Cambridge Audio A500 Amplifier

[ronbryan]

This A500 stereo amplifier uses obsolete SAP15N and SAP15P five pin darlington transistors as the output pairs. The SAP15P on one channel has failed, probably caused by shorted speaker wires.

The similar, later, SAP16N and SAP16P devices, which are still available from some suppliers, seem to have identical characteristics apart from having a base-emitter resistor of 100 ohms which is returned to the 'cold' side of the internal 0.22 ohm emitter resistor, rather than in the case of the SAP15N or SAP15P, a 75 ohm returned to the 'hot' side of the emitter resistor.

Can anyone confirm that the SAP16 complementary pair are an acceptable 'drop in' replacement for the SAP 15 pair and if any modification, such as external 0.22 ohm emitter resistors is deeemed to be necessary.

Ron

[Gillian]

Hello Ron,Is it that you have the SAP16N and 16P devices in stock.
Otherwise the SAP15N and 15N are avcailable on ebay with the following link with 69p carriage: http://www.ebay.co.uk/itm/SANKEN-SAP...item338a0b555d
Hope This Helps
Gill

[ronbryan]

Hello Gillian

Thanks for the Ebay link - I missed that one. In fact I have already ordered the SAP16 series from Profusion Audio Services in Essex, because the Ebay links for SAP15s that I had previously found were in Hong Kong and would not ship to UK. Those far eastern suppliers also had me concerned about whether they were selling counterfeit devices.

I would still like to know if others have used SAP16s as SAP15 replacements.

Ron

Make your own darlington pair(s) ??

[ronbryan]

Merlinmaxwell

The SAP16 devices that I have ordered are darlington pairs already, it's just the minor difference in the thick-film base-emitter resistor value and circuit position (see post #1) that made me wonder if I will have any trouble with the substitution. Hopefully someone will have tried it and can let me know how they got on.

Ron

['LIVEWIRE?']

To the best of by knowledge the SAP16N & 16P devices are 'drop-in replacements for the 15P & 15N.

[mhennessy]

You'd need to look at the overall circuit diagram to see if that resistor change would make any difference, but in general I'd expect it to be OK. Be careful to check the bias, of course - consider a lamp limiter or current-limited bench power supply or similar for the initial testing.

These SAP transistors seemed like a brilliant idea 15 years ago, and lots of people rushed to use them. Unfortunately, the emitter resistors proved to be rather less reliable than the silicon. As a designer, I tend to be a bit conservative, and tempted as I was, I did avoid these devices - I'm glad I did

[ronbryan]

Thanks Livewire and Mark for your replies.

Because Sanken bring out both ends of the internal emitter resistor in the 5 pin SAP package, so that you can measure the bias, it is possible to fit external emitter resistors of any wattage you like and ignore the internal one, subject to pcb space being available.

As the fault was caused by a speaker lead short, rather than full power operation, I'm going to try using the internal resistors first. It does seem that a couple of 4A fuses in the power rails are insufficient to save the output stage from shorts though!

Ron

[mhennessy]

If anyone is thinking of trying that, be aware that not all of the SAP range have die bonding wires that are able to pass the full collector-emitter current. Also, when the internal resistor lets go, it often cracks the plastic packaging - so the transistor is toast anyway. So with that in mind, external resistors is something to incorporate at the design stage - though of course a sensible designer would switch to a conventional transistor at this stage

The sense connections are principally for current-limiting, something which isn't always properly implemented in Cambridge Audio designs. It would be worth checking the operation of that - it's possible that the I-limit detect transistors will be subject to rather too much base current when the emitter resistors go open.

You probably already have it, but I've attached the schematic...

Good luck - I'm sure the '16s will be fine...

Mark

[ronbryan]

I've fitted the new SAP16s to the faulty channel in the A500 and they work perfectly well in place of the original SAP15s. I checked the overload protection transistors Q215 (MPSA43) and Q216 (MPSA93) and they were undamaged, so I put them back. A fair amount of dismantling of the chassis is required to get at the solder side of the pcb - I found the rear panel needed to be removed from all of the many pcb mounted phono and speaker connectors to gain access.

The quiescent current adjusted to the 60mA value in the manual (13mV across 0.22 ohms) although I had to make a small adjustment after a half hour warm-up. I checked the DC offset at the output terminals - it measured at just under 7mV, about 5mV more than the other channel, but still pretty small. I think this would be due to offset imbalance in the input transistors rather than the new output pair, but I did not investigate further.

Ron

[mhennessy]

Excellent news

Originally Posted by ronbryan

I checked the DC offset at the output terminals - it measured at just under 7mV, about 5mV more than the other channel, but still pretty small. I think this would be due to offset imbalance in the input transistors rather than the new output pair, but I did not investigate further.

I agree entirely. Anything less than about 50mV is considered to be good for an amplifier with no trimpots or servos, so no problem here.