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.

Specifications for this model are given as


The RIAA was unconventional given the number of separate stages involved, before the final volume control. Nevertheless, similarities with the P40's are obvious. A feedback pair has unity gain at DC and an AC gain of 6.5. A separate wafer on the input selector shorted unselected inputs to ground to reduce crosstalk. RIAA correction is given by a later stage which gives unity gain for other inputs.


Care was taken to ensure that prior to any switching, a resistor grounded the decoupling capacitors so that no DC shift, due to leakage, produced transients. The preamp volume control used linear controls to achieve a 'log' law because these were more easily matched. In the centre position gain was 20dB from maximum.


The mono switch could have used one switch contact between two associated resistors in series with each channel. The balance amplifier gave gains of between 1.3 and 12.5 times. This was followed by an LED overload indicator (green -20dB, red clipping) and the cassette and tape 1 outputs which were loaded with a 4k7 resistor to prevent overloads.


The filters and tone controls came next. The high pass filter gave about -6dB @ 60Hz and the tone controls ±15dB at 40Hz and 20kHz respectively. The low pass filter gave a notch centred on 27kHz, roll-off starting at about 3kHz and peaking again outside the audio band at about 70kHz. The Q was switched to provide different slopes, 'steep' giving -17dB and 'gradual' giving -7.5dB at 20kHz. The tape 2 monitor and power amplifier volume control followed these.


In the power amplifier, a differential pair drove an emitter follower fed by a constant current source which reduces the current swing on the voltage amplifier. This improves cross-over distortion (particularly at high frequencies) compared to the more common boot-strapped resistor arrangement which cannot act as a constant current sink at the crossover points.


The output devices were particularly robust being chosen for their large chip and die mounting area (200W, 90V, 30A, 2MHz), and were run from ±39V rails. The designer, Stan Curtis, used the same devices in the later Lecson AP3 and ETI System A. Note the use of a zener to clamp the feedback's AC decoupling cap in the event of gross output shorts to the supply rails.

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.

Contact me
especially if you want additional content to this page
or if you find any links that don't work. Don't forget
to add the page title or URL. Take care!

Back to index, sound, tips or home.