The Realistic SA150 amplifier
5 July 2017

A long overdue declutter of the workshop found this neat little Realistic amplifier. It had been around since about 1990. After a dust off and clean, it actually worked so here are a few results.

SA150 front

Front view

Very neat appearance with source selector, tone and balance control, headphones and speakers on/off button.

SA150 rear

Rear connections

Two line level inputs marked Tuner and Tape, a phono input and a tape loop output. Speaker terminals also on RCA connectors.

Frequency Response

The frequency response of the amplifier varies with the position of the volume control. Above about 60% rotation, response is flat, but a progressive loudness curve is introduced at lower control settings. When the control is below one third rotation, bass is boosted by 12dB and treble by 8dB. The 'tone' control is strictly a treble cut facility. For all response graphs below, the tone control is at maximum.

Response at near maximum rotation of the volume control: Gain is 26dB and scale is 5dB/division.

SA150 response flat

Below, we have the response at low gain setting:

SA150 response low volume

Finally, the response of the 'Phono' input switched to 'Mag' at low gain setting:

SA150 response mag input

Note that the bass loudness boost of 12dB still exists on this measurement.

The phono input is very curious. Response approximately follows the RIAA curve within about 3dB. That part is OK, but there is a wee switch marked 'Mag/Cer' associated with the phono input. When switched to 'Cer' the gain and response does not change from the 'Mag' position. All that does change is the input impedance. On 'Mag' it is 100kohms and on 'Cer', impedance is only 4.5kohms. That impedance for a magnetic cartridge is OK; they usually work with about 50kohms. However, if you wanted to use a ceramic cartridge (and you shouldn't anyway) but that low impedance will seriously load the cartridge, which would be expecting a load of at least 1Mohm, and a flat response. I was not able to try this with a cartridge but possibly, just possibly the low impedance and the equalisation curve together will make a flat response, however even if it did, this is bad practice.

Power Output and Distortion

Into 8Ω, maximum continuous power is 1.25 watts with both channels driven. For one channel only, this increases to 2.5 watts. Rated at 1% distortion.

For 6Ω loads, maximum power is 1.05 watts both driven, and for 4Ω loads, max power is 0.8 watts with both driven.

Concurrently with reaching 1% THD, a raft of AC mains harmonics appears, meaning that the limiting factor is the capacity of the internal power supply. This power supply limit also explains why the power is lower for lower impedance loads.

Sensitivity

Tape and tuner inputs are set for the same gain. Full power as noted above is reached for 212mV rms input with the volume control at maximum.

Summary

The SA150 is a low power stereo amplifier that could be useful to drive pc speakers, or small, very sensitive bookshelf speakers. Recommended speaker impedance is 8 ohms. Alternatively, this unit could simply be used as a headphone amplifier.

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