The ACF-2 is an All-in-One design that holds both the B+ and heater power supplies and two Aikido-ized cathode followers. In other words, the ACF-2 is a unity-gain buffer for those who do not need any extra voltage gain, but do need more current delivery than a passive line stage would allow.
The ACF-2 uses a bipolar power supply and two triodes per channel to greatly improve the cathode follower's PSRR figure. In addition, the bipolar power supply allows us to DC couple the ACF's input and, possibly, to eliminate the need for an output coupling capacitor. If the power amplifier holds a coupling capacitor at its input or if the amplifier uses a tube at its input, then the ACF-2's output capacitor may not be needed. With DC-coupled, solid-state power amplifiers, on the other hand, any DC offset, even a few millivolts is too much and an output coupling capacitor is required.
The ACF-2 circuit is quite simple. The top triode defines a cathode follower with a DC input and an active cathode load, which is made up from the bottom triode and its cathode resistor. These two triodes and their matching cathode resistors balance each other almost perfectly, nulling the power-supply noise from the ACF's output. To better help you understand why this should be so, imagine two identical resistors wired in series and spanning a bipolar power supply two voltage rails. Because both resistors are equal in value, the voltage division is one half. Thus if the two rail voltages differ only in polarity, the voltage at the resistors' common connection will be 0 volts. One half of +125V and -125V is 0V; in other words, the midpoint between +125Vdc and -125Vdc is 0V.
The same math holds up to the presence of noise on the bipolar power supply rails, which means that as long as the rail noise is in anti-phase and their amplitudes match, the output noise from these rails will equal zero at that same connection. In other words, the ACF-2 greatly improves on the cathode follower's already fine PSRR. In addition, the ACF-2 produces a tad bit less distortion than comparable cathode follower by using the triode’s own nonlinearity against itself.
The obvious tube choices are the 6CG7, 6DJ8, 12AU7, 12BH7, and ECC99. But many other tubes could be used. The only stipulations are that the two triodes within the envelope be the same and that the tube conform to the 9A or 9AJ base pin-out. The higher the mu, the closer to unity gain. The higher the transconductance, the lower the output impedance. Other possible choices are 6H30, which would deliver a low output impedance that could drive capacitance-laden cables; and the little-known 5963 & 5965, which were design for long life in computers. In other words, the list of possible tubes is a long one: 6AQ8, 6BC8, 6BK7, 6BQ7, 6BS8, 6DJ8, 6FQ7, 6GC7, 6H30, 6KN8, 6N1P, 12AT7, 12AU7, 12AV7, 12AX7, 12BH7, 12DJ8, 12FQ7, 5751, 5963, 5965, 6072, 6922, E188CC, ECC88, ECC99… Note, however, the 12B4 and 5687 cannot be used with this PCB.
The ACF-2 PCB also holds the heater raw power supply and voltage regulator, the LD1085 low-dropout adjustable voltage regulator. The regulator can be set to an output voltage between 6V to 25V, but the assumption is that a 12Vdc output voltage will be used for the heaters, so that either 6.3V heater tubes (like the 6FQ7 and 6DJ8) or 12.6V tubes (like the 12AU7 or 12AX7) can be used. Both voltage types must be used exclusively; for example a 6GC7 in one channel and a 12BH7 in the other channel cannot be used at the same time. The ACF-2 holds three jumpers that can place the two heater elements in series or in parallel.
The ACF-2 PCB requires a power transformer(s) to energize its two power supplies. When set up with a full-wave bridge rectifier topology, the heater power supply power transformer secondary must offer at least 1.8 times more current than the heaters will draw. For example, two 6CG7s will draw 0.6A @12.6v, so the heater power transformer must be able to sustain an AC 1.08A current draw. In addition, with sine waves, the AC voltage equals the peak voltage divided by the square root of 2, i.e. 1.414. Thus, a 10Vac sine wave peaks at 14.14V; a 6.3Vac, 8.9V. In other words, a sine wave that peaks at 14.14V will produce the same amount of heat in a resistance as a 10Vdc voltage source would produce in the same resistance; thus, we label the 14.14Vpk sine wave as being 10Vac. Thus, in order to get the 16Vdc a 12.6V heater voltage regulator requires an input voltage equal to sum of 16V and the rectifier loss (about 2V) divided by 1.414, which is roughly 12.6Vac.
The high voltage power transformer must also follow the same rules. Thus, to achieve 340V (+/-170Vdc) of raw DC voltage, before the two RC filters, the high-voltage transformer secondary must deliver (340V + 2V) / 1.414, or about 240Vac center-tapped. And if 50mA is required, the power transformer must be rated for 50mA x 1.8, or about 90mA. Such a transformer VA rating would equal 33VA. The high voltage secondary must be center-tapped (or consists of two secondaries that can be placed in series, thus creating a center-tap). Do not use an autoformer type step-up transformer, as the primary and secondary must be isolated from each other.
The heater power supply can accept a configuration as either a full-wave bridge rectifier circuit or a full-wave voltage doubler rectifier. Configured as a voltage doubler, which is a good choice when the secondary voltage is only 6.3Vac, capacitors C28 & C29 must be placed in series by being rotated 90 degrees clockwise.
The ACF-2 is a perfect partner for the Logitech Duet or Touch (or the old Squeezebox unit), as these products offer an easy way to set up a WiFi networked audio system. In addition, they use high-quality DACs and remote-controlled volume attenuation. What they lack, however, is a tube-based audio section.
The optional part package includes all the parts needed to populate the PCB (including two ceramic tube sockets and the standoffs and O-rings), except the tubes and coupling capacitors. The capacitors are all high-quality parts from Nichicon, Wima, Panasonic... Twelve solid-state rectifiers, including four HER108 and four MUR420 ultra-fast types; one LDO LD1085 3A voltage regulator. Includes many redundant resistor values for use with many types of tubes and RC filtering configurations.
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 6 by 6 inches, with five mounting holes, which helps to prevent excessive PCB bending while inserting and pulling tubes from their sockets.
Includes 16-page user guide, which holds schematics, explication, and design examples.