This a nice picture of a Samson XP308i.
Shortly after I’d dropped it off a cliff. Or maybe just before. In this case it wouldn’t have made much difference because it didn’t work.
I know this might sound Neanderthal, but I don’t like switchmode power supplies. Uggy-wuggy-grunt. There you go, I am Neanderthal. I’m Stone Age enough not to run my business on bullshit and subterfuge, which currently shrinks my playing field down to the size of a postage stamp.
So what is wrong with switch mode power supplies? More to the point, what is a switchmode power supply? And also why is it? And what is the alternative?
All these riveting issues you’ve fallen asleep over, will be rivetingly detailed in our next week’s riveting issue, called ‘How to sell something you haven’t got.’ No, not really…… onwards and inwards.
This is a picture of the main bit of a switchmode power supply. The finned thing is the heatsink, upon which is screwed a switching regulator (in this case a Fairchild KA1M0880B power switch which is the nearest device you see screwed to it. The next one along is a high current diode and the one furthest away is a mosfet transistor which beefs up the output from the switching regulator to supply the amp circuitry. There are few different ways to design these things but the idea is that the mains input is directly rectified to produce something like 330 volts dc. This an impractical voltage as it stands. It is too high to do anything with for a semiconductor amp, and also, d.c. is not readily manipulated to produce the various different voltages that the amp needs; in this case probably +15 volts, -15 volts, 5 volt digital supply, and then maybe +30 volts or so, and -30 volts ish.
So we’re starting with a voltage from the supply that is way too high, and we can’t easily modify it.
An orthodox power supply has an isolating mains transformer. The primary side is connected to the mains supply, and there is no direct connection to the electronic gubbins in the amplifier. The secondary side of this transformer has several windings (or coils of wire, if you like) wound on the same lump of metal as the primary winding. By having various sized windings, many different voltages can be available, and the whole idea is relatively simple.
The switchmode idea has no isolation from the mains, it goes straight into a bridge rectifier and comes out of that as high voltage d.c. as we’ve already said. So where do the voltages come from? Well, the switching regulator we mentioned ‘chops’ the d.c. and produces a high frequency (in the Samson case 75 kHz) and this can be transformed to give the various voltages by much smaller transformers, because of the high frequencies involved.
So what was wrong with the traditional power supply, that we need to do away with it?
One of these things you might have already guessed. Wait for it……..a switchmode……
IS CHEAP!!!!! Yayyyy! Even including the little transformers a switchmode like this is unlikely to cost the manufacturer more than a few quid. On the other hand the ‘old’ transformer arrangement is probably ten times the price to build.
Another big plus for the switching supply, and especially in the case of toy p.a. stuff like this Samson is, is that it’s lightweight. A traditional power supply is a lot of things but it is definitely not that.
So why don’t I like them? It’s fair question. Might it be that I don’t like anything less than fifty years old? Well………….alright, you got me there. But I do like ideas that save the good old planet’s resources, and it does save a lot of copper and steel by not building a big transformer. But……………………
Switchmode has many problems that I don’t like. First (whatever they say) I see a lot that are blown to hell. They have many protection devices in there (overvoltage, overcurrent, overtemperature, overblown claims of longevity…… well maybe not that) but they still seem to hit the rocks very regularly. Second, there is no isolation between you and the mains supply. I Don’t like that. Third, it doesn’t have the same inherent filtering of mains noise of an isolating transformer. Fourth, as with many unnecessarily ‘complicated’ things there is a lot more to go wrong. Fifth, (again, whatever they say) the switching noise generated inside the unit will leak into the mains supply. We don’t need more crap in the mains that we already have. Just one of these things, well, ok; but a hundred thousand, say? It just seems to be a step in the wrong direction, to me.
I’ve actually led you astray, here. (So what’s new?) Because this Samson Expedition thing I haven’t fixed. Yet.
This is where I get into another ‘Pet hates’ tirade. The pic shown is of the sheaf of paper I have so far assembled in the interests repairing this amplifier. Why did I need to do this? It’s because of the increasingly secretive nature of the so-called ‘free information society’. After spending several hours on the web searching for schematics for the XP308i, I was forced to draw the conclusion that there isn’t any.
I’m sure there are a bagful of bullshit reasons for this, but the obvious one, you rarely find mentioned. Samson don’t want these things repairing. Just a guess, you understand.
Being a very bloody-minded person, that, I find is a great encouragement to figuring out what it’s all about, how it works, and fixing the damned thing. I’ll bring news of this job in due course, but just as a hopefully useful aside, to anybody unfortunate enough to get stuck inside one of these:- Beware of applying too much heat to the tracks, they’re very flimsy. Check that you have 330 volts dc out of the mains bridge rectifier. Check, that from pin one on the KA1M0880B(that’s the nearest pin to the edge of the heat sink) to the negative pin on the main rectifier, you have a significant ac signal. Don’t use a grounded scope (or anything else grounded) on this switching part of the circuit, as it is all referred to virtual ground. If none of this makes any sense, probably best not to go there.
We will be back on this amp, we will fix it, and I will let you know how it was done. And hopefully Samson, and all those of a similarly secretive fraternity will get it as a sound kick up the arse.
Time for tea.