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Baxandall 2 - last updated 26/08/06 21:34:50



The following is a usenet posting made by Mark Garvin in 1995 - it covers the operation of the Baxandall tone stack in some detail.

Repost of article follows:

TONE CIRCUITS, PART 3. Fender controls vs hifi controls.

I've received a couple more email queries about tone controls,
so I figured it would be best to answer them here.

Fender-style controls are used by many amp manufacturers because most
guitarists prefer treble and bass boost (which is to say they
prefer midrange cut). The Fender-style tone stack CANNOT provide
treble or bass cut (in other words, no true mid boost).

One way to regard boost circuits is that all frequencies are
attenuated--then that attenuation is cancelled for a range of
frequencies. The amount of broad-band attenuation is called
insertion loss, and is a necessary evil of any circuit that
provides a boost.

The standard Fender midrange control does not really boost just
the midrange--it boosts ALL frequencies. In other words, it
decreases the insertion loss. This is why the value of the
midrange pot cannot be increased much past 10k or so: it will
render the treble and bass controls ineffective. This is perceived
as mid-boost because it does cancel some of the midrange cut, but
it cannot be a true boost because there is no way for the mid
control to elevate midrange levels any higher than bass or treble
levels. In fact, increasing mid control will actually boost the
bass and treble a bit.

Most tone circuits are centered around a midrange frequency at
300hz to 500hz. The Fender controls' center frequency (approx 300Hz)
is close to the fundamental pitch of a guitar's open high E string.

By contrast, hifi amp tone circuits are usually centered around
1KHz--well over an octave higher. For guitar, the 1KHz tone circuit
would provide a more effective control of bass frequencies, but
at the expense of the rich, hard-hitting spectrum under the
control of the treble knob. A 1KHz treble control would affect
only harmonics of guitar frequencies.

However, the standard hifi tone circuit DOES provide treble cut
and bass cut. Conversely, this provides an apparent midrange
boost. When I refer to the 'standard' hifi tone circuit, this
usually means the 'Baxandall' circuit, credited to inventor
Peter Baxandall. Maybe this post is appropriate at this time,
because Baxandall has just died recently. We have him to thank
for much innovation in tube amp design.

Following is one of Baxandall's contributions to tone circuitry:

mgarvin1.gif (1972 bytes)

[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.

----------------- BASS 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.

mgarvin2.gif (1276 bytes)

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 C2/R3.

This circuit can be related directly to the Fender tone circuit:
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.

----------------- BASS and 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.

mgarvin3.gif (1173 bytes)

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 circuit useful.

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.

Mark Garvin
Composer/Design Engineer
New York City


End repost