|Rogers HG 88 MKII stereo integrated amplifier|
The Rogers HG88 Mk II amplifier is the latest version of the HG88 reviewed in THE GRAMOPHONE for October 1960 by my colleague G.E.H. While the appearance remains substantially the same, many detail improvements have been made and the few minor criticisms made by G.E.H. no longer apply. For instance a separate on-off switch has been fitted in place of the original balance/on-off control. The volume control is now a ganged carbon potentiometer instead of a stepped resistor and the output per channel has been increased to 12 watts.
I am often asked whether there is any disadvantage in having control unit and amplifier built on one chassis. The answer is, of course, that it depends on the layout design. The amplifier under review is divided neatly into three parts: control unit, main amplifier and power supply, with adequate screening between sections. No trouble arises therefore from the proximity of power supply components and high gain input circuits, and the resultant compact unit is extremely attractive in appearance. Ivory coloured knobs of sensible size are neatly grouped on the brass panel and all junctions are clearly marked in black. The teak case is simple without being severe, and adequate ventilation has been achieved without sacrificing appearance. Where the chassis model is used the makers recommend that the amplifier should always be mounted with the front panel in a vertical plane so that ventilation is not impaired.
Each channel has five stages preceding its ultra linear output stage. The first valve is an EF86 and negative feedback is applied over this stage to provide equalisation. The signal then passes to one half of an ECC83 with which are incorporated the tone control and filter networks. Balance is effected by variable negative feedback over the next triode section, half ECC83, and after further amplification the triode section of the ECL86, acting as a divided load phase splitter passes the signal to the push pull ultra linear output stage. The record outlet socket is taken via a capacitor from the anode of the first stage. Power supply is by valve rectifier GZ34 followed by a resistance capacity smoothing network. A click suppressor is fitted to the on/off switch. Pick-up inputs function as follows: switch to magnetic, input sensitivity 4 mV equalised to RIAA, impedance 50-l00kc/s, input to Disc 1. By changing input to Disc 2 this becomes: input sensitivity 80 mV equalised to RIAA impedance 1 Megohm. With selector switch to crystal, inputs are: Disc 1, sensitivity 6 mV, impedance 50-l00kc/s, Disc 2 sensitivity 150 mV, impedance 1 Megohm, both these inputs being flat. Input preset potentiometers control the gain from the pick-up. There are two compensated and two flat inputs for tape per channel. Tape 1, compensated for CCIR has a sensitivity of 4 mV into 100 kohms. Tape 2 again CCIR has a sensitivity of 25 mV into 500kohms. Tape I flat and Tape 2 flat have input sensitivities of 25 and 150 mV respectively.
On test the amplifier showed only small deviations from the specification. Frequency response at one watt level taken at the ratio input socket was -3dB at 30 c/s and -2dB at 20 kc/s. Power output was well maintained at the lower frequencies, 10 watts being available at 30 c/s. The range of the tone controls measured: Bass -14dB to +12 dB at 50 c/s. Treble -14dBto+12dB at l0 kc/s. With the filter controls set to 5kc/s the response fell to -12 dB at 12kc/s, and set to 12kc/s, -l0dBat 20kc/s. The balance control had a range of ±3 dB. Cross talk measured 60 dB at 1 kc/s. RIAA equalisation was within 2 dB in the range 30 c/s to 18 kc/s. Final checks on the amplifier were made by connecting it to two Quad Electrostatic loudspeakers using a Decca ffss pick-up, results being excellent in every way, as indeed the test figures indicate. Workmanship is of a very high standard, in fact the internal wiring is some of the neatest that I have seen.
For a stereo amplifier in the medium price range this gives full value for money with every facility that one is likely to need now or in the future.