McIntosh MC2505 – The Solid State Amplifier That Change the World

As any vinyl enthusiast will agree, the pure audio enjoyment of this hobby will only go as far as the limitations of your system.  While everyone has varying opinions on what is the most integral link in the audio chain; cartridge , phono preamp, speakers, one thing that cannot be denied is the importance of your amplifier.  While a great deal of vinyl/audiophile are fans of the true-analog warmth that is generated by tube amplifiers, many of us are resigned to a world of solid-state amplification.  Whether you are tubbed or not, one thing is certain, and that is the high-quality build and analog compliance of vintage gear.  When it comes to vintage solid-state amplifiers there are few than come close to the greatness of the early McIntosh amps.

McIntosh MC-2505 Solid State Amplifier

[wordbay](McIntosh,Amplifier,MC2505)[/wordbay]

The History of McIntosh

In the mid 1940’s Frank McIntosh headed a small firm specializing in the design and development of radio and broadcast stations. Being on the forefront of this burgeoning industry he saw the many shortcomings of the professional equipment of the day. Most notably was the need for high-powered, low distortion amplifier. Hiring engineer Gordon Gow as his assistant the duo researched and developed a landmark amplifier. The amplifier was so far superior in output and distortion specs it was at first met with disbelief. The audio test equipment of the day was not designed to test an amp of such exceeding performance characteristics. But the proof was in its performance. Word of mouth spread and demand grew thus Frank McIntosh & Gordon Gow had new day jobs and McIntosh Laboratory was born. Hindsight confirms the company introduced a new era of professional audio excellence and they were amongst the first to market professional equipment to the consumer. Thus nurturing the high-fidelity industry and spearheading it for years to come.

For more than six decades McIntosh has maintained dominance in the high-end audio industry. They are the only surviving “foundational” high fidelity equipment manufacturer around today, still headquartered in Binghamton, New York, with operation and manufacturing carried out in the same method of the originators. McIntosh’s permanence in the marketplace is owed to its history of phenomenal components, all adhering to the same design criterion set forth by Frank McIntosh and Gordon Gow; superior sonic performance and unmatched reliability. Every component was created with capacity so great that it will never be operated at or near its stress point. A good reason why an estimated 80% of the McIntosh components produced since 1949 are still in use today and what’s more amazing is these same components are outperforming modern counterparts with the benefit of years of technological progression. When you buy a McIntosh you’re buying a lifetime component!

The Emergence of the 2505

When introduced in 1967 the McIntosh MC-2505 was a revolutionary amp in many respects and served as the blueprint for all future McIntosh amps to come. Not only was it McIntosh’s first solid state amp ever, it was also their first to have a full glass panel faceplate, precision calibrated output level meters, the panloc mounting system, and it was the 1st sold state amp to incorporate McIntosh’s famed Unity Coupled Autoformer for full rated output at 4,8, or 16 Ohm impedances. Its meter layout and chassis design also influenced the look of the emerging solid state amplifier in general, as other equipment manufacturers were quick to try and follow the leader. But most importantly the most defining aspect of the MC2505 is that it was one of the very first solid state amplifiers to actually sound good, and it easily set a new standard of performance excellence in its era.

 

 

Today the MC2505 is still loved for the same reasons it was 45 years ago. It has a timeless sonic signature that make listening to music so easy to do. From top to bottom the amp has a free flowing smoothness that is in contrast to the harshness of so many solid state amps of its era. Its sound is liquid with a lot of punch and solidity at the bass end and very defined mid and high frequencies. Some of the many features and functions of the MC2505 include;

Three Separate Power Supply Sections
One positive and one negative high current supply is used for the output stages. The other positive supply is used for the driving amplifier stages. All supplies are full wave and use silicon rectifiers. Adequate filtering is used to assure an absolute minimum of hum. The power output stage filter capacitors have very high capacity, which allows full power output below 20Hz. The power transformer is very generous in size and runs cool, even under heavy use.

Two Stage Amplifier & Two Stage Preamplifier Sections
each individual amp/preamp stage is efficiently overbuilt in order to provide tremendous power with the very least amount of distortion. The build quality is so great that it will never reach high stress levels, thus lasting indefinitely

Push-Pull Output Section
The output section is arranged as a series push-pull amplifier. The power transistors used in the output section are hand selected for their high power dissipation capability, wide frequency response, and large “safe operating area”

Oversized Anodized Heat Sinks
The power transistors are mounted on the large heat sinks to insure they will continue to operate at low temperatures

McIntosh Autoformer
Unlike any other amplifier, the Autoformer allows the 2505 to deliver full power at all speaker impedances, you are not penalized for your choice of speakers. There is no danger of exceeding safe limits or overheating. Another advantage of the Autoformer is the 25 volt output for constant voltage distribution system. With the 2505 several sets of speakers can be operate independently throughout your home. The Autoformer also protects the speakers from damage in the event of amplifier failure. Should a direct current component appear at the amplifier output, it is shunted by the low DC resistance of the Autoformer, instead of passing through the speaker voice coil, which could damage the speaker or even cause a fire.

Sentry Monitoring Circuit
Prevents failure of the power output transistors due to excessive mismatch of the output. When the 2505 operates normally the Sentry Monitoring Circuit has no effect on the signal passing through the power amplifier. If the power dissipation should rise above normal operation, the Sentry Monitoring Circuit restricts the drive to the output transistors. The Sentry Monitoring Circuit acts instantaneously for any input signal or load combination. This arrangement assures complete circuit reliability

Temperature Sensing Circuit
If amp temperature increases due to a shorted speaker, or restricted ventilation, an automatic temperature sensing device turns off the 2505. The device operates automatically at a preset temperature. The 2505 will turn on again when the temperature returns to normal

Dual Power Level Meters
for each channel a highly accurate peak meter is provided. Because of unique circuitry these meters are able to provided instantaneous and precise changes in musical program with an accuracy rate of over 98%. Additionally a four position (off, -20, -10, 0) meter range switch is provided to correspond with gain level.

SPECIFICATIONS

Power Output     Stereo: 50 RMS watts continuous per channel into 4, 8, or 16 ohms both channels operating
Harmonic Distortion     Stereo: Less than 0.25% at 50 watts output from 20 Hz to 20 kHz both channels operating. Typical performance is less than 0.1% at rated power. Distortion decreases as output power is reduced
Intermodulation Distortion     Stereo: Less than 0.25% if instantaneous peak power output is 100 watts or less per channel with both channels operating for any combination of frequencies 20 Hz to 20 kHz.
Frequency Range     20 Hz to 29kHz +0, -0.1 dB at rated power.
15 Hz to 60 kHz +0, -0.5 dB at rated power.
10 Hz to 100 kHz +0, -3.0 dB at one-half of rated power.
Hum & Noise     90 dB or more below rated output.
Output Impedance     Stereo: 4, 8, and 16 ohms
Input Impedance     200,000 ohms
Input Sensitivity     0.5 volt. Level control provided for higher input voltage
Power Requirements     117 volts AC 50/60 Hz, 75 watts at zero signal output. 250 watts at rated output.
Semiconductor Complement     26 silicon transistors. 27 silicon rectifiers and diodes
Dimensions     16″ W x 5 5/16″ H x 13 1/2″ D
Weight     38.5 lbs