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.
Neil Williams has kindly shared the experience of restoring a P60 which I've taken the liberty of adding below.
Well, here goes. I've attached some photos of my P60. It's pretty obvious what they all are so I won't add any explanations.
As to "how I fixed my P60" here are the details.
After using the pre-amp section to copy some vinyl records to CD, I noticed that the volume was way down one one channel. Using the various inputs and outputs to feed signals from a CD player into the amplifier I determined that: 1. Both power amps appeared fine 2. Channel A was quieter than channel B between the tape 1 input and the tape 2 output. This means a problem in channel A in the tone control section. 3. Channel B was quieter between the cassette input and the tape 1 output. This means a problem in the volume, balance or equalization stages.
I only had a clunky old AVO meter which has minimum ranges of 2.5V AC and DC so it's hard to trace audio signals. I was able to compare DC voltages on all the transistors between the two channels and they were basically the same. I also checked the impedance of all the transistor connections as described in page 15 of the service manual and found nothing abnormal there.
Without an oscilloscope, I had to come up with a way of tracing audio signals. The P60 is a great tool for this as the tape 2 input splits the amplifier into two halves. I used a normal phono lead plugged into the tape 2 input with a short length of wire taped to the centre of one of the input plugs and a speaker plugged into the output of that channel. I didn't have a signal generator either (I do now - I downloaded an app for my iPad which works fine) so I used a CD player plugged into the cassette input. This method does affect the DC levels in the amplifier as it places the volume control (47K) between the test point and 0V. Use of an electrolytic capacitor on the test lead stops this, but I didn't see any problems without it.
By touching the lead on various points in the circuit and comparing the signal level between channels, I was able to determine that C13 was bad on channel B.
The other fault was a little harder to track down. Following signals in the tone control section (which necessitated removal of the front bezel to get at the potentiometer connections on the circuit board) showed one channel consistently quieter than the other. I could not find a fault in this channel and eventually realised that the fault was in the channel with the louder signals! This brought me to the conclusion that it was probably C22 at fault since in normal operation the path through Q6 would attenuate the signal in the tone control circuit. Checking C22 showed that it was open circuit.
I replaced these two capacitors and the amplifier worked fine. I have since replaced all the electrolytic capacitors in the pre-amp and LED operation circuits (I found that C36 was faulty, too, as one of the leads was rotted away!).
I also replaced R101 in the power LED circuit as the original component was burned and the LED only glowed very dimly. I looked for a 1W resistor but could not find one so I used the 1/2W value that was originally there. This component in under rated and should really be 1W.
I now have a digital voltmeter which would have helped the testing enormously along with the iPad signal generator app.
Deciding to replace all the electrolytic capacitors as the first step would have been a good move, too! Hindsight is always 20 - 20!
I hope this is useful. Feel free to edit the above as much as you want. The photos are now yours to publish.
Best regards and thanks again for the very helpful P60 service manual.
By the way: there is a P60 circuit diagram floating around on the web which incorrectly shows C44 across R1. It is really across R3 as your information and the service manual show.
No problem, Neil. Any time. An excellent demonstration of deductive logic with limited resource. It's often an idea to consider replacing all the electrolytics in the signal paths of an amplifier of this age since these are the likeliest components to fail first and it makes life easier doing this in one go rather than repeatedly disassembling the unit. Then specifying a high temperature, high frequency type will increase longevity and (theoretically) improve performance too. Thought also could given to replacing the quiescent current carbon skeleton pots with sealed, multi-turn cermet types and the inclusion of a soft-start.
Of domestic gear of this vintage I am torn between this amp and the Armstrong 600 series particularly because of the latter's tuner (623/6, designed by Ted Rule) for which I built a number of AM loop antennae specifically for Luxembourg (208m) and pirates. I think, however, the P60 has an edge, not only with circuit concept but especially with styling. Maybe one day I'll own one.
Notes - PK. The distinctive input selector knob was supplemented by the unique indicators on the preamp level.
The PCB's 'upside-down' position was determined by there being no room on the rear panel for the input sockets. It is arguable that internal air-flow around the components would be reduced, but this was essentially limited anyway.
This arrangement with the sockets directly mounted on the PCB rather than the chassis, whilst reducing wiring, could prove problematic with the plastic sockets prone to breakage if care was not taken fitting connectors. A similar arrangement was used in the complex Beomaster 6000 making access more awkward, the (hefty) unit then having to be lifted when change was required. I recall one unhappy customer whose precious wooden top had been deeply scratched by the metalwork when doing this.
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