Ashdown ABM 500 Evo Mk 1 from early 2000s. Any info?

[Albert Burton]

Can anyone help with any info on this early version of the ABM Evo please?

I have a guy who is suffering with his Evo Mk1 overheating and I have tried everywhere to get even basic info on the circuit. He is trying all of the obvious things like cleaning heatsinks and fans, checking both fans work as they should, output devices correctly mounted and tightened, output device solder joints sound. I originally assumed it was a Class D design but he reckons not. Without confirmation I am not convinced as its rear panel just has that feel of a Class D layout but I could easily be wrong on that.

I have tried accessing the Ashdown schematic area on their site but can't even contact them to ask for permission via a link to their Dropbox site without being the owner of the amp and having access to its serial number.

Can anyone out there give me any info at all. Is it Class D? Is the schematic available? Is this a common problem with that amp?

 

[Jim C]

I just had a tech question for Ashdown last week.
Sent an email through their site and (of all people) Dave Green responded within a few hours!
Top shelf group that Ashdown, I'm confident they will help get this resolved.
OTOH, not a project for anyone untrained.
The weight of the amp alone will let you know it's not a modern Class D deign.

I'm not aware of overheating to be a known problem with this amp.
Some models had issues with the mute switch either working, or allowing a little signal bleed when activated.

 

[Albert Burton]

Thanks for the advice Jim. Glad to hear the Ashdown has good support.

I'm genuinely hoping that this is a standard Class A/B type as that is easily fixable no matter what. I have worked on Class D before but it is a pain in the rear with its wound components and even active digital parts. Everything is usually SMD too which, though it is workable, makes for another level of pain in the rear. My attitude to that nowadays is the same as the original manufacturers, change the board out! And don't worry, I do have plenty of experience with this type of equipment right from design downwards.

I'll get on to them direct as you advise. The thing which threw me was that their system would not accept a message without the amp serial number being filled in and that I don't have as the amp belongs to someone else and is with him at the moment. Thanks again for your help, it's always appreciated.

 

[Jim C]

Glad to help and I am nearly positive that this series are class AB amps.
It sounds like this will be easy enough to trouble shoot and repair based on your experience.
I used this form to get to Ashdown and was contacted by Dave.
Ashdown Contact Us

 

[eccles77]

ABM 500 are all class A/B as far as I`m aware. Ashdown are super helpful and one of the reasons I have used them for years. If you are in the UK they usually sort it for free, yes they are that good!

 

[Albert Burton]

I messaged Ashdown yesterday through their site explaining where I am sitting but they haven't had a chance to reply yet. I'll keep you up to date with anything that comes back. In the meantime I managed to get a copy of the Mk2 schematics and they are indeed Class A/B just as you all said. If the Mk2 is A/B then the Mk1 will of course be the same.

For anyone interested it's a very standard power amp circuit with two differential BJT stages and a triplet of output pairs of BUZ901P MOSFETs. It shares the same circuit with the 300W version which is only missing one of the output pairs. There are no frills circuitwise other than things being beefy.

One awkwardness is that the bias current in the output stage is set by a plain old 220R preset with no voltage multiplication applied around it and no provision for limiting of current. Usually you arrange the bias circuitry so that it fails to a lower current not a higher one. In our case the current is increased by increasing the resistance setting of that preset. That leaves a potential for overheating as the preset contact ages as it will increase its resistance naturally. I suspect this of being a potential source of issue given the age of the amp.

The output devices themselves, being MOSFETs, have no source resistors, (the emitter resistors in a BJT output stage), which stabilise the current via local feedback. MOSFETs do not need this but it means we have no way of easily measuring the current passing through each transistor. The bias will most likely be set by reading the output MOSFET gate source voltages.

Let's see what Ashdown come up with in terms of advice and what the owner finds when he gives things a physical clean up.

 

[eccles77]

Bear in mind that it is a holiday weekend in the UK, Friday to Monday so they may be longer getting back to you.

 

[Albert Burton]

Yes, I'm a UK guy anyway Eccles. Easter Bank Holiday only covers the long weekend of Good Friday and Easter Monday so anytime after that. I have been assured that Ashdown support is second to none so I'm pretty confident they will at least get back to me quickly. The amp is a legacy one so I may even get the Mk1 schematic from them. If not the Mk2 should be very close in terms of its power amp.

 

[Albert Burton]

Just to round this thread off. Ashdown got back to me on Tuesday. Prompt or what? They were extremely helpful and supplied me straight off with the correct schematic for the Mk1. It's pretty much standard for all of them I think but it was from the horse's mouth. Yes, it's a fairly frill free Class A/B power amp with 3 pairs of complementary MOSFETs on the output. There are no source resistors to measure voltage across for bias current as with BJTs.

They acknowledged (sort of) the difficulty with the method of setting bias, (that point about it failing to a high current), but there is nothing that can be done of course. They advised me that their recommended way to set the bias is to use an oscilloscope to monitor the output with a 10kHz sinewave input set for 8V pk2pk into an 8ohm load. It is then a simple job to increase bias until the crossover steps are just removed on the scope.

So yes, my thanks to Guy Morel their Service Engineer who was very patient and remarkably open and helpful throughout. Ashdown are genuinely top notch with their support.