What is Power Scaling?
• Power Scaling is a "methodology" and a performance
goal. The goal is to achieve the same tone as our loud sound but at
a much lower volume. The method of achieving this can be one of over
sixty distinct approaches, each with countless variations. There are
many approaches that reduce power but do not achieve the full benefit
of Power Scaling.
Other designers made attempts to
achieve Power Scaling, but never quite got there. London Power refined
and fully developed the technology to allow the maximum power of an
amplifier to be dialed down to whatever level a player needs. It was
first used on the amazing London Power STUDIO
amp.
Isn't this just like a speaker
load box?
• Not at all! "Speaker
load boxes", "speaker emulators," and "speaker attenuators" are all
forms of attenuation that are interposed between the output of a power
amp and the speaker. They work for some people but are notorious for
sounding "buzzy" at high attenuations.
A speaker attenuator forces your amp
to be run flat out producing its full power all the time. The power
that is not needed is thrown away as heat, with the required power
going to the speaker. It is quieter than full-tilt, but now the speaker
is isolated from the amp and cannot interact with it, so some tone
is lost.
The key to Power Scaling is that it
is applied to the power output tube stage itself, and so comes before
the output transformer. Power Scaling allows a dynamic power range
of 40dB. Most speaker attenuators alter the tone before they reach
8dB reduction. -8dB is just a little bit quieter than full blast;
-40dB is literally a whisper.
Wouldn't a 'master volume' do the same
thing?
• Only in specific situations. If you only play clean or you
only use preamp overdrive or distortion tones, then a 'master volume'
will satisfy you.
Power Scaling is the best solution for those
players who incorporate some amount of output stage "effect" in their sound.
This effect can be some clipping, heavy clipping, or just that cusp of compression
you get in a tube power amp approaching clipping. Power Scaling allows you
to live at that cusp or beyond, but at ANY loudness you need.
So Power Scaling will help my overdrive
sounds. How clean will a Power Scale amp play?
• All London Power Power Scaling amps are designed to provide
smooth, sweet clean sounds up to their limit of power.
How does Power Scaling affect tube
life?
• With the POWER SCALE dial set
to any setting less than maximum, tube life will actually be extended.
In accelerated tests, power tube life is as long as that of a preamp
tube ... up to 100,000 hours if the tube is not mechanically upset.
This is one of the benefits of the
full Power Scaling effect, referred to above. Output transformer
life is also extended as it is subject to much lower voltage stress
even with fully squared output signals and unexpected load disconnection.
Can I run a Power Scaled amp without
a load right into a mixer?
• Yes. It is perfectly safe to do this, although you lose the
benefit of frequency shaping provided by the speaker and the interaction
of the output stage with the speaker.
The POWER SCALE
control reduces voltage and current stress on the output tube, so even at
a fully saturated distortion output, the tube is under less stress than it
would be subject to in, say, a 3W amp.
Couldn't a low power tube amp do the same
thing?
• No. A low-watt amp only has one compression point, one cusp
of distortion and one loudness level through a speaker.
Power Scaling amps can play both louder
and quieter than amps of less nominal power. And, the compression
point stays in the same position relative to the cusp point for all
settings, allowing the touch responsiveness to remain consistent.
How is this possible? Is the circuitry
complex or expensive?
• In technical terms, all that must be accomplished is to keep
the "transfer curve" of the amplifier the same. The transfer
curve is just the relationship between the input and output signals,
but as we know, tube amps respond differently to different size signals.
This trait is due to the transfer being not straight and not uniformly
curved.
Think of the transfer curve like a mirror. A flat mirror that is
parallel to you will reflect your image perfectly and full size. If
you move the mirror away, the image is smaller but still perfect.
A tube's transfer curve is like a slightly curved or rippled mirror.
In this case the image is slightly distorted, but this is exactly
what we wanted - it is why we chose a tube amp in the first place.
So, moving the mirror farther away, reduces the size of the image
but it is still perfectly imperfect.
Electronically, it is very simple and inexpensive to achieve this
goal. The diversity of electronic circuits allows countless circuit
approaches to be implemented by different designers, with greater
expense added or bulkier components used. In the end, it is all Power
Scaling.
Does that affect the output impedance of the amplifier? People
on the net say Power Scaling changes the tube plate resistance.
• Because the shape
of the transfer curve is maintained, the plate resistance of the output
tubes is maintained, as well. So, the output impedance of the amp
does not change even though much less power is available once you
dial down.
Can Power Scaling be added to any
tube amp? Is the circuitry large?
• The circuits are simple and small, and are easily retrofitted
into existing tube amps, but... The tech doing this installation must
be very good at mods - not just a good repair tech as these are two
different skills.
Depending on the amp, some heatsinking
or a fan might be required to cool the Power Scale circuit.
Does that mean the Power Scale
circuit is just converting the unwanted audio power into heat?
• No. If you only need 3W of power then only 3W is produced;
if you need 29W, then you get 29W. The simplest Power Scale circuits
are soft regulators, and as such, they divide the voltage available
from the raw power supply between the amplifier and itself. In that
voltage sharing, there will be some waste heat.
For POWER SCALE control
settings between fully clockwise and about 12-o'clock, there will
be waste heat from the Power Scale regulator. At settings between
minimum (counter-clockwise) and 12-o'clock, the regulator runs cool.
Meanwhile, as audio power is reduced, waste power in the power tubes
goes down in direct proportion, which increases their reliability.
How does Power Scaling compare to Maven Peal's Sag circuit?
• Power Scaling allows
the player to go down to whisper levels, which for the most part is
unusably quiet. If you have a detuned speaker cabinet or any other
design that extends dynamic linearity, then you can play below a normal
speaking level. With London Power's Power Scaling design approach,
the player can choose how quiet to play with typically 44dB of dynamic
range - that is 100W down to less than 0.01W.
Maven Peal's designer chose the half-Watt lower power limit for their
Wattage control, so that the player would not encounter the point
where the matching open-back combo's speaker loses its tone at lower
volumes. To put this into perspective, most guitar speakers produce
90-100dB of sound with just one-Watt of input. Half as much input
power only drops loudness by 3dB, so you are at 87-97dB of sound,
which is a loud party level!
So, even though Maven Peal has amps that go from 100W down to 1W
(20dB range), and 20W down to 0.5W (16dB range), those lower power
levels are still fairly loud. To some players, they are "quieter"
or "quiet enough". But it is obvious that a power range
that is smaller than these - say, 100W down to 5W - is even less useful
at just 13dB dynamic range. The quietest level is still too loud for
most players.
"Sag" is an inherently signal-dependent effect exhibited by all amplifiers
with conventional power supplies. A regulated supply with a very beefy
transformer will exhibit much less sag. Sag is merely a voltage drop
under loading, that effects the attack of a note. Once the supply
sags, its stays sagged and then power limits the entire signal. Maven
Peal products use stiffly regulated power supplies to reduce noise
and thus have no inherent sag effect. Sag must then be added by letting
the signal modulate the supply reference. Despite the potential of
the system, the limits imposed by the designer restrict the range
of sounds, both in the sag effect and the possible power reduction.
Power Scaling supplies have a soft-rectifier sound and do not tighten
up as you dial down. Diode noise is inherently filtered, and you can
play as quietly as you want. "Sustain" is the sonic quality
of sag and London Power uses various Sustain circuits to increase
this tonal characteristic.
Do I have to buy a London Power amp to get Power Scaling?
• No. There are many amp brands
licensed to use our technology and trademarks. Each builder begins
with traditional or original audio circuit ideas enhanced by Power
Scaling. You can also have an amp you already own fitted with Power
Scaling by a recommended installer. See our Links
page, or the Choosing a Variable-power Amp
[currently being updated] page.
POWER
SCALING Q & A
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