Cambridge Audio A5 Repair

[TaylorD93]

Hi everyone,

Im hoping i might be able to get some advice for a Cambridge Audio A5 amplifier i have been repairing.

Bit of back story, in my Electronics/Hobby room, I was using a Cambridge Audio A1 mk3 SE amplifier up until a few weeks ago it worked fine and then the TDA1514 popped in it, after string of seemingly counterfeit devices, i gave up trying to fix it. So i dug out the A5 Amplifier, which was also faulty (bought it as Spares repair).

When i first bought the A5 last year, initially it wouldnt power up. I opened it up and found a popped fuse on the negative supply rail. Replaced the fuse and powered it up on a Dim bulb and it seemed to be fine, the bias Potentiometers for the output stage (100R single turn type) did feel really scratchy, so i set the bias to 13mV as per a service manual for the A500 (very similar model, identical output stage transistors etc). Then powered up without the dim bulb and the only remaining issues were that it seemed slightly noisy (hiss) and a "zipper" sound through the speakers when the amp was switched off, which i traced to the Preamp stage NE5532 oscillating as the PSU collapsed.

Last year for some reason i believed that re-capping the amp would help the Hiss and ordered up the parts.


So fast forward to this weekend, Ive replaced the bias pots, recapped all electrolytic capacitors (Rubycon/Panasonic) except a 1uF Non-Polar on the Input filter to the Main stage, and swapped out the NE5532 for OPA2134 instead.

After that, i again powered it up via the Dim Bulb, roughly set the output bias current 13mV across the SAP15 output transistor S and E pins (5 pin Darlington type Transistor), repowered it directly on the mains, and then finely adjusted the bias current. which is a bit of a back/forth as the change in bias current certainly has an effect on the temperature of the SAP15 internal resistor which then causes some degree of drift while it "re-climatizes".

The "zipper" sound has gone now (hooray) but ive since had the amp on at low volumes for the past day n half, and im noticing that there is still a constant hiss from the speakers, admittedly you can only really hear it when youre head is up close to the speakers, but its rather annoying.

The Hiss is on both channels, but the Volume of the hiss doesn't vary with change in the volume knob, so it must be in the main stage of the amplifier or PSU.

My initial suspicion is ive not quite got the Bias voltage correct and its introducing this hiss, but im not sure that is actually a logical suspicion, or if this is too be expected as a characteristic of a discrete semi Amplifier, or not? I can say with certainty that the now dead A1 Mk3 (TD1514 IC model) had practically no hiss at "idle" prior to failure, but again that is an actual Power amplifier IC rather than discrete semiconductors.

Does anyone have any advice to tracking this type of hiss/noise down?

There are some minor differences between the A5 and A500, but i have attached the A500 schematic (easily available online, so hopefully no issue sharing it)

thanks in advance,
Taylor