Studiomaster 1500E power amp

This is as far removed from the current versions of Studiomaster, as politics is from sanity. They’re the same colour (Henry Ford’s colour; “any so long as it’s black”) and…errmm….they’re the same colour. That’s about it.

This a picture of the inside (?) of a ‘proper one’. It’s about here that the longsuffering reader pulls out every hair on his/her head because it sounds like I’m ‘talking down’. Not true. Anybody who fixes amps for a living is in no position to talk down to rodents, even. Some of my best friends are rodents. Most of them don’t know it, however. We digress. Back to picturesque landscapes of the inside of an amp.

In various places on this blog (mostly under ‘information’; a much maligned term if ever there was one) there is a lot of stuff on ‘what does what’ in an amp. This is more to do with ‘where does what’.

We’ll start with the circular lump that is unmissable. This is the mains transformer. This is often abrieviated to’TX’ and used to be called a ‘tranny’ in the days before transistor radios, which were also, not a little confusingly, called ‘tranny’s’. Don’t know where that came from. Anyway this is a particular design of transformer called a ‘toroidal’ and it has a particular property which is very useful in this situation (this situation being cramped.) It has very little magnetic flux, external to the transformer itself. This is very desirable in these circumstances, as magnetic flux at 50 hz is picked in up the electronics (especially in the preamp stuff) as hum; something we definitely want to keep down to a minimum. The reason it is there at all is to take the mains voltage in, and transform it to relavent voltages that the amp needs.

Across to the left of that are two flat green pcb’s (printed circuit boards; and no I’m not talking down as my best friends are rodents etc.etc). This actually looks like one pcb but they are two identical ones side by side. These are the power amp assemblies, and the bit we can see is the back of the switching modules. More of that in a bit. In between the tranny (?) and the power amps are some black cylindrical things which are smoothing/reservoir capacitors and are part of the power supply section most of which is on that pcb in the middle with the capacitors on it.

This little pcb is mounted on the back of the amp and houses the input jacks/XLR’s and has the preamp chips on it. These might be TL 072 chips (or sometimes BA4558) and the purpose of it is to amplify the input signal to a level that the power amps can deal with. just under that are a couple of spade tags which are fitted to the main bridge rectifiers; we can’t see those too well, but they rectify the transformer voltages to d.c. for the power amps. The power amps have two seperate voltage levels (pos and neg 45volts; and pos and neg 95volts) and there are one each of these rectifiers fitted to the bottom chassis. The flat cables you can see are ribbon cables and carry the signals to the power amps.

Here we can see one of the power amps. This top pcb is the switching amp, and this is physically connected via heat sinks to a lower pcb which is the power amp. The top board sorts out the voltages (between the 45v and 95v mentioned earlier) that go down to the power amp section. The flat black piece of plastic to the left of the power amp is the top of the fan which drives air through the heat sink tunnel.

The 1500d, 1500E, 1600E, and 2000E all look identical, except that the 2000E has two fans. If you take the top off any of the powered mixers (vision, horizon; 1200, 1500,) the bottom of the amp again, looks exactly the same. Some have the transformer higher up and the rectifiers lower. The very early models of the Powerhouse look different. There is no voltage switching pcb; it’s just a flat board where you see the green pcb’s. To get the 250-350 watts out, the switching approach wasn’t necessary.

Repair isn’t necessarily cheap. If a power amp goes down on you, it’s generally a rebuild of both boards of whichever side, left or right. Not cheap, but a lot cheaper than buying something of a similar quality, these days.

So what was the point of all this, other than to postpone my taking the dog for a walk? Just that, if we insist on getting lost inside one of these things, it’s nicer to do it with the odd recogniseable landmark.

Tea time.

How to be (electronically) Daft and Clever at the Same Time?

“Is it possible?” you might ask. I’d have to say that in my opinion, not only is it possible, it’s normal; and it’s the prime reason why I didn’t volunteer for membership of the human race; personally, I’d rather be in the Oxford and Cambridge boat race, and I can’t swim.

“What’s he on about now?” Well, it’s just a sneaking suspicion, you understand, but it seems to me, that the brainier folks are, the less sense they have. Does this sound horribly familiar? Well, let’s stick with our usual ground; it is supposedly an electronics blog, after all; and how did this ramble come up at all, you might wonder?

It came up, unsurprisingly, with an amp. It was an Arcam Something-or-Other. This gear is so expensive that it’s difficult to find out exactly how expensive it is. Let’s just say they’re likely to charge ground rent on the dealer’s doormat. Oddly enough, I had another reminder, a few years ago, of the ‘lack of sense’ brigade for almost the same reasons, but from utterly the other end of the market spectrum; it was a little Laney guitar combo. If the Arcam was at the covered end of the market, the little Laney was definitely at the other end cleaning out the latrines. But the thinking behind them was disturbingly similar. It’s all around the ‘new is better’ philosophy. This also, in a round-about way, could be read as ’choice is bad for you’,  kind of ‘better’.

Virtually all the technology we might find in computers, mobiles, games consoles etc, is called ‘surface mount’ and is the successor to ‘through hole’ technology. A surface mount resistor (say a 603) is 600 thousandths of an inch long and 300 thousandths of an inch wide. That’s not very big. A 120 pin microprocessor chip, you could fit on a 10 pence coin. That’s not very big either. Without these things, computers, laptops, mobiles and all the rest would not exist. Depending on your point of view, that might be a blessing or a curse, but the point is that the purpose of surface mount technology is to be small. That’s it, ultimately. 

Now, back to the Laney. I opened this combo up and searched around for the amp. At first thought it was a wind-up. At one end of the amp was a pcb about as big as the palm of my hand. And the rest of it was a desert.  They’d miniaturised the whole amp down to a six inch square piece of pcb. In fact, taken as a whole, this amp that was in a usual- sized combo box, could have been fitted comfortably in a small jiffy bag; but for the fact that we needed speakers and a transformer etc. Considering that power electronics are happier with the space and bulk to dissipate the heat generated, the idea of cramping a bagful of this stuff in the corner of an amp that  has room-a-plenty to do it all comfortably and make service and repair that much more accessible, broaches the question “why?” And, uncharitably, ‘Daft?’

The answer is “Cheap”. A surface mount pcb is flow soldered (dipped in a bath of solder), populated by a pick- n- place robot (that puts the bits on it) and it’s more or less finished. However, try desoldering after a fault develops. The tracks (copper interconnections) are thin enough to peel off with the heat of an iron, (or hot air gun), even the first time, but the second time? Very unlikely you might get a second shot at it. So you send it back to the manufacturer, who might reflow the pcb (take off the components and redo the whole thing) or sling it in the bin, because the copper won’t stand a second go.

Make no mistake, the folks who design this gear are clever people. But sensible?

And finally to the Arcam. This was doubly strange, to me. Inside was a surface mount pcb. That was most of it. It had a microprocessor on it which did a load of sophisticated protection. Unfortunately, the output devices had fried; so that didn’t work. It also used an old technology in a new package. The output devices were darlington bipolar transistors. Great idea, give a lot of power gain in one device package. But they have a problem, as do/did all power bipolar devices. The control element (the base connection) unlike a mosfet, is not electrically isolated from the primary current path (base to emitter). When they fault, they often clean out the drive circuitry. Not to mention that one device going short will often take out the rest of the output devices. Mosfets often don’t do this, the gate insulated from the Drain-source current path, and being  happy to share out the power between the rest of the devices. The mosfet is a much newer technology than the bipolar, so why use bipolar? Sounds like new wine and old bottles? Well, I, and a lot of other folks also, think they sound better. A mosfet can do some strange things at the top end of the signal. But we’ll leave aside the subjective, for now.

Here’s the ‘Daft’ bit. We have, in a very expensive amp, a surface mount-populated preamp/drive pcb. Not a straightforward repair item, and one that many would refuse to attempt, especially in the state of the one I saw. This pcb ‘state of the art’ (ugh) pcb drives the (very old school) bipolar output stage that in a fault situation is likely to get obliterated. This is a ‘shove new insides in the old box’, and for what reason? Somebody has a new ‘electronics design software’ package, and has looked at it with the regulation hi-tech blinkers. Only a guess.  

A lot of upmarket gear also trades on the ‘made in uk’  boast. There should be a difference stipulated between ‘assembled in UK’  and ‘manufactured in UK’. It ain’t necessarily the same thing. Tea time, however, is.

                                  I’d actually finished this piece of silliness, and a little story came to mind that you might enjoy, which is, in a very circuitous way, relevant. Possibly.

I used to have a workshop across the road from a junk shop called ‘Aladin’s Cave’, and would, from time to time, insinuate myself for the odd cup of tea. On this occasion Tony had just been out to collect a piece of junk… errm, equipment; which turned out to be a tv set; quite a nice one that he’d ‘acquired’ at a ‘good price’ and was very pleased with his acquisition. He switched it on in the midst of the mayhem that was eternally ‘Aladin’s Cave’ and nothing happened. At all. I thought that I might repay his long term tea generosity and volunteered to have a look inside, hoping for maybe a blown fuse.

We took the case off and found a half brick taped to the bottom. He’d paid forty quid for a half brick in a nice case.

And so to…the JBL Aeon.. again

This has been not unlike taking a shortcut to Blackpool via Bombay. I think we’ve covered most of the problems of the Universe, without actually solving any. Some days are just like that.

The fault that we had so much difficulty in describing (see previous blog on Aeon if confused. If not confused also see previous blog, should you wish to be) actually came down to a faulty gain/level pot on the back of the cabinet. This where we get into levels of sillyness that should be experienced to be believed. That’s not a recommendation, you understand.

All we have to do is replace the pot. OK? No, not ok. The pot has to fit on the small pcb in the pic, which is mounted on the back of the cabinet. OK? No, sadly, not ok. Our first problem is to get one of these pots. They’re about 50 pence and five legged cows are easier to find. My first port of call was CPC, then RS, then Farnell. Now, Farnell actually had one of these. In fact they about a million. They’re called P140 pattern pots and used a lot in current equipment, which is presumeably why you can’t get them. The only problem with Farnell was that they wanted to charge me £16 (that’s, say, $28) to ship this component (which weighs less than my wallet: that puts it well into minus figures) about 100 miles. I could have launched it to Pluto for less.

Needless to say, that wasn’t an option. So I sought  out JBL parts suppliers. They were in US and you couldn’t buy the 50 pence part, you could only buy the complete pcb assembly. They didn’t say why; maybe some common philosophy with Farnell  somewhere. Anyway the pcb was about $50 or so, and shipping as per Farnell would have been at least double the gross national debt. I let that go.

I eventually did it for nothing (not counting the years of toil it took to do the mod).

Here’s how. The pic below shows a hole (amongst other things). This is where the original pot fitted. Just above that are four screened cables. These were soldered to the pcb pads to which the original pot was soldered. 


This is where the cables go to. I drilled a hole in the back of the cabinet and fitted the pot I had hanging around on the shelves

(dual gang as the original spec).

If you ever get a chance to read ‘Zen and the Art of Motorcycle Maintenance’ just bear this in mind, particularly where he proposes fixing his friend’s BMW bike with a bit of coke can; much to the perfect horror of his friend, who prefers to spend a fortune on the correct part. Even though it’s more or less the same thing with ‘Coke’ written on it instead of ‘BMW’. No cigar for guessing which way I’d jump.