Lowering feedback in KIT34 KIT6550 KEL84

[FAQ Team]

Feedback

Feedback improves the figures for noise, distortion, output impedance and bandwidth. In many ways it is a good, or even essential, part of an amplifier.

The designer will have designed enough gain into the amp to allow as much feedback as needed to meet the target specification. How much is actually needed in practice though will differ from system to system. In addition feedback is sometimes used simply as a tool for lowering gain to a desired sensitivity level.

It follows that it can be changed moderately without causing any great problems to see if a lower level offers a better sound in a given system. Many people find a lower level can bring more air and spaciousness to the soundstage and a more natural presentation. Too little however results in a messy presentation and poor bass control.

Probably the best place to start is by lowering feedback about 5dB and having a listen. This seems a modest amount but is actually nearly a halving (-6dB) in voltage terms. The user should be aware that noise and distortion will rise and sensitivity will increase by this same dB amount. Frequency bandwidth will also narrow a little but these are well specified amps and none of these will be very noticable in practice.

Lowering feedback in KIT34 KIT6550 KEL84

The operating conditions for the first section of the ECF80 are set by the cathode resistor which is also the feedback resistor. To lower feedback we need to increase the value of this resistor but, at the same time, keep the resistance the valve cathode, Pin 7, sees to ground at the original value. This is easier than may be thought; increase the cathode resistor then connect another resistor between Pin 7 and ground. For this extra resistor choose a value which will make the original cathode resistor value when paralleled with the new value cathode resistor.

KIT34 and KIT6550

Change R45 and R46 from 560R to 1k2. Connect 2 more resistors each of 1k from both Pin 7s to ground. Change C21 and C22 to 2.2nF

KEL84

Change R32 and R33 from 750R to 1k5. Connect 2 more resistors each of 1k5 from both Pin 7s to ground. Change C16 and C17 to 3.3nF

Resistors of 0.6w rating in carbon or metal film are suitable. The capacitors mentioned are the high frequency compensation components and will need to be a little lower value than standard so easily available preferred values are suggested here. Use high quality polystyrene, silver mica or polypropylene film caps. If you have test equipment the value can be adjusted using a Square Wave signal and oscilloscope for the best SW without under or overshoot but, soundwise, it doesn't seem to make a great deal of difference.

You could then go on to try any feedback level you wish, even removing it all if only to have a listen, using this same method.