Cambridge Audio 'P' series amplifiers.The P40 appeared in 1969, followed by the P50 in 1970, the P60 in 1975 and then the later P110. Output power was indicated in the model numbers, eg; P40 - 20+20W, etc. The P50, to some, was rather crude but a solid build with a surplus of connecting wires and large components whilst the P60 had a rather more professional finish with everything mounted on PCBs with no modifications, both being hand built. The P50 also had an unusual active volume control using variable feedback in the first stage of the preamp giving high overload margin and low noise, an approach often seen in guitar preamps, which was retained in later designs. This was useful for matching a variety of input levels.
Unfortunately, a number of errors and omissions occured in the source material relating to component values for the P40 and hand-rendered service manual (1/6/75) intended for both the P60 and P80. If the reader has access to information that the author has been unable to obtain, this would be appreciated. The P40 data has been kindly supplied by Aren van Waarde. PDFs of the P50/P110 and P60 service manuals are available on request.
The P40's RIAA stage differed from the norm having a high impedance, flat, inverting, variable gain input (>x21 max). This was followed by an eq stage (x15 DC) that remained virtually unchanged in later models, the common emitter configuration being a fundamental building block in these amplifiers.
The power amplifier differed also by employing a bistable to cut the bias under over-I conditions and a paralleled driver stage.

Balance was achieved by varying the feedback impedance, seen again in the later JLH 80W mosfet. The author would not recommend this approach which ties both amps with a single control, fixed and independent networks being considered safer and more reliable. The use of bistables and 1N914 signal diodes was a familiar hallmark of this series.The P60 was still available until the early '80s at a cost of about £289 with the matching T55 tuner at £169, both being rather expensive for the time. These designs have proved to be quite reliable, the use of the DIN input convention and the loud switch-on transient through the speakers being the predominant short-comings. The latter could be overcome by using a soft-start which could be fitted inside the unit.
An unconventional but high quality build, some considered the placement of volume, balance and mono controls before the tape outputs as inconvenient. However, the facility for up to three tape decks to be used, the tape 2 output following the lo-filter and tone controls, could be useful.







The over V/I protection was noteworthy in that a FET was used to reduce the input V in proportion to the output current.

This protection only turns on at the equivalent of 50W into a 2 ohm load and when it does it only adds a moderate distortion (0.2% typically) as distinct from clipping (V limit = 75V, I limit = 5A). The small capacitor prevents protection 'spikes' from appearing on the output and a LED drive circuit is triggered. Rated, by the author, to be one of the most succinct and successful systems seen.A notable addition to the Curtis 'stable' and favoured by many users who returned to it's use when later, more modern designs gave rise to disappointment. A main smoother is likely to fail first, which can take a rectifier with it.
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