Thanks for the dozens of kind notes letting me know you appreciated the mini series on the MCA. I'll work on telling more in depth stories about the development of our products over the past 50 years.
Any requests for specific products? Reply to this email if you do.
Fellow audio engineering nut, Gary Gallo asked me to be a little more specific about the circuitry that evolved from this adventure and head banging exercises. While the schematic is no doubt buried somewhere in my hand drawn archives (by the time Terri and I bought back PS Audio in 1997, sadly, all the documentation we had so lovingly kept on our products had found its way into a landfill), what I can tell you is probably going to be somewhat Greek to many. Sorry.
When you want low noise and you are OK with low input impedance, one of the best ways to get it is to use the transistor's emitter (rather than its base) as the input. In a moving magnet circuit, you would most certainly use the devices high impedance base for the input, but in a moving coil circuit we need an input of only tens of Ohms.
So, the basic circuit was a single emitter coupled MPS 8599, its base tied to ground through a huge electrolytic (to allow proper DC performance) and its emitter serving as the input through a 100Ω resistor. Its collector likely had a 3kΩ resistor to B+ giving it a gain of 30.
Interesting points here. The MCA had zero negative feedback, relying instead on the tiny bit of emitter degeneration for its linearity (which is fine since the signals are sooooo tiny).
To keep the hum and noise from the power supply out of the signal we introduced the idea of an outboard power supply feeding an AC voltage into a diode bridge arrangement on the MCA board. To keep hum down further, I employed PS Audio's first use of a star ground—this is where every ground point in the circuit has a separate PC board trace going back to one point, the heart of the star (have a look at the photo and you can see the individual traces going back to the one star point right where the two black, one red and one yellow ground wires are located).
You can also see how we provided little gold pin sockets for placing the cartridge load resistors. This practice stayed with us through many PS phono preamplifiers through the PSIV and V.
Lastly. note how the traces under the board aren't like what you see today: precision, straight lines. That is because back then, I designed all the PCBs by hand with tape and sticky pads. Today, of course, everything is handled in a computer and looks like Mr. Spock designed it. But, back then, it was free form art!
A real original. Thanks for reading.
