John Broskie's Guide to Tube Circuit Analysis & Design



 25 January 2005

Aikido enhancement  
The aikido amplifier allows for an easy sound enhancing control: a sonic width control. I have covered the topic of the sonic width controls before, so I won't go into great detail here on how such a control works. But I will comment on how desirable such a control is. Neither recordings nor listening rooms are created equally. Many of the classic recordings from the fifties and sixties were purposely mixed with too much separation between the channels; the motive is easy to surmise: stereo was new (and expensive) and the customer expected to hear separation and separation is what they got. Alternatively, few rooms are big enough to hold big speakers as far apart as we would like. Thus, electronically increasing the sound field's width might help in tight quarters. And, most certainly, headphones sound much more natural with a bit of channel blend.

Implementing such a control requires no more than a three-position toggle switch and two resistors: one position for wide, normal and narrow. In spite of the low cost, this control makes a huge difference in the sonic presentation, even non-audiophiles will readily hear the difference. The puritanical audiophile can easily opt out by selecting the toggle switches center position, which will allow the absolute purity of the impure signal pass without alteration (people who love pure signals should never see music being captured, amplified, processed, mixed, compressed, expanded, equalized, phase inverted, encoded…).


Above is the aikido amplifier without the width control and below, with a blending resistor bridging left and right channels.

Above is the aikido amplifier with a width-expanding resistor spanning left and right channels. How large should these resistor be?    

A good starting point would be to use the cathode resistor value to widen the sound stage. With 6SN7s and 2k cathode resistors, a 2k width-expanding resistor gives about a 20% negative bleed between channels. For positive blending of channels, a 50k produces about the same amount of blend. But feel free to experiment. The best solution might be to use a rotary switch with many positions, so that a variety of blend ratios could be switched in and out. The toggle switch, on the other hand, is easy to implement:

An added trick is to limit the positive blend to the high frequencies (via a small-valued capacitor in series with the blend resistor), as the ear requires less separation at high frequencies that it does at low frequencies.





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