TECHNICAL REPORTS By P. WILSON, M.A. E.A.R. Microgram, Type A750. (Price 30 gns. inc. P.T.)

Specification:

Record Changer : Collaro, 3RC531 ( speed) with Studio " P " Pickup.

Amplifier : 3-stage, plus rectifier with two negative feedback paths and one positive feedback path. Output 3 watts.

Controls : Volume, bass boost, treble boost. Valves : Mullard ECC83 (Twin), EL84, EZ80.

Speaker : Goodman io in. elliptical.

Cabinet : 8 in. by 15 in. by 164 in., weighing 28 lbs. Finished in leathercloth with loudspeaker and vent grills in cream plastic.

Before I ever played this Microgram I was convinced that I should be able to recommend it without qualification for the type of use for which it has been designed. So I have given it an exceptionally thorough test so as to be sure that my bias in its favour would not warp my judgement. It has come through everything with flying colours.

If I had set out myself to design such an instrument I should have chosen the same record changer and pickup, the same speaker, the same type of cabinet, and the same sort of amplifier design.

Since last September I have had the Collaro 531 autochanger and Studio " P " pickup in almost continuous use (apart from times of sleeping of course !) and have not modified in any respect the high opinion I formed of them. If I were chocsing a type of loudspeaker for this type of cabinet it should be this elliptical ro in., neither more nor less. If I were choosing a form of cabinet it would have this type of facility of bass reinforcement by Helmholtz resonator action and lid sounding board effect. In my amplifier I should have both bass and treble controls either of this type (which was originally due to Mr. Voigt, I believe) or of the Baxendall type. And I should have had one of these two types of negative feedback, though I probably should not have thought of adding the second type or the positive feedback, at any rate to start with. I must say at once, however, that these additions are distinct advantages, particularly in ensuring good transient response and substantial loudspeaker damping so as to extend the range in the bass.

I have tested the range of the whole equipment (loudspeaker and cabinet included) and find that it extends from 30 c/s in the bass to over it kc/s in the treble, though it is falling gradually below about zoo c/s and above about 9 kc/s.

This is a very remarkable performance for an instrument of these dimensions and calls for the highest commendation. It is considerably better than that of most radiograms in use to-day.

I fully expected to find a good performance in the treble range, but I never thought for a moment that it would be possible to produce such a massive tone from a smallish portable. With the bass control set at full boost (so that there is a lift of over to db at too c/s) and the treble at about mid setting, I will challenge anyone to guess from an adjoining room that only a portable player is in operation, and that even on heavy orchestral passages.

For standard records, too, I like the way in which good quality can be maintained with comparatively little scratch ; though here I think I should have been inclined to provide less than the present maximum lift ( to db at 9 kc/s) and give rather more treble attenuation; but perhaps that is because I have a library of old records.

All in all, I have no doubt that this instrument will take its place and remain for some time to come in the lead of portable reproducers.

Sound Sales DX Plus Four Radiogram. (Price L47 tos. plus kis its. 5d. inc. P.T.) Specification:

Circuit : 6-valve comprising frequency changer, IF., detector plus L.F., push-pull tetrode output, rectifier. Negative feedback. Output 8 watts. Valves : 6K7, 6K8, 6Q 7GT, 2 x KT6i 5z4.

Radio : Medium Wave-200 to 550 metres. Long Wave-800 to 2,000 metres. Variable selectivity.

Motor : Collaro Type AC3/544 with Studio " P " pickup.

Controls : Tuning (with large circular dial calibrated in wavelengths), volume, wavechange, Radio/gram., selectivity switch, tone (for records only), motor speed, on/off switch.

Loudspeaker : to in. m/c, in horizontal bass reflex compartment in lower portion of cabinet, with diffuser on front of cabinet operating both for loudspeaker and for vent of enclosure. Provision for 3 ohm extension speaker.

Cabinet : Massively built, finished in walnut veneer. 32 in. high by 31 in. wide by 18 in. deep.

In the above specification I have described the cabinet as massively built. It is in fact the most substantially built cabinet that I have seen for some years. This, coupled with the ingenious bass reflex system (or phase inverter, as the makers call it), means that coloration by cabinet resonance is reduced to very small dimensions.

The first impression one gets of the quality of reproduction, indeed, is of a lively, clean and brilliant tone, and a bass response of very little " hangover ".

Tests of frequency response showed a substantial output down to 50 OS, tailing off to about 30 c/s ; but I could find no special frequency from there upwards which I could identify as the main L.F. resonance.

At the other end of the scale the response began to fall off (on LP records) at a frequency between 9 and so kcis, but was appreciable even at it kcis.

Between these two extremes the response was particularly smooth with no harsh overemphasis anywhere. It was therefore no real disadvantage for most records to have only a simple treble attenuator as tone control ; the Studio " P " pickup itself takes care of the equalisation requirements as between the majority of " 78" and LP records.

On radio the performance was sans peur et sans reproche, as the ancient ballad has it. With the large circular dial identification of stations by means of their wavelength is very easy notwithstanding the absence of names. One end of the " hand " moves on a continuous wavelength scale and the other end picks out the principal stations by means of markings on the dial in the form of small circles in which the wavelength is inscribed. Calibration was particularly accurate.

The facility of a variable selectivity switch is a distinct boon. For it enables the best quality to be obtained at the position of minimum selectivity at times when reception conditions are good (e.g. local stations in daylight) coupled with a means of reducing interference when conditions are not so good. Some evenings I made use of this switch with advantage even on the London Regional service.

With the phase inverter, the output is ample for any room of ordinary size, and at all reasonable volume levels there is never any cause for the slightest anxiety. Though as a rule I prefer to have the loudspeaker in a separate enclosure, such as the "Sound Sales Inverter Speaker" on which I hope to report next month, the system adopted in this radiogram is a very useful compromise for those who wish to have a single unit.

Sound Sales Frequency Modulation Unit. (Price L15 os.)

Having very little time in these days to experiment with the building of special tuning units for myself, I have been looking round for a good F.M. tuner at a reasonable price which I could buy for my Owls use. Fortunately before I had invested in a much more expensive unit, Mr. Wellington of Sound Sales Ltd. told me that he had this one in hand and has given me the opportunity of a thorough trial ; so that I can now report on it as soon as it is available for the general market.

Let me say at once that it fulfils all my requirements, both as to performance and price. And it is very compact, measuring only tri in. long by 44 in. high by 4 in. deep and requiring only a cut out of 9i in. long by 4 in. to facilitate simple panel mounting.

It is calibrated and tunes from 86 to too Mc/s. Both volume control and autcmatic gain control are incorporated, and there is a three-position switch for A.M./F.M./Mod. The latter position is useful for tuning in the F.M. transmission accurately ; one first tunes to the lowest beat note on MOD. and then the signal is accurately tuned in the position F.M.

A few months ago the B.B.C. announced their programme for new stations in this " Band II (87.5-100 Mc/s). No fewer than 5r transmitters at 19 sites are in contemplation to reinforce the services given by the existing medium and long wave stations ; of these a substantial number will be frequency modulated. So one may take it that within the next few years a F.M. service will be available for most parts of the kingdom. In the meantime those of us who live in the S.E. corner of England will, I hope, continue to have the advantage of the experimental transmission from Wrotham in Kent.

At present, with this Sound Sales Tuner I can receive in S.W. London strong signals from Wrotham on the experimental aerial supplied with the tuner. This is a simple dipole with each arm 30 in. long mounted horizontally at a height of about 20 ft. I erected it in about 20 minutes.

Only the Home Service transmissions have been available so far on a frequency of 9 r .4 Mc/s, the Third Programme transmissions on the 93 Mc/s frequency having been temporarily discontinued. I feed the rectified output from the tuner into the pickup terminals of my amplifier, but as soon as my new pre-amplifier is ready I shall of course have a special position on the input switch for this tuner.

The power pack of my amplifier has ample margin to supply the 30-40 rn.a. at 230-250 volts and 6.3 volts at 1.5 a. L.T. which the tuner requires, so there is no anxiety on that score. But even if I had not that margin it would be well worth while to have a separate power pack for the purpose ; and I understand that Sound Sales will supply one for L4. Jos. extra, which will still keep the price of the whole equipment at a figure lower than any other that I have yet seen.

I have no doubt at all (and indeed can demonstrate it without question at any time when transmissions are on) that the quality obtainable from this equipment is considerably better than that obtainable on the ordinary wave bands even in the best conditions. The limit is in fact set by the ordinary amplifier and/or loudspeaker. And there are practically no whistles or other forms of interference.

Listening to one of these F.M. transmissions is therefore just as thrilling and absorbing as listening to a first class LP record used to be (and can even now be, on occasions).

The tuner has functioned perfectly during all the few weeks that I have had it. There has been virtually no frequency drift except during the first ten minutes or so after switching on, that is, whilst the valves were heating up ; and I never expected to avoid that, at any rate without a much more elaborate and therefore expensive instrument.

For the future, therefore, you may count me as one of the fans of" F-M via A-Z ". I would not dream of listening to an ordinary transmission when this alternative is available.

Wharfedale Super 12/CS/AL Loudspeaker. (Price k 17 tos.)

Specification:

Magnet : Alcomax III with flux density of 17,000 gauss, in. diameter centre pole. Total flux 190,000 gauss.

Speech Coil : 8/ to ohms or 12/15 ohms. Wound with aluminium wire.

Cone : Straight-sided but with eight concentric corrugations beginning at a distance of about 2 in. from coil. Material soft paper but with bakelised apex and cloth suspension. Surround resonance below 6o c/s.

Chassis : Die-cast with bakelised " spider" device external to speech coil for centring.

Frequency Response : From 30 c/s to over 14 kc/s.

I am often asked in these days which loudspeaker I consider to be the best, regardless of price. It is of course a question which really cannot be answered just like that ; for so much depends on how it is to be used.

At present my own preference is to have a multiple speaker system with an electrical cross-over at a frequency not higher than 1,000 c/s. I recognise the problems involved, particularly in the region of the cross-over(s), but with care I believe that they can be dealt with reasonably well. On the other hand the advantages of having each unit operating in its optimum conditions are so considerable, particularly in regard to non-linear distortion, which I have long regarded as more vicious than non-uniformity of frequency response, that I would not readily sacrifice them.

But I recognise that this preference is related to the volume at which I am accustomed to listen in my rather large drawing room. For smaller outputs it is, I find, quite possible to obtain just as impressive, and possibly even more agreeable, results from a single speaker suitably mounted either on a special baffle or with reflex loading.

For the two sets of circumstances, I should recommend quite different loudspeaker units, and within very narrow limits I should vary my advice even for the single loudspeaker system according to the conditions, e.g. as to the sensitivity required in the bass or the treble.

Fcr my own conditions at home, the Super 12/CS/AL is the best single loudspeaker I have yet tried. It does not give quite such an impressive bass response (from an infinite baffle) as some of my other speakers, but its sweetness as one goes up the scale is something to be marvelled at ; and it has none of the " paperand-comb " quality on the top strings which is so common a fault of otherwise quite good speakers. When reflex loading is used even the relative weakness in the bass disappears.

I like, too, its handling of transients— obviously a result of the high flux density and the quality of the centring device. The latter pleases me for another reason. Being based on a three-point suspension, it is co-planar and therefore truly linear in its effect at right angles to the plane of support—which so many other devices are not.

I haven't measured the intermodulation introduced, but I am quite sure from listening tests that it is very small.

I have really enjoyed putting this speaker unit through its paces. Whilst it has borne out in several respects the conclusions I had formed from its appearance and design, it has taught me not a few lessons, particularly about the response at each end of the scale.

For ordinary listening conditions at moderate volume it has almost made me give up my preference for the multiple system with its deeper, thrilling effects in the bass and its shimmer in the treble. I would much sooner have it than one of those systems to which I have sometimes been introduced in the name of " High Fidelity ", which turns that shimmer into the rattling of a bag of beans or French nails.

So, I come back finally to the conclusion I expressed earlier : that for certain conditions of home listening I prefer it to any other single unit I have yet tried. And those conditions are, I believe, more usual than not.

Pye Amplifier PF9i and Pre-amplifier PFgzA. (Price 40 gns.)

Thanks to the enterprise of our old friends, Messrs. Imhof, I was enabled to give this latest "high fidelity" amplifier a thorough test as soon as ever the first products were off the stocks and ion.-' before they were publicly announced at the Press Conference on Easter Tuesday.

Mr. Layton was so enthusiastic about it when he rang me up on the phone that I knew that here was something exceptionally good ; for by now he must be almost as blasé as I am about these things. However, as it happened I was already aware of the history of its design by my old friends of pre-war days, Pamphonic Reproducers Ltd. (which has now become an associated firm of Pye), and of its eager adoption by the major company. So Mr. Layton's verdict did not surprise me.

All the same, I was glad to have the opportunity which he provided of having the early model to test at home at my leisure.

Let me say at once, and before going into a detailed description, that the combination has taken its place in the very front rank. I cannot truthfully say that it is clearly superior to any other amplifier system commercially available, because that is something one could not say without very elaborate side-by-side comparison. But it has all the usual desirable features as well as a few which are claimed to be valuable and which " some others haven't got ".

Let us then look at the specifications : P.FoI Amplifier.

Circuit : Three stages with "long-tailed pair " phase reverser in second stage, and push-pull triodes in output stage.

Positive feedback from output transformer to first cathode to reduce output impedance virtually to zero.

26 db negative feedback from output transformer to first cathode to ensure linearity. Phase corrector between grids in output stage so as to ensure stability.

Sensitivity : 0.4 volts for 12 watts out. Frequency Response : Flat from 2 CiS t 160,000 C/S.

Harmonic Distortion : Less than o.r per cent at 12 watts i kc/s.

Output Damping on Loudspeaker : Adjustable from 35 to infinity.

Output Power Rating : 12 watts, but peak power pulses in excess of this can be handled with negligible distortion.

Valves : ECC35, ECC33, 2xKT66, GZ32.

PF9i A Pre-amplifier.

Circuit : Three stages plus cathode follower output.

Fixed correctors for various types of recording.

Bass and treble boosts and attenuators of the Baxandall type.

Steep cut filters at 4 kc/s, 7 kc/s and 12 kc/s with slope of 22 db per octave.

Controls : Input selector for microphone, magnetic pickup (three positions), crystal pickup and radio/tape. The three positions for the magnetic pickup bring into circuit different equalisers for British LP records, for American N.A.B. recording and for British 78's.

Bass, varying from 15 db boost to 12 db attenuation.

Treble, varying from 12 db boost to 15 db attenuation.

Steep cut filter switch with four positions Off, 12 kc/s, 7 kc/s and 4 kc/s.

Volume.

Valves : 2 X ECC40.

Sensitivity : From 3 to 120 m.v. according to input, for 0.5 volt out : (microphone, high impedance, 3 m.v. ; magnetic pickup, 20 m.v. for 78, 15 m.v. for LP ; crystal pickup, 120 M.V.).

The pre-amplifier may be either plugged in direct to the amplifier or connected to it by means of a long cable. In my tests I used the direct connection.

Good wine needs no bush ; and clearly any instrument that fulfils this specification can need no praise from me. All I need say, therefore, is that I am satisfied that the model which I have used for several weeks has regularly behaved according to plan, except in one particular which I will mention in a moment. My principal measurement tests were made with a square wave oscillator and oscilloscope. For all frequencies from too c/s to 15,000 c/s the output when connected direct to the plates of the oscilloscope proved to be of better shape than that claimed in the Pye booklet. I had to go well above 20,000 cis before I got any sign of high frequency ringing and the wave form was good well above there. One can say with confidence, therefore, that all the claims about high note response are fully justified.

It was only when I came to look at the wave form below about 50 c/s that anything unusual appeared, and I was unable to satisfy myself that there was an entirely linear response in the deep bass. My own amplifier incidentally was better in this region, but not so good on the 15 kc/s square wave.

I don't know what the reason was for this minor departure from the makers' objective. I should judge however that in actual playing conditions it showed itself in a slight hardness in the bass notes, nothing more. Certainly my own amplifier (which has 32 db of negative feedback and no intentional positive feedback, but has a low impedance input through cathode followers to the output valves) showed a definite softness in response down there whilst being equally sharp to transients.

But this is perhaps being a little hypercritical. I mention it only because the amplifier is so good that it merits an approach of the most critical kind.

There is still a little room for improvement even in this amplifier, though one has to go into minutiae to find it !

The pre-amplifier is a beautifully flexible instrument. Though I myself would have included somewhat different pre-set correctors, that is only a matter of taste and not of criticism. I thoroughly approve of the general design and particularly that of the output stage with volume control.

All in all, I believe that even the most hardened cynic would be greatly impressed by the quality obtainable in a large variety of conditions from this amplifier and pre-amplifier. It is definitely ahead of at any rate most loudspeaker combinations.

At the Press demonstration, the loudspeaker that was used was one that has just appeared on the market. That, too, has been designed by our Pamphonic friends and is a worthy example of their skill. It has a 15 in. bass unit mounted in a vented chamber which is claimed to be capable of handling more than 15 watts of 35 c/s steady tone without distortion ; and coupled to this is a special elliptical unit with aluminium voice coil and voice coil former which is mounted so as to avoid beaming effects. There is also a "room matching switch" which has the object of providing a means of adjusting the tonal balance to the particular room or the idiosyncracies of the listener. It behaved itself very well indeed at the demonstration and I am looking forward to a more detailed test in a few weeks time.

Yes, the Pamphonic " Victor" loudspeaker has great promise. I shall not forget the reproduction by this combination of the Nixa record of Walton's Belshazzar's Feast for a long time.