Friday, July 31, 2015

Watts Up With That?

Let’s talk power for a minute. An amps output is measured in watts, which generally determines how loud the amp can play. The Memphis Mini produces an honest 6 watts of power when measured accurately. Not all wattage claims are measured the same way.

We run a 120Hz test signal into the amp while it is driving a dummy 8 ohm load. Much of the sound of a well-played blues harp is low, so we use a low test signal. We send the amp’s output to a dummy load – not a speaker -- so the impedance is stable across all frequencies. A speaker will change its impedance unpredictably as the frequency and volume change, so getting an accurate wattage reading will be nearly impossible.

The standard method is to increase the power until audible distortion occurs, and record that level as the nominal watt rating for the amp. But since we are using a dummy load there is no sound. We take the measurement to a higher level of accuracy: We record the watt rating at the first visible appearance of distortion of the sine wave signal while viewing it on an oscilloscope.

You might wonder why we measure the clean signal when amps are usually played with lots of great sounding distortion. Good question! The method described above is the only way to arrive at an honest number that can be used to compare one amp to another. There are ways to juice up the signal and make grossly exaggerated claims about watts, but that does consumers no good at all.

Not all the breakup we love comes from the sine wave distorting its shape. It starts with the element in your microphone and ends with the cone in your speaker. A good vintage style tube amp rig is an organic thing with many moving sonic parts, including the rectifier tube and tone caps. Understanding wattage ratings helps the consumer make better choices.

Okay, so what does all that stuff mean? Well, it doesn’t mean that a 12 watt amp is twice as loud as a 6 watt amp. In fact, it would require a 60 watt amp to deliver a sound that is perceived as twice as loud as the 6 watt amp. I know, it sounds crazy but it is a fact. As with many things we perceive, the measurements are logarithmic. The 12 watt amp would be perceived as barely louder, and an 8-watt amp – for example – would be no louder at all.

Of course, all this talk about amp volume depends on the sensitivity of the speaker. The Memphis Mini uses a Weber Signature ceramic speaker that is efficient and punchy. Six solid watts into a high quality speaker like the Weber make the MM amp surprisingly strong. It happens every time the MM is played: People are amazed at the bigness of the tone.

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