Any respectable successor to the SOHA must replace the opamp with a good discrete buffer. There are quite a few discrete buffers to select from. These are a few:
- Diamond Buffer
- PPA Buffer
- JISBOS Buffer
- Stacker Buffer
The main feature that any hybrid buffer must have is a high input impedance so that the buffer doesn't load the tube. The raw PPA buffer is pretty good. The unmodified JISBOS is aroung 1MΩ and the Stacker is more than 3MΩ. The Diamond buffer, which will not have high enough Zi for this amp.
I thought about using the Stacker buffer, but it's too complicated for this amp (good for the Stacker though because the Stacker is a high-end amplifier). The JISBOS Zi falls off too fast with frequency because of the GD and GS capacitances of the input JFETS. The PPA buffer must be servoed to work properly which leads directly to the Stacker buffer design. I also designed a single ended buffer and with some encouragement from the Stacker team, I decided to use this in the SOHA II. Everybody likes SE buffers. There seems to be something magical about fully class A operation, even if the amp heats your house in winter.
A Cool (or maybe hot) SE Buffer
The SOHA II SE Buffer is a CCS loaded emitter follower. Nothing new, except that the design is targeted at creating a high Zi. To get a high Zi we must have current gain in the buffer which requires at least one Darlington pair. Like this:
Emitter Follower SE Buffer
The 820Ω resistor keeps the input BJT in class A for all signal conditions. The “C” class BC550 is selected because it has a high HFE, which increases the current gain and, thus, increases the Zi. The O/P BD139 is intended to run at about 100mA to give the buffer the ability to drive very demanding loads without cutting off. This means that the CCS has to handle 100mA too. Which means that the CCS will also have a power transistor in it.
The High Current CCS
The simplest BJT CCS would be a power BJT with an LED from its base to the negative rail and an emitter resistor to set the bias. But, a single power BJT has poor characteristics for a really good CCS. We need to partner the power BJT (needed to handle the current) with a high gain BJT to improve CCS behavior. There are several ways to do this, such as a cascode arrangement. However, the simplest CCS and the one with the lowest voltage drop is a ring-of-two pair of BJTs. Like this:
Follower with High Current CCS
The CCS has a theoretical dynamic resistance of about 37kΩ, staying flat from 1Hz to about 100kHz and dropping to about 25kΩ at 1MHz. This dynamic resistance would be low for, say, a plate load, but is good enough for a follower whose Zo is only a few ohms. It is more than 100 times larger than the resistance of typical high Z headphones.
The trimpot should adjust the CCS from about 60mA to over 100mA so that the builder can set this for the headphones in use. Some will only need 60mA idle current and this will reduce heat generation in the O/P stage.
This buffer topology will get very close to 1V from the rails, in this case, about 14V peak.
A Servo that Stays out of the Signal Path
What's missing from the buffer now is a way to bias the base of the input BJT. How ever this is done, it must be with a high value resistor to preserve a high Zi. The buffer also needs a servo to zero the DC at the output. We can solve two problems if we can return the servo to the input with a high value resistor. This is where the Darlington stage helps us. Because of the high current gain in the Darlington, the base current in the BC550C is very low, typically just a few μA. This low current means that we can use a 1MΩ from the servo to the base with just a volt or two drop across the resistor. An opamp servo's range will only be from rail to rail so the nominal voltage drop across the return resistor has to be much smaller than this to give the opamp room to account for variations in components. There is nothing special about the servo. It looks like this:
Adding the Servo
The servo sets the base bias of the input BJT to set the DC offset at the output to zero. Because of the 1MΩ resistor and the high Zi of the Darlington pair, the Zi of this buffer is about 950kΩ to above the audio band. This value is high enough for any tube that will be used in the front end. The 18kΩ resistor forces the opamp to always draw current, keeping it in class A mode even though the base current is very small.
The 470μ capacitor on the positive rail gives the follower a reservoir to draw from during high current swings. We don't need one on the negative rail because the CCS, obviously, keeps the current through the negative rail constant.