The Basic Amp Topology
The Embedded Hybrid Headphone Amplifier is based on a conventional and widely used solid state amplifier topology. This topology, which has found its way into millions of amplifiers, has three stages. These are shown in the block diagram below.
The three stages are: a differential input stage, a voltage amplification stage, and a current amplification output stage. Negative feedback is returned to the differential amp. This feedback serves two purposes. The first is signal feedback to reduce gain, second to reduce THD, and third to extend “audio flat” bandwidth. The second is to provide a DC sense voltage to the differential amp for zeroing the DC offset at the output.
This topology, in its simplest form, is embodied in the simple schematic below. Many enhancements have been made to this circuit to improve linearity and reduce other bad effects.
The circuit is, more or less, an opamp. The differential amp is typically sourced with a constant current source. The voltage amplification device is also loaded, through the VBE multiplier, with a constant current source. The VBE multiplier sets the idle current in the output stage.
The negative feedback goes to the inverting input of the differential amp through a voltage divider through a large capacitor to ground. The purpose of the capacitor is to ensure that the DC feedback loop for zeroing offset can utilize the full open loop gain of the amplifier. The servo needs this gain to achieve near zero offset. The signal feedback loop is essentially grounded and is, therefore, set by the ratio of the resistors in the NFB voltage divider.
This circuit can be made symmetrical by using a unity gain current mirror on the inverting portion of the differential amp to translate the other signal phase down to the bottom rail. This change is shown below.
This is a basic transconductance amplifier with a symmetrical drive of the output stage. The new diode on the first stage balances the voltage drop caused by the BE junction of the current mirror BJT.
Creating the EHHA
The Embedded Hybrid Headphone Amplifier is based on this last circuit. Typical rail voltages for a design like this are 30V or more. With 30V rails it is reasonable to replace the BJTs in the input stage with a pair of triodes also forming a differential amplifier. For example, a 6GM8 will happily operate with 25V on its plates. In fact, we can sub this tube directly for the BJTs, as shown in this schematic.
Note how this topology differs from the typical tube/transistor hybrid. Unlike the conventional hybrid where the tube stage is completely separated from the solid state circuitry by a coupling cap, the tube in an EHHA is “embedded” in the circuit. The EHHA is DC coupled front to back; no coupling caps. The tube runs from the same rails as the transistors and is integral to the feedback circuit.
There is one negative effect of inserting a tube into the input stage. The tube will have less gain than the BJTs. The mu of the triodes is far less than the hfe of BJTs. Thus, the gain of the amp is reduced considerably. Furthermore, the transistors are typically loaded with small resistors, around 1k. These small load resistors further reduce the gain of the input stage. Thus, the open loop gain of the amp goes from a typical value of 4,500-15,000 (depending on emitter degeneration resistors in the differential pair) to more like 1,500-2,500 (depending on cathode degeneration resistors). For a headphone amp, where we only need a gain of 5-10, this is a good change. But, the reduction in open loop gain also affects the DC servo gain. A gain of 1,500 is not enough to sufficiently zero the output, especially for headphones where a few mA of DC current affects the sound quality.
To compensate for the loss of DC servo gain an EHHA needs an additional booster servo to zero the output DC adequately for headphones. The simplest form of this booster servo is shown below. The servo adjusts the operating point of the output devices by controlling the positioning of the VBE multiplier with respect to ground.
The capacitor in the feedback loop is no longer needed since the amp is no longer depending on the open loop gain to zero the DC output.