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Amplifier Technology – What are the differences and which is right for your system?

Amplifier Technology – What are the differences and which is right for your system?

If you generally buy your stereo system as a complete unit, perhaps from a local big-box store, then you might not be too concerned over what's inside your boxes and how it all comes together. But if you're the type of person who 'shops for separates' then this article may be useful to you.

A person who builds a stereo system from scratch, piecing it together one component at a time, ultimately has a greater challenge on their hands than the person who walks into Walmart and leaves with a new stereo system in one box. But the challenge of putting together your own system from scratch can be infinitely more rewarding. 'How so?', you might ask. Well, by purchasing your system as 'separates' you can fine-tune the sound to suit your specific tastes.

But where do you start?

Quite simply, the only sensible place to start is with your choice of loudspeaker. [Hyperlink to an article on speaker selection]. But what if you already have a set of speakers and you're looking to get more out of them? In this case, you should be looking towards the component which drives the speaker – the power amplifier.

Power amps come in all shapes and sizes, including 'integrated amps' where they are combined together in a single chassis with a preamp. What follows applies to all power amps, regardless of their status as 'stand-alone' or integrated.

A power amplifier is the driving force behind the loudspeaker. It receives a low-voltage input signal from the source component (a CD player, for example) and it increases the amplitude of the signal to a point where it can drive the loudspeaker to audible volume.

Over the years there have been many different innovations in amplifier design, many leaps forward as it were. It all started with 'vacuum tubes' back in the 1940's. Then in the early 70's came the transistor amplifier, now we have digital amplifier technologies involving Class D, Tripath and Ncore designs.

I will be exploring the different designs at a more detailed and technical level in upcoming articles, but for now, I just want to give you the basics to help you make a more informed buying decision.

 

There are many parameters by which amplifiers are measured and judged, these include:

  • Bandwidth

  • Efficiency

  • Gain

  • Linearity

  • Noise

  • Stability

 

No amplifier will perform perfectly in all areas of performance, which means they mostly sound different and mostly have applications to which they're better suited.

So for the sake of simplicity, let's group together the different amp technologies into three broad categories, then see which is going to be best suited to your particular application.

Category 1 – Vacuum tubes (or 'valve' is the term commonly used in the UK and Europe)

Category 2 – Solid State (Includes all variations of transistor designs, mosfets, Jfets etc)

Category 3 – Digital amps (includes Class D, T, Ncore etc).

Category 1 – Vacuum tubes

Many people believe that vacuum tube technology disappeared with old CRT TV's in the 70's. Nothing could be further from the truth. There are still a large number of manufacturers designing equipment based on these 60+ year old circuits, and still a viable number of manufactures making the vacuum tubes (many based in Russia and China)

'Tube amps' as they're often termed, can offer a more deep, rich and musical soundscape, particularly through the midrange frequencies where some of their sonic attributes are most apparent on vocals and stringed instruments.

They suffer inherently from a little frequency extreme roll-off, and from a low 'damping factor', so generally they're not suited to driving larger diameter cone drivers in bass/woofer applications. At the top-end of the frequency spectrum they can sound a little rolled-off too, just lacking in that last bit of musical air and detail.

But in terms of shear musicality they're hard to beat.

Tube amps tend to be better suited to driving more efficient loudspeaker systems, where 25 – 50 watts is sufficient to drive the speaker to ample volume. Higher-powered tube amps are available but they tend to be expensive.

Maintenance is a definite factor when considering the purchase of a tube amp, the tubes require replacement after a certain period of use (typically 1000+ hrs) and re-tubing an amp can be expensive.

For – Sheer musicality and midrange presence/liquidity. Beautiful on vocals and strings.

Against – requires an efficient speaker design in most instances. Maintenance can be expensive. Not great at the frequency extremes.

Reputable vacuum tube amp manufacturers based in the United States include -

Conrad Johnson

Cary Audio

Manley Labs

VTL

Atmasphere

 

Price range new - $500 and upwards.

 

amplifiers for use in a stereo system

Category 2 – Solid State (Includes all variations of transistor designs, mosfets etc)

Though grouped together here for the sake of simplicity, the various solid-state power amp circuits/designs are quite different. Most common circuit variations are:

Class A

This type of amp circuit generally results in a more linear performance at the cost of efficiency. Class A amps can come quite close to matching the virtues of tube amps, but their inefficient operation results in them running quite hot, even in stand-by where they are actually using their full rated output.

 

Class B

This circuit is more efficient than its Class A counterpart but suffers from crossover distortion which can appear in the audible range.

 

Class AB

Class AB is somewhere between class A and B. It has higher efficiency than class A with less distortion than class B.

Solid-state amps have a high damping factor, when compared to vacuum tubes. They are more capable of controlling the speaker load and exerting more control over the larger cone woofers used in bass reproduction. They are more inclined towards a 'neutral' presentation, that is they do not generally exhibit the artificial warmth and bloom of a vacuum tube amp.

For – More consistent through the frequencies. More control over bass frequencies. More linear. More compatible with inefficient speaker systems.

Against – Class A amps can run extremely hot and use full power in stand-by. Not as 'musical' as tube amps, lacking the midrange frequency warmth and bloom. Can sound quite sterile, an artifact of there being on the cool side of neutral.

Category 3 – Digital amps (includes Class D, T, Ncore etc).

A class D amplifier is actually a switching amplifier where the power devices (usually MOSFETs) are operated as binary switches, much like you see in the bit stream coming from an audio CD.

Class D amps have a high damping factor and are used widely in powered subwoofer applications where they exert a vice-like grip over the higher mass, higher diameter bass cone drivers.

Class D amps are generally the most highly efficient of all amp designs, as such, most of the energy put into the amp is converted into power to drive the speakers, and very little energy is lost to heat - they run extremely cool.

Many people consider Class D amps to be deficient in the reproduction of high frequencies. Many of the earlier generation digital amps were described as harsh in the treble region, with 'ringing' anomalies detectable at the upper frequency extremes. With Digital amp design still in its relative infancy, there continues to be some steady gains in sonic performance.

For - efficient, runs cool. Small/compact/light-weight, many use switching power supplies which are small and light.

High damping factor and good control over dynamic cone woofers. Can be relatively inexpensive when compared watt for watt with other amp topologies.

Against - Hardness in the upper frequencies is a commonly reported issue.

Practical Applications

So there you have a rundown of the various design topologies. But how do you go about selecting an amp for your application?

If you're an 'audiophile' whose primary goal is the pursuit of sonic perfection, then you're going to want to go out and listen to some different gear. You can arrange an audition with a local audio equipment dealer, but only do so if you're somewhat serious about buying from them. In other words, don't use their showroom facility when you fully intend to buy your gear online.

Choose a reputable and knowledgeable dealer and have them demonstrate each of the amp designs listed above – tube, Solid State and Digital. Then let your ears be the judge.

Talk to the dealer about your musical tastes, how loud you like to listen, practical constraints such as the layout of your listening room etc.

Use their expertise to make your journey that much less fraught with danger! Yes, there's danger involved, it comes mostly from buying equipment which is incompatible, then having to sell it on at a financial loss.

Shop carefully, and if you're still in need of more information, then read my FAQ article here:

 

Amplifier Choices – some of the questions I've been asked quite frequently over the years.

As I touched on at the end of my last article [links to article above], you can make some fairly expensive mistakes when assembling a competent 2-channel stereo system. Components need to be coupled together so that they perform 'synergistically'. At a basic level, this simply means that component A works well with Component B and C and so forth.

But at a more technical level, it means finding components which work together with an electrical synergy. This involves matching input and output impedances so that your gear works within its intended design parameters, and it means selecting power amps with the required capacity to drive a specific speaker. But more of this later. Let's take a look at a few questions asked by others' and see if they might provide the answers to your own. And please, by all means, contact me if you have a specific question which is not answered here. If you take a look at my blog there may be a suitable place to post a question, if not, then just send me an email.

Q – What's the difference between a preamplifier and a power amplifier?

Great question! Think about it this way. The signal coming out of your CD player, or off your turntable cartridge or from your Satellite receiver is a very low-level signal. In terms of voltage it's perhaps only in the range of a half volt or less. The signal is too small to feed into your power amplifier so it needs to be increased in magnitude, or 'amplified', which is the primary function of the preamplifier.

Of course the signal also needs to be variable so that you can adjust the volume of your system, so when you turn the volume control up or down on the preamplifier, the signal voltage is raised/lowered before sending it off to the power amplifier.

Now the preamplifier also serves the function of providing switching between the different source components in your system. A modern stereo might have inputs from a CD player, a Cable receiver, a digital music server and a turntable. All of these sources need to be connected and have the ability to be switched in and out of the circuit, this function is performed by the preamplifier

With the task of managing the source-switching and increasing/decreasing the signal to affect volume covered, we must now get the signal out of the preamplifier and into the power amplifier. This is done via two L/R outputs on the rear of the preamplifier which are usually labelled 'pre-out'.

We then connect via cabling, either RCA single-ended or XLR balanced the preamplifier outputs to the inputs on the power amplifier.

The power amplifier now has the task of further amplifying the signal to the point where it has sufficient voltage and 'gain' to drive (via an output device like a transistor) the speakers.

In some cases, both the preamplifier and power amplifier can be combined into a single box (read my review on the Conrad Johnson CA200 here / Link).

And that's it. So to summarize, the preamplifier handles input switching between sources and provides control over volume (and sometimes channel balance), and the power amplifier provides the power to drive the speaker load.

Q – I'm looking to put together a system but I have space restrictions and I want to 'hide' the components as much as possible, what do you recommend.

A - In a Home Theater application you may need to place your amps in smaller equipment racks which may even be fully enclosed as with a audio/video cabinet. In this case you're going to want to stay clear of tubes and Class A solid-state amps as they run too hot.

An efficient Class A/B design or a Digital amp would be ideal for a HT application or any application where you need to hide equipment inside a cabinet.

But one other consideration is how you access the equipment to switch it on and off. There's a neat trick here which many manufacturers are adopting – a 'trigger' input for detecting signals. If your equipment (amps, usually), have a 12v trigger input, then you can connect a preamp or processor to the 12v trigger input on your power amp, and the preamplifier/processor will automatically wake up your power amp when a signal is detected and automatically 'put it to sleep' after a few minutes with no signal.

This means your power amp can be behind a door which you'll never need to open!

Q – My speakers have large diameter dynamic cone drivers and the bass sounds really 'loose' and boomy, what can I do?

A - You have two potential issues to think about, and in all likelihood, both of these will be part of your problem to some extent.

Firstly, you need to look at room acoustics. The lower frequencies emitted from the bass drivers on your speakers are notoriously problematic. They can create 'standing waves' which cause the bass to sound wooly, just as you describe.

Secondly, you need to look at how your speakers are being driven by your amp(s).

If you think about how a cone driver works for a moment, it basically has a piston effect where your amp is pushing and pulling on the cone (via energy into the speaker voice coil). Larger diameter cone drivers have more mass and more inertia and require more energy and more control to be exerted by the power amplifier to force the cone driver to start and stop in reaction to the audio signal.

Amps with a low output impedance can typically exert more control over the speaker driver, these amps are said to have a 'high damping factor'. Tube amps do not have this high damping factor and have difficulty with higher inertia cone drivers, such as woofers. Digital amplifiers inherently have a very high damping factor, hence their suitability to driving large diameter subwoofer applications.

So take a look at your speaker system – if it has 10” or 12” dynamic woofers, a Digital amp might offer the best solution for you.

Q – I'm looking for an amp to drive my High Efficiency loudspeakers, what should I be looking at?

If your speaker system is 96db or higher, it's classed as a 'high efficiency' speaker. High efficiency speakers require less power to drive them to full volume. This type of speaker lends itself to vacuum tubed applications where amps tend to be designed with a lower power rating. There are various circuit topologies and tube types which yield differing sonic signatures and differing power outputs, but I won't get into that here. If your speakers are efficient, and you have the time and money to invest into vacuum tube amps, then the sonic rewards can be quite significant.

I'd recommend that you obtain an amp from a dealer and try it in your home. Look for a dealer who stocks tube amps from reputable manufacturers. These would include:

  • Conrad Johnson

  • Audio Research Corporation

  • Manley Labs

  • VTL

  • Balanced Audio Technology (BAT)

  • Cary Audiophile

Those are a few of the big players, then there are many smaller 'cottage industry' manufacturers to consider too.

 

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