[NOTE: Use a DC-blocking cap to couple this circuit to a tube plate].
You've probably seen this circuit before in stereo amp schematics.
A close look at this circuit will reveal that the bass and treble
controls are almost opposite. The Bass pot (R2) and Treble pot (R5)
are both large values compared to the impedance of surrounding components,
so when viewing the circuit in simplified form, they can be ignored.
Visualize what is happening with the bass control: an R-C (resistor-
capacitor) mid-cut (bass-boost) circuit is formed by R1/C2 when the
bass pot is at max (wiper at the top). When the bass control is turned
to minimum, C1/R3 form a bass cut circuit. Actually, it keeps about
the same amount of midrange cut as before (flowing thru C1), but bass
frequencies are shunted to ground thru R3.
The converse occurs with the treble control. This time, R4 serves
as the resistance in both RC networks. At the max treble setting,
C3/R4 cut mid and bass freq's (actually shunting them to the bass
control which takes care of mid cut, etc). High frequencies pass straight
thru C3 unimpeded. At min treble, C4/R4 still shunt bass and midrange
freq's to the bass control, but now treble frequencies are shunted
to ground thru C4.
and TREBLE BOOST (MID CUT)
Consider what the circuit looks like with bass and treble at max.
Keep in mind that both circuits use relatively high value pots, so
they can be ignored for purposes of illustration. I've also omitted
other components which are incidental to this setting.
It is usually easier to read tone control circuits from right to
left. C3 allows treble to pass straight thru, but diverts bass and
mid freq's thru R4. At the junction of R1/C2, the bass frequencies
take the high road, while most of the mids are shunted to ground thru
This circuit can be related directly to a simple tone circuit that
provides boost only: C3 is the 250pf treble cap, R4 is the 250K treble
pot, R1 is the 100k fixed resistor, C2 is the .047 mid-cut cap, and
R3 is the 6800 ohm resistor which prevents too much mid-scooping.
All other components are incidental when controls are at max.
TREBLE CUT (MID BOOST)
This is where the similarity to the Fender control ends. When the
Baxandall controls are set to min, the following circuit results.
This has no equivalent Fender setting.
Now, C4 immediately shunts treble freq's to ground. Bass and mid
pass thru R4. At the junction of C1/R3, mid frequencies pass thru
the cap, but bass freq's are diverted to ground. Treble and Bass cut
== Mid Boost.
Why don't guitar amps use the Baxandall circuit? Some do, but again,
most guitarists prefer bass and treble boost. Also higher component
count. And it's best to minimize the number of components in the signal
path. The Fender-type circuit is simple, and if it does what you need,
then stick with it. Some jazz or blues players and those looking for
a bit more control may find the treble cut or 'mid boost' of the Baxandall
Please try to post follow-up queries, since I will not have time
to reply to separate email questions. This post has already taken
long enough. Darn ASCII diagrams.
Anyway, I hope this has helped to answer a few of the email queries.
Sorry for the delay.
New York City