The Armstrong 700 Series

The units were:

730 Pre-Amplifier
732 Power Amplifier

The 700 Range was a dramatic change from previous Armstrong ranges. During the middle 1970's it became clear that the Japanese manufacturers were taking a steadily growing share of the UK market. Hi-Fi was also becoming a standard domestic item. The Japanese were able to offer very good value for money for mass market equipment as they could manufacture in very large quantities. Armstrong, along with many other well-established UK Hi-Fi makers, found they were in the position of being ‘family firms’ having to compete with vast international corporations.

In general, the best way forwards was to move ‘up market’, and that is what Armstrong did with the 700 Range. The receivers (combined tuner-amplifier units) were abandoned. Instead, the new range were designed to be what we might now call ‘high end’ equipment with a level of performance (and a price) well above the mass-market level. The design aim for the 700 Range was therefore: accuracy even when playing at high power into ‘difficult’ loudspeaker loads.

The picture above shows both the 730 pre-amp and the 732 power amp. Here, the preamp sits on top of the power amplifier. The units were designed with separate boxes and power supplies for high performance and flexibility. However the underside of the lid of the power amplifier had a mu-metal layer to ensure that the performance would not be compromised if the units were used one on top of the other.

Moving from left to right in the above picture, the controls were: on/off push-button, 16dB output level attenuator push-button, balance control, volume control (large knob with the attenuation levels marked in dB), tape output selector switch, and input selector switch. The pair of selectors meant that it was possible to record one input (or from tape to tape) whilst listening to another input.

730 Pre-Amplifier Specifications

Rated Output	1 V rms into 600 Ohms	Max output	10 V rms into 10 kOhms
Frequency Response	20Hz - 20kHz ±0·2dB	I/P Overload margin	40dB
SNR (LP)	80 dB (mm/mc)	SNR	92dB
Distortion	< 0·005%	Crosstalk	better than -60dB
Sensitivity	0·25 mV (35 Ohms)MC	2·5 mV (47k) MM	150 mV (50k) others
Disc input cap	switchable 60 / 280 pF	Attenuator	-16·5dB
Width 440 mm	Height 54 mm	Depth 300 mm	Weight 5 kg

As had become the fashion by the end of the 1970's, the pre-amp had no tone controls or filtering facilities, nor even a mono button. A balance control was included along with a volume control as this was felt to be useful. The volume control was a very high performance detent stepped attenuator unit made by Alps. This gave accurate volume steps and kept the stereo balance accurate to better than 0·5dB even down to -60dB attenuation. The pre-amp had a very high overload and output headroom capacity. It could drive up to 10 Volts rms sinewave into a 10kOhm load although the rated output was 1 Volt rms into 600 Ohms. To avoid overloading sensitive power amplifiers the output level was switchable.

The above picture shows the rear view of the pre-amp and power amp. It shows that the 730 had a number of inputs and outputs These included two tape inputs and output (tape monitor). There were also three switched mains (IEC) outlets. The disc (which in those days meant LP!) input had a switchable sensitivity (moving coil / moving magnet) and loading / capacitance. The 732 was unusual for a high power amplifier in having two headphone outputs as well as binding posts for loudspeakers.

732 Power Amplifier Specifications

Rated Output	200 Watts cont. 20Hz - 20kHz. Both channels driven 8 Ohm loads
Distortion	< 0·02% 20Hz - 20kHz, any level up to rated output
Frequency Response	20Hz - 20 kHz ±0·1dB	Sensitivity	1 V rms for rated output
SNR	100dB or better	Crosstalk	<-60dB
Width 440 mm	Height 86 mm	Depth 300 mm	Weight 12 kg

Armstrong always used to include a “feedback and comments” form along with the guarantee card and asked new users to fill this in. The most common request for an extra facility was a pair of headphone outlets. So, in the Armstrong tradition these were provided.

The above picture shows a prototype 732 being tried out in the house of one of Armstrong's directors (Barry Hope) in the late 1970's. By then the external appearance and most of the internal details of the power amplifier were settled, but the pre amplifier was still to be designed. Since headphone sockets were provided a loudspeaker switch was also included. This was operated by the push-button near the headphone sockets to the right of the unit. The on/off switch is the push-button to the left. The speaker switching was via a high quality, very heavy duty relay. This also delayed loudspeaker connection for about one second when the unit was switched on so as to avoid any possibility of a loud ‘bang’. This would have been harmless, but might have worried the owner.

In fact, although rated at 200 Watts, the amplifier was even more powerful than this implies. At 1 kHz, both channels driven continuously, the amp could deliver 240 Wpc. For short tone bursts it could provide 280 Wpc into 8 Ohms. It would also deliver around 300Wpc both channels driven simultaneously into 4 Ohm loads. In practice, with musical signals it would provide unclipped peaks of ± 85 Volts and ± 50 Amps and work happily into loads down to below one Ohm! The power amplifier employed no active form of current limiting or protection circuit as these might have affected performance. Instead it was designed to provide accurate output waveforms even at high levels into ‘difficult’ loads. It was also built with highly rated devices to make it near to indestructable in use!

Content and pages maintained by: Jim Lesurf
using HTMLEdit and TechWriter on a StrongARM powered RISCOS machine.