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BA312+

Single Opamp

I don’t see much Sansui, but I was working on a Sansui 881 a couple months ago and wanted a BA312. I didn’t have one, but I found the original data sheet and thought, boy that’s simple. Initially, I thought about using an opamp, but the BA312 is NOT an opamp. Yes, it has lots of gain, a high impedance + input, but the − input is a low impedance point. The BA312 is an all N-channel design for single sided power supplies as high as 42 volts.

I googled low noise transistors and found a website in Europe that advocated several ZTX series transistors and chose the ZTX853 because it was available. I laid out the board and ordered the parts. I also ordered an original F-2617 microphone pre-amp board used in the Sansui8080/9090DB including an original BA312.

This microphone pre has too much gain so I skipped the first transistor stage. So testing only the BA312 in the Sansui 40dB gain design, the performance was surprisingly good for something this old and simple.

Then I dropped in my BA321+ board, and the performance is virtually identical.

The footprint is the same but a tad taller than a cap.

So the ZTX part won’t be available. How about the KSC1845?

I drew up a new PCB because the base is not in the center like the ZTX part. We built 10 and tested them. LOW NOISE WINNER! The KSC BA312+ is better (82uV) than an original BA312 (93uV) and almost as quiet as the ZTX version (79uV).

Distortion is just 0.0070% at 2 volts output and almost entirely benign second harmonic.

Amazing, the input referenced noise for -121dBV is 0.89uV.