Tuesday, November 24, 2009

Much Ado About Slew Rate

Given that most audio equipment manufacturers seldom – if ever – publish the slew rate ratings of their audio amplifiers, how important does an amplifier’s slew rate rating affect its performance?

By: Ringo Bones

After having the good fortune of visiting a good number of hi-fi shows in the past, I do find it odd that an overwhelming number of audio amplifier manufacturers seldom – if ever – publish the slew rate ratings of their preamplifiers and power amplifiers that they are trying to sell. Of probably thousands of brands and manufacturers out there, I only discovered six audio amplifier products so far that published their slew rate ratings. They are: 1.) Creek 4330R Integrated Amplifier at >40 volts per microsecond, 2.) Audio Research VTM200 Monoblock Power Amplifier (vacuum tube-based) at 40 volts per microsecond, 3.) Audio Research VT200 Power Amplifier (also vacuum tube-based) at 25 volts per microsecond, 4.) Simaudio Moon Rock Monoblock Power Amplifier (solid-state) at 160 volts per microsecond, 5.) Headroom Reference Amplifier Module – a power amplifier for headphones – at 1,300 volts per microsecond, and the 6.) Cinepro 3k6SE Six-Channel Amp with a slew rate rating of 65 volts per microsecond. Given that an amplifier’s slew rate rating is probably the last thing on a typical hi-fi audio amplifier buyers’ mind, what is slew rate anyway and how much of a factor does it play in a typical audio amplifier’s performance?

Slew rate is defined as the rate of change of the output voltage of an amplification circuit. Typically it is expressed as the maximum rate at which the output voltage of an amplification circuit can change in volts per microseconds. If an amplifier, for example, has a slew rate of 1 volt per microsecond it would take 10 microseconds for the output voltage to change from 0 to 10 volts, regardless of the rate at which the input voltage changes. Slew rate ratings also define the bandwidth capability of a typical amplifier design. An amplifier can only amplify a sine wave without distorting it up to a certain frequency as dictated by its slew rate rating. If a sine wave is of too high in frequency for an amplifier to process without distorting it given the amplifiers’ slew rate rating, that amplifier would output a triangular wave signal of higher amplitude instead of a sine wave. Triangular waves contain mostly odd-ordered harmonics by the way.

Audio engineers who would rather “listen” to their amplifier with 10,000-U.S. dollar Fast Fourier Transform analyzers instead of their very own ears tend to disdain in measuring the slew rate of vacuum tube-based amplifiers – especially single-ended triode amplifiers with vanishingly low to zero negative feedback. Because the squashing transfer function typical in these amplifier designs can decrease the odd-order harmonics in the signal they are trying to amplify. A triangular wave being fed into a tube amp’s input may end up looking like a sine wave with a higher magnitude at the output. A good thing about this typical idiosyncrasy of vacuum tube-based amplifiers is that it is not frequency dependent, unlike an audio filter with fixed frequency response. This is probably the primary reason why tube amps – especially ones that don’t use any negative feedback – sound musical and might – with the emphasis on might – give a misleadingly high slew rate rating in comparison to solid-state designs. Despite of all this, how does slew rate play a factor when it comes to the everyday use of our audio pre and power amplifiers?

If you’re fortunate enough to have peeked into a typical high slew rate hi-fi power amplifier, you might have noticed that the “apparent” complexity of its circuit layout and design just to achieve a high slew rate rating may seem a touch of glorious excess, but it is eminently practical. Today’s audio formats – if you’ve already managed to outgrow the typically compressed dynamic range of on-line digital music downloads – demands instantaneous power in order to amplify them realistically as Mother Nature or God in Her infinite wisdom intended.

Since the mid-1990s, the SMPTE movie-soundtrack specifications now permit special effects to peak 20 decibels above the average dialogue level, thus requiring 100 times more amplification power than is needed for dialogue. Using the watt-meter setting of my multi-meter, the average dialogue levels of the DVD of the movie Independence Day in my listening room with my audio system required only 1 to 3 watts of power. But during one of the loudest part of the movie – when the alien saucer blew up the White House – my main amplifiers generated a peak of 250 watts for probably half a second.

This performance – paradoxically - was produced by my 70-watt rated power amplifier equipped with a 300-watt power supply sporting high-speed Schottky Diode / Schottky Barrier Diode rectifiers with Rubicon Black Gate capacitors. This is at a sound pressure level in my listening room that doesn’t preclude casual conversation given that my main speakers has a sensitivity of 93 watts at 1 watt (2.87 volts) at 1 meter. What I’ve learned so far is that not only do you need clean power, you also need it fast in order to produce current music or other audio formats realistically. Though in my case, I tend to gravitate towards the sensitive loudspeaker route. Nonetheless, slew rate ratings is a very important factor indeed when it comes to the realistic sounding playback of recorded music - and movies - in a domestic setting.