Introduction: I've always had an interest in the McIntosh MC-250 ever since I learned it used a transformer to couple the output transistors to the speakers. This seems to be one of the traits that make tube amplification great, so when I had a chance to buy a 250, I went for it. My McIntosh MC-250 was bought at a local record store for a bargain price of $325. The chrome and overall condition is excellent with only a few scratches on the chrome and autoformers. Looking inside mine, it appears that all components are original except for the power supply diodes. It has a big muscular look which works with my three tenets in life - sex, power and chrome (ha!)
There certainly is a tube mindset, not only in the schematic, but in the physical execution. Rumor is this amplifier was voiced to sound just the venerable MC-240. The 250 looks much like a MC-240 chassis with solid-state guts and the power devices mounted on top.
After my purchase, I initially hooked this little beast up to my UREI 813A speakers which are brutally honest in their presentation. To tell you the truth, I didn't expect much from this amp except for the classic grainy solid-state sound from the days of yore, butI was suprised by the high quality sound coming out. Well, the autoformers seem to make a difference - compared to my tube amplifiers, the treble seems slightly rolled off but the midrange is very warm and forgiving. Bass control is excellent and this certainly sounds more powerful that its rated 50Ws. This amp is heavily biased into Class B and can produce big peak power when needed. Soundstaging and depth is definitely a little flatter compared to my tube amps, but the McIntosh 250 excels in rock music with big dynamics.
History: The McIntosh MC-250 (metered version is the 2505) was McIntosh's first foray into the field of solid-state amplification. They certainly held off for awhile since the amp did not come into production until 1967. Most of the hi-fi companies about that time had left vacuum tubes forever. It could be argued that this sudden shift of technology led to the downfall of many of these companies. Early solid-state technology was tenuous at best and the first generation amplifiers were not known for their reliability. McIntosh wanted to get it right the first time. Of course McIntosh is a company that continues to follow the beat of their own drum. The MC-250 has proven to be a reliable beast, having survived the tortures of time with its own unique circuitry - notably the Autoformer and the Sentry Monitor which continue to be used in their top-of-line models.
The autoformer is controversial, here is how Roger Russell explains it -
Transistor power output circuits can match 8-ohm loads directly. This eliminates the need for the output transformer for most manufacturers. However, output stages that are designed to operate into an optimum load of 8 ohms can double or quadruple heat dissipation when operating into 4 or 2 ohm loads. At some frequencies, speakers rated at 8 ohms can dip as low as 4 ohms. Some 4-ohm systems can dip even lower. This mismatch can cause the amplifier to exceed its thermal dissipation limits.Conclusion: So if you're looking for some cheap and reliable amplification, check out the McIntosh 250 or its big brother the 2100. The metered versions (2505 and 2105) look prettier, but my manly (ha!) instincts prefer the industrial look. A perfect piece for any mancave or swinging bachelor pad. This amp now sits on my living room bookcase providing hours of daily musical enjoyment.
On the other hand, if an amplifier is designed for an optimum load of one or two ohms, a low impedance load would be no problem. However, less power would be available for a speaker having 4 or 8 ohms impedance.
The unique McIntosh output autoformer was the answer. Since McIntosh output stages were connected in a single ended push-pull circuit, one side of the output was always connected to ground. They were typically designed to work into an optimum load of 2.1 ohms. The matching autoformer was connected directly to the output. In the MC2505 amplifier, the matching output was for 4, 8 and 16 ohms. Other impedances became available in later amplifiers. Full continuous amplifier power could be delivered to each of these loads. There was no danger of exceeding safe limits or overheating.
The autoformer also protected the speakers from damage in the event of amplifier failure. Should a direct current component appear at the amplifier output, it was 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.
McIntosh autoformers continued to be used in the "top-of-the-line" amplifiers. They were all designed and manufactured by McIntosh. Although the autoformers added extra cost, weight and took up extra space, they assured a safe, optimum match to a variety of speakers and speaker hook-ups. They were constructed and performed in the McIntosh tradition of excellence.
Although the autoformer provided an efficient match between the power transistor output and a variety of load impedances, a short circuit at the amplifier output or a load that was much lower than the selected autoformer tap could cause excessive current to flow in the output transistors. To complement the new transistor amplifiers, the McIntosh Sentry Monitor circuit was developed which prevented destructive current levels from occurring under any conditions. This circuit sensed the dynamic operating time, voltage and current of each amplifier output stage and controlled the current flow, confining it to non-destructive limits. The arrangement assured complete circuit reliability for all load conditions. The Sentry Monitor did not limit the rated power output available from the amplifier in any way. McIntosh power amplifiers continue to use the Sentry Monitor circuitry.
McIntosh is an American icon and they continue to make their amplifiers at their Binghamton, New York plant. The new ones are bloody expensive, but you can own one of their older amplifiers for a decent price. Do your ears a favor and check 'em out.