Indie IT 70 combo

This just an edgewise fit-in between looney JBL Eon bookends. Just preserving my insanity, you understand.

The IT 70 has an output stage that I prefer not to think too much about. This attitude, I’m afraid, is the price of being an old git. Don’t do it, it’s bad for you. Anyway, as usual, your first question is likely to accompanied by the bemused scratching of the head and the “What’s he on about?”  I have problems with a power amp that is as big as an adolescent  black clock. Especially as they used to take up half of your front room.  

“What is he on about?” Well, screwed to the heatsink INSIDE the amp (quite an interesting feat for the heat; it probably gets out up a ladder) is a device with a lot of legs (even more than a black clock) and that is your amp. It’s about half the size of a book of matches. And if you’ve ever tried to replace one, you’ll find a number of matters all come together to make life intolerable. It will be a double sided pcb, and the tracks will be rather short of the thickness of the foil they wrap kitkats up in. Which means that if you get a soldering iron anywhere near them, they peel off and wave bye-bye. Now, unfortunately, to desolder anything you have to get it hot. I rest my case. 

 Now, you all think I’m just after a cheap laugh; but whatever you read in these blogs is, sadly, based on fact. Here’s the proof of the pudding.

This is what the amp looks like on the outside; quite a nice looking thing. 

   

 This is it on the inside. Have a good look at that silver ribbed thing in the middle. That’s the heat sink that gets rid of the heat from the power amp.

This is the power amp. You see the little flat black thing on the silver heatsink? That’s the power amp. This is progress.

Just to try to be a bit less bitchy, there is a common fault with these amps that is relatively simple to put right. The overdrive channel radically underdrives to the point of not doing anything at all. If you look at the Heatsink/rat trap thingy, at the very left of the brown pcb is a white block. This is a 5watt wire wound 120 ohm dropper resistor, and its where the anode voltage of your valve comes from.  It’s not unusual for it to go open. It’ll cost you fifty pence, the whole thing unscrews on four screws, and you solder a new one in. WITH THE AMP UNPLUGGED!!!!!!!!!!!!

I feel better now. It’s been a bad day. Tea time.

JBL Power15

It seemed to me that there might be a lot of useful stuff to be gleaned from this job. Maybe firstly, how to describe what’s wrong with something. Sounds simple, but is it? This gentleman said his JBL speaker was buzzing. Could I get a buzz out of it? No, I could not. Maybe if he’d said something like ”It sounds like a parrot farting”? But no, that wouldn’t help me either, having never heard one of those colourful avions fart. So, it’s not a simple matter after all. What sounds like a rattle, sounds like a scrape to somebody else, sounds like a frog in a thicket to somebody else, sounds like the retching of a rodent to somebody else…… you get the idea; it’s subjective.

 

This looks a lot like a piece of headgear Kiss might have worn in the seventies. Would have gone well with the 15″ heels. What it actually is, (I am assured) is the back, of the front of a JBL Eon cabinet. It shows a complete piece of cast alloy, in which the chassis of the woofer, and the horn, are both part of the casting. This is a brilliant idea, for two reasons. One is that the baffle (that’s the flat board at the front of any speaker cabinet  that has all the speakers screwed to it) is as solid as your probably going to get, and as vibration in the baffle is a major source of both distortion and inefficiency, that’s a big plus. The other thing so (far as JBL are concerned anyway) is that once the jig has been designed and built to cast these things, they can be knocked out like shelling peas. Easy, and relatively cheap.

There is an inherent problem with small cabinet design, however. That is that they are small. What am I talking about? I had a guess at how much volume of air this cabinet would contain (when not in a lot of bits, obviously) and it would have been a lot less than 2 cubic feet. Should that matter? It depends on your idea of what bass should sound like. If you think that good bass sounds like a barn door banging in a gale, then there’s no problem at all. If however you want, say, a double bass, or a cathedral organ not to sound as though they are being played at the bottom of a lift shaft, (tricky with the organ, anyway) then it does matter, a lot.  It’s to do with something called ‘PORTING’, and (leaving aside jokes about gout, which will be totally lost those under bus-pass age) this means the way a cabinet can have a hole (port) in it to increase the bass.

There are two distictly different designs of cabinet; infinite baffle, pressurised, totally enclosed, are all descriptions of the type without a hole in. Why we can’t just say ‘the sort without the hole in’ you could perhaps answer better than me. The other sort, wait for it……… is the sort WITH A HOLE IN. How cool is that? This is known as a TUNED CABINET and is more efficient than a pressurised one, with one big ‘if’. It has to be tuned to a Bass frequency that agrees with the lowest frequency that bass speaker can handle without something called ‘frequency doubling’.

All bass speakers have a characterist called ‘CONE RESONANCE’ which is governed by many things but is (basically) the lowest frequency at which the speaker can produce clean bass. With a good 15″ speaker like the one in this cabinet, it might get down to say 30+ hz. If we tune the cabinet to a frequency close to this (anything much above produces this doubling phenomenon in the cabinet) we might need a vent area of say 3 square inches (I could do this in plastic money, but I’ll stick with the Neanderthal) . So far so good. As the vent area increases the bass resonance gets higher, and we’ve said why that’s not a great idea. If the cabinet volume (overall size in other words) is increased, then the port size increases for the same resonant frequency. It’s not a simple ratio but that’s not the point. Here is THE POINT. A small vent produces a high bass peak (as all vents do) but it is restricted to a very tight range of frequencies. Which means that if you’ve got a recording of a barn door in gale, it will sound great; but if you happen to have a nice recording of  Paul Tortellier playing the second Bach cello suite, the chances are you’ll really enjoy that low G. You’ll have to because you won’t hear any of the other notes. And that’s the problem. A small cabinet produces a very restricted bass frequency response. It’s a natural law, and although you can bury it in bullshit, it won’t, annoyingly, go away.

Conversely, a totally enclosed, infinite etc.etc. (without a hole in) cabinet, has no such peak in the bass, and a much flatter response throughout, but it isn’t as efficient, so you get a lot less noise per watt (it’s called SPL or ‘sound pressure level’) than with a ported system.

I’ve got way off the beaten track here; this was supposed to be about a fault this speaker had developed. More of this then, tea time. Er, I mean, next time.

Well! What about crossovers,then?

Yes, you’re right. I’ve said I’d be doing crossovers and I haven’t. But here we are, back in the frame. “Everything comes to him who waits.” Incontinence, overdrafts, road accidents, mummyfication. Even bird shit if we wait long enough in the same place. I could go on. And on….and….

Alright, the first question, which can be levelled at most things, is “Why do I need one?” Were I trying to sell you one, I might come up with something like:-

“Wow! I mean, so cool. That colour! It’s just you! It’s this week, I mean, well, what do I mean? But you gotta have one. Well, I mean, it’s the end, man. Plus its got the right name; y’know ‘CROSSOVER’ how cool is that? PPhhheeewww!”

Wow! I mean, its enough to send me running to my creche. Well, it just won’t do as an explanation. Here’s a more factual, and yet still emotive reason. Without one of these little wigetty crossover things in it, your nice hifi speaker will become an incendiary device. Or in extreme cases, a projectile. So they are (whatever colour) necessary.

The above schematic diagram (sounds good for start) is the simplest you’re going to find, and it consists entirely of the capacitor C1. So, we might ask, why do I want to spend a second mortgage on a crossover if I can buy a capacitor for 50 pence? This is where I do my ‘Old Git’ impression and say something horribly wise  like “When the Sun is in the West, the rice pudding sinks fast.” Or some such.

The capacitor of a suitable size (this depends on a number of factors but it’s often between 6uF and 10uF), will allow high frequencies through to the horn, and block bass frequencies. So what else do we need? Well, nothing really, but the problem is that the bass speaker is still trying to deal with the high frequencies (because there’s nothing to stop them) and so it soaks the high frequency power away from the horn, but doesn’t actually do anything. Those frequencies are too high for the  bass speaker to respond to. So a lot of the high frequencies are just dissipated in the bass speaker as heat. That’s where this simple crossover falls down. It’s just not efficient. It also has a slow roll-off per octave, which sounds vaguely erotic, but isn’t. I’ve led a sheltered life.

This is the next simplest crossover. We’ve added a coil, L1,(an inductor if you want to be posh) in the line to the bass speaker, and this does two things in this application. L is the letter that is usually used to signify an inductor, or inductance. It’s just a coil of wire, on some sort of former. It does the opposite to the capacitor. The capacitor passes high frequencies and blocks low, and the inductance passes low frequencies and blocks high. So, by putting L1 in line with the bass speaker, we block the frequencies that are meant for the horn, from getting to the bass speaker and being soaked away. This, then, is a much more efficient arrangement. The L1 does another thing as well as that. It builds up the resistance (‘impedance’ if you want to be even more posh) of the signal path to the bass speaker, and therefore pushes more signal up to the horn. A double whammy, then. The way the frequencies are dished out (high to horn, low to bass) form a ‘crossover point’, and its about here that things get paranoid. To enjoy doing graphs I think is the sign of a sick mind. Please have a look at the graph below that I drew. (YAAHHHGGGGHHH HEE<HEE<hee)

And just to show how far over the wall I’ve got, I’m going to  EXPLAIN IT!!!! Woooo!

The bottom (horizontal or X axis) of the graph is marked off in frequencies. Notice that it doesn’t go 10,20,30,40 etc. It goes 50,100,200,400. So each marker is twice the previous one. This is a logarithmic scale, and is the only one that makes sense in this context, because the markings, when done like that, are in octaves. The blog previous to this one hopefully sorted out that issue. Down the left (Vertical or Y axis) of the graph we have markings in dB. It stands for DECIBELS and it’s a comparison thing and that’s also a logarithmic thing. Leaving that aside with the rest of the washing-up, the important part for us, is that 6dB is a half. So what?

Now we’ve got to look at the line marked ‘woofer’; it starts at 0dB and that means the full output of the amp. It’s a form of passive low pass filter, and theres a fair bit about filters here and there on the blogs. So the woofer gets all the output until it gets to the rolloff point, where it takes a sharp dip. When it dips past -6 dB, thats half the output level. -12 is a quarter; -18 an eighth. Each one of these points corresponds to a frequency on the X axis and the rolloff point we’ve selected happens at 400hZ. Beyond that it drops off (rolls off) at the rate of 6dB per octave, because it dips by 6dB for every doubling of the frequency, or octave. So we’re not going to hear much from the woofer beyond 1.6 kHz or so. However, the tweeter, which has the high pass filter, rolls off at 6.4 kHz and isn’t doing much at all until it gets to around -12 dB, which is the crossover point. This point happens where the woofer and the tweeter have the same power directed to each of them, and beyond that point the tweeter takes more and more of the power and the woofer, less.

That’s about it, except to make the point that these are idealised in this graph, and what really happens is a lot less clear cut. But the ideas are sound; it’s just that ideals are great but we’re stuck with reality. Never quite been able to hack that stuff. 

TIME FOR TEA.