch out for is that the capacitors (both the larger value and the smaller value) may be made out of several discrete components so that the designer could get a special value.

A discrete-BJT feedback stage is modified in a very similar way: the larger-value capacitor is strapped (bridged out of circuit) and the lower value of the two capacitors is removed from circuit. If you keep hold of the lower value capacitors (or tape them to the inside of the unit for safekeeping), the preamp' may easily be returned to its original design if you wish to do so at a later stage.

Op-amp circuits are similar too, as illustrated below.

The Pspatial Audio Groove Sleuth Preamplifiers

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Finally, we present our own solution which results from the equipment we developed for testing Stereo Lab.

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