The Phono Box E is a phono preamplifier for MM (moving magnet) cartridges: it amplifies the turntable signal and applies RIAA equalisation so that it can feed a line input, which helps with amplifiers lacking a phono input.
Phono Box E: the circuit
Many current amplifiers have no input intended for a turntable. The Phono Box E goes between the record player and a line input, AUX or similar: it takes the cartridge signal on two RCA sockets, raises it to line level with 40 dB of gain and hands it on through two more RCA sockets. With 5 mV at 1 kHz going in, typical output is 500 mV.
The input presents a 47 kohm load with 120 pF of capacitance. The circuit, whose principle draws on the work of Dr. Sykora, uses surface-mount parts chosen for low noise: the manufacturer quotes an A-weighted noise floor of 88 dB, harmonic distortion below 0.05% and a departure from the RIAA curve held within 0.5 dB between 20 Hz and 20 kHz.
Two choices guard against outside interference: a metal screen inside the case and a separate DC supply, with an 18 V external adaptor in the box. The small footprint, 120 x 32 x 100 mm, lets it sit close to the turntable. Input and output RCA sockets carry gold plating. In use it draws 20 mA and at idle it consumes less than 1 W.
Specifications
- Type: phono preamplifier for MM cartridges
- Input load: 47 kohm
- Input capacity: 120 pF
- Gain: 40 dB
- Typical output: 500 mV at 1 kHz from a 5 mV input
- Background noise: 88 dB, A-weighted
- Harmonic distortion: below 0.05%
- RIAA accuracy: within 0.5 dB from 20 Hz to 20 kHz
- Input: two gold-plated RCA sockets
- Output: two gold-plated RCA sockets
- Power: supplied external adaptor, 18 V / 500 mA DC
- Current draw: 20 mA DC, under 1 W at idle
- Size (width x height x depth): 120 x 32 x 100 mm, 106 mm including sockets
- Weight: 269 g, adaptor excluded
- Finishes: black, white
