I don't know about you, but in my life I have discovered that in order to move forward past a seemingly insurmountable problem there was a requisite amount of time required for me to beat my head against the wall. Over and over again, I keep trying different angles of the same thing until I either give up or try something different.
Fortunately, with enough pain and flattening of the skull, I sometimes manage to pull my head out of its dark place and step back far enough from the problem to take a fresh look.
When it came to designing a low noise moving coil amplifier, that meant meant pulling back far enough to see more than just the "obvious" source of noise, the amplifying device itself. As mentioned yesterday, regardless of what device I used in that position I was having no impact on lowering the noise. Ergo it must not be that device.
Duh.
If it wasn't the amplifying device responsible for the noise there were only so many other choices: power supply, resistors, capacitors. Other than the chassis, there wasn't anything else left.
Spoiler alert. It wasn't the power supply nor was it the capacitors. No, in fact, it was the lowest cost parts in the circuit itself. The damned resistors—and no, not the quality of the resistors, but their values.
Turns out that there's a simple truth to a low noise phono circuit amplifying a moving coil cartridge. The resistive values have to all be very low. Mine were all the opposite.
In a moving magnet phono preamplifier, the resistor values have to be reasonably high: 47,000 Ohms at the input. A moving coil cartridge, on the other hand, wants to see the opposite. Most MC loads are 100Ω; often less.
With the 20/20 clarity of hindsight, this simple fact should have been obvious to me. But it took forever to come to the aha! moment. The noise was coming from the high resistance values of the circuit. I had simply mirrored what I knew to be a great sounding MM circuit into a MC circuit and then focused my repair efforts on what had worked for the one.
Simple as it may sound, this journey took me close to a year of head pounding against the proverbial wall. In fact, had it not been for a casual remark from a Santa Barbara-based engineer/reviewer (whose name I cannot remember) I might still be looking 40 years later.
Tomorrow we'll wrap this little bit of history up with a quick post about the chassis and chrome plating.
