Clarke & Smith stereo integrated amplifier 655
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Vintage test report


Taken from 'The Gramophone' October 1963

The Clarke and Smith Stereo Integrated Amplifier, Model 655 is of American styling, attractively finished in charcoal black case, satin aluminium panel with black overlay and silver knobs. It is suitable for free standing or cabinet mounting. The overall height is only 4 inches, giving it a neat and delightful appearance. There are five control knobs which from left to right are: input selector switch having six positions, bass, treble, balance and volume. All except 'volume' are concentric controls so that individual tone control can be applied to either channel. The balance control is in fact two separate gain controls, and having set these for correct balance, volume is then adjusted by a ganged master control. The only other controls on the front panel are three slide switches for: scratch filter, tape monitor and stereo/mono switch. With all controls at level, the high frequency response has been reduced at 6 dB per octave above 25 Kc/s, while at the other end of the scale a resistor capacitor network attenuates frequencies below 30 c/s. The six position input selector switch provides for tape head (CCIR), magnetic pickup, radio, crystal pickup, auxiliary (spelt in full) and microphone. Reference to the specification shows that the RIAA corrected crystal pickup input has an input sensitivity suitable for modem ceramic types such as the "Deram" or Connoisseur. The tape monitor input is suitable for tape recorders or decks having a separate replay head and enables A/B tests to be made between the signal fed to the recorder and the recording itself.

Each channel of the preamplifier section consists of five triode stages. The first, an ECC83, accepts inputs from tape head, magnetic pickup and microphone. An unusual feature of this input circuit is the inclusion of small radio frequency chokes, to prevent break-through should the user be in close proximity to a television transmitter. The next triode takes the amplified signal from the sources mentioned and in addition provides a less sensitive input for crystal pickup, radio and auxiliary. Then follows a voltage amplifier and cathode follower. From the latter equalization is applied in negative feedback form to the cathode of the second stage. The circuits in the last two triode stages provide for tone controls, scratch filter, balance, volume and cathode follower output. The main amplifier section is a conventional arrangement, having in each channel one double triode ECC83 followed by two EL84s in ultra linear push-pull. Overall negative feedback is applied from the output transformer to the cathode of the first section of the ECC83. HT supply is from a bridge metal rectifier followed by resistor/capacitor smoothing. Another bridge rectifier supplies DC to the heaters of the valves in the preamplifier stage.

Bench tests on the 655 proved to be merely the routine work of checking and agreeing with the makers' specification. Frequency response, both channels, measured flat from 40 c/s to 18 Kc/s, -1dB at 30 c/s and -1 dB at 20 Kc/s. The power output was 11.5 watts per channel at 1 Kc/s, 10 watts at 10 Kc/s, 10 watts at 50 c/s falling to 8.5 watts at 30 c/s. Measured at the magnetic pickup input the frequency response curve adhered so closely to RIAA that I measured it twice! The largest error that I could find was -1 dB at 30 c/s. Operating the scratch filter produced: -4 dB at 7 Kc/s, -10 dB at 15 Kc/s and -14 dB at 20 Kc/s. Bass and treble controls measured exactly as specification, as did the input sensitivities, with the exception of the crystal pickup input. I measured this at 45 mV for 10 watts output whereas the quoted figure is 60 mV. This deviation would pass unnoticed in operation as it represents only the slightest anti-clockwise turn on the volume control for correction. Hum and noise also conformed admirably, and the manufacturers are correct when they state that the amplifier is unconditionally stable.

One additional feature on the amplifier that I have not mentioned so far is the provision of a centre channel output socket. The voltage for this is derived from the output transformer, a resistor network acting as voltage divider taking equal amounts from each channel, so that it may be amplified and fed to a third and centre loudspeaker. This is, of course, American practice and I am reliably informed that it is seldom, if ever, necessary in this country!

I have had the 655 on various tests, both measurement and listening, for over three weeks. My original fears that two 10-watt channels in such a small space might produce problems due to overheating have been dispelled by running the amplifier for 12 hours continuously. The performance throughout has been most impressive, the facilities available all that one could ask, and the price extremely modest. A similar amplifier/control unit not integrated is made by Clarke and Smith. The preamplifier is model 656, priced at 26 gns., and the 10-watt per channel main amplifier model 657 costs 24 gns. The design is identical to the one I have described, so that those who like the control unit to be separate from the main amplifier can invest in this and be sure that it will have the same excellent performance.