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Digging into Fast-Response Amps

Fast response amps of yesterday and today: Vox AC30 and Mad Professor CS-40 are compared

In the September 2010 issue, we began exploring the subject of fast-response amplifier designs with a column on Hiwatt heads (“Mo’ Watts—A Nostalgic Look at Hiwatt”). Let’s continue this discussion using the same amp we cited before as a classic example of fast-response circuitry—a vintage Vox AC30. But this time we’ll compare our reference AC30 to a more modern example of a great fast-response amplifier—the Mad Professor CS-40 head. By the way, if you haven’t yet had the opportunity to play a fast-response amplifier, you’re in for a treat: Experiencing the sheer speed of picked notes rushing through the speakers can change the way you think about guitar.

Let the Fun Begin

A vintage Vox AC30 produces a very springy sound because it has more output-tube compression factored into its design than the Mad Professor CS-40. This compression is one of the reasons a properly maintained original AC30 is so much fun to play. By contrast, the CS-40 uses the least amount of output-tube compression of any modern guitar amplifier that I know of. However, both these amplifiers are very fast and very dynamic. And, oddly enough, despite the difference in output-tube compression, both have a hauntingly familiar and similar feel.

I’m writing this column in Boston, where I’ve been able to compare stellar examples of both amps side by side. A close friend of mine and I have been having a blast listening to his CS-40 driving a ’90s Matchless 2x12 extension cabinet. Sitting right next to this setup was his favorite candy panel 1963 AC30 Top Boost combo. For our sonic hijinks, we also had several vintage and modern guitars on hand.

The Vox Vibe
We played the AC30 first because we were both familiar with its seductive sound (and touch). We also started with the AC30 because we both agreed it represents the first historical example of a fast-response guitar amplifier. The first guitar we plugged in was a great-sounding 2006 Les Paul. This combination gave us a sound and tone that instantly reminded us of Mike Campbell, longtime lead guitarist for Tom Petty and the Heartbreakers. Okay, that’s not a bad thing is it? The AC30 loves to be set at (or just slightly under) the point of breakup. This is where that opening chord to “A Hard Day’s Night” sounds like a choir!

Though the AC30 has a healthy output, there is a point where you get a creamier distorted tone as you raise the amp’s volume control. Cranking the volume from there won’t actually make the amp any louder—that’s where the output-tube compression comes into play.

Going Mad

Moving along, my friend and I then switched amps and set the CS-40’s controls to yield a clean loud volume. Wow! We immediately noticed that the CS-40 was producing an amazing shift in distortion texture and character as we dug hard into the strings. The resulting effect was very refreshing. By varying our pick attack, we could play the same two notes and produce a sound that was quite like a trained human voice going from a soft whisper to a scream. This particular sound was more akin to a solo singing voice, whereas the AC30 sounded like the aforementioned choir—a different sound entirely!

Both amps are a joy to play because of their speedy response and touch-sensitivity. But they also have several differences. For one thing, AC30s (and other Vox models, come to think of it) will not do jazz guitar much justice, in my experience. This is because the Vox’s midrange frequencies are voiced higher in comparison to Fenders and other popular amps. Now, the CS-40 can do jazz sounds incredibly well, and it’s the only modern design I’ve played that can nail that old mid-1950s jazz guitar tone that Herb Ellis achieved with the Oscar Peterson Trio.

Here’s the kicker: Moving into distortion— from slightly dirty to heavily saturated tones—is a walk in the park for the CS-40. Sonically, it’s an extremely versatile, Swiss Army-knife amplifier, and we had serious fun dialing up an array of distortion textures.

While comparing the AC30 to the CS-40, I realized that vintage amps were designed to be “clean.” In the old days, it was common for an amplifier company to warn you in the owner’s manual that if you heard any distortion, the amp was too loud and you should turn it down. Compare that to the CS-40, which is designed to produce a full spectrum of tones, from sparkling clean to massively distorted. These two models illustrate how amp circuitry has evolved over the years to keep pace with guitarists’ demands.

Next month, I’ll be back with more gear adventures. Meanwhile, thanks to my friend, Milton Reder, owner of Rear Window Recording Services in Brookline, Massachusetts, for giving us beyond-generous use of his world-class facilities, and for providing the instruments and amplifiers we used for this month’s sonic explorations.

Dean Farley
The chief designer of “Snake Oil Brand Strings” (, Dean Farley has influenced contemporary string design and is a great source of guitar, amp, and gear lore.