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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.


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 McIntosh MI-350 and MC-3500

Of the numerous tube audio amplifiers produced by McIntosh over many years, the mighty McIntosh MC3500, or its industrial version the McIntosh MI-350, designed by Mile Nestorovic in about 1968, is unsurpassable - in my humble opinion, the greatest commercial tube amp ever made.

This amp is derived from the 1949 original McIntosh unity coupled design shown above and was produced by McIntosh between 1968 and 1971.

The MC-3500 and MI-350 offer beauty in style and finish, functionality, reliability and performance.

A blend of art and science.

The MC-3500 and MI-350 amplifiers were produced by McIntosh between 1968 and 1971.

This was the golden age of tube audio - at the pinnacle of its commercial development. However the world changed very quickly soon after with the introduction of solid state amplifiers. Their low cost, comparable reliability and lower weight soon displaced high powered tube amps. The result was that although the McIntosh patent was granted in 1971 its commercial purpose was limited by evolution in a competing technology.

The combination of patent protection and solid state power transistor developments meant commercial interest in this technology lapsed.

Nonetheless, the MI-350 and MC-3500 stand for all time as monuments to the technological possibilities and listener rewards of high-fidelity vacuum tube audio.

Tragically, due to competitive forces and technological development, a comparative solid state professional quality amplifier having identical specifications to the MC3500/MI350 can be purchased in 2012 for as low as one quarter of the 1971 price of the Macs. Allowing for inflation over 40 years that is a tremendous cost reduction over time.

When the current 2012 resale value of around USD$12,000.00 - as determined by the market - is considered, then the ratio of used MC 3500/MI350 to brand new solid state amp is something like 100 times. The audiophile market obviously thinks this expenditure is justified.

However notwithstanding published specifications, there is no comparison of sound quality. Solid state amplifiers just do not cut it for audiophile performance and the cost of the MC3500/MI350 is entirely justified by its performance. There is simply nothing like it on this planet.

If you do not believe that claim then prove it for yourself.




350W RMS continuous. 500W RMS Maximum.

Frequency Response: 20-20kHz (+0 -0.5dB).

Total Harmonic Distortion:  0.15%.
Intermodulation Distortion: Unmeasureable - ie behold measuring instrument threshold

Hum and Noise Levels: -95dB.

Output - Rated Load Impedances: 1, 4, 8, 16, 50 and 64 ohms.
Note: Rated Power Output for 1 ohm load is 300W and 4 Ohm load is 320W
Output Load Circuits: Can be isolated from ground - ie "balanced output".

Internal impedance: less than 5% of Rated Load Impedance.
Input impedance: 200k.
Input sensitivity 1.1V RMS

Anodized gold and black panel. (Note: Some after-market replacement panels were supplied in natural aluminium anodised finish).
Input connectors: RCA and BNC (front and rear).
Gain control.
Meter Range switch: off, volts, dB, dB-10 or V1-V8.
Power circuit breaker: on or off.
Output range switch: 1 , 4,  8, 16, 50, and 64 ohms with corresponding voltages 17.3, 35.8, 53.0, 75.0, 132 and 150V.
Dual banana output connectors.
Meter: Monitors output voltage, dB or cathode current for each output tube.
Heavy duty drawer-pull style handles.

Line voltage switch: 117 or 125V.
Output barrier strip.
Input terminals.
Sub sonic filter switch: In or out.
Cooling fan.

8-6LQ6/6JE6B output,
Solid state full-wave bridge rectifier.

DIMENSIONS: 19" Wide rack mounting x 10 1/2" High x 17" Deep behind front panel.

WEIGHT: 125 lbs

400W at zero signal
900W at maximum signal

Sold from 1968-1971.
1971 retail price USD$1099.00



Generally as for the MC-3500 but with the front panel anodised natural (silver) aluminium.

The MI-350 also has an optional plug-in input transformer to support balanced input loads of 50, 250 and 600 ohms - in addition to the high-impedance inputs as per the MC-3500.

My correspondents have not been able to identify any other significant variants, however there were some minor changes made to the on-off switch/circuit breaker setup in the early models.

The parts lists indicate the power and output transformers are the same in both models.

The MI-350 has a variable output impedance selector switch knob on the front panel that enables output loads of 1 - 4 - 8 - 16 - 50 - 64 ohms. (Same as for MC-3500)

A vital element in the design of this amplifier is the 6LQ6/6JE6B "compactron" vacuum tube. This tube has some special features such as novar compactron construction, 30 watt plate dissipation, 200 watt short-time plate dissipation, extra plate cooling fins for cooler operation, 5 watts screen grid dissipation, 2.3A dark heater (the 6JE6C has a 2.5A heater).

As far as I can determine to date, there is no practical difference between the two models.

One view is that the MI-350 was actually produced first for the industrial, recording, public address and specialty applications markets, then the MC-3500 produced as a later variant for the hi-fi market. This might explain its industrial design approach and construction quality.

At the time of its introduction to the market there was nothing else like it.


McIntosh MI-350 Amplifier - Front Panel

(Compare with the pics below)


McIntosh MC-3500 Amplifier - Front Panel


McIntosh MC-3500 Amplifier - Internal View with Top Cover removed
Note the chassis layout re the driver stages - the driver tubes are shielded by the electrolytic filter capacitors
The output transformer is on the left-hand side presumably to provide right/left weight balance.

Review: Courtesy of "Stereo Review" Magazine, November 1969 Issue

The advice regarding installing a fuse in the loudspeaker circuit is interesting. If the fuse blew at full output, the owner could expect to face a truly horrendous repair bill. These days, installation of a core-balance circuit breaker into the output circuit and controlling a contactor in the mains supply might be a smarter move.


The unity coupled MI-350 and MC3500 amplifiers were designed by Mioljub R. Nestorovic in about 1968.

Given the ground-breaking electronic circuit advances incorporated in this amplifier, together with his attention to detail, it is likely it may have been on his mind for some time before that.

Those conceptualised thoughts combined with market demand for high power with high quality reproduction created a conjunction of forces that provided commercial opportunity for such a device.

On 17 February 1969, an application to patent the design was lodged with the US Patents Office - Application 799867.

US Patent 3573648 was granted 6 April 1971.

The patent describes the design as a "High Power Audio Amplifier Having Feedback Provided By A Winding Co-filar With An Output Transformer Primary Winding".

This description tells us much about the design paradigm and together with the text of the patent suggests it was conceptualised as an extension of the original Frank McIntosh unity coupled design of 1953 - US Patents 2477074 and 2646467.

But Mile went further, also creating voltage amplifier and driver stages that of themselves were unconventional cutting edge at the time.

The Patent also refers to prior patents of Corderman 1959, Romander 1959 and Wolcott 1963.

For a detailed explanation of how the output stage works refer to the Patent text and drawings.

However it is important to note that these amplifiers are much more than an electronic circuit on paper.

Great attention has been given to component selection, component layout, chssis construction and, of course, the transformers.

Performance can only be described as "awesome".

The end result is an object of beauty and style worthy of a prominent place as the centre-pice of an audiophile hi-fi system.



Rear View:



Cooling Fan:





In the mid to late 60's when this amplifier was designed, the availability of electrolytic capacitors was limited in capacitance range, voltage range, physical size and cost.

Modern capacitors use more advanced materials and different construction techniques, thereby offering more grunt in a smaller package. Compact computer bus grade capacitors offering high capacitance values and higher operating temperatures are now readily available. Both performance and reliability can be simultaneously improved.

Consequently the power supply can be upgraded to deliver more energy to the output stage.

One mod that could save a bundle is to reduce the B+ to 450 VDC, thereby allowing 450 VDCW/500 VDCPeak caps to be installed.

This small change enables the caps to be in parallel instead of series, thereby easily doubling (or more) the available capacitance to the power supply circuit in the space available.The 6LQ6/6JE6B screen-grid voltage should be left as-is. The reduction in continuous power output would be small however dynamic music performance (PMPO) will be increased dramatically.

The only practical limit to capacitor value is the available physical space under the chassis.

The same applies to the voltage amplifying and driver stages, where some styles of modern coupling capacitors offer improved sonic characteristics (to subjective personal taste of course).

Driver stages also benefit form large filter capacitors, which prevent voltage sag during dynamic peak signals.

The 8 uF and 12 uF polarised electrolytic capacitors coupling the output transformer windings may be beneficially replaced with high-quality non-polarised motor start caps.

Also about 100 uF of non-polarised caps installed in parallel with the main filter caps provides non-polarised sonics to the mid-range and highs where harmonics are more audible.

Tread your own path.

MI-350 UPGRADE project by ZED AUDIO

McIntosh MI-350 Amplifier - Front Panel

Here are details of a major upgrade and refurbishment project upon a pair of MI-350 high fidelity tube audio amplifiers performed by Stephen Mantz of the ZED Audio Corporation.

Click to view

MC-3500 Schematic

MI-350 Schematic

Roger Russell's McIntosh History

Dave O'Brien's McIntosh Amplifier Clinic


The mighty MC3500 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 19 August 2012

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