The cost of a free lunch

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Engineering products is a series of choices and compromises. The first rule designers learn is there is rarely a free lunch. One design choice that solves one set of problems often leads to another. And the greater the complexity of the system the fewer free lunches.

I described in yesterday's post how power amplifiers with a switch to choose between between stereo or mono work. The technique is called bridging and it is simple to build. All the design engineer must do is provide a means of flipping the phase of one amplifier channel. This is typically handled in the input stage where the addition of a phase inverting circuit does the trick nicely. It can even be cleaner if the designer uses a differential pair on the amplifier input. This type of stage has two inputs: normal and inverted. Once flipped over, the output of one channel goes positive while the other goes negative. Between the two outputs voltage is doubled and wattage to the loudspeaker is quadrupled. But this is one of those situations where the free lunch of an easy switch feature comes with a hefty price tag.

For starters designers have the problem of power supply current. Stereo power amplifiers have enough current capacity to drive both channels to rated power with little left over. A 100 watt per channel amplifier is powered by a supply capable of 200 watts, 100 for each channel and not much more. When you bridge the amplifier the output voltage doubles but the supply cannot provide four times the current, so instead of 100 becoming 400, we run out of gas early and find ourselves limited to 200. Amplifiers that produce high voltage but clip early because their power supplies run out of steam are not ideal. But this is how the majority of switchable amplifiers are made.

Yet another problem with the bridged amplifier is impedance. Unless it is designed to be robust into low impedances, the amp can run into trouble in bridged mode. This is because from the amp's perspective whatever load the loudspeaker presents, 4 Ohms or 8, the impedance seen by the amp is cut in half. Thus, a bridged amplifier presented with a 4 Ohm loudspeaker is actually asked to drive a 2 Ohm load. To make matters worse speakers do not have constant impedance, they dip and rise at different frequencies.

Of course I am speaking in generalities. It is entirely feasible to over-build a stereo amplifier with many more output transistors to handle lower impedances, and super-sized power supplies to keep from running out of power, but it is rarely done. More often than not, switchable stereo amplifiers are designed to satisfy the needs of stereo owners first, mono wannabes second. A good marketing technique perhaps, but a less than optimal solution for greater performance by investing in two power amplifiers, surely.

Yet, there is a third way to build a mono amplifier superior to the two practical examples I have previously described: purpose built single channel amplifier stage, and bridged stereo pair.

Let's take a look tomorrow.

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Paul McGowan

Founder & CEO

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