|Goodmans Maxamp 30 integrated stereo amplifier|
The entry of a well established manufacturer into a new field is always an interesting and sometimes exciting event. It is over 30 years since Goodmans Industries started the manufacture of high quality loudspeakers and enclosures. Many used advanced designs and often set a standard that other manufacturers imitated in due course. More recently, the Maxim loudspeaker enclosure has created quite unusual interest in the design parameters of wide-range miniature enclosures, and this design has also been copied throughout Europe. Such small enclosures are fundamentally inefficient transducers compared with large loudspeaker, units and enclosures and therefore a larger power output amplifier is required for a given sound output.
The illustration adequately shows the simplicity and elegance of Goodmans' first entry into the field of amplifiers. Housed in a neat wooden cabinet of similar dimensions to the Maxim enclosure, the front panel is made from Danish silver with engraved and black filled lettering, and a chromium-plated surround is used to locate the whole amplifier in the cabinet. The two major controls use large control knobs, the upper one selecting the input source - auxiliary, tape, radio or pickup - whilst the lower control is for volume. The three smaller knobs control the bass response, treble-response and balance between channels. Each knob has a metal machined centre adequately marked for position and has tapered smooth sides, a feature I prefer to fluted knobs which are difficult to clean. To the right of the control knobs is a vertical bank of press-buttons which, reading from top to bottom, control the following functions: LF filter, HF filter, mono/ stereo, LS out, and the mains on/off switch. Almost invisible until the set is switched on is a miniature indicator lamp, and below this a jack for headphones.
The back panel carries six pairs of phono sockets for dynamic or ceramic cartridges, radio, tape, auxiliary and tape recording output. Horizontally mounted is a substantial moulding carrying six screw terminals for 4, 8 or 15-Ohm loudspeakers, with alongside the mains voltage adjuster and an AC mains voltage outlet for tuner or gramophone motor. The whole unit is positioned and retained in the wooden cabinet by means of two angle pieces and two screws, the back being left open to assist cooling. Removing the angle pieces reveals a most interesting form of construction. A U-section cadmium plated steel frame forms the top, back and bottom of the chassis, and two matt black, perforated panels are hinged to the chassis and retained by three screws. The side panels each carry one main amplifier with the power transistors directly mounted on the metal panel, which acts as an efficient heat sink. Below the top of the U-section is a printed board carrying the two preamplifiers whilst the back carries the mains transformer and the base, the power rectifier and electrolytic capacitors. Most components are readily accessible, the three printed circuit boards being retained in spring clips. Also revealed are three slow acting fuse* to protect the AC power circuit and each output circuit against short-circuit.
RCA silicon transistors are employed, the output stage using direct coupling in a transformerless Class B configuration, with a temperature stabilized biasing circuit. Starting at the input phono sockets, the appropriate signal goes to the input selector switch, which is mounted very near the phono sockets and the input circuits of the preamplifier printed circuit board. RC coupling is used between the first and second transistors, and negative feedback between the pair introduces the appropriate frequency correction circuit and also controls the overall sensitivity. The well-known Baxandall type of tone control is used between the second and third transistors and is followed by the ganged volume control. Also in this part of the circuit are the HF and LF filters. The HF filter comes into operation when the push button is depressed. It gives a flat characteristic up to 8 Kc/s, above which the HF response falls to give a cut of 16dB at 20 Kc/s. The push-button operated LF filter produces a cut of 10dB at 20 c/s with an ultimate rate of 12dB/octave.
After the filters and the volume control, the signals are passed to the two printed circuit boards on either side of the chassis, which also carry the power transistors. A two-stage transistor amplifier passes the signal to a pair of driver transistors and a stabilizing circuit. The output pair of transistors feed the loudspeaker and operate in transformerless Class B, the output being designed nominally for 8-15 Ohms impedance and a 2 Ohm power resistor is introduced when 4 Ohm speakers are used. A 1 Amp anti-surge fuse protects the output transistors against accidental short circuit, but it is necessary to remove the amplifier from the cabinet and unscrew the appropriate side to gain access to the fuse.
The following measurements were made with non-inductive resistive loads and confirm the manufacturer's specification.
An interesting feature of all these measurements is that, at any power of 1 Watt level or higher, they are considerably better than the manufacturers' figure of less than 0.4% for 15 Watts into an 8 Ohm load. Further they illustrate one of the features of transistor amplifiers that at very low power the cross-over notch between the output pair introduces more harmonic distortion than one obtains at higher levels - the reverse of that obtained with valve amplifiers.
From all these measurements it will be seen that the makers have been more than modest in their published specification. To check the overall performance with gramophone records, measurements were made with the EMI series of TCS test records, a Shure M.55E cartridge mounted in a SME 3009 arm, Thorens TD 124 transcription turntable and output voltage measured at the "tape output" phono sockets. The measurements followed the RIAA characteristic reasonably accurately down to 100 c/s but then remained at a constant level. To check that this was not due either to the record (TCS101) or the cartridge, a Wayne-Kerr audio oscillator with a 50K output impedance was substituted and this confirmed that below 100 c/s the compensation in the amplifier was less than that required for the RIAA curve. However a slight increase of the bass control re-introduced the required 5dB loss between 110 and 50 c/s.
As one would expect from the curves, the LF filter has a gentle roll-off below 200 c/s with a maximum of 11dB at 20 c/s, which would assist in suppressing rumble from a turntable. Similarly the HF filter rolls off above 7 Kc/s and is useful for reducing interference on radio or surface noise on older gramophone records.
The phones jack is a valuable asset when one wishes to listen to programmes without the use of loudspeakers, or for monitoring purposes when making a tape recording. Tests were made using AKG K.50 stereo headphones and very satisfying results were obtained.
This first entry of Goodmans Industries into the amplifier
field deservedly created considerable interest at the International Audio
Festival and Fair. It is one of the quietest amplifiers I have used, has
more than adequate reserves of power for all domestic conditions even
when used with modern insensitive loudspeakers, and a discrete elegance
of design that reflects credit of all concerned.