Not exactly ‘hifi’, not even stereo, but still a true Sansui, by sound, by buildquality, by everything. A rarity in my country, because in the late 1960-ies, a Sansui would not have been the obvious choice for a 100 Volt PA amplifier to drive a few dozen ceiling mount speakers in a small supermarket.
The owner of this one has a small recording studio and asked me to convert Mic 1 and Mic 2 inputs to not only accept a balanced microphone signal, but also to have phantom power for condensor microphones. I fitted two new 2:1 audio transformers to replace the original 1:10 ones and agreed to sacrifice the RCA connector for the phono input on the back panel to fit the on-off switch for the phantom power. The line amp turned out to be very noisy, so I had no choice but to replace all the small transistors and some of the composite carbon resistors, mainly on the input side, where they contribute most to the noise. The power amplifier has two transformers in the signal path, one interstage transformer to drive the two power transistors, and an output transformer, to allow for various output impedances, from 4 Ohms to 125 Ohms, to drive 100 Volt speakers. I chose not to replace any of the original lightbulbs, since they were all working fine. Once they start to fail, they will be replaced by LED. For now, I kept as much of the original amp as possible.
Today the owner let me know that the sound of the mic preamps rivals that of many ‘high-end’ preamps for musicality and clarity. I converted the inputs on the back panel to be direct outputs for the four preamps, but it can still be used as a mixer amp with a summing output via the line stage. Essentially the mic preamps consist of only two transistors. Proof once more that less is more.
The microphone preamplifiers; only two transistors per channel, the metal can Hitachi on the left is original, the 2SC2240 is the only new component. Deliberately chose to not replace the capacitors, to keep the amplifier and its ‘sweet Sansui sound’ as much intact as possible.
The two new microphone balancing transformers.
The PA100 has both a signal level meter that monitors the preamp output level as well as a small monitor speaker with a seperate volume control to monitor the speaker output.
Simple yet elegant solution; a small metal lip, attached to the volume control rod, connected to the potentiometer rotates away from the lightbulb (in the black sleeves) thus illuminating an amber bead in the front panel, indicating the being active of that channel.
