Preamps, power amps, power supplies PART 5

I think we really need some diagrammatical stuff soon. But as I can’t find a pencil in this dump, I’ll leave it for now.

We finished up last time discovering that you don’t get summat for nowt. (Something for nothing in English). And this, in amp terms anyway, means that all this power we have coming out of the output stage, which has multiplied the input from your guitar by maybe a hundred, say, has to come from somewhere.

This ‘from somewhere’ is the power supply, and it’s the function of this that we need to get to know about.

For a start, the stuff that comes out of your wall socket at home, is of no use to your amp, as it is. It comes out of there as ac. These two letters stand for ‘alternating current’ and what that means is that the stuff that comes out of the socket continuously alternates between positive and negative, 50 times per second, and our stuff (UK mains voltage)alternates about 336 volts positive and the same negative, which works out to a value of 240 volts rms. The rms bit is another serious source of bullshit that I’d really like to bomb out of existence, but this isn’t the place to do it. I might finish up generating my own bullshit, and there’s enough around without me joining in.

Rms is actually a really useful term if we steer clear of all the rubbish. Without going into it in any depth, if you connected a 9 volt battery (which is a dc, means direct current, or non-alternating current source) up to, say, a bulb, you would get exactly the same light/power out of it as if you shoved an ac voltage of 9 volts rms into it. That’s where the idea of rms (which means ‘root-mean-squared’ and is something we’ll look at somewhere else) came from.

The reason for that agreement between ac and dc, is largely a historical one. If we go back to the 1950’s and before, the ‘mains voltage’ (in UK at any rate) that came out of your wall socket was d.c. at a lethal 240-250 volts. When the system began to be changed to ac, it was important to be able to have some means of comparing the output voltage/current in ac to the equivalent as it existed in dc. That way, your 1kw (dc) electric heater was still 1kw (ac rms) after your home supply had been changed.

Phew, that went on a bit.

Anyway, back to the plot, if I can remember what it was.

Inside your amp, is a transformer; almost, but not quite, always. If it hasn’t got one, it’s either battery driven, or goes back to the 1940’s (ish) and was designed to operate on ac or dc. They were called ‘universal’ at the time, and were, if they developed a particular fault, universally lethal. Or it might have a ‘switch-mode’ design. This is a relatively new technique and if I get onto that subject I’m going to sound like an old git pretty quick. I don’t like them.

The transformer is the first bit of the power supply in your amp, and it has two functions. It completely isolates the amp from the mains voltage, (that voltage is connected to the ‘primary’ or input side, of the transformer), and also modifies (transforms) the voltage that goes into the amp. This ‘secondary’ voltage comes out of the ‘secondary’ side of the transformer, and it can be more volts (often in the case of valve gear), or less, maybe, if we’re supplying semiconductors. The stuff coming out of the secondary ‘winding’ of the transformer (they’re called windings, because they are actually coils of wire wound on a chunk of laminated metal, called a ‘former’) is still ac, and as such is no earthly use to the amp as it stands. It has to be ‘rectified’. It’s alright (I suppose) calling an amp a ‘double’ or ‘triple’ rectifier, like Mesa Boogie do, but if we aren’t told what it is or does, then it’s bullshit again.

This is what a rectifier does. It converts ac to dc. That’s it. That’s all it does.

It can do it in a variety of ways, using a number of different devices and circuits, but the voltage goes into it ac, and comes out dc. This dc voltage is connected to the dc ‘rails’ in the amp, that often used to be solid copper ‘rails’ at one time, but now are usually tracks on a pcb (printed circuit board), and their function is to carry this dc voltage around to various places in the amp.

Why do we need dc voltages? Well, when it comes right down to it, the valve, the transistor, semiconductors generally, and mosfets, all do the same thing. They are all devices to control the flow of current.
There is a particular element in each of these devices which more or less defines what it does. In the case of a valve, (this is simplified: there are a lot of different valves) this element is called the grid; for a bipolar transistor, it’s the base; in a mosfet it’s called the gate. This, in each case, is the ‘control’ element.

By applying a small voltage, say from your guitar pickup, to the grid of a triode valve (say an ECC83: this is a actually double triode but most folks know its name. It’s a 12AX7 in American-ese), we can control relatively big currents and voltages going through the valve. These currents/voltages come from the voltage rail(s), and so for these relatively big currents to accurately reflect the small signal we applied to the grid, we need a stable and noise free supply voltage. This is what the power supply, supplies, and is carried to the valve via the rails.

So, what comes out of your amp, is just a controlled version of the voltage rails as supplied by the power supply, and controlled by the valves or what-have-you, in your amp.

The kettle calls. The next blog will be concern itself with clarifying all the previous stuff, by drawings or pics or whatever. Then we might look at the ‘schematic diagram’, that piece of black art that shouldn’t be.

Preamps, power amps, power supplies PART 4

So, we worked out during the last blog, that the power amp’s main function is to drive the speaker cone, and that is for the sole purpose of moving some air. Everything that happens inside the amp, (preamp or power amp,) exists in a dimension that we can’t experience. There is no doubting that it’s there, but, so far as we’re concerned, it may as well not be.

If we really wanted to get philosophical about it, how much else is out there that we can’t know about? But that’s another place we’re not going; that way lies madness!

As soon as the speaker cone starts its act, we can hear the stuff that is happening inside the amp.

To do this, the power amp supplies the speech coil of the speaker cone with electrical current. This output current reflects more or less exactly what has been happening through the amp, but bigger. The output current of the power amp when it goes into a fairly normal speech coil (I’m avoiding impedances here; that’s the ‘ohms’ thing) would be 5amps, or 20 volts, to push out 100 watts. So that’s a big increase over your half volt guitar signal at the input of the preamp.

 So why don’t we just stick the preamp output straight onto the speaker, as it can be more or less the same sort of voltage output, as the power amp?

We can walk into a lot of serious navel-gazing here, as we get involved with complex impedances, impedance matching, phase angles, harmonic filtering, and a whole raft of stuff we can well do without, considering that this is a blog and not a design engineering seminar.

The simple answer is that speech coil has to be supplied with significant levels of current, and a preamp just won’t do that. Stick a speaker on the end of a normal preamp, and its output voltage will be dragged down to almost nothing, because it has very little current capacity, and that won’t drive your speaker. This phenomenon is due to something called the ‘internal impedance’ of the amp, and is another complicated subject.

The big difference between the output of the preamp and the power amp is that the power amp has a power stage designed to supply a hefty current to the speech coil of a speaker. It almost invariably does this using a technique called ‘push-pull’, and this goes for nearly every type of output stage; bipolar transistor, mosfet, valve, and even integrated circuit. The only power amps that don’t use this technique are the ‘class A’ designed amps.

 And here we get to another chunk of bullshit, because the term ‘class A’ is often used a marketing ploy, to suggest that ‘class A’ is ‘The Best.’ Which it isn’t, necessarily. It would also be a near physical impossibility to build a class A amp at, say, 100watts.  So for small amps of a few watts, a class A output stage is probably the best you’re going to get. But the output stage on 15watt class A valve amp would probably weigh as much as a complete Fender Twin; and that’s serious heavy.

Again, the reasons for this go too deep for our blog. Virtually all power amps from the very biggest down to just a few watts are rated at some variation on class B. Could be class AB, class AB1, Class AB2, class B. They’re all push-pull operation, and the difference lies in the cross over point and the biasing that sets that point.

Class B and its variants are all push pull (sometimes called ‘open ended’), and that term means that, roughly, if your output stage consists of  say four devices (might be valves, or mosfets or bipolar transistors), two of those will push the positive part of the signal, and the other two will pull the negative part of it. Between these two polarities is a zero point (also called a nodal point) where the two ‘cross over’. It’s this cross over point that sets the class of the amp. Class B has least overlap, class AB the most. The more overlap, the more current the output stage has to cope with, and the bigger the devices are needed for what is effectively less power output. In the case of valves, it’s the transformers that get bigger.

 That’s more or less it for the power amp. A cuppa calls. I’d like to do some graphical stuff to try to make some of this a bit clearer. Maybe next time.

We get to the power supplies next. Here’s an interesting thought. We shove in a signal of next not very much, in the input and it comes out at the other end at a 100watts.

 So, we’re getting something for nothing, aren’t we? Well, no, life just don’t  seem to work like that. Not for me anyway. So where does all this power come from? Don’t miss the next gripping episode of Batman. Or something.

Preamps, power amps, power supplies PART 3

We’ve got as far as the power amp, and we could suppose, then, that these two things form some kind of a chain, and this they do. The original signal from your guitar etc., goes through the preamp and then into the power amp, with the phase splitter (or driver) between the two.

 So what does the power amp do, then?

We can’t hear electronic signals. Unless on some sort of subliminal psycho-wavelength that nobody knows about. But we won’t go there.

Whatever it is that happens inside the amp, it has to get out of the amp, and move some air. This is the only way anything inside the amp is going to be detected by human hearing. Steve Vai could play as many notes to the bar as he likes into his amp, but nobody is going to know about it, unless there is a speaker stuck on the end of it shifting a serious amount of air.

 To move air requires power, however we do it. Why? To get the idea of that, we need to look at a speaker and how it works.

 The bit of the speaker that we can see from the front, is usually a cone –shaped looking thing, and it’s called …wait for it… the speaker cone! Tricky stuff, eh?

To shift air, this cone moves backwards and forwards, and pushes and pulls air. The principle is very like what happens when you overhang a ruler off the edge of a desk, and smack it one. It generates a wave, and it does this by the principle of ‘compression and rarefaction’ and we can hear it. Not Steve Vai exactly, but we can hear it. All that this means is that, when the speaker cone pushes forward, it compresses the air in front of the cone, and when it pulls backwards, it has the effect of sucking or rarefying that same air. This means that, the wave in front of the speaker doesn’t actually go anywhere. It just moves back and forth, and we hear the result by the way the air molecules interact. It’s the same as wave motion in water, more or less.

Anyway, this movement can’t happen on its own. It has to be supplied with power, just as the ruler did when it was smacked to start it off.

Behind the bit of the speaker you can see, (the cone,) are the bits you can’t see, which are the magnet and the speech coil. It’s the way these two things are arranged together that makes the noise, and also it’s these that take power to make the movement that makes the noise.

You can actually see the magnet, from the back of the speaker, but not the operating bit of it. The magnet is the circular lump of metal at the back of the speaker, which is welded or bolted to the back of the speaker chassis (sometimes called the basket). There is a circular gap in this magnet (called the speech coil gap) at the other side, that you can’t see, in which a circular tube of cardboard or plastic material moves in and out. Wrapped onto this is a coil of wire. This is the speech coil, and the cardboard former (tube) fits onto the back of the cone. This is the powerhouse of the speaker. If you apply some voltage to the speech coil, say with a battery, this produces a magnetic field that reacts with the magnetic field of the big magnet on the back of the speaker, inside of which the speech coil rests.  

 The result of this interaction between these two fields, is to tend to push the speech coil away from, or pull it further into, the speech coil gap in the main magnet. It’s the same principle that you get if you put two bar magnets together. Two of the same polarity (say two Norths) will tend to push apart, and two opposites will pull together. The direction the speaker coil pulls or pushes depends on the way we connect the battery, and also the direction that the speech coil has been wound onto the former.

All that, and we haven’t got to the power amp yet. Well, we have now.

To get the speaker to produce a wave, or sound, we have to get it to alternately push and pull. A pure wave (a sine wave) will move the same distance forward as back, and in the same time, but that isn’t always the case. 

The power amp’s job is to supply the power to move this speaker cone, in order to simulate the sound of the guitar you shoved into the preamp. That must seem like years ago. As electro-static, and electro-magnetic signals, travel at not far short of the speed of light, it wasn’t all that long ago.

 Well, I managed to get through to the job of the power amp before the tea pot called. 

The next one will be down to problems that the power amp has to overcome, in order to produce movement in the speaker cone, and therefore, sound.

Preamps, power amps, power supplies PART 2

More useful(?) things for musicians to know about electronics.

The most important bit that the 1st blog tried to get across, was what it is, in essence, that a preamp does. And all there was to that, was that it makes whatever signal (note, chord, or whatever ) you put into it, bigger at the other end; and the purpose of that is to come up with a signal that a power amp can work with.

Although it can be persuaded to do a lot of other things, that is what it’s for.

One other subject we need to touch on before we leave the preamp, is gain.

The thing most musicians associate with the ‘Gain’ is distortion, (or overdrive, or sustain; they’re all versions of the same thing), and the knob that is usually on the front of your amp that says ‘gain’. It’s actually not that at all.

Gain is simply the size of the output of the amp, divided by the size of the input. Put another way, how many times the amp multiplies what it is going in to get whatever it is coming out. So if the gain of the amp is two, if we stick a 1 volt signal into the input, 2 volts will come out of the output. If the gain is four, 4 volts will come out for the same 1volt input.

We could, about here, start into another way to quantify that, with a familiar phrase called ‘db gain’. This is a much misunderstood term, and it would be far too easy to get, yet again, into areas of deep bullshit, which I seriously don’t want to do. Most of the music industry does that a lot better than I can. I hope.

Very briefly, a db figure says something about gain. As a ‘for instance’, 6db gain gives twice as much out as went in: so a gain of two. I hope I can get into a bit more detail as we go along, but these are early days, yet.

There are many different designs of preamp, and I’ll do what I can to expand on this in due course, but at the end of most preamps is a ‘phase splitter’ of some description. In the case of a semiconductor amp, this is generally part of the power amp circuit, and would be termed ‘the driver’ part of the circuit.

In the case of the valve amp, this is almost always (Except in ‘class A’ amps: this is another serious heap of bullshit that needs a good kicking. We’ll get to that also, but probably much later.) a separate circuit involving one valve. It might, or might not, have gain. Early phase splitters had unity gain (same in as out) or even slightly less. But they were in general, very accurate, and that accuracy gets less with increase in gain. A phase splitter with gain, is an arrangement which varies very little from amp to amp, manufacturer to manufacturer. You Laney LC30 that cost a few hundred quid, will have a phase splitter circuit with more or less the same stuff in it as your Mesa amp for a couple of grand (and some). Most amps use this system. Some Ampeg amps don’t; they use an early version and an extra gain stage.

The purpose of the ‘phase splitter’, (It’s not a ‘driver’ in a valve amp. It doesn’t necessarily ‘drive’ anything), is to split the phase of the signal that goes into it from the preamp. I have to avoid explaining this too far, for fear of the bullshit factor. I will get to it at some future time though. Suffice it to say that most power amps deal with the signal in two halves, the positive-going half, and the negative-going half, and it needs some form of device (the phase-splitter for a valve amp or driver for a transistor one) to supply these two halves of the signal, usually.

Well, we got the power amp. Eventually. I’ll try to explain what that beast is for, and what it does, (and sometimes why it doesn’t) in the next lot. But now, it’s tea time.

Preamps, power amps, power supplies PART 1

Useful things(?) for musicians to know about electronics.

Well, you’ve got to admit, the idea is alright. But why bother?

Let’s face it, the musician (especially the ‘electric musician’) gets stitched up by the ‘music bizz’ maybe more than anyone, anywhere.

Why? Because when bullshit abounds, it affords every opportunity to blind those poor sods (the long-suffering musicians) with pseudo-science. I’d just like, in my own small way, to level the pitch a bit.

So, where to start? How about the preamp/power amp/distortion/overdrive thing?

What are they, what do they do, and how do they do it?

It doesn’t matter what amp you have (valve, semiconductor, or even pulse modulated chopper amp) they have only three functional chunks in them. A pre amp, a power amp, and a power supply. This is just as much the case for a 5k front of house pa., as it as for your little Watkins Dominator combo. And neither does it matter who built it; Jim Marshall, Leo Fender, Dick Denny (for Vox), or any of the no-name computer designed stuff of today.

So what do these three bits do?

The preamp.

This is the first piece of electronics that your guitar signal finds, inside the amp. It’s also the bit that has all the gain, master, bass, mid, top, presence, etc., knobs associated with it, on the front of your amp.

Its main function is to boost the signal that comes in through the input jack, which will, probably, be only a few hundred milivolts; from a guitar pickup for instance (a fraction of a volt; say a half). The preamp has to boost this signal, to a level that will be useful for the power amp to use, which could be anywhere between say 5volts to maybe 80 volts. I’m not getting involved with rms, peak to peak, program, peak, or any of the other ways to quantify the size of a signal, until I’ve got round to explaining those things. Otherwise it’s just bullshit again, and I don’t want any of that on these pages. There’s enough out there already, without me chucking some more on.

The preamp has one more, and very important, function. Maybe the most important, so far as the musician is concerned. It’s this bit of the amp that has the filter section, sometimes called EQ., or equalization. This basically alters, manipulates, the sound that comes in from you guitar or whatever. This is also the bit that sets the overall output volume, and also gives you the opportunity to adjust any sustain/distortion you might want to add.

Switchable preamps are fairly common these days. This means that you can set one preamp for a sound, and adjust the other(s) for a quite different setting, and switch between them. This design of amp has then, pretty obviously, more than one preamp. But they all do the same thing, essentially. They build the signal up to a level so that the power amp can work on it.

 That’s it for now. I’ll do a bit more after a cup of tea.

More about ReVamp

Roger Pattison, Revamps Valve Amplifier Repair Specialist

Roger Pattison, Revamp's Valve Amplifier Repair Specialist

An improbable history of ReVamp and it’s owner….

Could also be construed as a text book on the Art of Yawning.

In the first place, it should be noted that yawning is a lost art of the early Mayan tribe of Brixton. This was a branch of the real lot in South America, and they only got there by tagging a lift on the back of the Golden Hindquarters, not knowing at the time that it was also lost. But that has only peripheral connections with ReVamp. Can’t think what they are, but there definitely were some.

 Anyway, shortly after the Columbus bit, a singularly misguided person in 1963 called Roger Pattison, decided in his infinite wisdom to get into electrical engineering as an apprentice. Against all the odds, as with many apprentices of the time, he survived this. During this ill-advised 5 years in an annex of Hell, called Robinson and Sons Electrical department, the emphasis shifted, subtly, to industrial electronics. Partly because the company was acquiring electronics gear, hand over fist, but mostly because there was nobody else there daft enough to want to get involved in it.

He finished up seven years later, with three City and Guilds Technicians Certificates and an H.N.C. in industrial electronics and a headache. This was (and is) a source of considerable embarrassment, as it was pretty much all out of date, more or less as soon as he had finished it. Such is life.

 After that (Yawn), in 1971, he became involved in the musical instrument side of things via various retail shops, the first being Hudson’s of Chesterfield. This proved to be a minor annex, of some institution for the mentally unhinged, and the aforesaid Roger Pattison soaked up this atmosphere like a sponge. He afterwards moved (1976)( Yawn), to the major annex of the asylum which was Davidson Electronics, a company of (seriously deranged) development electronics engineers, near Sheffield. This more or less completed his training in electronics and lunacy, although he has developed the latter as a sideline since that time.

 After (Yaaawwwwnnnn) that, he pursued his musical ambitions, having been a musician from the age of two and a half weeks (Roger studied the Harry Lime Theme on goat bell for forty years. Unwisely.)

Starvation prompted a return to electrical engineering when the demand for goat bell virtuosi dropped off a bit, and he worked as electrical engineer for the Weekly News Group in North Wales up to 1996. At this point the company had been taken over by Trinity International, who had this philosophy of running a business without any people, so everybody (ish) got made redundant.

By this time Roger had achieved the heady heights of the obscurity of being forty nine years of age. It is common knowledge that folks of that age are as useful as soggy MDF, so rather than suffer the embarrassment of being told that a lot, he scraped his £1.39 redundancy package together, and started ReVamp.