Monday, August 8, 2016

McIntosh C26 Pre-Amplifier

McIntosh C26 Pre-Amplifier 

Here we have a McIntosh C26 Pre-Amplifier that can in for a rebuild from a customer.  The C26 was sold from 1968 to 1978. The first preamplifier to feature a glass plate to match the MC2502 amplifier. The C26 featured two unique controls at the time for speaker phasing, helping to correct placement and center channel marked at -6dB/VS LR, the C26 also featured speaker outputs and a combined volume/switch potentiometer with integrated AC shielding at the contacts, a fairly revolutionary design in attenuators. To finalize a quality potted transformer was mounted to a chromed out chassis.

Power Supply - Centre CH. Assembly
The simplistic power supply is regulated by the 75V zener D303 to the NPN Q302 2N3738 transistor and the 75V filtering stage. The diode smoothing capacitors were replaced with Vishay PP type .01 and C304/305 multipack restored @ 220/220MFD 200V ratings. On the PCB mount the 100MFd electrolytic was replaced with a low impedance high temp (105C) PW Nichicon and the smoothing films to quality MKP and WIMA high grade film capacitors.

The centre channel assembly for specially recorded albums and additional output, which was set a -6db less than l/R output had the .47/.22 coupling films upgraded to a high grade MKP and WIMA film capacitors.  The Brass attenuator and phase controls were cleaned and lubricated to protect the phenolic.

AF Stage Assemblies

The C26 af stages were populated on two symmetric PCB’s on top of the chassis. All of the films were removed and upgraded to a high grade MKP and WIMA polypropylene film capacitors along with the tantalum and electrolytics to a audio grade Elna Silmic II capacitors.  Initially small signal driver pairs were updated with NOS BJT’s I located, with the batch I was able to match within about 5-7% at best.



Audio Notizen

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