This one I’m going to do in more than one part. You’ve got admit that was, for this blog site anyway, sensible.
In the Houses of Parliament, you will find the repository of all wisdom, sincerity, truth seeking, beneficence and self-effacing personal sacrifice. Well, you can’t be sensible all the time, eh?
The Mackie 808M is quite a lethal package for a small box. At 600 watts rms per side, that’s a lot of watts; and it’s not fan cooled. But it has a nice chunky, cast alloy heatsink on the back, that you could probably chuck a few burgers at and double as a Barbie when used in anger. There’s one born every minute.
The reason for this serialised blog, is twofold. First, if I keep going too long, it’s only goldfish that will be with me at the end. Mostly because they think it’s still the beginning. Secondly, it broaches the subject of switched rail supplies, and that could do with a piece of its own. Any conventional class AB amplifier (which is most of them) rated at over, say 300 watts rms, is almost certainly going to have switched rail supplies, and that means that, it has an extra pair of voltage rails (often at twice the voltage) in addition to the usual + and – rails.
So what? Well, from a practical pint of view, a fault in the power amp can look very serious, and in fact it’s the switching that has gone down. It might make the difference between a parts bill of say £40+ to one of say £4-ish. Not to mention a nasty shock with the labour costs. We’ll have a look at the Mackie, then. (You don’t say! I thought you were going to waffle on about goldfish into next week.)
This is the front of the Mackie 808M…..(Duh!). Unless you’ve had a serious bad time with whoever sold to you, there will be something behind it.
The ‘something behind it’ looks like this.
Unless you’re a victim of one of those dubious types that infest ebay, in which case it will look like this…..
You won’t get any problems with distortion with this latest piece of electronics from ‘Wonderfully Clever Inc.’
Let’s assume for a minute that the inside of yours isn’t a house brick. Referring to the pic above that, there are two components marked ‘High rail switching mosfet’ and ‘low rail switching mosfet’.
The basic idea behind rail switching is that the mosfets (in this case an IRFP150N and IRFP9140N) switch the higher rail voltages onto the power transistors MJL21194. So what? So that the higher rail voltages give a much higher peak to peak output level before clipping. So a big increase in power output capability.
The fault in this amp was that the output distorted, it also had a dc offset; not a lot ( about a volt or so) but enough to know that all was not well in Offset Voltage Land.
It turned out that the low side mosfet switcher had shorted. So the voltage to the high side was about +45 volts and the low side rail was -90volts. Quite enough to upset the output dc offset and produce a lot of a-symmetrical distortion. This fault was cured by replacing a fiver’s worth of mosfet. Un….fortunately, there were other problems, but at least I wasn’t tempted to take the output stage to bits.
I’ll do a bit more in detail about rail switching in another blog soon; but I have to take my goldfish for walk.
And here’s a little aside, just while you put your bedsocks on. I had a repair shop for few years in a little town on the North Wales coast. I had to close it after a visit from somebody called ‘Bailiff’ and somebody else called ‘Taxman’. They both wanted the same thing. Called ‘Money’. I digress. Just across the road from my estimable establishment was an emporium called ‘Aladdin’s Cave’ run by Tony. He came across on this particular day. He’d just bought in a TV set (for sixty quid if I remember right), which he had, unwisely, not tested before parting with his money. We sat in front of this machine on his counter, twiddling every knob and switch we could see, and nothing happened.
I took the top off and inside, completely alone and taped to the bottom with gaffa tape, was a house brick. That was an expensive brick.
And now to put the kettle on.