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Phase Accurate, Audio Amplifiers

Amplifier pic I have developed an experimental high quality, precision audio hi-fi amplifier designed with a completely different set of criteria from conventional commercial hi-fi amps. As most naive hi-fi buyers compare harmonic distortion specs between amplifiers and influence their choice on little else, manufacturers compete by using large amounts of negative feedback to reduce their total harmonic distortion (THD) to absurdly low levels, typically less than 0,05%, sacrificing transient response. This is actually ridiculous, as the ear finds it impossible to hear harmonic distortion below about 5%, and in fact creates significant (and measurable) internal harmonic distortion of its own. Even excellent loudspeakers produce 1 to 5% THD at low frequencies! The result of all this excessive negative feedback is that when a transient signal with a short rise-time is applied to the amplifier, the input stage(s) overload for a brief period of time, until the transient arrives at the output and the correction signal is fed back to the input. For a simple transient, such as a step function, the result is merely a slowing down of the step at the output. If, however, the input consists of a continuous tone, plus a transient, then the momentary overload will cause a loss of the continuous tone during the overload period and the creation of instantaneous dynamic intermodulation products. These specs are never specified in commercial hi-fi amplifiers, as they are difficult to measure, usually very poor, and sound terrible. . Some designers now hold the view that in current amplifier designs, harmonic and intermodulation distortion levels are so low that transient effects are the main cause of audible differences between designs, and the area in which the most improvement can be made.


If an audio signal is to pass through a linear system without distortion due to phase effects, then the phase response (i.e. the difference between the phase of output and input) must be proportional to frequency. This simply means that all components in a complex wave form must be delayed by the same amount of time. If phase distortion is present, then different components of the wave form are delayed by differing times, and the result is to change the shape of the wave form both for complex tones and for transients. This causes a loss of accuracy in depth perception in the reproduced sound, making it impossible to place the source of an instrument in three dimensions.


Why is accurate phase performance important? Have you ever tried to tape record a conservation in a room full of talking people? While you are there, you can hear every word, but when you play the recording back there is so much background noise and other conversations you can't make out a thing. What happened is that your 2 ears heard the persons conversation with all the phase delay information perfectly, were able to position them and the sound source in space and disregard the others. The tape recorder captured them all without preserving the phase relationships and thus the direction and distance of the sound sources.


Our amplifier takes this into account and using a different approach, produces a spotlessly clean sound with remarkable detail. As a result of this attention to phase control, dc coupling and bandwidth, a listener is able to perceive depth in front of, and behind the speaker system, as apposed to conventional left-right stereo. Two leading hi-fi dealers have remarked that the amplifier sounds at least as good as the best "top of the line" models costing tens of thousands of Rands. One of the amplifier's most remarkable features is the way it looks, but it's more than a pretty face. If you would like to hear one, contact Graham at (cell) +27 (0) 82-419-1334 or email me right now. The colours of the casing are anodized, and you can select one or more to match your decor.


The big surprise is the price, (which varies with exchange rate and features of course), but is around US$1500.
The specifications below show only the levels of distortion we have been able to measure on an HP Spectrum Analyser with a dynamic range of 60dB (0.1%) The TID products are unmeasurable on this instrument and so they are shown here as <0.1% until we can make lower level measurements. When comparing other amplifiers, remember very low THD claims may point to shocking (and usually unquoted) transient performance.

Amplifier Specifications:
Power Output (<1% THD)40W(RMS)
Max Power Output (1% THD)40W(RMS)
3dB Freq Response12 to 35KHz
Harmonic Distortion< 0.3%
Static Intermodulation Distortion< 0.1%
Transient Intermodulation Distortion< 0.1%
Noise Factor> 80dB
Input Impedance47kOhms
Output Load8Ohms
10º Phase Response10 to 25KHz


These measured specifications are very conservative, and if anything, are probably better than quoted. I have been listening to this amplifier for over 10 years and it never ceases to impress me with it's clean and crisp sound.