Lecson AP3 MkII

The second Lecson product was the AP3 amplifier housed in a taller case, compared to the AP1, and with a cooling fan at the top. This amplifier was fraught with problems and production had to stop shortly after Stan Curtis joined the company. This could have been a financial disaster for Lecson because there was an abundance of parts in stock yet few amplifiers were being sold. Curtis made a few changes to the AP3 design so that a trickle of products could leave the factory and then created a new design in a matter of weeks. This was the AP1X which could be described as a de-tuned AP3. The cooling fan was removed, a new smaller power transformer was fitted together with modified versions of the original AP3 circuit boards. The end result was a 70 watt per channel amplifier which sounded good and was very reliable. The AP1X quickly killed off most of the AP1 sales which was good news from the factorys' viewpoint. However dealers were asking about the AP3 and something had to be done.

Curtis designed a totally new circuit board which used high power Motorola transistors; two pairs of MJ802 and MJ4502 per channel. These transistors did not need any conventional short-circuit protection because they could cope with 30 amp peaks of current. However, in order to get a high power output a higher power input is required and the existing E-core power transformer was insufficient. Curtis had come from Cambridge Audio which was the first company to use toroidal transformers in audio equipment, so his solution was to use an efficient toroid but a very unusual one. This transformer was tall compared to the usual flat doughnut shaped toroids of the era. The only company who could make such a transformer was Plessey, a specialist in defence equipment, and the resulting transformer was 'very, very good and very expensive'. However, it made for a powerful amplifier and a typical AP3 MkII could deliver 140 watts with less hum and noise, compared to the 120W of the MkI. Modifications were made to the protection circuit as well. The AP3 MkII proved to be very successful in the market and as reliable as the AP1X.

Radical then, inside and out, this is probably one of the most memorable of amplifier designs. Matched with the AC1 preamp.

The power amplifier (1976) is a directly coupled inverting class B design using a fully complementary output stage of series connected transistors and gives a power output of around 150 watts per channel (600W short-term peak).

(No component values were available with this schematic. Do you have any, or more interestingly, would anyone like to suggest some? C6 and C7 (missing) assumed to be associated with Q4 - Q7.)

Although adequate performance was given by the Quad 303, Cambridge P-Series and the near theoretically perfect Crown DC300, new speaker designs offered more in terms of low-impedance. The use of very lively speaker loads, such as the notorious Linn Isobariks, could present problems where previously there were none, especially if used with 'low-loss' speaker cables. A number of approaches were built with output stages intended to be capable of dissipating all of the energy from the power supply.

This design mounted the output transistors in series effectively doubling the Vce of these thus increasing the safe operating area (SOA, see graph - Beomaster 4400), the output transistors chosen for their large chip and die mounting area (200W, 90V, 30A, 2MHz). The quiescent current was set at 40mA. Although the author expects to be corrected, it is ventured that although similar arrangements date from 1964 with valve equivalents, Motorola, in 1976, announced a similar arrangement using BD364/BD369 transistors (Vce = 50-80V, 25A) using 6 per amplifier.

Capable of delivering 20A to the load (in terms of current, the BGW 750 could deliver even more), the distinctive cabinet, which split into two halves, contained two identical amplifiers, power supply (±59/63V, 6m8F smoothing on each rail for both amplifiers, 10A fuses for each). A separate module carried the Zobel networks, output chokes, headphone attenuators, DC offset protection, delayed switch-on and thermal protection circuits including a 62.5°C mains trip and 45°C fan switch. One stereo system seen used one amplifier per speaker, each driven in bridge mode by a circuit similar to that below.

A truly innovative overall approach that still inspires admiration, by Stan Curtis who was also responsible for the Cambridge P60/P80 ('75), the Mission Electronics Voltage Amplifier ('77) and the ETI System A ('81), amongst others.

Reliability was an issue for some, innovation perhaps over-riding good practice. For example, the mounting of the output devices could have been better, notwithstanding the minimal physical gap (<1mm) between devices carrying up to 130V between them. These could have been set at 45° with a slightly different track layout giving a >10mm gap, avoiding the danger of destructive short circuits and the overlapping of insulating washers.

The reasoning that a high power rating will be more rugged is sound, especially at elevated operating temperatures. However, the thermal resistance between ambient and junction with this layout must be high. A larger mounting plate with an H or U shape (and a copper one at that) would have increased the heat-flow to the sink. Does anyone have any data?

A lack of heat-sinking has been commented on by others who worked on the Cambridge P series, for example. Perhaps the estimated survivability of these 'tough' transistors in these operating conditions was optimistic.

Iain Mackays' 'A FEW NOTES' covers the repair and cloning of these with good photographs and component values. A copy of a PDF of this is available on request. Pitfalls described can be avoided with some tips on setting up a power amplifier.

This design differed significantly from, say, the AP1 (J. R. Stuart), a symmetrical amplifier with two differential input stages and single output pair (2N3442/BDX1880 - 117W, 140V, 10A, 500kHz), a circuit diagram of which, with component values, is available on request.

A close 'living relative' to this approach, bar the output configuration, is Michael Bittners' SymAsym5.

Google search for Lecson AP3.

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