Recently we were caught up trying to find a match for a friend’s blown tube amplifier – we won’t give the make here except to say it begins with an F and it is a 25W amplifier with one 12″ driver. We are suffocated with all makes of amplifier listed on the net but one thing which became more than apparent was the lack of control on pricing internationally, varying from $200 to $1000 for similar units.
Yes, the more expensive unit was 75W with two 12″ drivers but did it warrant the price tag? I am not a musician but I do claim to be knowledgeable on audio amplifiers – there are no-named brands, let’s call them lesser known brands out there that perform just as well as the known brands at 2 to 3 times the price. My rule of thumb when looking at an amplifier rated at a certain power (normally the over 200W units) is to pick it up. The weight of the unit inside, if we are not cheated by an added steel weight or two (that’s another story) can give you a quick guesstimate as to what power one can expect.
Good amplifiers do have more than sufficient cooling and do have more than enough power from the supply to drive all the power amplifiers simultaneously over a lengthy period of time – read: your home theatre system is not designed for disco use. Yes, good quality audio amplifiers cost more because the power supply needs to be hefty and the main ingredient here is the mains transformer. Ever picked up the old Phase Linear amplifiers rated at 200W + 200W and compared it to the cheaper imports of a similar power rating. The power supply and protection circuitry plays a vital role in what power to expect – there are thermal cut-outs, there are temperature sensing devices which reduce drive to the audio power block and there are electronic switched which cut power to the amplifier when voltages to the loudspeakers are dangerously high or just pure DC.
This does add weight to the audio amplifier but nothing compared to the mains transformer. An older, stripped down 200W+200W amplifier I have at home has a transformer which weighs 6Kg. I have a 300W + 300W audio amplifier (lesser known brand) which weighs 10Kg, this includes mixer desk and cabinet. Both did or do deliver the power output rated by the manufacturer, one with both channels driven simultaneously and the other guaranteed only one channel.
Audio amplifier ratings are very ambiguous and sadly the lesser known brands are more likely to overrate their equipment. NAD, Yamaha plus many other brands don’t. Neither does Fender or Peavey. This does not mean you are stuck with a white elephant. When coming to musician choices I am not a fundi suffice to say you need to be careful of head room. Hi-Fi Audio amplifiers are designed to, or at least should be designed to cover the audio spectrum of say 20Hz to 20Kz and the power rating tested and shown at what frequency it was tested to give these results. High quality amplifiers are tested using spectrum analysis and this will be available in the specifications – manufacturers will give this to you as well on request. NAD, Yamaha are cases in point. Fender, definitely.
Well, what does this have to do with head-room? High amplitude multi-frequency signals should be reproduced without a mushing of sound – so if your amplifier volume is set to 80% it may not faithfully reproduce the input signal at the given rating by the manufacturer – in other words the reserve power is just not there, comparing apples to oranges. I don’t know if this sounds confusing? A 100W amplifier from manufacturer A may not have the same response as that from manufacturer B. Something is amiss. This is often caused by an inferior power supply, mains transformers are the most expensive single item in an amplifier, torroidals are horrendously expensive. If I know the amplifier is going to only be used for home theatre use then I can take a chance and put a smaller transformer in because not all channels are driven equally hard and at the same time. Alas, this does not go for PA equipment. Very high powered audio equipment makes for heavy equipment. I am not a fan of switched mode power supplies for audio use although there are some very reliable units out there.
Another problem with getting that amplifier with less headroom (comparing apples to oranges again – same spec reputed manufacturer A against unreputed cheaper model B) is the very real problem of voice coil burn. The loudspeaker voice coil is designed to move in and out and is cooled by the air inside this cavity. Sustaining a level of distortion through the loudspeaker means the driver voice coil is not moving in and out cleanly around the magnetic cavity but rather at a rapid uncontrolled rate which does not allow proper cooling. Yep, a 25W amplifier can burn out a 50W loudspeaker in this very manner. In this same manner a 50W audio amplifier can be played at 40W into a 25W loudspeaker often without damage. Don’t try this though – the voice coil is much thinner than that of a 50W driver, there is also a narrower air gap. I have played an 80W audio amplifier into a 25W speaker with absolutely no distortion at way passed the loudspeaker rating – it did burn out the tweeter though (3W).
Getting back to the power supply again. I apply the formula (Vcc x Vcc)/(8 x RL) where Vcc is the supply rail voltage and RL is the load impedance of the loudspeaker to get to the results below.
An amplifier is rated at 30W RMS into 4 Ohms and supply rail is 12V DC. Fact or fiction? 144/32 = 4.5W. Yes, common car audio myth. However the car audio amplifier often has a stepped up voltage supply. PAs don’t or rarely use stepped up power supplies unless you live in a 110V country where very high powered audio can exceed 200V. (+100V and -100V split rail supplies). Getting back to base again, most amplifiers do not use regulated power supplies so an 80V supply will drop to around 60V under full load (about 75% regulation). So at 80V the amplifier has the potential of delivering 200W into a 4 Ohm load but in fact it is under 120W, more like 100W. When that voltage drops to 60V you can bet that the distortion is highly audible – the bass is going to be horrific, this will modulate all frequencies in the rated spectrum. By regulating the supply you need to beef up the audio driver and output transistors. This is a glaring problem in audio amplifier design – unfortunately this comes at a cost. A very big cost because you need to push up the rating of the transformer as well. Plus additional costs for cooling the power supply and additional power transistors (they are cheap). Car audio is regulated however through the switching circuits designed to crank up the DC voltage – you are very limited when using only 12V as your main supply rail to the output transistors.
So when looking at the summary of facts if you are a power user do spend that extra cash to get yourself something which does what it is supposed to do. In ending off, as our title implies visit the website Musician’s Friend – the pricing is very realistic and it is based on well known gear. If you do see a cheap import compare the pricing of that Peavey you had in mind to the lesser known brand. Often you may end up paying hard earned cash for an unfamiliar brand compared to similar priced well known brand like Behringer, Peavey or JBL. I have worked on quite a few Peavey amplifiers and if one just looks at the power supplies in these units they are under-rated. All the necessary precautions are taken into account for long and durable use. A good audio amplifier fails often because of abuse, wrongly rated loads or unregulated generators.
Before I get bitten by hard core audio fanatics I deliberately left out RMS, peak, music power and PMPO as terms of usage.
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