Taken from Hi-Fi News, April 1958

HIGH Fidelity amplifier design seems to be dividing into two well-defined groups. First, the separate preamplifier or control unit with a power amplifier intended for hiding away and then table top models complete and ready for use.

General Description

The Pamphonic 1004 falls into the latter category. Glossy black knobs adorn a handsome metal cabinet which can safely be stood anywhere on its protective rubber feet.

The output stage of this amplifier uses two 6BW6s in ultra-linear push-pull. These valves are the equivalent of the well-known 6V6, but on a Noval base. A 12AX7 double triode is used in the phase splitter. The output transformer has a tapped secondary so that the negative feedback which is taken from the whole winding need not be changed when adjusting for another speaker impedance—moving a small plug at the back of the amplifier to 3.5 or 15 ohms being all that is required. The feedback which is taken to the cathode of the first triode in the phase splitter measured 21 dB at 1 Kc/s.

Preceding this is the built-in preamplifier, which uses an EF86 at the front end, followed by another 12AX7. The power supply has a contact cooled rectifier, a component which did, in fact, keep remarkably cool. In addition to the usual selector switch and tone controls a knob marked "Contour" is provided, which also operates the on-off switch.

The back of the amplifier is liberally provided with plugs, sockets, and various adjustments. These include mains voltage selection from l00v to 250 v A.C, a special plug to adapt the
PU input to suit a particular cartridge, auxiliary power and tape recorder outlets, and on the side of the chassis a hum-dinger pot.

Electrical Performance

Power Output

Frequency Response
Measured at the radio position with the tone controls flat this was 2dB down at 30 c/s and 30 Kc/s.

Tone Controls
The effect of these, as can be seen on the graph, was an engineer's delight, cut and lift being exactly equal and the flat position being just where it said on the dial.

Record Equalising
Three curves are provided, those selected being up to date and sufficiently accurate.

Stability
The amplifier would stand a 12 dB increase in feedback and was stable on open circuit. Square wave response, however, was not particularly good, showing some ringing, and capacitive loads of about 0.5.µF produced instability.

Background
With short-circuited input, figures between 52 dB and 62 dB below 10 watts were obtained. In this connection no noticeable improvement could be detected when the hum-dinger was moved from the mid position, which would correspond to the conventional centre tap.

Contour Control
The effect of this on the frequency response is shown in the graph. The control is marked on the front at points labelled 0, -l0dB, -20dB and -30dB. A reasonable method of operation would be to set it at 0, turn the gain control up until the sound was very loud and then use the contour control to reduce everything to a more acceptable level. The leaflet provided suggests that the resulting sound will have the same apparent frequency response at all levels The reason why this does not seem to be the case in practice can be seen by comparing the measured curves with those based on Fletcher-Munson's well-known curves for equal loudness. These were obtained by plotting the difference between the 50 dB, 60 dB and 70 dB curves and that for a maximum reasonable loudness of about 80 dB. Large quantities of treble boost do not seem to be justified.

Comment
The Pamphonic 1004 gives a good impression from the moment it is removed from its substantial packing, since a proper descriptive booklet complete with circuit diagram and even spare fuses are provided. This impression is borne out by the electrical performance, which follows the specification with great accuracy, and by the listening quality which is of the highest standard. The general construction is also excellent, a combination of conventional and printed wiring being used. The variety of co-axial connector employed, however, is more likely to be popular for its cheapness than its ease of unplugging. Apart from the contour control the only other piece of minor criticism concerns the overload capacity of the radio input. Most tuners are going to need some attenuating to ensure that the signal level never exceeds 650 mV. These details, though, comprise nearly all the fault which could be found in a piece of equipment that is definitely first class audio engineering and excellent value for money.

J.C.L.

Before publication this review was submitted to the manufacturers, who returned it without comment - Editor.