eXStatA Schematics


Complete Solid State Amplifier

One additional challenge in an electrostatic amplifier is the resistors. Resistors have a working voltage rating (just like capacitors) which cannot be exceeded. Most of the 1/8W-1/2W resistors that we commonly use are rated at 250V. In a stat amp, however, many of the resistors will see more than this, some will see the entire rail-to-rail voltage. In this amp this will be 600V.

There are two ways to handle this problem. The first is to use resistors with higher voltage ratings. The other is to put lower rated resistors in series so that the total working voltage exceeds what the series chain will see.

However, the higher voltage resistors have fewer available values and the ones most commonly available are 5%. For this amp I've chosen to use the second approach using 1% resistors in series. The full schematic shows all of the resistors that are actually needed on the board.

eXStatA Full SS Amplifier Schematic

Complete Amp Showing All Resistors

The schematic is for one channel. The part numbers correspond to the board except that on the board they are designated L or R since both channels are on one board.

 

Complete Hybrid Amplifier

The hybrid amp is nearly identical to the solid state amp. Two transistors are removed and replaced with two triodes and two grid leak resistors. Everything else is the same. For the triodes the prototype team recommended the 6S4 deflection triode. They seem to be generally in good supply even though they are strictly NOS tubes.

eXStatA Complete Hybrid Amplifier Schematic

Complete Hybrid Amplifier

The schematic is for one channel. The heater bias resistors RH1/RH2 are only on the left channel. The part numbers correspond to the board except that on the board they are designated L or R since both channels are on one board.


The Power Supply

The power supply has three parts: the primary split 300V supply, a low voltage adjustable bias supply, and a high voltage adjustable bias supply. The split 300V supply is a slightly higher performance version of the simple CCS/Shunt design:

eXStatA 300V Power Supply

300V Split Supply

The shunt regulator has an additional stage compared to the simple version in the design section. This additional gain gives the supply an output impedance of about 1Ω. This Zo would be high if the amps drew lots of dynamic current. But they don't. What we have is simple, but good.

The voltage divider for the cascode devices uses zeners instead of a resistor to get a flatter response over the audio spectrum (and beyond). The half-voltage divider supplies the base voltage for both stages of the regulator.

The amps (both channels) draw about 30mA from the positive rail and 35mA from the negative rail. The supply is designed to shunt about 5mA-6mA per rail which means that the postive rail CCS is set for about 36mA and the negative rail CCS is set for about 41mA. This leaves about 3-4mA flowing in the MJE power transistors which sets their power dissipation about 1W each. A good compromise. The amps don't require any more shunt current because, as noted, they don't generate much dynamic current draw.

The 1μ output capacitors ensure that the high frequency response of the regulator does not degrade at higher frequencies.

Bias Supplies

The bias supplies each use one side of the 250-0-250 secondary. This gives each supply about 250VAC to work with. Each supply is fed from a variable voltage divider that provides the full 240VAC down to 120VAC.

The low voltage bias supply uses a half wave rectifier. The DC voltage from 125VAC to 250VAC is about 168VDC to about 350VDC. The actual voltage will depend on the exact regulation of the transformer and the line voltage conditions. But the range will be roughly in this territory.

The high voltage bias supply uses a voltage doubler which, when fed with 125VAC to 250VAC, makes about 350VDC to about 680VDC.

 

Maxed Power Supply

It is possible to improve the performance of the power supply by substituting CRDs for R13 and R14. This substitution gets the Zo of the amp to around 1/4Ω. This additional performance is not absolutely necesarry because of the way this amp works, but the option is offered for those who always want the maximum performance.

eXStatA Maxed 300V Power Supply

Maxed 300V Split Supply

The CRDs provide a much higher load for the MPSA gain stages increasing their gain and, hence, increasing the performance of the regulator. These devices are still numbered R13 and R14 on the boards, the the resistor silkscreens also have diode markings to show the correct orientation.