Crank up the Volume [I]

Not being in the same class as the famous or infamous Jeremy Clarkson, of whichcardoIdrivetoday fame I’ll try to elaborate by saying that I’m a serious listener but not so keen reviewer – all manufacturers bring out a sound system which can beat the next manufacturer’s easily but never the listener. Who cares if you the have Philips Dolby XX Minus 5 or the lastest Marantz ZX 55 Spitfire – even more so, “my husband has the latest Wharfedale 22″ Incredibles and the biceps of Arnie” – before buying a sound system, don’t go by the ad, drive it! If you are prepared to pay serious money for a sound system don’t even bother about listening to your mate, the salesman – listen,listen and listen again. Oft been said – take your favourite DVD or CD with you and listen to it. Be careful of the sound room factor though – do they have one? If not you are in the wrong place. Do they have one and is it similar to yours? If not get them to test it at your home.  This doesn’t work with entry level systems by the way.  You may be in for a drive by shooting with this idea in mind.

Well today we are actually going to be talking about something different – the amplifier.  I’m going to mention only two words in today’s exercise: 

Ceiling and Match.

My definition of amplifier ceiling is the power that your amplifier can reproduce a level of sound that is comfortable to the ear but yet when there is a transient – the reproduction will be clear and give you palpitations, night sweat, raised blood pressure and humping a hollywood star. An amplifier must always have room for plenty more – unfortunately the computer home theatre systems that put out 200PMPO aren’t going to cut the grade. I have heard and read about the different ‘new fangled methods’ that one derives the power output of an amplifier to get to PMPO but most of them are thumb sucked or utter bullls**t – go with the tried and trusted RMS power delivered into a load of such and such. To cut to the chase there are only three ways – continuous power, RMS power (a colleague once mentioned that he overheard a sale person saying that it meant “Real Mean Sound”) and lastly, a good old fashioned way of testing – pick the damned thing up***.  If it’s heavy, you’re half way there – as long as someone hasn’t weighed it down with a lead weight inside.  That’s another story entirely.

1) Continuous power/RMS. OK, let’s look at the following reasoning:  Your home theatre amplifer can deliver 600 Watts RMS.  Is this all channels driven or only one mulitplied by the amount of channels the amplifier has? The specifications should state whether simultaneously or not.  Because your home theatre is not designed to reproduce all inputs at the same level at the same time I qualify this as being reasonable; the home theatre system is not designed to put out 600W RMS continuously, which gets to point 2)
2) I feel so heavy now. The amplifier should be able to sustain the output power continuously…. which means what?  Power transformers and power supplies are the order of the day. Regulated power supplies, big, rugged transformers and reservoir capacitors the size of the wart on your nose. Hey guys, get a life! A good power supply is one of the most important parts to your amplifier.  The ceiling (mentioned in column C, paragraph Z, agent X) relies on this more than anything.  A power supply which drops it’s output rail voltage will affect the output – and the bass is going to be so severely affected you’d want to throw it out the door after one session with Pink Floyd, or with Michael Jackson’s Beat It. There’s a fairly well known formula for calculating power of an amplifer; note though this is with a non-bridged format power amplifier – we’ll get to that later:  Vcc(squared)/8RL; power rail voltage to the power devices divided by 8 times the load resistance.  See what a difference your amplifier would make if the supply voltage fluctuates? The voltage drops, the potential power output drops – and if it drops to a degree where there is mains ripple on the supply rails, well then you lose the race. In fact so many amplifiers distort so badly when hitting the upper limits that you won’t be worried about the bass.
3) Your suspension is damping out my dear. Good loudspeakers are meant to be driven – don’t fool youself that you bought the best sound system to impress your wife. My wife hates listening to loud music unless it’s Robbie Williams and I know she isn’t listening to the music. Well, unfortunately for you guys out there wanting to hear about the damping factor – a very crucial part to the dynamics of your sound system, and here we are in fact only going to be covering the impedance of the loudspeaker – the damping factor comes in a little bit later although what we cover here is all relevant.  I deliberately covered the aspect of power output first and then the weight. (cunningly disguising the power supply as being the main topic). What about loudspeaker impedance? What is impedance firstly? Simply put this is the resistance measured when an alternating current is applied to …. a load in this case i.e. your loudspeaker. In electronics we call this reactance. A loudspeaker has a magnet and a voice coil, bingo – you have it! The voice coil is an inductance which has a direct current resistance – but applying an alternating current to it the ‘resistance’ rises as the frequency increases. And sadly, as Whitney Houston hits the high registers so unfortunately your loudspeaker impedance start to rise. Remember that little formula we mentioned earlier on – that one with the square of the supply rail voltage divided by that other stuff…. well now you can see that as the frequency goes up the loudspeaker volume starts dropping because the ‘resistance’ gets higher. Or does it?  Ummm, now something intersting starts to happen between that crossover network and your hearing.   Wait for the next article.

Ciao
The Lounge Lizard

***I’d be weary of buying something like an audio amplifier with a switched mode power supply – it will be a nightmare to fix and secondly this article is going to lead to you – yeah, you, building your own high performance amplifier and SMPS will not be covered.   We will show you how to re-design a computer power supply to deliver a symmetrical +/- 40V at 5 amps. What a bonus.

Part II