Feedback Tone Control
December 1958 Radio-Electronics Article

December 1958 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

I have to admit to not being real certain why I selected this article on feedback tone control for posting. Although it is a good write-up on some simple audio frequency filtering circuits, and the principles can be applied to any frequency, it is most likely this page was marked because it contained an electronic-themed comic on it. Oh well, Melanie already scanned and OCRed it for me, so you might as well go ahead and read it.

Feedback Tone Control

By A. V. J. Martin

High-fidelity tone controls through feedback

This elaborate tone control using a separate feedback chains found in some Marquett French receivers. The theoretical circuit is shown in Fig. 1. The af voltage from the anode of the preamp is applied to the grid of the power amplifier through a divider made of two 470,000-ohm resistors. The grid thus receives only half of the af voltage. However, a low-value capacitor is connected in parallel with the first 470,000-ohm resistor, effectively short-circuiting it at high frequencies and producing an important treble boost.

This arrangement is completed by a feedback chain around the power stage. A four-position switch modifies the effect of feedback. To make things clearer, the simplified diagrams Figs. 2-a, -b, -c, -d, show what is the actual circuit for positions 1 to 4 of the switch.

In position 1 (Fig. 2-a), the feedback chain is a simple 2.2-megohm resistor, giving an overall feedback ratio of the order of 10%. The high frequencies are boosted by the coupling circuit so that this is a treble-boost circuit.

In position 2 (Fig. 2-b), a series R-C combination appears in the feedback path. It reduces by approximately 50% the feedback at low frequencies, which becomes 5%. At medium frequencies, you obtain the full 10% feedback. At high frequencies, there is the boost due to the coupling circuit. This is then a bass- and treble-boost circuit.

In position 3 (Fig. 2-c), the circuit is identical with Fig. 2-b, except for the fact that the shunt capacitor in the coupling circuit now has the lower value of 250 μμf. The treble boost appears at higher frequencies. The bass and medium frequencies behave as in Fig. 2-b. This then is again a bass-and-treble-boost position, the treble boost coming into play for the higher frequencies.

In position 4 (Fig. 2-d), the circuit differs from Fig. 2-c by the connection of a 50-μμf capacitor between plate and grid of the power stage. This causes a strong feedback at high frequencies, but does not modify the behavior of the circuit for bass and medium frequencies. This is then a bass-boost-treble-cut circuit.

The simplified response curves included in the diagrams give a rough idea of the effects of this clever circuitry.

Posted  January 6, 2015