Single Ended Global Negative Feedback

[chunkitup]

Now I have recently built a Jose style preamp with a single ended el84 power amp. The preamp is awesome but I need a little more bass and I would like to add in some neg feedback (depth mod). I was thinking about taking the feedback from the 8ohm tap and return it to the cathode of the el84. Values and ideas would be greatly appreciated.

[R.G.]

When there are three or more time constant in the forward path of an amplifier between a negative feedback point and where it's fed back to, there is a solid potential for gain-phase oscillation. A transformer is two time constants all by itself. So using capacitor coupling from output to the cathode is going to be chancy businesss; at least, it may well need some sincere work to keep it stable. Failing that, the forward gain from cathode to output may have to be limited.

This is a constant problem with solid state amplifiers, as each transistor in the forward path contributes a pole (oops, sorry - time constant) at high frequencies because of the device capacitances. Without compensation, all opamps would oscillate. The compensation is severe. Generally, there is a pole down at 1 Hz to 100Hz to cause the gain to decrease below unity before two more poles contribute their phase shift.

[printer2]

Not sure if you have enough gain in the output section into the cathode.

[loudthud]

Not sure if you have enough gain in the output section into the cathode.

You may just be able to connect the ground side of the EL84 cathode resistor directly to the 8 ohm tap without attenuation. Feedback factor (amount of gain reduction) might only be 6 to 10 dB.

[printer2]

You may just be able to connect the ground side of the EL84 cathode resistor directly to the 8 ohm tap without attenuation. Feedback factor (amount of gain reduction) might only be 6 to 10 dB.

That is interesting. Doubt if you would need more than 6dB.