Setting the Gain
The EHHA has been designed to provide flexibility for setting some important amplifier parameters. The process for selecting these settings is describe in the Instructions section. That description refers to the information in the tables in this section.
Setting the Closed Loop Gain
The closed loop gain is the actual operating gain of the amplifier. The table below shows the decrease in gain with increase in R13 for both versions. The amp is loaded with 1kΩ, R22 and R23 are open.
These are theoretical values taken from simulations, but they are probably close to real values and are certainly accurate as to the trend of the data. After you set the closed loop gain, select values for R22 and R23 that give the amount of NFB that you want to have by setting the open loop gain.
Setting the Open Loop Gain
The OL gain of the amp is determined by the hfe of the transistors in the VAS. Changing these transistors with ones of different hfe will change the gain. Additionally, a pair of resistors, R22 and R23, have been designed into amp for reducing the gain of the VAS stage by reducing its output impedance. A direct reduction of open loop consequently results. Though not required, their inclusion has the effect illustrated in the tables below.
Choosing a value for R22 and R23 also sets the amount of negative feedaback. This is illustrated in the tables below.
These numbers are for the stock transistors (2SC2705/2SA1145) with the amp loaded with 1kΩ.
Again, these are theoretical values taken from simulations.
The Mosfet version has higher OL gain than the BJT version when R22, R23 are left out. This is because the Mosfets offer a higher impedance to the VAS devices. But, as R22 and R23 decrease, their relatively low resistance dominates the output impedance seen at the VAS stage outputs so that the OL gain in both versions becomes approximately the same. Values of R22, R23 less than 10k are not recommended because the bias point of the VAS stage will be upset.
The design center for the EHHA is the 6GM8/ECC86 low voltage twin triode. But, the design also includes some flexibility for higher voltage tubes that can fit into the operating parameters. These include 6922/6DJ8 and 6H30.
If you are sure that you'll only be using 6GM8s you can eliminate the trimpot R6. Operationally good 6GM8s will have enough balance between the two triode halves not to need the trimpot. The pad for the trimpot can just be jumpered over. Change the values of R5 and R7 from 47Ω to 100Ω.
6922, 6H30 and other compatible tubes
Tubes designed for higher operating voltages than the 6GM8 will tend to have more triode imbalance at low voltages. To compensate for this possibility install the trimpot R6. See the Instructions page for steps in making out of balance tubes work.
Beware that the heater current for a single 6H30 is approximately 800mA. Increase the size of the heater transformer according.
DC or AC? Series or Parallel?
There is always some debate about running heaters AC or DC. The EHHA is designed to run satisfactorily using AC heaters, reducing the complexity of the heater supply to a single, small transformer. In the vintage days of tube equipment, where creating a low voltage DC supply was very difficult, nearly all equipment, including phono preamps, were run with AC heaters. Manufacturers and builders simply employed good heater wiring techniques. The tubes themselves are fairly immune to heater-cathode hum leakage. Thus, the EHHA will operate with AC heaters with no ill effects as long as the tubes are in good operating condition.
However, a simple DC heater supply can be made using a 6.3V transformer, four Schottky rectifiers (1N5822), and a 10,000u/16V capacitor. The supply may also need a small dropping resistor in the event that the heater voltage is too high. Beware that using a DC supply like this will prevent tube rolling except among tubes with the same heater current requirements.
Since the EHHA boards are single channel, it is possible to run the heaters in series using a 12.6V/500mA transformer. Several prototypers have used this approach.
Another advantage of using 12.6V is that it is easier to make a regulated 12.6V supply from a 12.6V transformer. This is because the AC peak voltage from 12.6V is nearly 18V, leaving enough headroom for the necessary drop across a low-dropout regulator. See the Wiring and Ground section for 12.6V heater wiring diagram.
In essence, then, you should get excellent performance from the EHHA no matter what heater supply you choose.
Modifications for a Power Amplifier
With a few small modifications/additions the EHHA can be converted to a 20W power amplifier. These are:
Changes to the schematic are shown in red.
The changes to R20/R21 adjust the Vbe multiplier to account for the reduction in the values of R32/R33. Also, the range has been limited so that the O/P stage cannot be set for idle currents above 500mA.
The changes to R32/R33 reduce the voltage drop across the follower resistors when they are passing amps instead of milliamps.
The zobel network compensates for the inductance of the speakers and should be attached right at the output jack on the chassis.
The mosfets will dissipate much more power in a power amp. They should be attached to off-board heatsinks that have a thermal spec of about 0.5C/W.