Taken from 'Hi Fi News' October 1963

AT the present time living standards are probably higher in America than anywhere else, but this prosperity is reflected in the high cost of products which have a large labour content. This helps to explain why British hi - fi equipment has for many years managed to compete with the ‘home grown’ products the other side of the Atlantic. True, at one time we were ahead in design, but because of the lower production cost amplifiers in particular could stand heavy freight charges, etc. and still remain competitively priced. Now, however, this trade has assumed a two - way aspect which keeps manufacturers on their toes and pleases the customers - although the thought must horrify British economists!

The Third American
So we come to the Fisher XI00, the third American amplifier to be reviewed in these columns during the last few months. It is priced at just under £60, and in view of its specifications - plus the higher cost of manufacture - this must be considered very reasonable. The name Fisher is, of course, well known among audio enthusiasts all over the world, and Avery Fisher himself is one of the pioneers in this field. As far back as 1937 he was producing Sound Systems using amplifiers with negative feedback and speakers in acoustically designed enclosures. More recently he has been responsible for some unique features in FM tuner design for stereo multiplex systems and also for a range of high-performance speakers.

The XI00 was specially designed for this country and has a lower power output than the average American amplifier (I note that Fisher have just released a 150 W model!) but the rated output of some 20 W per channel is still quite high by our standards. These are 'music power' figures, but nevertheless the X100 delivers a good twelve healthy sine - wave watts. Styling and presentation follow the current American trend, with the right - angled panel finished in brushed brass and knobs trimmed to match. The lettering - which is in brown - is very tastefully carried out and the appearance of the whole facia is most attractive. It is, in effect, divided into two sections, and the control layout is as follows: top half, tape - gram equalisation slide switch, two filter switches (high- and low-pass), low-level control for the left-hand channel, ditto for the right-hand channel, tape monitor switch, centre channel switch, and the loudness contour switch. From left to right on the bottom section, we have the input selector, mode selector, concentric bass controls for both channels, treble ditto, balance control, and finally the master volume control. The input selector switch has the following functions: mag 1, mag 2, tuner, and two auxiliary positions, while the mode switch has positions for stereo, stereo reverse, channel A, channel B and mono pickup.

Circuit details
The first valve is an ECC83 double triode, with tape and gram equalisation effected by a feedback loop between the anode of the second triode and the cathode of the first. After this stage comes a level control, then the high-level inputs, low-pass filter and another ECC83 double triode. Between these two triodes is a passive tone control network and the rumble filter. Following this stage - in other words the fourth triode - is the volume/loudness control, balance control, and then the driver and output stage. The driver valve is another double triode, this time a 7247 with the second section functioning as a conventional anode - cathode phase-splitter feeding the two EL84 (7189) output valves.

Feedback is applied from the output transformer secondary to the cathode of the driver section of the 7247, and the combined cathode current of the output valves is used as a source of DC heater supply for the ‘front end’ valves. Resistance-capacity smoothing is employed and the rectifier is a GZ34. A divider network is connected between the output stages of channels A and B to form a centre, or ‘phantom’ channel. The output valves are pentode-connected instead of the more usual ultra - linear arrangement, and operate at a fairly high anode voltage (about 350 V anode to cathode). Thus the power output is near the maximum for this type of valve. Sockets are provided for the Fisher ‘Dynamic Spacexpander’ which can add reverberation to a tape recording or to a tape playback. Provision is also made for tape monitoring from a recorder, using separate recording and playback heads. Both magnetic and ceramic pickups are catered for, and an indication of the versatility of the XI00 can be gauged from the fact that no less than 14 input sockets are provided!

Power Output

Frequency c/s	30	40	100	1,000	10,000	15,000	20,000
Watts RMS	9	13	14.7	15	11.3	10.4	9.6

Power dropped off rapidly below 40 c/s and at 20 c/s it was down to 6.5 W. The feedback loop measured 25 dB and no instability was evident when this was increased to 40dB - the maximum possible without changing the circuit! Thus the stability factor is at least 15 dB, which is exceptionally good. Frequency response was within 1dB from 20 c/s to 13 Kc/s, dropping to -3.3dB at 20 Kc/s. and thence gradually rolling off. Square-wave definition showed no trace of ringing at 1 Kc/s and the rounding at 10 Kc/s (see diagram) reflects the deliberate attenuation in the high frequency response. Crosstalk was 34dB at 10 Kc/s and 46dB at 1 Kc/s.

The tone controls measured as follows: bass, a lift of 17dB and cut of 15dB at 40 c/s; treble, a lift of 15 dB and cut of 17 dB at 10 Kc/s. Filter characteristics are shown in the diagrams, and it will be noticed that the turnover point (-3dB) of the low-pass or 'scratch' filter is around 7 Kc/s and the attenuation is about 15 dB per octave. The rumble filter attenuates from 45 c/s with a cut of 15 dB at 20 c/s. Due to the use of DC heaters and careful design of the input stages, hum and noise were very low. Here are the figures (referred to full output):

Here it is worth noting that the input level controls for magnetic pickups are connected after the second triode and not right at the input stage as is customary. Gram equalisation followed the RIAA specification within 2dB and the tape compensation was found to comply with the American NAB standards. To obtain a fairly close match to the RIAA characteristics a slight treble lift will be needed, position 1 or 2 being found satisfactory. The loudness control switches in an R/C network which gives bass and treble lift when the volume control is turned down. No measurements were taken, but using the Quad electrostatic and Cotswold speakers it was considered that the sound was more natural, at any level, with the loudness control off. However, this is a matter for individual preference, and some compensation may well be useful for certain types of speaker systems which are demonstrably different at low volume levels.

Both the Decca ffss and EMI EPU100 pickups were used during the listening tests with excellent results. Plenty of power was available and the background noise was very low - almost inaudible a few inches from the speakers. The ‘scratch’ filter was quite effective, although obviously not so efficient as the more sophisticated variable types fitted to many British amplifiers. As far as styling is concerned, I would give the XI 00 full marks; I doubt whether even the Design Centre would find much to criticise. Summing up, I would consider this amplifier a very strong competitor in what is already a fairly crowded field and would have no hesitation in recommending it to the most discriminating.

A special word of praise is due to the clear and lucid instruction manual - especially for the ‘short form’ instructions for ‘the man in a hurry’ - although I feel this page might be even more useful for the woman of the house who usually dislikes to read technical information but just wants to know how to set the knobs to produce music!

G. W. Tillett