December 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. 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
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
Fig. 2 - Feedback circuits for each switch position of Fig. 1 and their effect on frequency response: a -
treble boost; b - bass and treble boost; c - same as b, but treble boost starts at higher frequency; d - bass
boost and treble cut.
Fig. 1 - Circuit of the 4-position feedback tone-control circuit.
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