The GlassWare Balancer is the inverse of the GlassWare Unbalancer. Where the Unbalancer circuit accepts a balanced input signal and delivers an unbalanced (single-ended) output, the Balancer converts an unbalanced input signal into a balanced pair of output signals. Balanced audio signals offer some real advantages, such as lower noise and easy phase reversal and no need for a phase splitter in a push-pull power amplifier.
The Balancer is basically a phase splitter with gain and low output impedance. A single-ended (or unbalanced) input signal enters the Balancer and a pair of balanced output signals appears at its output. Why would someone want such a feature? You might own a power amplifier with a balanced input or you might want to select the phase of the signal going to an unbalanced power amplifier or you might want to run one stereo power amplifier per channel. This last possibility is rare, but truly interesting. Many power amplifiers can deliver twice the wattage into a 4-ohm loudspeaker than they can into an 8-ohm speaker. By bridging a stereo power amplifier's outputs so the single speaker attaches across the two hot terminals, you can quadruple the wattage into the 8-ohm speaker. In addition, the power amplifier now behaves as a monobloc design, which can yield lower noise and improved stereo imaging. It this setup, XLR connectors are not needed, as a pair of stereo interconnects per channel is all that is needed. RCA out, RCA in, in other words.
Another possibility is to use the Balancer as the frontend of a push-pull power amplifier—either with the Balancer built in the push-pull power amplifier's chassis or external to it. Think about it: if a line-stage amplifier's balanced output signal is sufficiently high in amplitude, the push-pull power amplifier may not need an input and phase splitter stage; instead, the push-pull amplifier might consist of just a power supply and output transformer and a pair of output tubes. Of course, a global negative feedback loop is not possible in such an arrangement, but then not everyone wants global feedback. (Within the spare push-pull power amplifier, an added driver stage could be placed that would encompass a negative feedback loop.)
In general, the input tube should present a low mu for line stage use; for use as push-pull-amplifier frontend, a high-mu tube works better, such as the 12AT7, 12AV7, 12AX7, 5751, 6072. When a 12DW7/ECC832 is used, the 12AU7 section is used in the grounded-cathode amplifier position and the 12AX7 is used in the split-load phase splitter position. The input tube’s plate resistor can be either single resistor or two parallel resistors per plate, so two lower-wattage resistors can be used to make a higher-power plate resistor.
The Balancer uses two cathode followers as an output stage, as the cathode follower offers both low distortion and low output impedance. The input and output tubes do not have to be the same type or even share the same heater voltage; and the following design examples are not exhaustive by any means. For example, the 6H30 and ECC99 would make powerful output tubes and the 6DJ8 works well in a relatively low-voltage setup, where a B+ voltage of only 150V is used. (Low-voltage operation allows to use much larger RC filter and coupling capacitor capacitors, which will prove important when driving 600-ohm loads.) In general, we want high-transconductance triodes for the output stage, which are usually lower mu types. An excellent pairing is the 12AU7 as the input tube and the ECC99 as the output tube. The cathode follower's cathode resistors hold two parallel resistors per cathode, so two lower-wattage resistors can be used to make a higher-power cathode resistor.
This FR-4 PCB is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through heavy 2oz copper traces. In addition, the PCB is lovingly and expensively made in the USA. The boards are 7 by 6 inches, with five mounting holes, which helps to prevent excessive PCB bending while inserting and pulling tubes from their sockets. The Balancer PCB holds two line-stage amplifiers, which accept an unbalanced input signal and deliver a balanced pair of output signals. Thus, one board is all that is needed for stereo use.
The Balancer PCB comes with 20-page user guide that holds schematic and design examples.