One Channel Schematic
The EHHA was first introduced as a concept on Headwize. Several members of the forum contributed to the thought processes that evolved the amp to its present stage. As part of this process Snoopy (initials T.B.) collaborated with the author (runeight) to complete the design. The original EHHA used 2N5550/2N5400 high voltage complementary transistors. Another member of the forum, Steinchen, suggested using the fast, Toshiba 2SC2705/2SA1145 high voltage complementary audio devices, and this change was made as shown in the schematics.
There are two versions of the EHHA, one using BJT output devices (MJE243/MJE253) and one using MOSFET output devices (IRFZ24N, IRF9Z34N).
The 6GM8 twin triode is the differential amp input stage. Its tail is loaded with a constant current source set to approximately 3mA. The triodes have cathode degeneration resistors with a center trimpot to adjust for seriously out of balance tubes. The first half of the differential amp drives the upper VAS, Q5. The second half drives the lower VAS, Q6, through the unity gain current mirror made up of Q2, Q4, R4, R10, and R14. The symmetrical VAS ensures that the voltage will pull up and down with equal speed as Q5 and Q6 alternately hand off the work to each other. In contrast, the non-symmetrical approach, typical of most implementations of this topology, relies on the single VAS device to pull the voltage up and down leading to an asymmetrical swing.
The output stage is either an emitter or source follower biased using a standard VBE multiplier transistor. Both AC and DC feedback are routed back to the inverting input of the input stage through the voltage divider R12 and R13. The DC feedback partially adjusts the output DC offset and the Opamp servo does the rest of the job.
The servo booster mechanism was designed by Snoopy. It uses an Opamp to sense the output DC and a pair of current mirrors that control the operating position of the VBE multiplier. The Opamp adjusts the asymmetry of the currents in the complementary mirrors to compensate for the asymmetries in the components of the amplifier. The servo consists of Q10, Q11, Q12, Q13, the Opamp, and associated components.
Both circuits follow the same topology developed in the Technical section. The only difference is in the output stage where the Mosfets are provided with gate stoppers, R24 and R25.
R22 and R23 set the open loop gain of the amp by providing an additional load for the VAS devices. The schematics show a typical value of 33k. By setting the open loop gain, R22 and R23 provide another set of choices for values the in the feedback network, R12, R13. This gives the builder a wide array of alternatives for OL and CL gain. See the Tweaks section for detailed information on OL and CL gain.
The output stage bias design center has been set to 100mA. However, the bias conditions for the two types of devices are different. The fixed bias resistor in the VBE multiplier, R20, uses a different value for the different versions.
The servo circuitry tends to make the amp look complex, but without the servo the amp is fairly simple, looking like the symmetrical schematic with the tube input stage shown in the Design section.
Both circuits use two small compensation capacitors, C2 and C3, (often called Cdom) on the VAS devices to prevent high frequency oscillation by rolling off the gain at high frequencies.
The emitter/source resistors at the output, R32 and R33, are different for each device type too. The BJT resistors are set larger to provide more degenerative feedback to compensate for the BJT's positive temperature coefficient.
Decoupling capacitors have been placed along the rails to give the local circuit some charge storage capacity and to decouple the input and output stages of the amp. Since the rails are +/-30V, 78L15/79L15 regulators are used to supply the Opamp, including rail caps to prevent Opamp oscillation. The Opamp servo is non-inverting with two low frequency poles ensuring a smooth rolloff at subsonic frequencies where the servo is active.
The internal shield (Pin 9) of the 6GM8 is grounded through a small capacitor. This gives the buider the option of leaving Pin 9 ungrounded, grounded through the capacitor, or DC grounded through a jumper.
The EHHA is designed to work with any good quality +/-30V split supply. An excellent companion PS is the σ22 Tracking, Regulated Supply from Ti Kan (AMB). The amp also requires a small, 6.3VAC/1A heater transformer.