The RIAA Phono Preamplifier
This phono preamplifier for Moving Magnet (MM) and Moving Coil (MC) pickup systems is based on the legendary Douglas-Self-Design.
MM & MC
There are two pickup systems for record players:
Moving Magnet (MM) and Moving Coil (MC).
Moving MagnetMoving magnet pickups generate a small voltage by moving a magnet between copper coils. MM pickups have a very high inductance and impedance. The Phonos offers a choice of three capacitors to correct the linearity of the frequency and phase response. The capacitances are 150 pF, 220 pF and 330 pF. In the off-position the capacitance is determined by the capacitance of the cable.
The MC pickup is also a small electromagnetic generator, but with the magnets firmly anchored and the coils moving between them, i.e. exactly the opposite of the MM pickup.
The output voltage, however, is very low at only a few hundred microvolts and is therefore more sensitive to noise and induced hum. This makes preamplifier design a challenge, because the very low output voltage must be pre-amplified extremely noise-free. Phonos offers one of the most powerful phono preamplifiers for MC systems with up to 71.5 dB preamplification. Thanks to the 120V technology, the preamplifiers are extremely low-noise, linear and phase-locked.
MC pickups have a very low inductance and impedance.
This means that the effects of cable capacitance (from turntable to preamplifier) are negligible. The impedance, however, is relevant and can be set in six positions: 100, 220, 470, 2.2 k, 4.7 k, and 10 kOhm.
If there is any doubt as to which impedance matches that of the pickup, the impedances can also simply be switched through until the linear frequency response is acoustically determined.
The gain control can be used to determine the amplifier power, in other words, the output level of the Phonos.
There are three values to choose from.
In the “Norm” position, the gain is +46 dB for MM systems and +67 dB for MC systems.
With the switch set to “-10”, the gain is reduced by 10 dB (MM = +36 dB, MC = +57 dB). This is optimal in combination with standard HiFi equipment that works with a reference level of -10 dBV.
If the switch is in the “+4” position, the gain is increased by +4 dB (MM = +50 dB, MC = +71 dB).
So the output level is about as loud as that of modern DA-converters at a reference level of 0 dBfs = 15 dBu.
The rumbling of the turntable’s drive mechanism is also amplified by the high amplification of the bass range due to RIAA equalization in playback.
The Phonos offers a switchable subsonic filter to remove this rumble.
Gold-plated RCA sockets are available for connecting the turntable and for outputting to a preamplifier such as the Director Mk2 or the Elector.
A ground connection for the turntable is designed as banana plug socket. The ground lead can be either clamped or plugged in.