[Part 1 introduces the topic of grounding and "GND-think." Part 2 considers the ideal differential input. Part 3 looks at impedance balance vs. current balance, instrumentation amps and cable shielding.]
This article originally appeared in Linear Audio, a book-format audio magazine published half-yearly by Jan Didden.
H-pads attenuate the differential-mode component without affecting the common-mode component. At low volume settings, effective CMRR of the whole system may even become negative. H pads are out. A two-gang potentiometer will convert CM to DM unless matching is phenomenal. Other than that, CM impedance is directly determined by DM impedance.
For noise and distortion reasons you'd like a low-resistance pot; for CMRR reasons you'd like high resistance. This is going nowhere either. It turns out that there is no acceptable method of constructing a balanced passive volume controller. In fact, there is no sensible way to arrange a potentiometer in a differential fashion.
I have a double agenda in presenting this demonstration project. Firstly just to demonstrate how the "new" design methodology works in practice, but secondly to invite doubters to discover for themselves how a bit of rational engineering can produce staggeringly good sonics without resorting to boutique parts or boutique thinking. This is going to be the cheapest and best-sounding preamplifier you've ever built (Figure 25).
Figure 25: Complete balanced preamp/volume controller
The input stage
The input stage is a straight buffer implementing the improved input biasing network. I would have used Whitlock's input chips and implemented the capacitive bootstrap technique as well, except that the distortion performance is not good enough in my view.
The difference stage
As noted above, we're out of luck when it comes to wiring a pot differentially so we won't even try. Instead we'll be using the surrounding stages to reference the cold point of the variable gain stage. So between the input buffer and the variable gain stage we insert a difference amplifier. This is the circuit that'll confer CMRR to our little preamp, so resistor matching is of prime importance here. The output of the difference amplifier is referenced to the cold point of the volume controller.
The DC servo
I've always considered it the task of the preamplifier to remove DC. I've thrown in an unusual DC removal circuit that isn't actually a servo; in that, it doesn't measure DC at the output. Instead it's a 2nd-order low-pass filter whose output is subsequently subtracted from the signal.
Next: The volume controller