Audio DesignLine Blog
Remembering W. Marshall Leach Jr. (and his low-TIM amp)
Rich Pell
12/21/2010 3:04 PM EST
A major figure in the audio field, Dr. William Marshall Leach Jr., passed away recently. Anyone familiar with audio electronics design is sure to be familiar with his work.
A teacher of electrical engineering at Georgia Tech Institute of Technology, Dr. Leach was considered an outstanding professor by his students. He was also an author, and shared his knowledge and passion for audio engineering with a much wider audience through published magazine articles and with a book ("Introduction to Electroacoustics and Audio Amplifier Design").
In fact it was one of his early articles - published in Audio magazine back in the 1970s - that helped inspire me (and countless others I'm sure) to eventually pursue an EE degree. The article - "Build a Low TIM Amplifier" - described an audio amplifier design, complete with parts and board layout, that any DIY-inclined audiophile would find hard to resist.
At the time, transient intermodulation distortion (TIM) - thought to have been responsible for the "harsh" sound of many early transistor amplifiers - had become a major concern among solid-state amplifier designers. Leach's amplifier avoided the problem through, among other things, optimization of open-loop bandwidth and the judicious use of overall negative feedback.
The Leach amp was one of my first DIY audio projects and Professor Leach's article was a great help in learning about audio design. I also learned another lesson as well.
During one of my early listening sessions with the amp I was suddenly startled by a loud gunshot-like "bang!" followed immediately by the sound of a projectile hitting the ceiling of the room. As readers of this blog have no doubt already guessed, the mystery object turned out to be a small power supply rail capacitor that had been accidentally reverse installed. This of course was the fault of yours truly and not the designer of the amp.
Despite this minor incident (the amp was unscathed), the Leach amp (along with another DIY amp project - a single-ended Class-A amp designed by Nelson Pass) went on to serve me well for many years. Its designer will be sorely missed.
Comments, questions or suggestions? Email me at rich.pell@verizon.net.
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kendallcp
12/23/2010 4:59 AM EST
Yes, Leach was truly one of the giants of audio electronics history. The obsession with TIM triggered a new wave of audio design, but we eventually found out that it was just another name for the (predictable) modulation of gain caused by the gradual onset of slew-rate-limiting mechanisms internal to the amplifier. These days there's no excuse for building an audio amplifier that exhibits TIM, even if you want to use the traditional forms of amplifier circuit.
I had the 'gunshot' experience with an early technology lithium thCl backup battery connected up with a reversed charging protection diode. My head was inside the equipment rack at the time! Took ages for my ear to recover...
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Rich Pell
1/31/2011 4:28 PM EST
Here's "more on Leach and amplifier design" from an Audio DesignLine reader (who gave permission to include his name):
Yes I built all the Leach amps and respected his design skills. Interesting to note late in his career Leach took an interest in classic Dynaco tube amps and modeled them in SPICE.
Others to check out are the British engineers John Linsley-Hood and Malcolm Hawksford, USA designers Nelson Pass and Bob Cordell, and the tube guys from Audio Research.
You may wish to investigate Australian Hugh Dean (Aksaonline.com) who is making some interesting progress in tailoring the harmonic profile of power amps to emphasise even harmonics. His stuff is not cultic nonsense, but well-engineered, reliable, and proven by SPICE and measurement.
As we head toward the class-D era, for my money the best amps are still bipolar BJT designs.
Leach did much to further the art.
Julian Driscoll
Manager
DSCAPE
AES M16662
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