Ultimately, what we desire is an amplifier that is as quiet and as bass solid as the best solid-state amplifiers and as fluid and as lifelike as the best tube amplifiers.   

Which Way? T>SS or SS>T?
     While most tube hybrid amplifiers hold a tube front-end driving a solid-state output stage, the inverse is certainly possible, i.e. solid-state devices driving a tube output stage. Audio Research and Stax have made such amplifiers (as, I am sure, have many music amplifier companies, as it is the tube output stage's softer clipping that pleases the professional musician and it is the cheap IC front-ends that please the accountants).

      The IC's power supply is created by the current draw through the cathode resistor and zener, which might result in the IC latch-up at turn-on. This problem can be often solved by adding input protection diodes to the IC or by simply using a different IC.
      The problem with connecting a feedback loop to the plate is that the signal at the plate is not necessarily the same signal that will leave the transformer! For example, if no signal is present at the input, the feedback used will force the plate to mimic the ground's lack of signal. The power supply, however, contains a noise voltage that will couple through the transformer to the output, as the noise then becomes a signal. But if the plate held the same amount of noise (in the same phase) as the power supply, then the transformer would not see any difference to relay to the secondary.
    The circuit below hopes to overcome the power supply noise. It uses a separate negative power supply rail and the DC heater power supply voltage to power the Op-Amplifier. A small fraction of power supply noise is injected at the positive input (the tube inverts the phase) to develop that noise at the plate. Thus, the inclusion of the .03µF capacitor. This capacitor along with the 1µF capacitor defines a voltage divider. The ratio of this diver is made to match that of the ratio of feedback resistors. Therefore, we have by injecting noise into the amplifier lowered the noise at the output: not Zen, but Aikido applied to electronics.

Self powered hybrid IC front-end
single-ended amplifier with auto-bias

      The amplifier shown above uses an IC to provide the gain necessary to drive the EL84 to full output and to set the EL84's idle current. The Op-Amp strives to equalize the voltages on its inputs. Thus the Op-Amp will automatically adjust the output tube's idle current until the voltage divider (formed by the 1 meg and 750k resistors) yields the same voltage as the zener develops. (The quiescent current through the IC must be included in the calculation. Fortunately, most Op-Amps draw little current.)

Hybrid IC front-end single-ended
amplifier with power supply noise cancellation


www.tubecad.com   Copyright © 2001 GlassWare   All Rights Reserved