Flat response preamplifier

There are a multitude of options in obtaining a phono preamplifier with a flat frequency response suitable for needle-drop recording and subsequent processing in Stereo Lab.

These range from low-cost and DIY solutions to precision preamplifiers with laboratory-standard accuracy and dynamic-range provided by our partner company Phædrus Audio.

The various options are described below.

DIY options

Low cost

Groove Sleuth preamp

DIY (no preamp at all!)

At its most basic, you can experiment without any sort of phono preamp at all, and simply apply the output of the phono cartridge to a commercial external pro-audio interface like the one illustrated below.

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.

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

Moving magnet cartridges are more 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).

Beware: 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. This is the reason for the loading capacitor and resistor across the pickup shown in the lower diagram. This will ensure that the moving-magnet phono-cartridge is correctly laoded and the frequency-response optimised. If done carefully, these components may be secreted within the jack plug.

Roll-your-own flat-response 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 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 right 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. Most modern RIAA preamplifiers will be of this type.

DIY done for you....

For those vinyl fans who wish to discover the advantages of processing needle-drops in the digital domain, yet for whom electronics DIY remains anathema, Pspatial Audio have organised a fully-modified commercial RIAA preamplifier.

Called the Transform Media Non-Equalising Phono Preamp, the unit is supplied fully pre-modified and comes complete with a universal power-supply. The preamp is available in two models: the standard NEPP-1 for moving-magnet cartridges and the NEPP-1PH which includes circuitry for phantom power to Phaedrus Audio's PHLUX-II active phono cartridges.

To purchase a Transform media NEPP preamplifier contact Pspatial Audio sales, email: sales@pspatialaudio.com

Transform Media NEPP-1/ NEPP-1PH Specifications

• Size: 105 x 60.5 x 33mm (steel chassis) • Power: 12V DC, Universal plug top PSU included with UK and EU detachable heads • Power consumption: <1W; compatible with European Eco-Consumption directives. The unit is intended to remain energised all the time. • Inputs: RCA phonos • Outputs: RCA phonos and 1/4" TRS jack • Earth: Binding-post • Input level: 3.5mV RMS/cm (nominal). Suitable for MM (NEPP-1) or PHLUX-II (NEPP-1PH) • Gain: 37dB • Bandwidth: 30Hz to 30kHz (-1dB points) • THD: <0.05% • Equivalent Input Noise (EIN): 1.3uV RMS (-116dBu), Unweighted • Crosstalk: better than -60dB at 1kHz


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