'The Gramophone' July 1963

The Avel Dynaco Mark IV amplifier can be used with any conventional stereo or mono preamplifier. Its matching companion, however is the Dynaco PAS2 stereo preamplifier and a review of this follows. I am reviewing them separately because they are sold as two distinct units, and also because the main amplifier with its large power output may have certain applications not connected with the use of a normal preamplifier.

The specification of the Mark IV is so good that it represents a challenge to any self-respecting reviewer to find something wrong somewhere. Frequency response ±0.5dB from 20 c/s to 60 Kc/s - rise time 5 microseconds - power output within 0.5dB of 30 watts from 20 c/s to 30 Kc/s - completely stable on all capacitative loads - all this is an invitation to 'have a go’. I did and I cannot disagree with any of it. My plotted curve of frequency response looked like a drawing for a new motorway in the Fen district. My plans to upset a square wave with capacitative loads applied to the loudspeaker terminals failed completely and the power output at 1 kc/s showed a steady 44 watts. However, before my enthusiastic pen runs away with me, I must begin at the beginning, digressing only to invite readers to take another look at the price.

This is a hybrid amplifier. The mains and output transformers are British and the remainder American. Aveley Electric make the transformers, which are toroidally wound, impressive to look at, undoubtedly expensive to manufacture and contribute more than a little to the amplifier's excellent performance. Both transformers and chassis are finished in bright nickel and the ventilated cover in charcoal brown. At the rear, alongside the loudspeaker terminals, are an octal socket for power to pre- amplifier and a mains on-off switch. The circuit is fairly conventional but a few minor modifications have been made to the American circuit by Aveley Electric in the negative feedback line to suit their transformers.

A voltage amplifier stage, pentode half of a 7199 triode pentode, is directly coupled to the triode section, which is used as a phase splitter with equal loads in anode and cathode. The signal then feeds the push-pull output stage, a pair of matched EL34s in ultra-linear operation. Fixed bias is applied to these from a small metal rectifier fed from a tap on the mains transformer secondary. The power supply employs a GZ34 rectifier with both choke and resistor smoothing. All the components for the input stage, including the valve itself, are mounted on a printed-circuit board. Output stages and rectifier circuit are wired in the conventional manner.

Apart from the printed-circuit board, the wiring on my review model was far from neat. I could see no reason for this. There is plenty of room on the underside of the chassis to allow for something approaching Leak or Quad standard. Not that it makes any difference to the performance but I confess that I felt a strong desire to spend a couple of hours tidying it up. This was resisted, however, in favour of writing about it. Components throughout are of the highest quality and the metal work soundly and solidly constructed.

As I have already indicated, measurements showed the amplifier to be magnificent. The frequency response measured within 0.5dB from 20 c/s to 50 Kc/s. Power output at 1 kc/s measured 44 watts, at 20 Kc/s 37 watts and at 20 c/s 28 watts, good waveform being maintained all the time. Square wave response was excellent, stability under various capacitative ' loads complete and hum and noise negligible.

The triode pentode preamplifier valve type 7199 has no equivalent in this country but supplies are available from Aveley Electric in case of need. For listening tests the PAS2 stereo preamplifier was used, a Quad main amplifier supplying the second channel when checking the stereo performance of the control unit.

PHILIP TANDY.