Output Buffer

Using an off-the-shelf walwart necessitated some important design constraints. And working within 75mm x 120mm necessitated a few others.

Some hybrids use DC walwarts. This is the simplest way to supply DC power to the amp but it tends to limit the B+ to something less than what the walwart can supply.

However, if we use an AC walwart, as is used in other hybrids, then we have the opportunity to use voltage multipliers to create a higher B+. How high depends on the voltage from the walwart and the number of stages in the multiplier.

In this case I had settled on a 24VAC walwart as the starting point. There is an additonal limitation imposed by using a walwart. Since the walwart only supplies two wires there is no CT. And with no CT we cannot create a split supply for a complementary output stage. We could use an SE output stage as in the SOHA II but the power requirements for this are too high.

So, the two constraints of small space and AC walwart power require that the O/P stage be:

1. Complementary
2. Use TO92 size devices
3. Use a rail splitter to avoid using a large output capacitor
4. Minimize number of components

Number 3 further means that, since the O/P is direct coupled to the headphones, it has to be a self-zeroing buffer. This is because the input of the buffer will be coupled to the tube stage by a small coupling cap.

The simplest BJT complementary buffer is the diamond buffer with TO92 BJTs. BC327/337 complementary pair are easily obtainable O/P devices and BC550/BC560 do the same for the input pair. The operating point is should be about 20mA in the output stage. Since each device sees about 12V this makes for about 240mW power dissipation for these 600mW devices.

Simple Diamond Buffer

To minimize any possible problems due to hfe mismatch the devices should be all chosen from the same hfe class. For example, BC550/560 should be "C" class and the BC327/337 should be "25" class. Transistors shouldn't have to be matched, but class selection is a reasonable requirement.

Then next simplest improvement to the simple diamond buffer is to load the input followers with CCSs on their emitters. This helps to make the input followers more linear with consquently less distortion.

Simple Diamond Buffer with CCS Loads

This techique is used in the PPA buffer except that the CCSs are formed using current mirrors that are themselves sourced with a single jfet CCS.

The CCSs, because they are active devices, now offer us a way to introduce a DC offset servo. If we leave the upper CCS fixed but make the lower CCS variable we can use the bottom CCS's current adjustment to set the DC offset. Note that we don't have a bias adjustment in this buffer. We must use a fixed bias because there is no room for a variable one. A fixed bias will change somewhat with the actual devices hence an additional need to ues BJTs from the same hfe class.

What we need now is an adjustable CCS at the bottom that is referenced to 0VDC. We can do this with a current mirror driven by an opamp. The opamp will be the current source and the mirror will reflect this current to the bottom input BJT. Like this:

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CTH Self Zeroing Discrete Buffer

The value for R_off requires some thinking. The opamp offset servo only controls half of the input stage. The other half is fixed by the CRD. The input BJTs have their bases in series so their base currents must be identical. Thus, if they have hfe differences their collector currents will differ by the hfe differences. This cannot be otherwise because the base currents are forced to be the same. There are two regimes that determine the value of R_off.

Q_p(hfe) > Q_n(hfe)

If the bottom transistor has lower hfe then its collector current will be smaller than the upper transistor. To adjust for this the opamp will try to reduce the current in the mirror by lowering its output voltage towards the bottom rail. The opamp can drive its output almost to the rail so it can reduce the current as much as it needs to to compensate. In practice it can’t do this for any possible disparity in hfe, but it can do it for anything reasonable when the lower transistor has lower hfe than the upper one. In this case, therefore, the value of R_off is not particularly critical. In practice there will be a lower limit on this resistor value because the opamp's output cannot be connected directly to the mirror. If it is, there is not much resistance there to limit the O/P current from the opamp.

Q_p(hfe) < Q_n(hfe)

There can be a problem, however, when the top device has lower hfe. In this case the top transistor’s collector current will be smaller than the bottom transistor. But, in order for the circuit to work the top transistor, by design, must have a collector current equal to the CRD current. This means that the top transistor’s base current has to go higher (because it has lower hfe) and this means that the collector current in the lower transistor must go higher by the ratio of the hfe of the two devices.

Now the opamp will try to drive the mirror current higher by raising its output voltage. It is limited by the top rail. The maximum current that can flow into the mirror is approximately;

I_m = (V⁺ - V^- - 1.8V - V_em) / R_off

The 1.8V is for the two diode drops in the bases of the mirror transistors and for the fact that the opamp can only get to approximately one diode drop of its rails (three diode drops total). V_em is the drop across the emitter resistor of the mirror device.

V_em = R_em * I_m

Substituting V_em in the first equation and solving for I_m gives:

I_m = (V⁺ - V^- - 1.8V) / (R_off + R_em)

And solving this for R_off gives:

R_off = ( (V⁺ - V^- - 1.8V) / I_m ) - R_em

To go any farther we must make a few choices. First we choose a typical value for R_em = 220Ω and for the CRD current we pick 4.3mA (1N5313) to keep the input devices in a nice class A regime.

If we want the servo to be able to handle a 3X difference in hfe (more than the spread in the "C" gain class which is 400-800) then the mirror must be able to provide 12.9mA collector current to the bottom device. If the rail-to-rail voltage is 24V then we have:

R_off = (23.2V / 16mA) - 220Ω = 1578Ω

To give some additional headroom we select R_off=1.5kΩ.