PS AUDIO / Paul McGowan
Of all the questions we are asked by customers, finding and fixing hum is perhaps number one. Ground loops would be our first suspect, but a close second is hum and noise from turntables.
Ground loops occur when two pieces of interconnected equipment find themselves grounded at different levels (one slightly higher or lower than the other). The difference in ground potential permits a tiny noise current to flow and we hear hum or buzz. The biggest source of ground loops we see is happily supplied by the cable television companies whose grounding schemes differ from our home’s. It doesn’t take much to introduce a nasty buzz in the system—a problem solved with either an isolation transformer on the cable television or disconnecting it altogether. We have several tutorials in our How To section of the website if you need.
Turntable hum can be much harder to solve if you’re unfortunate enough to have a noticeable amount. That is because the task of amplifying tiny signals from phono cartridges to the relatively huge voltage swings necessary to power loudspeakers is impressive, to say the least. Gains of 1000 times are needed for moving magnet and 30,000 times for moving coils—and these gains are just to get it to the preamp where it is further amplified up to 10x by the preamp, and 30x more by the power amp. Considering these gobs of gain you get an idea of how difficult a task it is to quietly amplify the tiny currents from small coils of wire in the phono cartridge to the massive coils of wire in your speaker drivers.
In fact, it’s actually quite amazing just how little hum and noise we get in a phono setup. My latest video covers some of those issues in more detail. You can watch it here.
One of the earliest head-scratching design challenges I faced in the late 1970s was designing our first moving coil head amp. Every amplification device has noise. How does a design engineer build a device with a gain of 30x without adding more noise? Seems rather impossible and it’s one of the reasons engineers turn to step up transformers instead (they haven’t extra noise to speak of). However, I was determined to design an active head amp with little to no more noise. What I wound up with, for those among you that care, is called a common base transistor. Here’s a simplified schematic.
Instead of the input going into the transistor’s base (the “B” middle pin similar to the grid of a tube), the device’s “E” emitter is used instead. This is a very low noise configuration with one serious downside for most applications—the input impedance is quite low (it’s essentially set by the emitter resistor). But, guess what? That’s perfect for a moving coil which wants a low input impedance in the first place. Voila!
This simple circuit became the basis for our first moving coil amplification product, the MCA.