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In the December 1949 issue of US "AUDIO" magazine, Frank H. McIntosh and Gordon J. Gow, of the McIntosh Engineering Laboratory Inc. published a paper detailing their new concept in audio amplifier engineering.

The design shown had just been granted US Patent 2,477,074 on July 26 1949 in the name of Frank McIntosh.

Thus the conditions were right to reveal to the world their new and very advanced concepts for electronic circuitry and transformer engineering design.

The original 50W-1 (c 1949) with interstage driver transformer is my favourite - a true work of audiophilic engineering.

Here's How It Works - Courtesy Radiotron Designers Handbook (Wireless Press AWV) 4th Edition 1953

Commentary by F. Langford-Smith B. Sc. (1st Class Honours) Senior Member I.R.E. (USA) A.M.I.E. (Aust)

McIntosh further developed the unity coupled amplifier and in July 1949 applied for a second patent. In 1953 he was granted US patent 2646467

Model 50W1 Power Amplifier Schematic: Note the simplicity of the interstage transformer coupled design.

50W-1 Schematic.jpg



An extensive coverage of the McIntosh experience is presented at:

Roger Russell's McIntosh History

Berner's McIntosh Information Site  (select "compendium" tab for detailed information on specific models)

Peter Millett's McIntosh Technical Details and Service Information



One of the essential features of the McIntosh design is its specially constructed output transformer, as described above by McIntosh and Gow.

In 1956, not long after the original McIntosh design was released publically, Hugh Lockhart acting under the guidance of Professor Bereskin, developed an alternative design that used a trifilar output transformer instead of the more complex and costly McIntosh design.

At that point in time the McIntosh amplifier used a driver transformer as shown in the above schematic. Later models eliminated this device.

The Lockhart version uses a driver transformer also.

A key feature of the Lockhart design is the use of reduced screen-grid voltages to the 6L6 output tubes,,jintended to improve reliability and enhance tube life.

For details see http://www.tubebooks.org/Books/lockhart.pdf

The Lockhart-Bereskin design paper is reproduced here for convenience.

Some loss of performance is reported however it is not clear if this is due to the transformer characteristics or to the reduced screen-grid operating voltage and/or other modifications.

However, modern DIY builders can use 6L6GC tubes instead, eliminating the need to apply reduced screen-grid voltage in either the McIntosh or Lockhart designs. In the case of the Lockhart amplifier, operating the 6L6 plates and screen-grids at the same potential simplifies the power supply dramatically.

It will be clear from study of the MC-3500 design below that the MC-3500 and Lockhart designs have some common aspects in relation to output stage operation. Nonetheless, the MC-3500 offers superior performance at rated power output levels


The original 1968 McIntosh MI-350 and MC-3500 HIGH POWER HIGH-FIDELITY
select and click to view




The completely redesigned 2021 model MC-3500 MKII HIGH POWER HIGH-FIDELITY TUBE AUDIO AMPLIFIER       select and click to view




The mighty MC-3500 hi-fi audio amplifier and its industrial version the MI-350, were designed by Mile Nestorovic (pronounced Meela Nestorovich) in about 1968 while he worked with the McIntosh organisation.

This single contribution to the world must be recorded as one of the great events of all time in the art and science of audio engineering - not so much because the product excels in every respect in its own right, but for the hidden gems of innovation and evolution buried within its design and construction.

Design patents and copyrights at the time ensured the world could not learn the detail of this fine work, or its underlying design philosophies, so it has taken around 40 years for us to unravel the finer points of his design.

My understanding from correspondents is that the output transformers are penta-filar wound in the manner of the original McIntosh amplifiers for maximum coupling efficiency (and therefore transient response) and minimum leakage inductance, so are not conventional - thereby illustrating just how excellent an amplifier can be when designers strive for the ultimate.

Several years ago, Mile suffered a massive stroke, which sadly did not kill him, but instead left him a prisoner in his own body, unable to care for himself, unable to communicate save by blinking his eyes, but cognizant of his circumstance and his surroundings.

Mile needed intensive nursing home care during those years since, but although his family gave him all of the support they could, love and care were not enough to change his condition.

Finally his body gave up the fight and Mile passed to higher life on 1 May 2009.

May his memory live on through his great technical achievements.

Vale Mile Nestorovic.

For the record, Mile was born and educated in Yugoslavia. He was Christened "Mioljub" but he chose to use "Mile" after emigrating to the USA.

If anyone has further relevant information on Mile Nestorovic, please email to me at diyaudio@oestex.com





Copyright and intellectual property in these audio amplifier designs and circuits remains with their original owner.

Their inclusion in this page as reference materiel is not a license to reproduce or use them for any purpose contrary
to the terms of any original copyright notice or license.

These reproductions are presented for historical information and education purposes only and are not intended for construction.

No warranty is given of any kind as to fitness for purpose or performance or rating.

Their source is not acknowledged here because I am unable to determine their origin.

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This page was last modified 08 December 2022

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