|Avantic DL7-35 power amplifier & SP21 control Unit|
Avantic DL7-35 Amplifier and SP21 Control Unit
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.
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: -
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.