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Wednesday, June 20, 2012

A bright idea

In the early days, it was a struggle for the guitarist as venues grew in size and volume levels went up. Amps grew in power and were EQed to cut through the mix with brightness and presence features.

Many amps at this time had a "bright" channel. The brightness is often achieved by routing the treble around the volume control so in effect, the high frequencies are always passed, and the volume control only affects the lower frequencies. In other words, the cap is a high pass filter.

A good example is the Marshall Super Lead. There are two high pass filters in the bright channel (see schematic), but the cap across the volume pot is the major brightness contributor. At low settings, the tone is very bright, but around 5 on the dial, the tone is much more in balance. However with a 100 watt Super Lead, you're also really loud!

So the amp designers set out to solve one problem with this cap, and inadvertently contributed to the volume at which rock was played. It's not the only or even the biggest factor in the volume climb over the years, but if you look at the amps being used at the time and realize they need to be turned up for tone, you may start to see a cause and effect situation.

The Super Lead is probably the most obvious use of a high-pass filter, but the Vox AC30 with a Top Boost preamp and most of the Fender amps also employed it. Many of the Fender amps made the high pass filter switchable. (The Deluxe Reverb is an exception where the cap is hardwired into the Vibrato channel.) Fender also used a more moderate value for the cap so the effect wasn't as great as it is on a Super Lead.

For many people today the bright channel on a Super Lead actually does its job too well which is why the normal and bright channels are often jumpered together and run in parallel. There is also a simple modification to clip one of the leads on the cap to remove it from the circuit. You still have the second high pass filter, but it's a more subtle effect.

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