McIntosh C11 Vintage All Tube Preamplifier - Full Restoration

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General:

This McIntosh C11 received a full electrical restoration from our friends and experts over at Audio Classics.

This electrical restoration included a rebuild of the power supply with custom multi-section filter capacitor cans, replacement of every out of spec coupling capacitor, and installation of brand new gold-plated RCA connectors to address some intermittent grounding issues.

Condition is very good considering its age considering it was only made from 1961-1963! You can tell it was very well cared for over the years as there are no signs of rust or corrosion.

It is physically complete with the exception of the rear cover plate for the tubes.

Tube Notes:
This preamp includes an assortment of used vintage Telefunken ribbed plate 12AX7's. Two of the existing tubes were found not to spec in our testing, so we replaced them with ribbed plate Telefunken tubes from our in-house collection.

The tube placement was optimized for best performance on line and phono inputs.

Functional Notes:
These vintage units tend to have slight imbalances in their volume controls. A small compensation of the Right Channel output trimmer brought this unit into the proper range, giving great center image balancing throughout the listening range of the volume control.

Recommended Cables:

Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Phono Interconnects - Better

Kimber Kable - Flagship Tonearm Interconnects - Best

Please see our last photo or the owner's manual below for specifications:

McIntosh C11 Vintage All Tube Stereo Preamplifier 

The SkyFi Testing Process for Preamplifiers:

We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCB’s for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.

When we first power on a preamplifier we connect its RCA output to a Sencore PA81 Power Analyzer which simulates real world loading conditions and gives us an oscilloscope interface. The first order of business is checking that the volume control works smoothly throughout its entire range with acceptable channel balance. This is accomplished by feeding a 1KHz sine wave into one of the preamp’s line level inputs while monitoring the preamp’s output on an oscilloscope. We then switch to a 1KHz square wave to test the tone controls, loudness function, and filters where applicable. During this step we are watching for equal alteration of the test signal by both channels. This also helps us identify dirty controls that will need treatment. Once the basic line stage functions are verified, we test each input individually. This is especially important for devices that use relays to select their sources. If the preamp is equipped with a phono stage we test that as well. We use an inverse RIAA filter which allows us to feed a reference test signal into the phono input with the proper RIAA equalization and level. A square wave or sine sweep is used to verify that the device’s phono stage is faithfully reproducing the RIAA curve. If the preamp under test has balanced inputs and/or outputs these are tested as well.

We finish up our bench testing with a listening test with our bench amplifier and reference speakers. During this test we check for hum or hiss that may not have shown up in earlier testing. We also check that all of the tone controls and filters perform as expected. If the preamplifier has remote control functions these are also tested. Preamps with tube circuits or complicated power supply topologies are connected at our long term test rig for extended stress testing under real world conditions.