Taken from 'Hi-Fi News', October 1956

Avantic DL7-35 Amplifier and SP21 Control Unit
Manufactured by Beam Echo, Ltd., Witham, Essex, England Price complete, £55.

THIS amplifier, which I heard for the first time at the BSRA exhibition in May, is one of the best examples of the Mullard 520 (designed by D. Busby and the late W. Ferguson) I have yet tested. Considerable modifications have been made, of course; the original design allowed for only two record equalisation characteristics and had 4 input sockets, whilst the Beam-Echo version has provision for 4 recording characteristics and no less than 8 input sockets. Furthermore, because of American requirements, a "loudness" control has been added. This type of control is intended to compensate for the non-linearity of the human ear by introducing bass boost when listening at low volume levels, and is based on the well-known Fletcher-Munson equal loudness curves. There is considerable controversy over the desirability of such a control, but it is sufficient to state here that however carefully this control is "tailored" to the Fletcher-Munson curves, the results are bound to be a compromise, as these curves do not take into account the effects of distance. For this reason then, the majority of Acoustic Engineers in this country favour the use of the normal tone controls for compensation purposes as being more efficient and flexible for dealing with so many variable factors. Nevertheless, this must not he taken as a serious criticism of the Beam-Echo amplifier as the use of the loudness control is entirely at the discretion of the listener - in other words, "You pays your money and you makes your choice".

An interesting feature, which will particularly appeal to owners or prospective owners of tape recorders, is the inclusion of a 3-position monitor/record switch permitting the output from the preamplifier to be diverted to a tape recorder while using the main amplifier as a monitor if required.

Circuit Details

Three EF 86 valves are used in the preamplifier, equalising and sensitivity being controlled by a negative feedback loop around the first stage. The second EF 86 also operates in a negative feedback loop and precedes the passive tone control system and high and low pass filter network. The "loudness" control is included in the feedback loop around the final EF 86 from which the output is taken via the volume control to the main amplifier.

The main amplifier uses an EF86 for the first stage, and this is directly coupled to a ECCC 83. double triode cathode-coupled phase inverter, followed by the Ultra linear output stage using two EL 34s in push-pull. Test figures indicate that in the main the amplifier conforms to the original Mullard specification and are as follows: -

Power Amplifier

 Power Output: This shows a maximum power output of 29.3 watts r.m.s. between 45 and 5,000 cps., dropping to 23 watts at 30 and 11,000 cps, respectively.
 Distortion: A total harmonic content of 0.25% at 1,000 cps (for both units combined) was measured at an output of 23 watts.
 Stability: With the advent of the new electrostatic speakers, which present a large capacity load, a high stability factor is very important. The Beam-Echo past this test easily and was perfectly stable with capacitative loads up to 32 mfd, and inductive loads up to 200 mH. The normal feedback loop of 30dB could, in fact, be increased by 8dB before instability became apparent There was no sign of transient instability and square waves of 10 kc/s could be resolved with a commendable lack of transient "ringing".

 Frequency Response: This was within 2dB from 20 to 60,000cps falling to –6dB at 120,000 cps with a 5dB rise at 180,000 cps, thence falling rapidly.
 Hum and Noise: This was slightly better than –90dB relative to 20 watts output.

preamplifier

 Equalisation: In addition to the 4 record equalising characteristics as shown in the diagram, a further switch position gives a flat characteristic for use with certain crystal pickups which are self compensating, e.g., the Collaro Studio "O". The NAB curve is given a steeper roll-off than is usual, possibly because this is intended for use with early 78 recordings.
 Tone Controls: These are continuously variable. The bass control was found to give a variation of +14dB to –12dB at 50 cps and the treble control +l4dB to - l6dB at 10 kc/s.
 Filters: A variable slope control might be considered a worthwhile refinement, but the provision of 5 kc/s and 10 kc/s fillers attenuating at the rate of l2dB per octave represents a satisfactory compromise. The third position marked "flat" brings in another filter limiting the response above 20 kc/s in order to minimise the effects of resonances frequently met with in wide range pickups.   Loudness Control: The effect of this control is shown in the diagram, and it will be noticed that bass boost only is applied, consequently a slight treble lift may also be necessary.

The amplifier was tested with a wide variety of pickups including the Ortofon, Tannoy, Goldring 500 and the Collaro "Studio P" and came well up to expectations. Reproduction was characterised by a clean transient response and a feeling of a tremendous reserve of power typical of this type of circuit. Both units are well made and neatly wired, and I was particularly glad to sec that extensive use is made of high-stability resistors and low-tolerance condensers. Mr Lawson, the Beam-Echo Chief Engineer, is to be congratulated on a sound and workmanlike job, and a word of praise is also due to those responsible for the really comprehensive and beautifully produced instruction book.

G.W. TILLETT