Some thoughts on volume controls from Balanced Audio Technology (BAT), just to add to the discussion:
Remote Control: HIgh End Achilles Heel
Remote control of volume has rightfully been given a bad
name in high-end audio due to numerous preamplifiers that
offer such capability with mediocre sonic performance. In fact,
remote convenience is often synonymous with sonic compromise. The customer is caught in the middle trying to decide
how to balance these apparently conflicting goals.
Since remote control is available in receivers sold for as little as
$200, the issue is not one of cost alone but a tradeoff of cost
versus quality. Indeed, given the limited engineering resources
of most high end firms, many find themselves resorting to the
same off the shelf solutions used in “low end audio”. These are
generally integrated circuits (IC’s) with devices such as low
quality op amps placed directly in the signal path. Such
implementations are very inexpensive, as witnessed by their
presence in many low priced products.
Therefore, we have been surprised to see such solutions
appear in high end preamplifiers retailing for well over
$10,000. Another high end tube manufacturer uses a similar
integrated circuit based volume control in their vacuum tube
preamplifiers. Thus, even tube designs that have garnered
favor for the purity of their approach to amplification find
themselves corrupted by this desire for a quick-turn solution
to the remote conundrum.
The i-series Investigation
Balanced Audio Technology undertook a careful investigation
of remote volume control implementations. Our experience
with the development of the VK-5’s shunt volume control
taught us that the volume control implementation played a
critical role in determining the ultimate performance of a
preamplifier. The volume control can be thought of as a
gateway or gatekeeper within a preamplifier since any
information lost by the volume control is lost forever to the
succeeding gain block(s). Thus, we used our ears to tell us if
our theories were correct about the purity of alternative
volume control implementations. In all cases, the existing
VK-5 preamplifier’s Vishay™ shunt volume control was used
as the benchmark for listening comparisons. Without further
ado, here is what we learned.
Digital Volume Controls
Digital volume attenuators are becoming increasingly common
within high end audio. These are packaged as integrated
circuits that rely upon a digital interface to set volume levels.
Depending upon the specific chip employed, a variety of op
amps or other solid-state devices are usually found directly in
the signal path. Resistive elements used inside these chips are
diffusion-type integrated components that have unpredictable
sonic characteristics. The advantage of such an
implementation is that, as a packaged solution, it can be
plugged into an existing preamplifier design relatively easily.
Such parts also have very good electrical specifications
(e.g. dynamic range, distortion, range of attenuation).
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This allows the designer to build a very convenient and
friendly user interface. The parts functionality, coupled with
quick availability and low cost, make this a very tempting
proposition. We tried this solution with the VK-5 as our
investigation into remote volume control began.
The result confirmed our fears about such an implementation. Almost every sonic parameter of the VK-5 was affected.
The air and breath of life that the VK-5 brings to music was
replaced by a more congealed and flattened perspective.
Instrumental attack was softened and instrumental decay was
truncated. It was as if we inserted a lesser preamplifier inbetween the source and the VK-5. Referring back to the
volume control as a gateway, its easy to understand why.
Based on these observations, an integrated circuit digital
volume control implementation was rejected for our reference
products.
Multiplying DAC Volume Control
Another popular integrated circuit implementation is the socalled Multiplying Digital-to-Analog Converter, or MDAC.
As the name implies, the output of the MDAC represents the
product of two input variables: one being a binary number on
the digital port and the other - the reference analog voltage.
When using an MDAC as a volume control, the audio signal is
connected to the reference voltage input. Changing the
digital input will modulate the signal output, allowing for a
very convenient way of changing volume.
Most of the concerns mentioned in the previous discussion on
digital volume controls apply here as well. The internal
resistive ladder that is the very heart of complicated DAC
integrated circuits, is comprised of silicon diffusion-based
resistors and not, for example, metal film resistors, normally
used in high end components. In addition, such DAC’s
usually require that an external operational amplifier (used as a
current-to-voltage converter), another complex solid state
component, be inserted directly in the signal path.
Our listening evaluations confirmed a striking similarity to the
sound of the digital volume control. Our search continued.
Motorized Volume Control
In contrast to the two solutions described previously, in the
motorized volume control the motor and its electronic drive
circuitry are not part of the signal path. You can still use your
favorite volume potentiometer and, therefore, preserve the
original sound of your product. You can buy an integrated
motor/potentiometer package or create your own. Existing
units can be upgraded by simply adding the motor and control
board.
This convenience comes with some penalties. First of all, any
moving mechanical component in your system is bound to be
one of its most troublesome elements. Potentiometers, even
the best ones, will suffer from the resistive element wearing
out and its moving parts breaking down. The same is true for
the motor and gears. In addition, there is something exceedingly old-fashioned and antiquated about this solution.
The choice of motor is a very painful exercise. Low cost
models, of the type usually found in inexpensive receivers, are
of inferior quality for use in high end components (although
they are still used by some companies). High quality motors
and gears, worthy of a high-end product, can be very
expensive.
Implemented within the VK-5 prototype, the motor was
driving the four-section high-quality plastic conductive
potentiometer that is used in a shunt attenuator configuration.
This should be differentiated from a traditional motorized
volume control that does not incorporate the Vishay Bulk
Metal® foil resistors as a pass-through element (see pg. 8 of
the VK-5 white paper for more detail).
As expected, this solution did not have a negative effect on
the sound of the VK-5. Nor was there a positive improvement. The biggest problem with such a solution is that it is a
crude architectural dead end. With this implementation,
repeatable setting of volume level is practically impossible
(unless one embarks on a full-blown closed-loop design with
potentiometer position sensor in the feedback loop) and such
niceties as being able to offer a unity gain function are also
eliminated.
This became our solution of last resort. Although it satisfied
many of our objectives, we felt that Balanced Audio
Technology should set the highest standard possible in all
aspects of design. Our third objective of providing a modern
convenient user interface would not be satisfied by this
somewhat crude approach. We continued our search.
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BAT Electronic Shunt Volume Control
At this point in the investigation, it was clear that a markedly different approach had to be undertaken. The vision of Balanced
Audio Technology’s volume control began taking shape with the following key attributes:
1. The new volume control would continue to be
based on the shunt configuration. The Vishay
Bulk Metal Foil resistors would remain at the heart
of the VK-5i.
2. It would provide a substantial improvement in
overall sound quality due to elimination of the
plastic element potentiometer from the signal path.
3. It would be a fully electronic volume control,
alleviating any reliability concerns associated with
the deterioration or breakdown of a motorized
control.
4. It would use only the highest quality parts,
such as precision low noise metal film resistors.
5. It would be digitally controlled using an
onboard microprocessor. This would allow us to
design a convenient user interface and provide an
easy future upgrade path - requiring only the
change of one chip in the field to accomplish any
such upgrade.
6. The stepped attenuator would have a broad
range of adjustment. The steps should be small
enough in size to maintain the feel of a continuously-variable potentiometer.
7. The solution would be fully retrofittable to
allow the owners of even the oldest units to get
the full benefits of this latest upgrade.
The step size for the attenuator was chosen to be 0.5dB over
most of its adjustment range. This provides an essentially
continuous feel and ease of predictable gain setting, without
becoming burdensome in cost.
The choice of switching element in any stepped attenuator is
a critical one. While relays are being used in some high end
products, we found them to be undesirable for several reasons.
An effect best described as relay chatter becomes annoying on
some products when changing the volume. In addition, the
usual questions about the reliability of electromechanical
components remain. All this meant that carefully selected
electronic switches had to be found for this application.
Through extensive circuit analysis and simulation, the proper
choice of components and their placement in overall circuit
topology were determined. These findings were later confirmed in our listening tests.
Up until this time, we believed the VK-3 and VK-5 preamplifiers to be the most transparent units on the market. Somewhat surprisingly, the i-series remote volume implementation
removes a slight opaque film from the presentation of music.
Transient reproduction and detail resolution are all improved.
Needless to say we are very pleased with the sonic improvement.
The i-series User Interface
The microprocessor controlled volume attenuator of the i-series
makes possible some delightful convenience features.
For example, in addition to the common Mute function, the
VK-3i and VK-5i remote controls incorporate a very useful
FADE button, allowing for a gradual reduction in volume.
Few of us enjoy the abrupt termination of music caused by
using a Mute switch. So, while the Mute switch is still available, now you can also enjoy a soft gradual fade -
convenient for gracefully handling the common interruptions
to listening to music.
The i-series volume control allows for an instant recall of
up to 6 preset volume positions. You can use this feature to
preprogram your favorite volume positions or to compensate
for signal differences in various source components. Or simply
hop from volume to volume in 10 or 20 step increments....
The number 6 preset position is reserved for the UNITY
GAIN setting. Please note that in this position the signal is still
routed through the preamplifier active circuit.
