|Scott 200B integrated stereo amplifier|
The name H. H. Scott, although perhaps new in the UK, is one that enjoys great respect in the USA. In 1947 Scott introduced one of the first true high fidelity amplifiers, their 210A, which had many features that were later copied by others and which are to be found in many amplifiers today.
Their latest hi-fi amplifier, the 200B, continues the Scott tradition of value for money and a reliable performance. The Scott 200B delivers 15 watts per channel RMS, has high and low pass filters, tape monitoring, low volume frequency compensation and last but not least, separate tone controls for each channel. The front panel and controls follow the conventional pattern found in America and are extremely pleasing to the eye. The finish will harmonise with almost any furnishing scheme and the amplifier can be mounted either in a cabinet or on a shelf, in which case a rubbed wood case is available.
The circuit layout is very dean and there is plenty of space around the components. This not only means easy servicing but a longer life due to cooler running. The 200B was one of the coolest running amplifiers I have ever tested. This is undoubtedly helped by the non-magnetic electrolytic aluminium chassis construction.
Magnetic and ceramic pick-up inputs are fed into the first half of a 12AX7 double triode which is coupled via a radio tuner pick-up switch to the second half of the 12AX7. Equalisation is applied between the anode of the second half of the 12AX7 and the cathode of the first. The RIAA record curve was with ±1dB. The radio tuner input is fed directly to the second half of the 12AX7, the frequency response via this input being flat. Tape recorder output is taken from the anode of the second half of the 12AX7. The anode of this valve also feeds the next stage via the tape monitor switch. When switched to tape replay the signal goes direct to the third stage which is half a 12AX7 used in an integrated tone control and treble filter circuit. The anode of this stage also connects via a 'loudness' low level frequency compensating circuit and volume control to the triode section of an ECL86 triode pentode. The anode of this triode connects to a further ECL86 triode which is used as a phase splitter to drive the two output pentodes in push pull.
The power supply consists of a full wave rectifier using a GZ34 with resistance capacity smoothing for HT. A half wave semi-conductor diode provides DC bias to the output pentodes and there are also separate balanced 6.3v AC heater supplies for each channel. Each channel has a hum balancing control. A fuse is provided in the mains input and the pilot lamp is a neon which lights up only when the amplifiers have warmed up and are ready for use.
Tests were made to check the performance of the 200B under various conditions. The power output was 15 watts per channel at 1,000 cps; a very good power response indeed. Harmonic distortion was checked at various power outputs by using an input signal at rated input sensitivity and reducing the volume for each power level, i.e., normal signal conditions. The total distortion was almost constant at 0.75% between one and fifteen watts. However, at the higher power setting the distortion consisted mainly of higher order harmonics which would perhaps give the effect of slightly more audible distortion than would be apparent at lower power settings.
Hum and noise was measured as follows: Tuner -76dB; Pick-up -44dB; Tape -76dB. The -44dB of the pick-up input is not as good as claimed and was just audible during soft passages of music. The filter response curves can be seen from the graphs together with the ‘loudness' compensation at various volume settings.
A square wave gave a good wave form with a 5 micro-second rise time at which the amplifier was found to be completely stable. The range of the tone controls was +14dB to -13dB at 50 cps. and +8 B to -l0dB at 10 Kc. Balance between channels is by means of a dual concentric volume control and could be maintained to within ±1dB over its range.
Other features include a two selector control which selects the mode of operation of the amplifier. It has seven positions. Bal.L combines signals from both left and right inputs and feeds them to the left hand speaker only; Bal.R functions as Bal.L but feeds to the right hand loudspeaker. These two positions are part of the Scott patent balancing system; Mono is used when a stereophonic pick-up is being used to play mono records; Stereo is the normal position; Rev.stereo reverses the two channels; L.input feeds signals on the left input only to both speakers; and R.input functions as L.input but feeds right channel signals only to both speakers.
To balance the system using the Bal. L and Bal. R selector positions is very easy. By feeding the combined channel inputs to first one and then the other speaker, any change in volume or quality due to differences in speakers or room acoustics can be readily heard and the separate volume and/or tone controls adjusted accordingly.
A switch is provided on the chassis, as well as two sockets, for checking the output valve biasing. Simple instructions are given in the comprehensive instruction manual and 'spot on' matching of output valves is possible.
The treble or scratch filter provides a roll-off of about 4dB per octave above 3,000 cps, and is very useful in removing hiss and other high frequency noises from certain signal sources. The rumble filter response curve can be seen in the graph. It is not switchable, but cuts out all sub-audio frequencies and prevents intermodulation distortion caused by such things as turntables and recorders, which, however good, always produce some very low frequency noises.
The standard of workmanship is of a very high order (although one dry soldered joint did cause the volume control to fail), and it is obvious that a great deal of careful inspection and testing is carried out in the factory. Home tests included using an FM tuner and a ceramic pick-up, both of which provided very satisfying results. The only fault found with the 200B amplifier was the slightly audible hum when using the 'magnetic' pick-up input. This may have been due to a minor fault in the amplifier I tested and not therefore applicable to other models. F.M. sound was excellent, and even at loud volume no distortion was apparent when using the amplifier with Wharfedale W3 loudspeakers.
I can fully recommend the Scott 200B stereo amplifier,
which is distributed in the UK by A. C. Farnell Limited, Templar Street,
Edward Street, Leeds 2, who will be pleased to send further details on
request. The 200B amplifier complete retails at 82 gns.