Taken from 'Hi-Fi News', January 1965

FISHER is one of the most respected names in high fidelity in America - indeed all over the world. In general. Fisher amplifiers and tuners are not particularly cheap, but they do set a very high standard and appeal to those who want all the refinements and trimmings - and can afford to pay for them! The TX300 transistor amplifier costs $329 in America, and what with freight, import duties, etc., the selling price here is rather high.

What do you get for this money? First, a solid power output of nearly 35 W per channel, high and low filters, separate tone controls for each channel, provision for reverb unit and centre channel, inputs for tape heads, loudness switch and facilities for extension speakers. Oh, yes - I nearly forgot - sockets for 'phones.

There are in fact a total of 17 controls on the front panel arranged as follows: Top left, four push-buttons for tuner, tape replay, aux. and low-level inputs - each button having its own indicator light. Next we have a stereo-mono switch, bass and treble controls, then the volume control, which is combined with the on/off switch. The tone controls are dual-concentric with a clutch arrangement permitting independent operation if desired. At the bottom of the panel on the left is the selector switch, then the speaker switch, balance control, and at the right we come to a set of four rocker type push-button switches. These are tape monitor, high-pass filter, low-pass ditto, and loudness switch. In the centre, at the bottom, are the tape output and 'phone sockets. At the rear are the various input sockets, speaker connections with matching switch for 4-ohm and 16-ohm, AC power supply sockets and five - yes, five fuses.

Styling and presentation follow the conventional American pattern, with anodised aluminium panel and knobs, but even so the TX300 looks distinctly less clinical than most. This is probably due to the contrasting plastic push buttons and the choice of a warm golden colour for the panel. Whatever the reason, the net result is a very handsome - indeed elegant, appearance which will harmonise with any type of furnishing scheme. Incidentally, the Fisher nameplate on the panel is removable and it hides the pre-set input controls - a most ingenious and useful idea.

Inside there is ample room for all the components (the chassis measures 14 x 11 in.) and although no fewer than 43 transistors and diodes are used there is no overcrowding and servicing should be quite easy. Most of the components are neatly mounted on three circuit boards and the output transistors are all placed in a row along the back and covered by a metal cage.

Low-level signals are applied to the first transistor, an n-p-n silicon type (A1380) which is AC-coupled to a 2N2614. Equalisation is applied between the collector of the second to the emitter of the first. The next stage (to which the high level inputs are taken) consists of a 2N2613 emitter-follower and a 2N2614, forming the high- and low-pass filter circuit. Then comes another 2N2613 emitter-follower and the Baxandall tone control stage using a 2N2614. This is followed by the balance and volume controls with the loudness switch. After this are three more 2B614s and the driver stage which uses a larger transistor-type 2N2148. The driver transformer is fed via the emitter through an LF step network and the secondaries are coupled to four 2N2147s in a conventional transformerless push-pull arrangement using a 36 V negative and 36 V positive HT supply. As the four transistors are virtually connected in series across the two supplies the output at the centre will be at zero DC potential, so the speaker can be fed direct without the need for a capacitor. Feedback is taken to the emitter of one of the pre-driver 2N2614s.

The power supply is quite elaborate, consisting of a bridge for the two 36 V lines, plus another bridge and three transistors for a regulated supply to the rest of the amplifier - except the input transistor. This is an n-p-n type, remember, and so yet another bridge provides a positive voltage for it and also powers the tuner section on combined models. A further winding provides a low voltage for the pilot light and indicators. Fuses are located in series with each pair of output transistors and that is a special slow-blow type in the mains input circuit.

POWER OUTPUT
(per channel, both channels driven simultaneously)

The load resistance was 8 ohm (non-inductive). Using a 15-ohm load the power dropped by about 25%, as shown in the diagram. These figures relate to a mains input voltage of 240 V, 50 c/s, and as there are no tappings for lower input voltages tests were made with lower voltages as follows:

Frequency response was within 1 dB from 10 c/s to 40 Kc/s, falling to -10 dB at 100 Kc/s and - 30 dB at 250 Kc/s. Square-wave resolution was excellent, with no trace of ringing. Although the amplifier was stable with all values of capacitive load, instability was experienced when used with electrostatic speakers. Hum and noise were as follows (ref. 32 W):

Crosstalk measured 40 dB at 10 Kc/s and 52 dB at 1 Kc/s. Equalisation followed the R1AA recommended characteristic within 1 dB, and tape compensation conformed to the NAB curve for 7½ ips tape. The bass control gave a lift of 10 dB and cut of 18 dB at 40 c/s, and the treble a lift and cut of 15 dB at 10 Kc/s. Operation of the loudness switch introduces a bass and treble lift at low volume levels as shown in the diagram. The balance control permits the output of either channel to be faded to zero; this is of course by far the best method and is much superior to a limited control. The high-pass or rumble filter had a turnover frequency at about 50 c/s, with an attenuation of some 11 dB at 20 c/s; and the low-pass filter attenuated from 5.3 Kc/s at a rate of about 11 dB per octave, increasing to nearly 15 dB.

DUAL CONCENTRIC TONE CONTROLS
As mentioned previously, the tone controls are dual-concentric, permitting independent control of each channel, but they are so arranged that for normal operation they turn together. This flexibility may be useful if dissimilar speakers are used, but it is unlikely that good stereo will be obtained unless the frequency responses are - with or without manipulation of tone controls - within a decibel or so in the location frequencies of 300 c/s to 10 Kc/s.

For normal use, then, the controls are treated as ganged units and they were used in this way for the listening tests. I found the 'scratch' or low-pass filter most useful - not only for removing 'chromium-plating' on some records but also to render many FM transmissions bearable. In this part of the country (Yorkshire) the local transmissions are considerably inferior to Wrotham, for example, and many programmes not deficient in HF response suffer from distortion. Speakers used most of the time were the Wharfedale W2s, and as one might expect there was always ample power in reserve to handle peak outputs. Somehow, there is always a different feeling with a large amplifier - rather like using a more powerful car. Smoother and no sense of effort or strain - that sort of impression.

How about this 'transistor sound'? I must report that the TX300 sounded like any other first-class amplifier, although sometimes I did have the impression of crisper transients. What I can be certain about is this: the sound was always clean and easy to listen to at any volume level. What higher praise could you have than that? I was disappointed to find that the amplifier would not function with electrostatic speakers, but I modified the circuit and then obtained very good results. (We understand that the manufacturer or U.K. distributor will now perform this modification for any purchaser who intends to use electrostatic speakers - Ed.)

One word of warning - no protection circuits are used so shorting the speaker leads could have disastrous effects. This point is dealt with in the excellent instruction manual, which also gives a foolproof setting-up procedure whereby the output transistors are run at reduced power until it is ascertained that everything is connected in the proper manner. This man Fisher thinks of everything!

G. W. Tillett