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Every audiophile sooner or later hears the phrase "this [usually transistor] amplifier has plenty of current reserve for difficult loads". Exactly what does this mean? There are only a few interpretations possible, so it is easy to cover them. The first way of looking at the statement is in terms of the power supply in the amp. This makes sense, because reserve does imply the power supply. One manufacturer has even advertised current reserve of 80 amps! Is this 'reserve' being used at the speaker? Ohm's law, the basic formula of all electronics, states that there is a simple relationship between current (our 'reserve'), resistance (our loudspeaker) and voltage (which we have to have in order for the current to be present). Let's take a worst-case example. Suppose that you own a speaker with a one-ohm impedance (a very unusual speaker). So, in addition, lets say you really, really like Wagner or maybe Black Sabbath. Either way you need lots of current to drive that speaker, right? How about, as in our previous example, 80 amps? Plugging the values into the formula for Ohm's law (R = E/I) we get 1 = E/80, E therefore being 80 volts. A further extension of Ohm's law says that voltage times current equals power. 80 x 80 is 6400 watts- nobody makes an amp that big! So the rating can only be interpreted ONE way: The amount of current that flows through the POWER SUPPLY when it is shorted out for 10 milliseconds. That's the official spec. And there are a number of tube amps with ratings that high! So when you see the idea of 'current reserve' being bandied about, keep this in mind. Voltage vs. Current Amplifiers This leads directly to our second myth about current vs. voltage amps, usually the myth of transistor vs. tubes (tubes being the 'voltage' amps). Ohm's law is still around to help us out. The way this argument is usually heard goes something like this (and sounds a lot like the previous myth): "This amp has lots of current and is good for low impedance speakers..." or "...that amp has lots of voltage and is better for electrostatics". Again, the power formula saves us. Let's look at some easy examples. Let's start with a common load impedance, four ohms and drive it with a lot of power, say four hundred watts. Now if the above statement is true, a four hundred watt transistor amp will do better then a four hundred watt tube amp, right? With a leading question like that, obviously not. The power formula tells us that 400 watts is just that, tube or transistor not withstanding. But let's look at the actual numbers for a second. By working with Ohm's law and the power formula, we can derive the following (simple) equation: Power = Current squared times Resistance. Plugging in the values we get 400 = current squared times 4. The current is 10 amps. That's all. Suppose an 8 Ohm speaker. The current is roughly 7.071 amps. If the amp produces the power it must produce the current, if the amp produces the current it must produce the power. This fact is inescapable, but it is amazing how much misinformation is spread in its ignorance. Load Sensitivity This myth is slightly more subtle. You probably have heard: "This transistor amp is way less load sensitive then that OTL". Actually, the reverse is true. But how can a transistor amp be MORE load-sensitive then an OTL tube amp? Let's look at some typical examples. Let's say you have a high quality 150/channel transistor amp. 150 watts into 8 ohms, a reasonable amount of power, but if you have a four Ohm speaker it’s 300 watts. Nice. Into 2 Ohms, if the amp doesn't blow up or current limit, 600 watts. So what does the amp produce driving 16 Ohms? 75 watts. Into 32 Ohms its only 35 watts! This could result in serious problems were the speaker a typical electrostatic, where such impedances are common in the bass frequencies. This explains why transistor amplifiers are usually such a poor match for electrostatic speakers. This is what the right OTL can do into those impedances: 150 watts into 8 ohms (we knew that), 145 into four (less then 1/2db difference), about 80 watts into 2 ohms, but into 16 we have 149 watts, into 32 ohms 145 watts- so you see that as long as the speaker load is moderately well behaved, this particular OTL produces far more linear power over the same range of impedances, whereas the transistor amp is quite simply incapable of being linear at all! Why? When a recording is made, it is assumed that a linear system is to be used so that it is capable of recording the same energy at all frequencies. When we play it back, for best results the playback should be the same at all frequencies, too. If there are variations in the speaker impedance, this will not be possible with a transistor amplifier unless it has a lot of negative feedback (which most of them do), which has the additional effect of decreasing bass impact, restricting dynamics, foreshortening soundstage depth and increasing odd-ordered harmonic distortion. Thus there is no way that a transistor amp can be described as linear if it is subject to these problems and that is one of the reasons why transistor amps produce so many amusical colorations. The reason has to do with the vanishingly small output impedance of the transistor amp (here's some myth bashing for you). The result is that the transistor amp has what is called a constant voltage characteristic, not constant power, which is of course what a power amp should do! So, despite the fact that smaller OTL amplifiers don't like four ohm speakers, they are quite capable of giving you a more even power characteristic (read: flatter frequency response, all other things being equal), especially on a speaker with a wild impedance curve. Now the fact of the matter is if you want a tube amp, even if you don't want an OTL, you should avoid four ohm speakers. This is because output transformers using the four ohm tap will not give nearly the performance that the same transformer will do on eight ohms. Sixteen ohms is better yet. Fortunately, with all the Single-Ended triode amplifiers now available, the market for sixteen ohm speakers has improved quite a lot and they are once again (like they were in the fifties) available. Obviously a transistor amplifier is at a severe power disadvantage on a sixteen ohm load, which is why four ohm speakers became more common in the seventies and eighties. High-End audio is kind of like being at the carnival show. Have fun, but keep your wits about you.” |
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