Over the years it has become general knowledge that, over
and above “traditional” measurements of THD and IMD, there are other factors
responsible for the sonic character of audio components. The final sonic
signature is the summation, and more importantly, the interaction of many
ingredients including the choice of tubes (leading to the very popular practice
of “tube rolling”), passive selection (resistors and capacitors), component
placement, interconnect type and routing, grounding methodology, and power
supply design – the subject I would like to explore further in this paper.
The designer is presented with an infinite array of combinations and permutations of components and topologies that all factor into the final result. This is the reason for the never-ending debate over types of power supply – tube rectifiers or fast recovery diodes, regulated vs. non-regulated, choke input vs. capacitor input. Regardless of the topology chosen, all of these configurations will contribute to the voicing of the combination. It is common to see an improvement with simple component changes - as for example if the soft character of a tube rectified supply is combined with an audio amplifier circuit having “hard” character aberrations. To some this “less irritating” result may be acceptable but it is not accurate and it certainly can be improved. More often however the “one best solution” philosophy causes the changes to accumulate in the same direction causing the aberrations to add in like fashion and become intolerable. For example, the typical “cookbook” high voltage regulator exhibits a characteristic sound that is hard, grainy, and thin. Applying this power supply regulator to the above example will result in an accumulation of errors that is horrible. As a result, some knowledgeable audiophiles have categorically rejected “regulated power supplies”. Similar comments have been associated with solid state rectification. The truth is that the designer must carefully understand the contributory factors and must either eliminate those that are relevant or minimize those that cannot be eliminated. All these factors must be considered in the final design.
If one were to carefully examine virtually any vacuum tube
audio circuit, it becomes clear that the tube amplifies by controlling the
current thru an impedance – most times a resistor. Voltage appearing at
the input (between the control grid and cathode) causes the current flowing from
the plate to the cathode to change in response. The current is provided from
the power supply and flows from the power supply, through the impedance
(again usually a resistor), through the tube and finally back into the power
supply. The output is usually derived by sampling the voltage across the plate
impedance via ohms law (Output Voltage = Tube Current X Plate Resistor - and
notice this is theoretically a direct linear relationship). If everybody is
following their theoretical and simplified model, we will be provided with a
perfect sounding distortionless output – a perfect replica of the input only
higher in amplitude – the perfect amplifier.
Unfortunately, in the real world, all three depart from
perfection and as a result both individually and in total, contribute to the
overall “sound” of the total amplifier. The tube isn’t perfect by
definition – it’s output only approaches linear when operated under certain
strict conditions all hopefully understood by the designer. Resistors deviate
from linearity - mostly from the effects of voltage drop across its internal
structure, but nowhere near as significant as the tube. Finally, poorly designed
power supplies, in addition to injecting noise (hum or hiss), can deviate from
perfect (no change in voltage level nor current availability regardless of the
demand requested). Worse, the power supply can deviate in a manner that, of the
three, has the greatest effect on the amplifier sound. And note that these
artifacts do not show up on any traditional steady state measurements (THD, IMD,
or frequency response). Our listening tests have shown that the power supply
performance contributes more to the overall sound of the amplifier than any
other single factor - more than the tube choice, circuit topology (cascade,
cascode, SRPP, Hybrid, MU whatever) and more than the choice of passives. It
is clear that although these other factors do have a contribution, many end
users become engaged the art of part rolling (tubes, capacitors, and
resistors) are simply “tuning” their circuits to counterbalance and
compensate for poor power supply design.
When called upon by the tube, the power supply must instantaneously
deliver the exact (not more, not less) current requested through the resistor. It
must not differentiate if the demand is at 10 Hz or 1 MHz. It must do so
without any delay and it must do so without being sent into a reactive short
oscillatory condition (called ringing). Any failure to meet these
requirements alter the current expected into the resistor and will be heard
creating a characteristic “sound” to the circuit. That sound may be
hard, sterile, thin, or on the other extreme, soft, fat, recessed depending upon
how the power supply performs.
Each of the power supply components including the line
filter, transformer, rectifiers, choke (if utilized), and certainly the
smoothing capacitors and regulators (if utilized) all have a role in the sound
of the power supply. As the amplifier (tube and plate resistor) “looks back
into” the power supply it would like to see a pure DC voltage source of zero
ohms at any frequency. In reality it sees an impedance curve that varies from
milli-ohms to tens of ohms across the audible frequency range. The degree that
it deviates from flat in many “high end” audio circuits is frightening –
you would never purchase an amplifier that specified an equivalent frequency
response. We have found that the impedance frequency response of a power supply
is very close to the mirror image “sound” of the total amplifier using that
power supply. And as the power supply’s components change in value and
material of construction, the impedance is modified causing a direct effect on
the sound.
Now comes the regulator to the rescue. A regulator is an
electrical circuit designed to provide all the current needed as demanded by the
amplifier without any fluctuation in the DC voltage output – essentially a
zero impedance voltage source.
To do this regulators combine a DC amplifier with lots of
feedback – the feedback “watches” the output voltage and compares it to a
“reference” voltage (the regulator’s “input”). Should the DC amplifier
detect any change at the output via its feedback path, it is designed to (as
quickly as possible) restore the desired voltage. Notice that it can only
correct the voltage after it sees the change. What do you think happens
if the regulator responds slower than the audio signal causing the current
demand? Dull and sloppy sound. However this is rarely the problem since high
frequency solid state devices are readily available. Most times a far more
serious problem is triggered – ringing. Ringing happens when the
regulator overcompensates and sends in more current than requested.
To visualize ringing you need to imagine that most music is not simple sine waves but more often a complex collection of quickly rising pulse like waveforms. These steeply rising pulses of current demand, appearing from the tube’s plate, expect the power supply to respond in kind with exactly the current needed. Regulators operating with high speed high feedback DC amplifiers, when placed in this quick demand position, tend to overshoot their intended target – they quickly respond and actually deliver much more current that is called for. The equivalent of an “off sides” penalty from an aggressive defensive lineman. Afterward, and trying to recover, the regulator goes into a short burst of oscillation from supplying too much to too little and then again too much current. This goes on for a few cycles until it finally stabilizes. To you and me the “outburst” sounds like hardness and edginess.
CAE has elected to utilize high-speed wide bandwidth
regulators in all of our designs. Our “reference” product line employs our
patented high-speed wide bandwidth servo design. We are flattered to observe
that it has been “cloned” by others and humored by the fact that their
“subtle” changes (such as the substitution of a power MOS-FET in the pass
element) has compromised its damping resulting in ringing thereby giving it a
“sterile” signature.
Our design incorporates a floating 317 pre-regulator
that provides an ultra stable voltage reference and bias supply to a bank of
high speed proprietary operational amplifier slaves. (Many of our customers have
used these same op-amps in their crossovers and CD players telling us they are
“the most musical” they have auditioned.) Each high speed operational
amplifier is coupled to a bipolar transistor (not MOS-FET) to form an unconditionally
stable voltage source that exhibits near zero impedance over an ultra wide
bandwidth (from DC to well beyond 1MHz). More importantly the circuit is critically
damped to deliver instantaneous response to a step current demand without
any ringing whatsoever. We place a slave regulator at every stage and every
channel of the associated amplifier. In this way each regulator is responsible
only for the current demands of the stage supported. For example, our Sarah
reference phono preamplifier is comprised of three stages of amplification for
both channels (right and left). In this case six independent op-amp “slaves”
are incorporated into the design.
The result is that each stage of amplification is provided
a foundation and environment of complete isolation form adjacent stages (except
where the signal is supposed to couple – at the input and output) and
supported by a power source that provides the exact current requested, on time
and without overhang.
To us listeners, it sounds accurate, musical, three dimensional, and dynamic.