Power Supplies In Audio Digital To Analog Converters
Article By Benchmark Media Systems, Inc.
is an audio digital-to-analog converter. This application note explains the
power supply configuration inside Benchmark's DAC2
D/A converter. In part
1 of this series we discussed the importance of the analog
section of an audio converter. In part
2 we discussed the unique high-headroom digital processing chain
inside the DAC2.
The analog and digital systems each contribute toward Benchmark's overall
goal of transparent musical reproduction, but this goal can only be reached when
these systems are supported by a well-designed power supply system. Power
supplies can adversely affect the performance of these critical analog
and digital systems. As the resolution of D/A converters has improved,
power requirements have become more challenging. In many cases, the classic
solutions (linear power supplies, line-frequency transformers, and large banks
of capacitors) fail to deliver adequate performance in a D/A converter with
a 125 to 130 dB signal to noise ratio (SNR).
DAC2 Internal View
Switching Power Supplies
Benchmark has transitioned to switching power supplies because they
have the potential to be much quieter than conventional linear power
supplies (see Audio
Myth "Switching Power Supplies are Noisy" for a more detailed
discussion of this topic).
Conventional power supplies have large magnetic components
that operate at the AC line frequency (50 - 60 Hz). Magnetic emissions from
these components are the primary source of the AC hum that can be
heard in the noise floor of most audio products. In contrast, the DAC2 has a
natural "white" noise floor without any audible traces of AC hum. To
achieve a white noise floor, the AC line-related tones must be at least 20 dB to
30 dB lower than the noise measured across the entire audio band. The DAC2
has an A-Weighted SNR of 126 dB, while AC line-related hum is below -160 dB,
relative to full output. This means that the AC line-related hum is at
least 34 dB lower than the idle-channel noise of the DAC2
(easily achieving a virtually perfect white noise floor). This can be seen in
the following FFT plot taken from the DAC2
DAC2 Idle Channel Noise Spectrum
A similar plot for the DAC1
(from the DAC1
manual) shows that AC line-related hum measures -128 dB, relative to full
DAC1 Idle Channel Noise Spectrum
This comparison shows that AC line-related hum was reduced by
more than 30 dB by using switching power supplies in the DAC2!
This is why we say that it
is a myth that linear supplies are quieter than switching supplies.
Video Demonstration - Seeing Is Believing!
Two months ago, we released a video
demonstrating the magnetic immunity of star-quad
microphone cables. We exposed the cables to the stray magnetic fields
produced by a variety of power supplies, including some rather noisy low-cost
switching supplies. We also exposed the cables to the fields produced by a DAC1
and a DAC2.
produced magnetic interference, but the DAC2
did not. The difference? The DAC2
has a switching power supply that is optimized for audio application while the DAC1
has a traditional linear power supply. The video shows that the switching power
supply in the DAC2
is much quieter than the linear power supply in the DAC1. The
comparison is not even close! Sometimes seeing is believing!
Watch a short clip from this video and help put an end to
another audio myth!
Switching Power Supplies Must be Optimized for Audio
But, it is important to understand that switching supplies
must be specially designed to be quiet within the audio band. To do this,
Benchmark operates the magnetic components at frequencies ranging from 200
kHz to 1 MHz. This keeps the power supply noise well above audio frequencies
where it can easily be removed with analog filters, if necessary. More
importantly, the power supply interference is much lower when the magnetic
components are operating at high frequencies. For a given power requirement,
transformer size decreases as the switching frequency increases. The magnetic
components in the DAC2
are very small. The magnetic fields emitted by these components are
much smaller than the magnetic fields emitted by the large toroidal
transformer in the prior generation DAC1
converter. The following photo shows the large transformer used in the DAC1:
DAC1 Internal View
Power Supply Noise in Audio Systems
Power supplies can add noise to the audio through one or more of
Conducted interference is
caused by noise voltages that are conducted through the power supply
connections. Power supply output filters and bypass capacitors (capacitors
between the power supplies and ground) can help to mitigate conducted
occurs when a noise voltage is capacitively coupled from one conductor to
another. A power supply conductor may capacitively couple to adjacent audio
conductors. This type of radiated interference is easily mitigated with the use
of ground planes and shielded conductors. Shielding becomes more difficult
as the interference frequency increases. Switching supplies can emit more
electrostatic emissions than linear supplies, but with the proper choice of
switching frequencies, this noise can be entirely above the audio band. If the
interference is above the audio band, it can be removed with filters that will
have no impact on the audio.
In an audio product, magnetic interference is often the
most problematic because it is difficult to mitigate. Magnetic
fields radiated by a transformer can directly induce currents in sensitive sections
of an analog audio circuit. If this interference is line-frequency interference
(from a linear supply), it will fall within the audio band where it cannot be
easily separated from the audio. On the other hand, if this
interference comes from a high-frequency switching power supply, it will lie
above the audio band where it can be filtered out without impacting the audio.
Electrostatic shielding will not block magnetic interference.
Audio circuits can be designed
for immunity to conducted, electrostatic, and magnetic interference.
Immunity to Conducted
Audio circuits can be designed
to reject conducted noise voltages on the power supply rails. Power supply
rejection ratio (PSRR) is a measure of a circuit's ability to reject noise
voltages on the power supply rails. The audio circuits in the DAC1 and DAC2
converters are designed to have a PSRR of more than 100 dB at AC line
frequencies. We can test the PSRR by applying a noise signal to a power supply
rail while measuring the noise at the output of the audio circuit being driven
from the noisy rail. For example, if we apply a 1 volt 60 Hz sine wave to the
+18V or -18V analog supply rails in a DAC1
this will produce a voltage that is less than -100 dB volts (0.00001 volts) at the
analog outputs. This is a PSRR of 100 dB at 60 Hz.
Immunity to Electrostatic
Audio circuits can be shielded
to reject electrostatic coupling of noise voltages. Benchmark products use
6-layer printed circuit boards (PCBs) with a unique inside-out construction. The
outer two layers are connected to ground and provide a shield above and below
the signal traces that are carried almost entirely by the four internal layers.
For this reason, there are almost no signal traces visible from the top or
bottom of Benchmark circuit boards. Unused space on the internal layers is
also filled with a ground plane. Sensitive traces are surrounded on all sides by
ground plane or grounded guard traces. The outer edges of our circuit boards
have vertical connections (vias) between the ground layers that form an
electrostatic picket fence around the perimeter of the circuit board. Many
additional via connections tie all sections of the various ground planes into a
single integrated shield. This shield is highly effective at blocking
electrostatic interference from the power supply and other external sources.
This shield also prevents the emission of noise from digital circuits. For this
reason, Benchmark products will often pass EMI
emissions tests with the chassis cover removed.
Immunity to Magnetic
It is important to note that
electrostatic shielding does not block magnetic interference. Magnetic immunity
requires the use of expensive magnetic shielding materials and/or the use of
symmetrical geometric and electrical structures that cancel the effects of
magnetic coupling. Star-quad
XLR cables have a symmetrical geometry that
causes a precise matching of the magnetic interference on the + and - legs of
the audio interconnect. When these legs feed a balanced input, most of the
magnetic interference is removed. Benchmark has extended this technique to
interconnects that are internal to the printed circuit board. In our microphone
preamplifiers we connect the XLR inputs to the preamplifier circuit using
star-quad traces on two of the internal circuit board layers. We use the same
technique in reverse, inside the AHB2
power amplifier. High-current circuits and their high-current ground returns run
through a quadrupole structure of four traces. This symmetrical structure
cancels the magnetic fields that would otherwise be emitted by the
high-current traces. The XLR input signals on the AHB2
run through a similar quadrupole structure on their way to a
precision balanced input amplifier. The AHB2 also
includes two magnetic shielding plates that are fabricated from a material that
is designed to attenuate magnetic fields. The magnetic components in the DAC2
and AHB2 are
encapsulated in ferrite housings
that help to minimize radiated magnetic fields.
Audio circuits do not place a constant load on a power
supply. The loading changes with every musical peak and it is not unusual to
have some audio conducted through the power supply rails. If bypass capacitors
are placed near each audio buffer, the higher audio frequencies can be shunted
to ground in order to reduce the audio signal on the power supply rails. The
lowest audio frequencies must be removed by the regulators within the power
supply. Voltage regulators attempt to maintain a constant voltage while the load
has a distributed regulator system. There are 20 separate voltage
regulators within the DAC2.
Each is dedicated to one specific subsystem. This segregation minimizes
crosstalk between subsystems and eliminates the need to deliver regulated
voltages over long distances. Voltages are regulated at the point of load.
The ES9018 D/A converter chip has two voltage
reference inputs that have a PSRR of 0 dB (no rejection). These left and right
reference inputs are very sensitive to noise. Any noise on these
inputs will also appear on the output pins of the ES9018. To mitigate this potential problem,
we use a precision voltage reference, a multistage passive filter and a
high-bandwidth low-noise buffer to regulate each of these reference inputs. This
custom regulator is not the low-cost cookbook solution that you will find in
most other products that feature the ES9018. This Benchmark regulator is one key
to the low noise and low distortion delivered by the DAC2 converter.
The strategy within the DAC2
|Minimize sources of
|Keep the interference above
the audio band|
|Maximize immunity to
|Provide separate regulators
for each subsystem|
Switching supplies operating above 200 kHz minimize the power
supply noise and keep it out of the audio band. The analog circuits are designed
to reject noise on the power supply rails, and the rails include bypass
capacitors at every point of load. The printed circuit board uses a special
inside-out construction that provides a complete Faraday
cage for the audio circuits. Each critical subsystem has its own voltage
regulators. These regulators separate analog, digital, and clock loads to
prevent interactions. The left and right voltage reference inputs on the ES9018
are the most critical points in the system. These inputs are equipped with a
regulator that Benchmark created specifically for this task.
Never judge a D/A converter by the choice of a D/A conversion
chip. The power supply, the analog processing, the digital processing, and the
circuit board layout all contribute to the overall performance of the D/A.