I know these are justifiably popular amps (I now have 2) and there is a wealth of information on this forum about repairs and modifications but I would appreciate some thoughts on this.
Most of the issues with the A3i arise because it was 'pushed' to +/-39V rails from the originally spec'ed +/-35V designed by Mike Creek. Actually mine measure around +/-41V. They produce far more power than I need. So I am thinking of replacing the mains transformers and what I have currently available are 18-0-18 toroids. That would give a rail voltage of around +/-25V. Are there any problems with that - assuming I reset the quiescent current as per the service manual?
Thanks
Most of the issues with the A3i arise because it was 'pushed' to +/-39V rails from the originally spec'ed +/-35V designed by Mike Creek. Actually mine measure around +/-41V. They produce far more power than I need. So I am thinking of replacing the mains transformers and what I have currently available are 18-0-18 toroids. That would give a rail voltage of around +/-25V. Are there any problems with that - assuming I reset the quiescent current as per the service manual?
Thanks
Any regulated supplies might suffer if the raw supplies fall to low. Such things are very circuit specific though. Without seeing a full circuit diagram it is impossible to say.
Just looking at a circuit and I see an LM317 giving what looks like 26 volts regulated (an LM317 using 240 ohm and 4k7 voltage setting resistors) and so that is straight away a problem, you haven't enough raw input voltage available.
Also Zener supplies for an opamp would need the series resistors altering.
You would have to look at the relays as they seem to be from raw rails and also further Zener supplies in the protection area of the PSU.
So not a straightforward swap by the looks of it.
Also Zener supplies for an opamp would need the series resistors altering.
You would have to look at the relays as they seem to be from raw rails and also further Zener supplies in the protection area of the PSU.
So not a straightforward swap by the looks of it.
Thanks @Mooly - I have the schematic and can see the potential issues on the preamplifier side but I think I am happy to address those but I need to investigate the other voltage regulators in the protection circuit. The supplies for the opamp need attention - the load sharing resistors are under-specified and one has burnt out. So there is a bit of work required in any case. It will be a bit of a project!
There's actually quite a bit of of good, pro. advice on fixing the problems of the Cambridge Audio A3i already here. The designer, actually Alex Nitikin of Creek Audio at the time and known here as x-pro, had quite a lot to say about what went wrong in production and you may find it useful to know before diving in. https://www.diyaudio.com/community/threads/cambridge-audio-a3i-repairs-and-mods.122378/
There's also a more recent thread on transformer replacement by RS232 but there are others too. Just search A3i.
https://www.diyaudio.com/community/threads/cambridge-audio-a3i-repairs-and-mods.122378/post-6564628
There's also a more recent thread on transformer replacement by RS232 but there are others too. Just search A3i.
https://www.diyaudio.com/community/threads/cambridge-audio-a3i-repairs-and-mods.122378/post-6564628
I may be stating the obvious, but even if the amp is able to deliver more power than you need, as long as you don't turn up the volume, it wont.
And with a new (lower voltage) transformer/supply it will sound exactly as loud as before at the same volume position (as long as you don't clip it). Just saying.
Jan
And with a new (lower voltage) transformer/supply it will sound exactly as loud as before at the same volume position (as long as you don't clip it). Just saying.
Jan
Rather than chopping into the amp you could add an autotransformer on the mains input. Or maybe a 120->100 V transformer (assuming you're on 120 V mains).
Tom
Tom