Phono preamps

PHONO PREAMPS

Our simple guide to external phono preamps.

If your amplifier does not possess a phono input, you’ll need a Phono preamplifier to play LP. Pickup cartridges produce a very weak signal that needs amplification  before it as suitable for a typical integrated amplifier’s line input (Aux, Tuner, etc). All phono preamps also possess RIAA equalisation that boosts bass and cuts treble to compensate for the way records are cut. So a phono preamp performs two basic functions: amplification and equalisation.

Revived interest in vinyl means modern integrated amplifiers and even AV receivers are now being fitted with phono stages. However, external stages offer equivalent quality at the cheap end of the market, like the Cambridge Audio Azur 640P for just £80 or so, up to very high quality designs like the Leema Agena. Nowadays there’s no shortage of them.

Most external phono stages cater for Moving Magnet (MM) and Moving Coil (MC) cartridges, but a few cater for one or the other alone. Transistor designs, usually based on silicon chips, are least expensive, typically £60 up to £1000. Generally more expensive are valve (tube) stages, typically priced from £1000 up.  Valve stages give a more atmospheric sound, lacking the stark and somewhat mechanical presentation from  silicon chips.

Inevitably there are variants, like the ANT Audio Kora 3T which uses just three transistors and costs £750, or the highly tuned Anatek MCR that uses chips and costs £2200.

MM PREAMPS

Cambridge 640P rear

The simple and inexpensive Cambridge units are good exemplars of the breed. You simply plug the turntable leads into the inputs and take the outputs to the amplifier. There is little need to vary anything, but often tweaks are provided.

Load impedance for MMs is always 47k and rarely is it variable. It is often possible to vary load capacitance from 100pF minimum up to 400pF or more. Commonly, increasing capacitance above that of the connecting leads (i.e. 100pF)  will cause the upper mid-band to peak up, whilst higher treble dips, making the cartridge sound brighter. Precisely what affect changing capacitance has depends upon the cartridge (as a load) so whilst there will be sonic difference they may not be improvements. Modern cartridges tend not respond well to increased capacitance, whilst older designs do, so the ability to vary capacitance is an interesting frippery.

Quite important is gain. Moving Magnet cartridges need at least x100 (40dB) gain. More useful is x200 (46dB) or so, as this better suits insensitive amplifiers (400mV input sensitivity). Connect a low gain stage to a low sensitivity amplifier and you will have to turn volume right up after playing CD, then right back down again when going back to CD.

Noise in the form of hiss is almost always low nowadays. Hum is low too, even in valve phono stages which often possess d.c. heaters in the first stage to minimise hum.

MC PREAMPS

Emille Allure valve phono preamp

In outline MC preamps are the same as MMs but they have at least ten times more gain (amplification) to cope with the extremely weak signal from MCs. Simple ones retain the 47k load used by MM  cartridges too, into which MCs can work. However, the usual load for MCs is 100 Ohms. Because of the nature of an MC, altering load capacitance has no affect and neither does altering load impedance, except to very low values (<10 Ohms) which imposes electrical damping on the generator, reducing high frequency ringing. A very few cater for this.

Noise (hiss) in MC stages is an issue, because some MCs, like Linns, have very low output. They need super low noise preamps and even then slight hiss may be apparent if volume is turned right up. The input transformer commonly used in modern valve (tube) preamps gives a very low noise input, comparable with the best transistor stages, so good modern tube phono preamps have no weakness here.

BALANCED INPUTS / OUTPUTS

Aqvox 2Ci balanced input phono preamp

Currently, almost all phono stage are ‘unbalanced’ in their internal electronic topology, and correspondingly in the input and output connectors used, which are always RCA style phono sockets where one side is at ground potential. Now appearing are phono stages with balanced outputs (Leema Agena, Aqvox  Phono 2CI). This improves sound quality and is especially suitable for driving long cables to the amplifier.

The Aqvox Phono 2CI also has balanced inputs, currently a rare feature. Both the arm wiring and the cartridge must not have one signal line grounded as some have (e.g. Rega, Shure) to take advantage of this. Also, the arm must use a balanced output cable where both signal lines run inside a shield, instead of one line (green and blue)  being connected to the shield and, ultimately, ground. XLR plugs are used as connectors. Manufacturers of specialist arms, like SME, can provide balanced cables, at extra cost.

The use of a balanced line between cartridge and preamplifier is a good idea that needs exploiting. Unfortunately, it suits and therefore promotes the use integrated circuits (ICs), which is counter productive to sound quality. Balanced output lines are easily achieved by using balanced line driver ICs, Tube phono stages using a centre tapped input primary winding could best exploit balanced input lines but as yet none exist.

Aqvox 2Ci balanced-input phono preamp, with XLR input sockets at left.

Balanced wiring from the cartridge to the XLR plugs. Signal is separated from earth, eliminating hum.