Toshiba SB-420 Amplifier Review
Considering that the Toshiba SB-420 amplifier is in the cheapest group it has a very wide range of sensible controls, as well as offering a generous 42W into 8 ohms or 50W into 4 ohms with minimal distortion.
Two fixed impedance and sensitivity phono inputs are provided for magnetic cartridges, in addition to the high level inputs which consist of an auxiliary and a tuner input plus an additional low level input for a microphone. This input has its own level control such that the microphone signal can be added to the source in use.
All input sensitivities were reasonable and the overload margins adequate, but it is felt that the capacitive component of the two high level inputs is on the high side at 360pF and that this could cause high frequency losses with some input sources.
Two sets of loudspeaker terminals are provided, with front panel switching for either or both sets of loudspeakers, plus an off position for headphone listening via the front panel headphone jack socket. The amplifier has more than adequate capability for driving single sets of 4 ohm loudspeakers or two sets of 8 ohm loudspeakers used simultaneously.
Whilst the noise level from the inputs was to a high standard, it is unfortunate that the output noise at the worst case volume setting was poor, and this was particularly objectionable when listening on headphones.
Whilst the power bandwidth is a bit restricted when working into 4 ohms, it is reasonable when using 8 ohm loads, and the very low harmonic distortion and intermodulation distortion at half power and at 1W output more than compensate for this shortcoming.
In addition to the inputs mentioned above, there are two tape input/output facilities, one of which has a DIN compatible socket. Tape dubbing in either direction is controlled by a three-position toggle switch, with a rotary switch controlling tape monitoring, the crosstalk associated with this switch and also the input source switch being to a good standard.
The volume control and its associated ‘muting’ attenuator which offered either 10dB or 20dB extra attenuation worked well without upsetting the amplifier balance, and the full range balance control had a satisfactory performance.
Both a high pass and a low pass filter are fitted, these being 6dB per octave filters with â€"3dB points at 15Hz and 8kHz respectively. In addition to these well selected filters, the treble and bass tone controls which are of the detent mechanism type offer a choice of both treble and bass turnover frequencies, these being selected by a three-position toggle switch which is associated with each tone control and which also provides a ‘defeat’ position.
Finally there is a mono/stereo switch and the loudness control which provided a large boost to both the treble and the bass.
Additional phono sockets at the rear of the amplifier provide access to the output of the pre-amplifier and the input to the power amplifier for connecting equalizers or four channel decoders, and a further pair of phono sockets allow the connection of a third tape recorder for recording the combined microphone and source signals.
From the point of view of construction standards, the electronics of the Toshiba SB-420 are not to the highest quality, with some untidy components and not the best workmanship so far as soldering is concerned. However components are properly identified for servicing, and having regard to the price tag this amplifier has a great deal to offer.