Optonica SM-4100

The Optonica SM-4100 is an Integrated Stereo Amplifier, made by the Sharp Corporation of Japan in the 1980s. It is a high-end vintage amplifier made to high specifications. You will most certainly find some of these amplifiers in civilised parts of Europe; however, the list will be short.

The power output uses the Sanyo STK465 chip capable of providing a maximum of 30-watts per channel; however, a respectable 25-watts from an 8 Ω load should be more realistic from this vintage old timer. This amplifier chip uses Sanyo’s IMST technology, which not only produces high quality audio amplification, but also provides power efficiency. These chips were very popular in their time and used in many amplifier designs. Although the STK465 does not have a modern equivalent, many audio enthusiasts manage to keep their vintage equipment operating with recycled parts.

This amplifier has an interesting design in that all the stages appear to be direct coupled. The final stage is semi-complementary, and the final output is (OCL) capacitor-less.

The input section of this amplifier uses the LF353N Dual Operational Amplifier chip in a PDIP package. This chip has wide bandwidth dual-junction FET inputs with high impedance, and therefore the input section to this amplifier is capacitor-less too. This means that the sound quality will be at its highest possible, and this type of circuit is rare to find these days.

The AN6551F is a dual operational amplifier IC with a built-in phase compensator. This chip is used in preamplifier circuits to create active filters for bass and treble. It can also be used to make a phono signal preamplifier. This is one of those wonderful inventions by the Panasonic Corporation of Japan, which will be difficult to find today. After hearing the sound from it, I figured that my ideal amplifier, that I am designing, would use one of these.

The bass and treble filter circuits also include some discrete components, utilising Mylar capacitors. I approximate the boost and cut to be around ±10 dB, simply by listening to it, however back when I was a young whippersnapper, I could detect ±0.2 dB variation. :-).

This amplifier also has stereo phono inputs for a record player and it sounds amazing when connected to my turntable. The RIAA curve deviation appears to be around ±0.4 dB :-). With just a cursory glance at the PCB, I detected 3-ICs, 13-transistors, and around 10-diodes.

This Article Continues...