McIntosh C-24
Overview
The C-24 is the preamp where McIntosh stopped warming up its line stage with tubes and went solid-state. When it appeared alongside the C-22 in McIntosh's own advertising of the mid-1960s, the copy drew the line plainly: the C-22 was the tube stereo preamp, the C-24 was the solid-state one. For a company whose identity was bound up in glass bottles and output transformers, putting a transistorized control center at the top of the catalog was a real bet.
The feature set was complete for the period. The C-24 carries two phono inputs plus a tape head input, feeding an equalization module that McIntosh built as a discrete plug-in assembly rather than a scattering of parts on the main board. Input sensitivity for the phono and tape head stages is 2 millivolts for 2.5 volts of output at 1 kHz, a conventional figure for a magnetic cartridge of the day. The line-level inputs (aux, tape, tuner) want 0.20 volts for that same 2.5 volts out. That is generous sensitivity, and it lets a tuner or tape deck loaf along well below clipping.
The measured numbers are where the solid-state argument gets made. Frequency response is spec'd at +0 to -0.5 dB from 20 Hz to 20,000 Hz, and distortion is held to less than 0.1% across that same band at 2.5 volts. A dealer would have put those numbers in front of a skeptical tube loyalist. Hum and noise sit 70 dB below a 10-millivolt input on the phono and tape head stages, and 75 dB below rated output on the line inputs. That is quiet enough that the noise floor of a good turntable would be the limiting factor, not the preamp.
The output stage is more interesting than the spec sheet first suggests. The main output delivers 2.5 volts with rated input, but McIntosh notes that up to 10 volts can be developed without distortion. That headroom helps if you're driving long interconnects or a power amp with unusually low input sensitivity. The tape output is a separate 0.20 volts with rated input, and a 10-millivolt phono signal produces 1.2 volts there, so recording levels are usable without an outboard step-up. There's also a center channel output at 2.5 volts with both channels driven, with its own level control spanning -6 dB to +6 dB relative to the main output. Three-channel playback was still a live conversation when this preamp was designed.
The tone and filtering controls are anything but token. Bass and treble each offer +18 dB of boost, at 20 Hz and 20 kHz respectively, and the C-24 includes switchable low- and high-frequency filters - roll-off below 50 Hz and above 5,000 Hz, or flat. The front panel carries a compensation switch, an input selector, tape monitor, mode selector, and separate LF and HF switches, plus speaker and phase switches. That's a lot of switching, and it's all mechanical. The parts list shows switch assemblies shared across several functions, with the input selector on its own part number. The volume and balance functions are handled by ganged potentiometers. The tone controls use separate bass and treble pots, available in both flat-shaft and knurled-shaft versions, which matters to anyone hunting for a replacement today.
Inside, the C-24 is a discrete transistor design. The parts list enumerates silicon NPN transistors across the audio path, with a pair of silicon rectifiers and zener diodes in the supply. The tone control and equalization stages are built as McIntosh's own modules, and the power transformer has a dedicated part number. A power supply delay circuit is called out in the electrical specifications. McIntosh used it to keep the output muted until the rails settle, which spares both the speakers and the listener a thump at switch-on. The main circuit board carries its own part number, and the service documentation tracks running changes by serial number. Capacitor values, resistor values, and even the presence or absence of specific parts shift across the production run, with early units differing from later ones in the equalization and tone-control networks. Anyone servicing a C-24 needs the serial number before ordering parts, because "C-24" alone doesn't say which revision is on the bench.
Press coverage was less a review than an ad campaign. McIntosh's mid-1960s advertising ran the same checklist of components month after month in the audio magazines, and the C-24 appears on it as the solid-state preamp option next to the C-22. High Fidelity (April 1965, p.35) and Audio (March 1965, p.47) both carried the same literature-request coupon listing the C-24 among the company's tuners, power amps, and integrateds. The pitch in those pages was durability rather than novelty. The copy in HiFi-Stereo (March 1965, p.3) made the case that all tuners, amplifiers, and preamplifiers sound good when new, and only McIntosh units keep sounding good year after year.
For collectors, the C-24 is the first solid-state McIntosh preamp, and that's a defensible reason to own one. The practical concerns are the usual ones for a sixty-year-old transistor unit. The delay circuit, the electrolytics, and the mechanical switches are the wear items, and the serial-number-dependent parts variations mean a service manual alone won't get you all the way there. Potentiometers are the real headache. The volume and balance controls are ganged assemblies on specific part numbers, and the tone pots came in two shaft styles, so a clean unit with intact controls is worth more than a cosmetically perfect one with a scratchy volume pot. Buyers should expect to recap, and should check which revision they're looking at before assuming any published value applies.
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