Amp Voicing
What is voicing?
... VOICING
your opinion. I’m VERY familiar with
that as I seem to do it in my articles every month!
... VOICING
– what Britney Spears did at the recent video awards show. She certainly wasn’t singing, and she wasn’t
even lip synching. Maybe whatever it is
she was doing can now be called
voicing.
... VOICING
– the decisions and resulting component choices an amp builder makes to tailor
the sound of an amp.
As much as
I’d like to write an article about Britney Spears, I’ll focus on the last
definition!
The
following discussion will dive into the guts of amp design. However, I’ll embark on this voyage with a
promise that you won’t need to open your calculus textbooks to understand
what’s going on (you do have calculus textbooks, right?).
Resistors and Capacitors
First some
background. Amps are filled with various
components ... transformers, potentiometers, resistors, capacitors, etc. In this article we’ll focus on how selecting
specific values of resistors and capacitors at specific points in an amp’s
circuit change its tone.
We don’t need to delve into the details
of resistors (see leftmost photo) and capacitors (the other photo at the
left). All we need to know is that they
are individual components that, when connected in various configurations, alter
the sound of the electrical signal from your guitar as it passes through them
... basically they can roll off highs or lows or enhance or diminish mids.
Even in a
simple tube amp there are multiple points in the circuit where a designer has
to make a decision as to exactly what value of resistor and capacitor to use so
that the signal passing through the amp will be altered to meet that particular
amp’s tone objective.
Of course
the tone alteration I’m talking about is not the only way that an amp creates
its particular sound. The choice of
tubes, the overall circuit design, transformers, speakers, etc. all have a
significant impact on the amp’s sound.
However, “voicing an amp” typically refers to the small tweaks that a
designer makes to dial in the details of tone shaping.
A Typical Tube Amp
The below
block diagram depicts a common layout of a tube amp. The guitar signal plugs into the input
circuit, wends its way through the amp and the speaker is connected to the
output transformer.
A few notes
before we get going:
1. The block diagram below depicts a
typical, fairly simple, push-pull tube amp.
2. There are a bunch of details not
shown in this diagram. However, it’s
enough for our discussion.
3. I won’t cover tone controls in this
article. If you’re interested, you can
refer to my earlier article “Tone Controls: Tone and Gain Sucking
Leaches?”
4. The term “gain stage” refers to a
section of an amp related to a tube or ˝ of a tube (some tubes come with two
tube sections in one glass bottle).
Since tubes are present to amplify the signal the word “gain” is
used. Our amplifier has four gain
stages: preamp gain stage #1 (˝ of a 12AX7 tube); preamp gain stage #2 (the
other half of the 12AX7); the phase inverter (uses both halves of a second
12AX7); and the power amp stage using two EL84 power tubes in our example.
5. We’ll leave the inner workings of the
phase inverter and power amp stages for upcoming articles.
OK, let`s
have at it!
The
first voicing opportunity relates to the input circuit. There are many input circuit configurations
out there in the amp world. Typically,
they consist of one or more resistors although some include a capacitor or
two. For those resistor only designs,
the input circuit resistor(s) interact with the tube’s own internal capacitance
and alter the frequencies that pass through to the first stage tube.
In its simplest form the purpose of the input
circuit is to make sure that the guitar signal is not ‘loaded down’ resulting
in an unacceptably weak signal entering the amp and to make sure that the
signal entering the first tube stage is properly coupled to the tube
input. The other purpose is to filter
out radio frequency signals that may be picked up at the guitar or on the
guitar signal’s route to the amp – that’s the primary spot where we stop your
local ‘easy rock’ station from playing through your amp (which would ruin it
and make it forever unplayable if it were ever to happen!).
Those more complicated input circuits
can be used as a first step in voicing the amp.
For example, the Ampeg B22X input circuit has a ‘normal’ input and a
‘bright’ input. The bright input circuit
has a different resistor/cap arrangement than the normal input circuit and as a
result quite significantly rolls of the bass frequencies of the guitar signal
thereby making a bright tone.
Each
of the preamp stage tubes has to be biased.
This is a commonly referred to term that I won’t bother defining other than
to say that it involves adding components to establish the operating conditions
of the tube. For our purposes all we
need to know is that for a preamp tube the bias circuit typically consists of a
resistor and maybe a capacitor.
The resistor is chosen to set the
tube’s operating condition. The voicing
decision involves the capacitor. The
first choice to be made is whether to add one at all. A premp tube without a cap in the bias
circuit will have less gain than one that does.
If we add a cap we increase the gain of that stage. Voicing enters the picture because we can
adjust the amount of bass that the stage passes on to the next stage by
altering the value of the cap. The
bigger the cap the more bass frequencies we pass, the lower the cap the less
bass we pass on.
So, by adding a cap and adjusting its
value we can filter more or less bass from the signal. Often designers elect to add a cap in the
bias circuit to only one of the preamp gain stages. This is typically done to tailor the amp’s overall
gain structure.
As
you can see, a cap is placed between each stage of the amp. This is called a coupling cap – it couples
one stage to the next. Here again we can
alter the frequencies that pass from one stage to the next. The smaller the cap’s value the more bass
frequencies we filter out of the signal and vice versa. Often this cap is sized so that it passes
through all audible bass frequencies, however it can be used to ‘tighten’ the
sound going to the next stage if the designer wants to take some bass response
out of the amp’s tone.
We usually hear about output transformers and tubes as being
the determinants of amp tone. While they
certainly are significant contributors, as we’ve seen above making a single
component change can alter an amp’s frequency response. Once tubes and transformers have been
selected, “voicing an amp” is a critical task (and one that is painstaking and
time consuming!). However, it is necessary
to REALLY dial in an amp’s character.
If you want to dive into the details of gain stage voicing I
highly recommend Richard Kuehnel’s fine books “Gutiar Amplifier
Preamps” and “The Fender Bassman 5F6-A”.
These are both exceptionally well written books that step through amp
circuit details with terrific clarity.
HIGHLY recommended for the amp lover that already has an electronics
background.
As always I’d like to hear from you regarding this article or
any amp related questions or comments.
Just send me an email!
Don Mackrill