This Kenwood LS-P9000X "Digital" speaker is one of a pair we recently reconditioned.

Although they were never what you'd call a landmark in Hi-Fi speaker design, they look and sound big and impressive - a real Japanese retro-classic.

Their owner had had them since they were new and kept them in excellent condition.

Obviously, these babies had been part of the furniture for so long that the lounge just wouldn't have looked the same without them.

When they finally failed, the prospect of having to replace them with someting new and different just had no appeal at all for their owner, so he contacted us to see if they could be fixed.

Many speakers of this vintage are discarded by people who assume they can't be fixed - but very often they are repairable and will continue to play for many more years after they've been restored.

The problem with these speakers was 'Rubber Rot' - a common issue with speakers that have foam surrounds.

Rubber is an organic compound - and it does eventually decompose.  

As the foam breaks down, holes will start to appear in the suspension, as you can see in this shot of one bass driver.

The rot in the other speaker was much more advanced and the foam surround had completely gone.  The surrounds are replaceable so after the rest of the system was tested to make sure there was nothing else wrong, the surrounds were expertly replaced with new ones.

The Kenwoods went straight back to work and ultimate test: the owner's kid's 21st. where they rocked the house!

This will depend on their type and condition.  The example above shows a common problem and a typical outcome.  You have to make the decision whether to repair or replace your speakers.  Sometimes, it's not just a matter of their replacement value, but what they're worth to you.  It'll only cost you a phone-call to find out more... so give us a call and we can advise you on the the best solution.

Modern consumer electronics is made more affordable by mass-production techniques. Printed Circuit Boards (PCB) are no longer hand-soldered (a very time-consuming process).

Hot solder is 'waved' onto the boards where it rapidly bonds with exposed metal pads and leads.

If you could slice through a PCB, the cross-sectional view of a wave-soldered component lead would ideally look something like this.

The secret of good soldering, is that all parts of the joint must reach the same temperature and cool at the same rate - otherwise, the solder becomes 'grainy' and the joint will be weak.

Wave soldering however is a very fast process and often there can be several joints that don't have time to reach the ideal temperature where the solder forms the strongest bond.

Components heating up in use and cooling down when turned off, result in expansion and contraction which can cause stress fractures in weak joints.

Magnification reveals an annular crack forming around this component lead. This crack will ultimately cause the unit to fail. At this scale, the crack is easy to see but in real life, they're usually too fine to be seen with the naked eye.

These shots were taken from the amp board of an active speaker. Active speakers (which includes subwoofers), generally have poor ventilation and then of course, there's added vibration...

Another pad from the same PCB clearly showing a gaping annular crack. This one has really opened up! If only they were all as obvious as this... usually, it takes some long, hard staring under the magnifier to spot them.

This is the sort of thing that causes units to shut down when warm - or to crackle and distort - or to need a 'technical tap' (AKA a good thumping) to make them work.

In this example, from the unit's power supply stage, you can see small globs of melted solder, burn marks from arcing and little 'blow-outs' around the annular crack.

The resistance of the stress fracture, in conjunction with the current flow has resulted in enough heat to re-liquefy the solder. Electrical erosion, compounded by vibration from the woofer did the rest.

It's a wonder the darn thing worked for long enough to get into this state!

This idealised drawing shows how a hand-soldered joint would look in cross section.

The increased contact area and density along with proper molecular bonding make this joint extremely strong, both physically and electrically.

Conclusion: There are many reasons an amp may shut down or go into 'Protection Mode' however one of the most common causes is failure of wave-soldered PCB connections.

We often see integrated amps and A/V receivers, perhaps a year or two out of warranty, that go into shut-down -- usually after a spell of hot weather or a loud party (or both). Once we have established that this is the cause, the faulty joints can be re-soldered by hand.

Solder connections which are hand-made by an experienced technician are rock-solid reliable -- and as a bonus, they usually sound better too!

Since the advent of e-bay, many Aussie audiophiles have taken advantage of second-hand goodies from foreign vendors. As a result, 240 volt conversions have become a fairly hot item. Hardly a week goes by that I don’t have at least one e-bay ‘bargain’ come into my workshop for conversion.

So, the answer to the question above is: “Yes, but with qualifications”.

Generally speaking, products which are marketed internationally will either be designed for multiple voltages – or at least, be designed to be convertible.

This beautiful classic Accuphase CD player is a prime candidate for 240 Volt conversion.

The designers clearly intended this unit to last for many years and they allowed for the possibility that it may change continents in its lifetime.

Underneath the chassis, Accuphase have provided easy access to the primary circuit and a even small colour chart to facilitate re-wiring.

Thoughtful design, superbly executed.

It's easy to see why this is one of my favourite brands.

Products targeted at a particular domestic market however, may not be designed to be converted. This doesn’t necessarily mean they can’t be... it just might be bit harder to do though.  At worst, these products can be operated on Aussie power with an appropriate external transformer.  If you’re looking at a model that was never released on the Australian market, it might very well fall into this category.

This magnificent Marantz 2130 came via e-bay.  It was made for the U.S domestic market so there's no simple way to convert it.  Also, its 'collectable' status would be compromised if its transformer was changed.  In cases such as this, we can supply an external transformer.

Occasionally, one comes across a product that is specifically designed to make voltage conversion impossible.  For some reason, the manufacturer doesn’t want the product being sold outside of their control, so they deliberately make it difficult to convert.

This still doesn’t mean that it can’t be done, but it does mean that the conversion will be unnecessarily expensive.  I have heard all sorts of opinions about this practice – most of them negative – but I’m not about to use this article as a forum to express my own.

Most times, 240 volt conversion is a relatively straightforward procedure.  In these cases, a standard charge of $120 applys.  Where a unit requires a special transformer, or the manufacturer has deliberately chosen to make conversion difficult, conversions must be quoted on a case-by-case basis.

So, if you have bought (or are thinking about buying) a foreign 'bargain' online, give us a call to discuss having it professionally and safely converted for Aussie power.

Because it doesn't know the words, of course! (boom boom)  But seriously, several things can cause a system to hum, in this article though, I shall address the eternal mystery of Ground-loops (AKA Earth-Loops). These are the major cause of hum (and other problems) in Hi-Fi systems, and probably the most common and widely mis-understood of faults.

Many audiophiles have tried curing hum by adding extra earth wires - sometimes tying them to the cold-water pipe - sometimes even driving copper stakes into the dirt... but in reality the cause is usually TOO MANY earth connections, not too few!

Usually, this comes from the fact that, in this country, the regulation mains-plug has 3 pins, Active, Neutral and EARTH.  With respect to any single electrical appliance, this is a good thing, because it adds an extra element of electrical safety.  

But when you connect several audio components together, they form a SYSTEM and it is important for this system to have a logical SYSTEM GROUND -- otherwise, it will just have a mish-mosh of un-planned earth connections, depending on your particular mix of components and their respective power cables.

Once the simple "Ground Rules" are understood, you will be able to banish ground-loops and their nasty effects from your system forever!  Once again, let's go back to the basics.

We like to think it's music going through our wires, but it's actually just an electric current flowing in a circuit between say, your CD player and your amp.  It's called a circuit because electric currents always follow a circular path (logically, not physically).  The electric current in this circuit is the 'analogue' of the music signal it represents.

The standard for unbalanced connections in Hi-Fi is the RCA plug.  With the exception of a small handful of European brands, just about everything connects with these.  The centre pin carrys the music signal which travels down the centre of the cable.  The outer ring completes the circuit by providing the return path, via the 'shield' which usually takes the form of a wire braid wrapped around the signal wire.

In unbalanced circuits, the return path is always the unit's 'Ground' - which means that the outer ring is ultimately connected to the metal chassis.

As mentioned above, the connection between your CD player and your amp is a circuit because electric currents always follow a circular path.

This electric current (the analogue of your music signal) travels down the centre of the cable and returns via the outer shield... or does it?

Electricity ALWAYS follows the line of least resistance -- and we know that the shield which is supposed to be the return-path for the signal is also connected to the chassis and the chassis is connected to the Earth pin of the mains plug.

So... what happens if the mains cable has less resistance than the shield of your RCA cable?  Well, it becomes part of the signal path - that's what!

The signal current returns, not via the shield, as it should - but via the mains cables which are connected together by your power-board - and in the process, it picks up interference from the mains, resulting in the noise that we know as HUM.

But hum is not the only potential problem here... remember, the Earth pin of the power plug, is also connected by the house-wiring to your TV, the fridge, the microwave, the computer, the power pole in the street, the house next door, the butcher, the baker... you get the drift.  In theory, in "a perfect world", the Ground is meant to be SILENT... but guess what?... it ain't a perfect world!

A ground-loop can cause your system to pick up all manner of electrical garbage - and in extreme cases, it can even result in instability and catastrophic failure.

So... What's the solution?
Consider the diagram on the right, and notice how it resembles a tree with a root, a trunk and some branches.  If your system ground is structured in this way, ground-loops are eliminated.  All units are grounded to the pre-amp via their interconnects, and ONLY the pre-amp connects to mains Earth, preferrably via the best power cable you have on hand.

Alternatively, this arrangement will work if you have a power amp with heavy mains cable -- you can use it as the 'trunk' of the tree.  Remember, this is a map of our system ground only, not the power connections or the signal path.

The general tree-shaped map still applys, regardless of your particular setup.

Removing any additional ground wires forces the signal currents to do what they should
i.e. to return via the correct path, the interconnect shields...

BUT

Before you take a hacksaw to the Earth pins on your mains cables...
make sure you know what you're doing

First, be aware that ground-loops are only responsible for hum about 95% if the time.  If the hum you are hearing is coming from inside a faulty component, no amount of mucking about with cables is going to cure it.  Hum can also mean you have a faulty cable somewhere.  So you should first try a couple of tests.

"The Hard Way" (recommended if all else fails)
To test where the hum is coming from, you can try disconnecting EVERYTHING except the power amp and speakers, and then switch on the amp and listen closely to the speakers.  If it has NOTHING ELSE connected to it, the amp should not hum... if it does, then the amp is faulty, pure and simple (so call me, I'll fix it, OK?).

If there's no hum, switch off, connect the pre-amp ONLY to the power amp and once again, switch on and listen to the speakers.  Keep repeating the above procedure, each time adding another component until you find the one that makes the system hum.

"The Easy Way" (try this first!)
You will need a very sophisticated piece of equipment - and you have to make it yourself!

Grab a common old double adapter (the pyramid-shaped ones work best) and a screwdriver.  Undo the screw that holds it together and pull the entire earth-pin assembly out and then put it back together.

Throw the earth thingy into the drawer of bits you're keeping in case they come in handy some day.

Using a CD marker (or similar), write "FLOATER" (or any scary message that'll stop your spouse using it on the kettle and the toaster) on the side.

Keep away from children.

Armed with your Floater and your new 'system ground tree', seek and destroy all ground-loops.  Start by isolating the source components first.

Don't forget that your RCA cables must now earth the floated components, so make sure they make good contact.

If your interconnects aren't up to the job, then you'll either need to upgrade your cables... OR

If just the RCAs are no good, talk to me about having your cables refurbished with some high quality plugs, like these gorgeous WBT Toplines (my personal favorite).

That annoying hum will soon be just a bad memory and you won't believe how good the music sounds against a black background of silence!

What's the real story on balanced lines?  Are they really so much better than 'normal' unbalanced ones?  Balanced lines were originally only seen in professional audio systems such as used in recording studios.  They were hardly ever seen in Hi-Fi systems except for a few high-end units which were built to professional standards.  

Over the last decade or so, balanced lines have become more common in domestic Hi-Fi with many components now offering balanced input/output options.

To properly appreciate the advantages of balanced lines, it is helpful to understand a little about how they work.

We like to think it's music going through the wires, but it's actually just an electric current flowing in a circuit between say, your CD player and your amp.  It's called a circuit because electric currents always follow a circular path (logically, not physically).  The electric current in this circuit is the 'analogue' of the music signal it represents.

The standard for unbalanced connections in Hi-Fi is the RCA plug.  With the exception of a small handful of European brands, just about everything connects with these.  The centre pin carrys the music signal which travels down the centre of the cable.  The outer ring completes the circuit by providing the return path, via the 'shield' which usually takes the form of a wire braid wrapped around the signal wire.

In unbalanced circuits, the return path is always the unit's 'Ground' - which means that the outer ring is always connected to the metal chassis.

This is called "Unbalanced" because there is only one signal line which is referred to the Ground which (in a perfect world) represents 'zero volts' or silence.  The significance of this will become much clearer when you compare it with the Balanced circuit (below).

The signal can be represented graphically like this...

The line through the middle represents zero volts, which is the same potential as the RCA plug's outer ring and the Ground (or chassis) of the equipment.  The wave shape represents a sound-wave as positive and negative motion of electrical current.

The same signal in a balanced circuit looks like this...

The signal is not referred to the chassis, instead, it is referred to its own mirror-image.  The music signal is carried by the difference between the two identical, but opposite electric currents... as opposed to the difference between one current and the unit's chassis.

Obviously, this 'balancing act' will require an extra signal line & therefore a different plug.

The standard connector for balanced lines is the XLR (sometimes called a "Canon plug") which has two signal pins plus a shield pin.

Unfortunately, XLRs are quite a bit bigger than RCAs but they're rugged, reliable and relatively inexpensive.

So, why does this extra line make such a difference?  Well, there are several reasons...

For one, the signal does not 'sit' on the Ground as it does in an unbalanced circuit -- remember, the Ground is also connected to the unit's chassis, which is also connected to the Earth pin of the power plug, so it's also connected by the house-wiring to your TV, the fridge, the microwave, the computer, the house next door etc. etc.  

As mentioned above, "in a perfect world", the Ground is meant to represent SILENCE... but the problem is... it ain't a perfect world!

But the really BIG bonus is a thing called "Common Mode Rejection" - do read on!

Common Mode Rejection is all about how the circuit behaves in the presence of unwanted signals.  Unwanted signals?  Consider this: Every wire is also an antenna for interference and no shield is ever 100% effective.

Remember, The music signal is carried by the difference between the two identical, but opposite lines... which implys that: if there is no difference, there is no signal.

Now, if one line sees some interference, the other one will too...  The graphic on the left shows what happens when a balanced circuit picks up a 'spike' of interference.  As there is no difference in the spikes, they have virtually no effect on the signal.

Another huge bonus that comes from the balanced connection is the abscence of ground-loops.  These are the major cause of hum in Hi-Fi systems, and probably the most widely mis-understood and common of problems.

As mentioned above, the connection between say, your CD player and your amp is a circuit because electric currents always follow a circular path.

The music signal which travels down the centre of the cable and the outer shield completes the circuit by providing the return path... or does it?

Electricity ALWAYS follows the line of least resistance -- and we know that the shield that is supposed to be the return-path for the signal is also connected to the chassis and the chassis is connected to the Earth pin of the mains plug.

So... what happens if the mains cable has less resistance than the shield of your RCA cable?  Well, it becomes part of the signal path - that's what!

The signal current returns, not via the shield, as it should - but via the mains cables which are connected together by your power-board - and in the process, it picks up interference from the mains, resulting in the noise that we know as HUM.

Because the balanced connection does not rely on the shield to return the signal current, ground-loops are not a problem.

If you're reading this, the chances are, you're already thinking about how you could make your system sound better than it does. The options available to you are as follows:-

Buy better equipment...
No explanation necessary here. Visa or MasterCard?

Improve the environment...
Acoustic treatments for your listening room, proper location of your system components, power conditioners etc.

Use better connections...
Better speaker wire, higher quality cables, gold-plated connectors. (You're not still using the wires that came in the box are you? Uh-Oh!)

Modifications...
OK, you're using the good wire, you've tweaked the lounge as much as the spouse can take, you don't want to buy a load of new stuff - but ehhhh... the thrill has gone? This is the time to talk about modifications.

When you bought this unit, you read the owner's manual and it said: "Congratulations, you just bought the finest audio component in the world." (they all say that).

Now, do you think that was written by the designer, who's under the hammer from the board of directors to cut costs?

Or was it written by some marketing doofus* who's under the hammer from the board of directors to make it look like they're not cutting costs?

Even if the above does not apply, there have probably been several improvements in component technology since the unit was first conceived.

Also, is the wiring inside the box of the same quality as your good cables outside the box? Or has the 'jewel in the crown' become the weakest link?

* (my apologies if you are a marketing doofus!)

That's like asking: "Will cosmetic surgery make me ugly?"

Only if carried out by someone who doesn't know what they're doing.

Same answer.

Usually, they can - depending on the exact nature (and extent) of the mod. I normally try to make mods reversible but again, it depends on how far you want to take it.

Wiring and component upgrades are generally reversible… anything which involves drilling holes generally isn't.

This depends on what you've got - and of course, what you'd like to get from it. So, just as a guideline, here are some typical results from expert modifications.

Improved low-level performance
Improved clarity at higher volumes
Tighter, more controlled bass
Sweeter, clearer highs
Better resolution of inner detail
Faster, cleaner transients
Improved stereo imaging and depth
Lower noise, hum and distortion
Increased efficiency and power

Call me!   Call me now!   But seriously, let's talk about what sort of mods will work for your setup. Occasionally, I'll get a punter who'll send me a unit with a note saying something like: "See what you can do with this" but I prefer not to work like that.

In order to suggest a sensible direction for your system, I need to know:

1. What is the weakest link (the obvious place to start)

2. Where you feel it needs improvement

3. The sort of result you'd like to achieve

This much will only cost you a phone call… (See? Talk is cheap!)

From here, I will assess the unit - a MOST important step!

I have occasionally seen cases where some 'expert' has attempted to modify a unit in an attempt to remedy a sonic deficiency that was actually due to a minor electronic fault in the unit! Sometimes, a simple repair is all that's needed to restore the proper level of performance to a unit that still works - but just sounds a bit 'ordinary'.

Next, I will prepare a quotation for you with a few options.

Again, depending on the unit and your expectations, a modification could include any (or all) of the following:-

Design review and treatment of inherent structural deficiencies
Substitution of standard parts with superior audio-grade components
Replacement of internal wiring with improved cables
Replacement of original valves (if applicable) with better types
Bypassing unused features which impact the signal path
Augmentation of components to alter their sonic attributes
Additional components to improve noise immunity and stability
Removal and replacement of worn or oxidized connections
Re-orientation of internal cabling to prevent unwanted interactions
Acoustic treatment and/or reinforcement of cabinet/chassis

"Are we going for the 'Extreme Make-over' or shall we just do the Botox today?"

I’m often asked about amplifier ‘class’.  If you hang around audio long enough, you’ll hear and read about; Class-A, Class-B, Class-D, Class-G and Class-H which are really just categories of amplifier design used in audio.

I have never seen a good, non-technical explanation of amplifier class (sort of like… if you can understand a paper on amplifier class, then you don’t have to?) so it occurred to me that only non-technical people ever ask what amp class means.

So, if you’re looking at an amp that says “Class-D” (or whatever) on it and you’re thinking: “What the heck does that mean?” then this article is for you!“

Although Class-A is theoretically simpler than Class-B, I think it’s actually easier to explain Class-B first.

Class-B amplifiers are the most common type on Earth, mostly because they’re traditionally the cheapest to make.  You have already heard several Class-B amps today, even though you didn’t know it – they’re in your clock radio, your TV, your car stereo, your computer, the P.A. at school or at work, the elevator, the shopping mall – to name a few.

Consider the simple wave shown below.

The line through the middle is just for reference. 

Imagine that the curves above this line represent the speaker cone moving forward and those below represent the cone moving backward.

Class-B amps always have at least two output devices (per channel), one which is positive and one which is negative.

The positive device switches on to drive the speaker forward – its contribution to the output is shown here.

And the negative device does the same job below the line.  This arrangement is often referred to as “Push-Pull”.

When there is a silence, the wave ‘flat-lines’ - the speaker is at rest and both of the Class-B amp’s output devices are switched off which saves both power and cost.

Now, for the bad news… The positive and negative devices are almost never exact ‘bookends’ so they don’t always jive together.  The result is “Crossover Distortion” – as shown in this (highly exaggerated) graphic.

The level of Crossover Distortion is constant, regardless of volume which means that when the volume is high, it only accounts for a small percentage of the signal.  When the volume is low however, Crossover Distortion begins to dominate.

Class-A” seems to have a mystique about it – probably because people associate “A” with the ‘perfect score’ and ‘the top of the class’ and maybe even ‘First Class’ but it ain’t necessarily so.

In a Class-A amp, the output devices never actually switch off which completely eliminates Crossover Distortion.  The downside of this is that the amp is constantly running flat out, sucking power and getting HOT – even during periods of silence!  Class-A amps may also be “Push-Pull” meaning that they have both Positive and Negative output devices (like the Class-B amp does) or they can be “Single-Ended”.

In a Single-Ended amp, only one output device does all the work – it only pushes ‘up’ and relies on the ‘weight’ of the load to bring it back down.  This is a very simple setup and produces no crossover distortion at all. 

The downside, once again, is that the device works all the time, even during silence.  Imagine the guy with the barbell holding it dead still, exactly half-way up and you’ll have a pretty good idea of what this amp has to do.  These amps are working hard all the time and as a result they get very hot, consume huge amounts of power and generally need some sort of elaborate cooling system.

Single-Ended amps can only produce comparatively small amounts of usable output power because they are so inefficient and it’s therefore too expensive to manufacture S.E. amps that can produce more than a few Watts.

Having said that though, Single-Ended amplifiers continue to enjoy cult-like support from a hard-core of devotees.  How do they sound?  Well, just about every mantle or console valve radio made up until about the late 1950s had a single-ended output stage and most folks thought they didn’t sound too bad at all!

Class-B is highly efficient, produces lots of Watts, runs cool and is cheap to make but has its own special type of distortion.

Class-A is highly inefficient, expensive to make, eats electricity, runs hot and produces lower Wattage but has extremely low distortion.

Class-B is ALWAYS Push-Pull.

Class-A may be Push-Pull OR Single-Ended.

Single-Ended is ALWAYS Class-A*
*(except in non-audio applications)


So, does this mean that Class-A is reserved for the rich and famous and the rest of humanity must live with Class-B?  Not completely – read on!

This ‘half-breed’ is the workhorse of ‘middle-shelf' Hi-Fi.  Not too expensive, not too cheap, not too hot, not too cold – you get the idea.  Simply put, Class-AB is a variation of Class-B with greatly reduced Crossover Distortion.  It works by having each of the devices in the Push-Pull output ‘just a little bit’ turned on.

Visualise if you will, the runners in a relay race.  The second guy that’s waiting for the baton doesn’t just stand there… he starts running just before the first guy stops, matching his speed, so the baton never actually slows down.

Now consider the following wave drawing.  The shaded band through the middle represents a small region in which the Positive and Negative output devices are both turned on.  

At low volumes, the signal stays within the shaded region so the amp behaves like Push-Pull, Class-A. When the volume increases past the top of this region, the bottom (negative) device switches off and allows just the top device to complete the rest of the positive excursion as per Class-B.

The amp then drops back through the Class-A region until the top device switches off and allows the bottom device to complete the negative part of the wave. The end result is that you get a little bit of Class-A purity at low volumes and power like a Class-B at high volumes. 

Added to the fact that it won’t scorch the carpet and you won’t need a second mortgage to own one, this has got to be a winner!

I’m skipping directly to Class-G now, just because it follows on nicely from the above discussion.

An amplifier which combines the desirable attributes of Class-A and Class-B (but does not suffer any of the disadvantages of either) is the ‘Holy Grail’ of amp design.  As seen above, Class-AB is a pretty good compromise but a compromise nonetheless.

So what’s the next logical step?  Roughly, the line of thought goes like this: “It’s not economical to make a really big Class-A amp… but what if we could make a tiny little Class-A amp that gets bigger when you turn it up?”  Sounds like a wacky idea at first but it can actually work.

So, we have a little Class-A amp that stays in control of the speaker at all times and it’s small enough that it’s not too inefficient or uneconomical to make -- but what happens when it runs out of power?  The Class-G circuit ‘turns up the gas’ and supplies the extra power to the little amp as and when required and then ‘throttles back’ again when not needed.

There have been many variations on this basic idea and it’s been called “Rail Commutation”, “Current Dumping” and goodness knows what else but it was Hitachi who called it Class-G (don’t ask why) when they released some fairly impressive hardware based on this idea back in the late 1970s.

Now, some hard-nosed engineers will tell you that there is no such thing as Class-G (it is sort-of an ‘unofficial’ class as it was named by a famous company as opposed to a bunch of faceless academics) but the term has become legitimate enough through common usage.

And before some bloody genius e-mails to tell me that rail-switching amplifiers can actually be Class-A, Class-B or Class-AB (or that Single-Ended amplifiers can also be Class-C or Class-D), I would just like to add:-

   1.  I’m trying to keep this simple, remember?

   2.  If you’re so smart, how come you’re reading this?

Class-H is really very similar to Class G.  The major difference is that where Class-G sort of ‘shifts gears’ only when the amp needs more power, Class-H’s power is continuously modulated by the music signal.  In other words, its power supply shrinks and grows to fit the power demand.

In terms of its circuit structure, it looks sort of like a small amp inside a big amp.  Class-H has recently become a large enough category to warrant a little more discussion.  Why? Because it is has become very cheap to produce so it is becoming very, very common.

Basically, Class-H is a good idea which could be used for good instead of evil… BUT in the wrong hands, it could also be used to squeeze quite a bit of extra power out of crappy single-chip amplifiers of the type that are used in low-end boom boxes and car stereos and thereby form the basis of some genuinely second-rate ‘Home Cinema’ systems.

So, if the whole thing comes in one box and looks too good to be true – it’s probably from the ‘dark side’ of Class-H and not something I would recommend.

Class-D has been around a very long time but has only recently made inroads into domestic audio systems.  Modern manufacturing techniques now make this once-impractical class quite economical, so expect to see a lot more of it in future. It really only has one outstanding advantage over the other classes and that is amazing efficiency.

Sometimes (incorrectly) called a “Digital Amplifier” Class-D operates a whole different way… so much so that I could write another whole article about it but this one has to end somewhere, so I’ll just give you the ‘executive overview’ for now. 

In essence, Class-D seeks to leverage the most efficient mode in which a transistor can operate i.e. ON or OFF.  You could think of a transistor as being a bit like the clutch in a stick-shift automobile in that, it runs cool when it’s fully engaged and also when it’s fully disengaged – but anywhere in between, it’s getting hot.

So, if you can find a way to chop up an audio signal so it just looks like a bunch of uniform on/off pulses and then ‘unchop’ it just before it gets to the speakers, you could make some fairly small transistors produce quite a lot of power.  The method used is called “Pulse Code Modulation” or PCM which is also a core technology of Compact Disc systems.

Up until recently, this type of amp was rare, and then only found in some industrial applications such as P.A. and instrument amplifiers but now that they’re easily mass-produced, they’re finding their way into boom boxes and the ‘economy’ end of home entertainment systems at a scary rate.

In theory at least, it is possible for a Class-D amplifier to sound very good but in practice, I personally find them a bit fatiguing and hard-sounding.

As far as I know, there are no Hi-End Audiophile amps that use Class-D.  I suppose that’s got to tell you something?

Conclusion: At the end of the day, if you find something you like and it's in your budget, don't let the class bother you -- but also be aware that things that sound exciting up front can also become fatiguing and even irritating in the long term. Classes D, G and H are becoming commonplace and look like good value, at least on the surface but be very sure about what you're buying.

Class-A will open up a tin of worms you may not wish to swallow... for starters, your choices in speakers will be limited to high-efficiency types to compensate for the low efficiency of the amps. This is especially true of Single-Ended Triode (SET) amps.

If you're prepared to live with the expense and other compromises though, Class-A is the cream of the crop.

Most of the amps I like are Class-AB and this is usually a pretty safe bet.