Hi Jeff
For many years, I’ve been playing hard
rock with British master-volume amps (i.e.,
Marshalls). Lately I’ve fallen in love with
the sound of a Marshall 1959SLP. Besides
enjoying the slight breakup distortion, I like
the amp’s great dynamic range. My picking pressure can make the amp go from a
clean, chimey sound with a “hollow” low
end that really cuts through to a great mid-rangey, crunchy, distorted sound. I’ve tried
to replicate this with master-volume amps
like a Marshall JVM or Orange Rockerverb
by reducing the gain, but I just can’t get
the same dynamic response. Do non-master-volume amps inherently have a better
dynamic response? Would class A amps
come closer to what I’m looking for?
Len
Hi Len
Nice observation. You’re correct that most
amplifiers designed with master-volume controls don’t have the dynamic range and headroom of non-master-volume amps, but it’s
more than simply a case of a master-volume
control being inserted into the circuit. Here’s
my opinion on the subject.
Let’s use your two-channel, four-input model
Marshall 1959 as an example. Say you want
to run the volume controls on channel 1 and
2 at high levels to achieve a bit of preamp
overdrive, but need to play at a less-than-ear-shattering level. (We’ll assume you’ve
jumpered channels 1 and 2 together, because
that’s where the magic lies in this amp.) If
we were to modify this amp and properly
install one of several different master-volume
circuits, you could lower your volume level
with decent results, albeit with a reduction of
dynamic range.
This loss of dynamics happens for two reasons: You’re saturating the preamp section,
and the master-volume circuit is preventing
the full signal from reaching the output stage.
If you crank the master volume to 10 and set
the volume controls on channel 1 and 2 as
you currently do, the amp would sound virtually identical to the way it does now. At this
point, you’d realize that the lack of dynamic
range is not entirely a result of having a
master-volume control in the circuit.
So what’s causing this lack of touch sensitivity
and extreme playing enjoyment? The basic
answer is “too much stuff!” Most master-volume amps are designed with additional gain
stages in the preamp section to achieve a considerable amount of front-end gain and produce a substantial amount of overdrive. This
usually means more instances where the signal
is amplified, filtered, attenuated, altered, controlled in some way, and then re-amplified and
treated all over again. More circuitry means
more places for the signal to get lost.
All of this detracts from the signal’s purity
and dynamic range. Some of the very early
“British”-style master-volume amps (such as
the Marshall 2203) are not as guilty of this
and can have some pretty decent dynamic
range and headroom, but the more current
amplifier designs are generally horses of a
different color. For me, this is a great argument in favor of “less is more” design. If
you want a big, open, responsive, chimey,
full-bodied sound, the more basic the circuit,
the better the result. As you’ve already found
out, your Marshall 1959 is a prime example of
this design philosophy.
Regarding your question about a class A
amplifier possibly coming closer to what
you’re looking for, first let me say I believe
the only true class A amplifier is one with a
single-ended output stage, such as a Fender
Champ. That said, while most output stages
that builders describe as class A can be
responsive and sound pretty dynamic, the
amp’s tone is also going to depend on the
design of its front end. Too much stuff in the
signal path could equal too little tone.
A quick clarification: One of the sentences
in my last column regarding speaker impedances read, “A higher number of turns in
the 16-ohm coil may slightly increase the
response of the speaker at higher frequencies due to an increase in inductance.” This,
unfortunately, was an error. While there is an
increase in top-end extension of a 16-ohm
speaker versus an 8-ohm speaker, it is due
to an increase in the DC resistance (DCR)
and not due to an increase in inductance.
While the increase in inductance could actually cause a slight loss in high frequencies,
the DCR is the dominant factor here, and its
effect increases faster than the inductance.
Sorry for any confusion.
Jeff Bober
Jeff Bober, one of the godfathers of the low-wattage amp
revolution, co-founded and was the principal designer
for Budda Amplification. Jeff has just launched EAST
Amplification. He can be reached atpgampman@gmail.com.