Class Act: The Pros and Cons of Amplifier Design
By John Roberts
Many manufacturers would like you to believe that there is only one "best" way to design a power amp. These companies usually make only one type of amplifier. Perhaps they believe their own hype, or more likely, they are limited to a single in-house technology or by a specific target market. In the course of designing power amplifiers for the different markets we serve, Peavey has developed products using radically different technologies. I will attempt to give an overview with specific pros and cons of the different approaches.
For convenience, engineers usually characterize amplifiers by circuit topology, the type of active components they use, load type, and even operating voltage: CLASS A, B, C, D, etc. Circuit topology describes how current is "steered," or controlled, within a power amplifier before it is delivered to a speaker load.
Class A is the simplest, most basic topology. Reproduction of music requires speaker motion both in and out. To do this, amplifiers must "source and sink" current. In a Class A amp only one direction of current control is used. To generate both directions of output flow, a constant current stage is subtracted from a variable current stage.
Pros: Since neither output stage ever turns off, device non-linearity and turn-on/turn-off time can be minimized or ignored, resulting in very low distortion designs.
Cons: As the maximum output is limited to the constant current stage, at idle this stage must put out full power and the variable stage must absorb this full power. Transformer, heat sink, and output stage must be sized for continuous duty at maximum power. Because of cost and the amount of waste heat generated by this approach, Class A only appeals to esoteric hi-fi designers, where lack of efficiency or price is no object.
Class B topology uses two variable output stages, one to source current and the other to sink current.
Pros: This topology overcomes the poor efficiency of pure Class A designs, only delivering power as needed. Transformer and heatsinks can be sized to match typical demands of music being reproduced.
Cons: Both output stages turn completely off, then on again, during each cycle of a waveform. Time delay and low-level non-linearities cause severe distortion, called "crossover distortion," during transition from source to sink output stages. This type of distortion is worst at low output levels. Pure Class B is only used in the lowest-cost, lowest-fidelity designs.
Class A/B topology, as you may have guessed, is a combination of Class A and Class B. Using two variable output stages like Class B but keeping them from ever completely turning off, you get near Class B efficiency with near Class A's low-distortion performance.
Class C topology combines active devices with resonant magnetic components for high efficiency at radio frequencies. This topology is not used in audio-frequency designs.
Class D topology uses source and sink output stages that consist of full-on or full-off switches. These output stages toggle from full sink to full source at a rate significantly higher than the highest audio frequency to be reproduced. The ratio of time sinking to time sourcing controls the audio output, with a 50% ratio delivering zero output.
Pros: Class D offers significantly higher efficiency than even Class B, which at 1/3 power is wasting more power inside the amplifier than it delivers to the load. Losses in Class D designs are limited to turn-on time of the switching devices and resistive losses in these devices and output filtering.
Cons: Class D amps require more complex circuit designs with extensive shielding and filtering.
Class G & Class H topologies are variations on Class B that use multiple source and sink output stages. Low-level signals are handled by one pair of output stages, while higher-level signals are handled by other pairs. Each pair is optimized for the power range it delivers.
Pros: More efficient amplifiers can deliver the same output power with smaller transformers and less heat sink.
Cons: Circuit complexity increases, which adds cost. Switching distortion similar to Class B's crossover distortion occurs at each output level transition.
Bridge Mode takes advantage of the fact that speaker loads can be driven differentially. Using separate amplifiers to drive both the positive and negative speaker terminals with opposite-polarity waveforms yields an effective doubling of the voltage swing for 4 times the power. It could be argued that this isn't actually a topology, as each amplifier can be any of the previously mentioned topologies; however, it warrants discussion.
Pros: Provides high power levels using lower-voltage components.
Cons: Increased circuit cost/complexity and inability to ground reference either speaker lead.
Transformers are often used in the output circuit of audio amplifiers to match the "real world load" to an impedance or voltage swing that is more comfortable for the amplifier. Step-down transformers are used with vacuum-tube amplifiers to match the large voltage swing and high output impedance of tube circuits down to speaker levels. Step up transformers are used to generate 70V and 100V constant voltage outputs used in the fixed sound/background music industry.
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.
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.
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.
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.
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.
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!
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!
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.
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.
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?