This is a grounded cathode voltage amp followed by an opamp buffer.

The tube stage:

The triode is fixed-biased by the grid-charge current that flows through the 1meg resistor.

The hot cathode always emitts some electrons that have enough energy to fly up to the grid.

The grid resistor closes the current path between grid and cathode so that the grid discharges across the resistor and following Ohm´s law builds up a negative voltage between grid and cathode.

This way of biasing works for many small signal tubes like 12AU7, 6DJ8, ECC88, 6922 but also for 12AT7, 12AX7 etc.

Output capacitor (C2):

This capacitor is very important to prevent feeding DC current to your headphones.

Warning: Never go without output cap, otherwise your headphones (and probably your ears) will be ruined.

You may use an electrolytic capacitor of 220uF for headphones above 32 Ohms or 470uf if your headphones are below 32 Ohms.

Some people put a small film cap in parallel with the electrolytic to have better high frequency response.

Important: Check the polarity of the cap. The + side of the cap goes toward the opamp output!

The bleeder resistor (R3):

This resistor goes to ground and will allow the cap to charge at power on, this helps to minimise the POP when you connect the headphones.:

You should connect / disconnect the headphones always if the amp is up and running.

The bleeder resistor should be about 10 times higher than the value of the headphone impedance plus the output resistor value.

The output resistor (R4):

The output resistor should be between 22 and 150 Ohm depending on which final gain the amp should have as the output resistor forms a a voltage divider together with the headphone impedance.

As a rule of thumb choose the low value for high impedance headphones (>=250 Ohm) and the high value for low impedance headphones (32 Ohm).

Leave a Reply