Class Act: The Pros and
Cons of Amplifier Design - Page 2
Devices
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Vacuum Tubes
Up until about 30 years ago, the vacuum tube was the only device
capable of delivering power reliably and cost effectively.
Tubes operate from power supplies delivering hundreds of volts
and require output transformers to deliver substantial power
levels.
Bipolar Transistors
The bipolar transistor has replaced the vacuum tube as the
workhorse of the power-amp industry, with single devices capable
of tens of amps and hundreds of volts. The transistor's low
output impedance is a much better match to speaker loads. The
biggest change in bipolar transistor technology in recent history
is the availability of power transistors in plastic packages.
These are essentially the same parts as before, mounted to
a metal substrate, but now surrounded by plastic instead of
the traditional metal jacket. These lend themselves better
to automation and will eventually replace metal in all but
a few applications.
Mosfet Transistors
The mosfet (Metal Oxide Semiconductor Field Effect Transistor)
has been around almost as long as the bipolar transistor but
has only recently become a significant factor in power amps.
There are two basic types: vertical and lateral. Vertical mosfets
are optimized for switching and popular in Class D designs.
Lateral mosfets are optimized for linear operation and utilized
in the output stages of conventional Class A/B designs. They
are not drop-in replacements for bipolar. They require different
approaches for driving and protection. They do not yield a "tube
sound," as some claim. When used "open loop" or with low negative
feedback, they exhibit "mosfet sound." Properly designed and
used within design parameters, an amplifier will not have a
characteristic sound. The positive sound qualities attributed
to vacuum-tube amplifiers are actually artifacts of non-ideal
performance and musical sounding overload characteristics beyond
electrical clipping.
New Devices
The only notable new device in recent history is the IGBT (Isolated
Gate Bipolar Transistor). This is a cross between mosfet and
bipolar, aiming for the best of both (high power gain of mosfet
with low saturation voltage of bipolar). These devices are
essentially designed as on/off switches, but at least one esoteric
hi-fi design uses these linearly. Currently, the big market
for these is in switching power supplies, and they may eventually
find their way into Class D amps.
 CS
800X
These are the basic components and techniques. Probably 99.9%
of all the amps you will ever encounter use some combination
or variation of these parts and topologies, with a good 90%
using bipolar transistors in a Class A/B topology. The CS® and
PV® series are classic examples of this, using Class A/B
topology and bipolar power transistors. The reason for this
is excellent performance, reliability, and cost effectiveness.
 DPC
1000
It's much more interesting to talk about the other 9.9%, so
here goes. Probably the most technically advanced amplifier
technology available today is Peavey's Digital Power Conversion
(DPC) series. While the original DECAs used a modified Class
D topology, the DPC uses a patented, improved form of Class
D called "Phase Modulation Control."
Instead of creating the audio signal by simple Pulse Width
Modulation or duty cycle variation of a square wave, Phase
Modulation uses two switches operating at the same frequency.
By controlling the phase angle or time difference between the
two waveforms and processing them differentially, the output
varies from full off (0 phase shift) to full on (180°). This
approach does not suffer from the difficult turn-on speed and
symmetry problems that limit the performance of conventional
PWMs. Vertical mosfets are the output device of choice due
to their ability to turn on and off very quickly. This series
of amps delivers on the promise made by Class D, with Peavey's
single-rack-space DPC 1000 putting out 1500 watts of peak music
power without becoming a space heater. Due to the complexity
there is a small price premium. But just ask any bass player
which he'd rather carry around in his rack, a DPC at 12 lbs.
or a conventional 1000W Class A/B amplifier... Enough said!
 VX
1.5
Next in the high-technology hit parade is Peavey's VX series.
These amplifiers use a novel variation on Class G/H topology.
The VXs use multiple power supplies for improved efficiency
over conventional Class A/B, but with a twist. Our design engineers
came up with a way to switch between the rails without the
significant distortion (switching) spikes typical of these
designs (patent pending). Now you can get 1500W RMS in two
rack spaces without having to grit your teeth every time the
drummer hits his cymbals; true hi-fi performance with typical
full-power distortion figures of less than 0.008% THD+N, and
full power at 20 kHz still below 0.04% THD+N. These numbers
beat most manufacturers' conventional Class A/B designs!
 Classic
120/120
For generations, musicians have been in love with the sound
of vacuum-tube power amps. Most tube power amps are basically
Class A/B designs with output step-down transformers. These
are undeniably "low-tech"; however, tube amplifiers like Peavey's
Classic® series are still very popular. It's difficult
to say exactly what it is about tube designs that people love
so much. The low damping factor, caused by the output transformer
and the relatively high output impedance of vacuum tubes, produces "pleasant
interactions" with speaker resonances. More likely, the vacuum
tube's unique characteristic overload (what, musicians overdriving
an amp?) is what pleases the ear. Although low-tech, tube amplifiers
are not cheap. Good-quality tubes get harder and harder to
buy and quality audio frequency output transformers are very
expensive.
Our next major family of amplifiers is Peavey's Architectural
Acoustics series. These are designed for fixed installations,
such as churches and auditoriums. These amps use conventional
Class A/B topology with output step-up transformers to deliver
70/100V to large "distributed" sound systems. By stepping up
the voltage, resistive wiring losses become less significant.
In fact, this is the same reason utility companies prefer to
send 20,000 volts across the countryside than good ole 110V.
In some cases bridged configurations are used to directly generate
70V swings, or in the case of emergency battery-powered units,
to get the signal swing up and resistive losses down. These
use conventional approaches that are optimized for their applications,
where reliability and cost are more important than portability
or size.
 PMA
70
Getting back to the unconventional, Peavey's AMR PMA 70+ uses
a unique variation of Class G/H to deliver momentary levels
of 100W from a 35W continuous-output amplifier. Unlike the
typical Class G/H, the PMA70+ doesn't use multiple fixed-rail
voltages. Instead, a specialized voltage doubler circuit generates
higher rails on demand. The amount of peak power available
is limited by the doubler's storage capacity and how fast you
recharge the doubler's capacitors. To better match the power
demands of typical musical waveforms, the boost capacitor is
only recharged at a rate that supports 60 watts RMS, and this
is thermally limited to no more than about 15 seconds of continuous
boost. If you've ever looked at music on an oscilloscope or
even on a peak-reading meter, you will notice that music only
hits its highest peaks for small fractions of a second with
average power levels 6-10dB below that. The PMA70+ delivers
momentary output equal to a 100W amp but with a size and cost
closer to a 35W amp. This amp is popular for driving headphone
systems in recording studios and for small near-field monitor
speakers.
 RP
500
Another studio/hi-fi offering is Peavey's AMR RP 500 Reference
Power Amplifier. Using proven Class A/B technology but with
lateral mosfet power devices instead of bipolar transistors,
this amp is designed for critical studio monitoring applications.
The RPA delivers tons of output drive current to deal with
even the most exotic (read poorly designed) monitor speaker.
State-of-the-art slew rate, distortion, and dynamic range insure
effortless reproduction of master-quality music. This approach
is slightly more expensive due to the high cost of the lateral
mosfets and associated circuitry.
If I haven't discussed your favorite Peavey amp, I mean it
no disrespect. Who knows what kind of amp we'll come out with
next?
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