Dynaco SCA50 Integrated Amplifier

About the SCA50

The SCA50 Integrated amplifier was rated at 25 Watts per channel. It was a rather more modern little brother of the SCA80. Rather than discrete transistors, the preamp section was based on the now obsolete TBA231 Dual Opamp. It was an externally compensated dual-opamp in a 14 pin DIP package, rather similar to the UA739 opamp.

The power amp section used a complementary symmetry output stage. The driver set up is modern-ish, but uses a bootstrap cap rather than a current source in the voltage amplifier stage. There is also no current mirror in the first stage, and no degeneration resistors in the input pair. So, one could do a lot better.

Here's a link to the assembly and service manual.

Tom P was kind enough to send me a scan of an advertising flyer for the SCA50:

• SCA50 brochure, front
• SCA50 brochure, back

A Few Random Technical Facts

• The original power supply caps were just a touch over 35 mm in diameter. 35 mm diameter is the closest modern size. You might have to wrap a few turns of electrical tape around the bottom of a new, modern capacitor that is 35 mm in diameter, to make it fit snugly into the cap. Although the manual shows a screw-terminal cap, my SCA-50 has snap-in caps.
• The original value was 4700 uF or 5000 uF at 35V. This reflected the change over in capacitor values that was taking place about that time. New capacitors could have higher voltage rating, and a bit more capacitance, perhaps in the range from 6800 uF up to 10,000 uF.

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