Empirical audio Overdrive SE DAC User Manual

Empirical Audio

Overdrive SE DAC

User Manual

© 2012

Overdrive® SE DAC User Manual

Critical – read all red warnings first to prevent equipment damage

1. Technology/Design

The Overdrive DAC incorporates our excellent low-jitter USB interface technology and
breaks new ground for D/A converter technology. The Overdrive design philosophy is
“less is more”, with one of the simplest analog paths ever devised. The Overdrive
provides solutions for the three most important issues with digital sources:

•

Jitter

•

D/A noise/distortion

•

Volume control noise/distortion

This high-performance triple solution is unprecedented and delivers a totally new sound
quality level. The Overdrive is a non-upsampling DAC (no ASRC) that has an ultra­simple analog signal path, reducing background noise and harmonic distortion to
unprecedented levels. It incorporates a preamp function that has never been tried in
ANY audio product before. This preamp function adds no devices whatsoever to the
signal path, leaving it simple and ultra-low noise. It has selectable digital filters that allow
one to tailor the sound to your personal taste. You can effectively eliminate them if you
want.

The sound of the Overdrive is simply indescribable and utterly magical. The clarity is
unlike anything you have heard before, particularly driving amps directly. Other DACs
sound muddy in comparison. The bass is tight and the detail and imaging is more than
first-rate, it’s breathtaking. Drums are rendered with more realism than any other DAC is
capable of. The leading and trailing edges and harmonics of percussion, strings and
woodwinds are delivered with true realism. Even with this level of detail and dynamics, it
still gets vocals right. The Overdrive USB DAC resets the bar for the term “accuracy” in
digital to analog conversion.

Empirical Audio Rev. 1.0

1.1. Performance

The performance of the Overdrive DAC is a result of several innovative design
breakthroughs:

1. Low jitter USB interface module

2. Ultra-Simple pure Class-A analog Path

3. D/A Volume Control

4. Ultra-Linear low-noise Output Stage

5. Fully Balanced Analog Path

6. Selectable digital filters

1.1.1. Low jitter Asynchronous USB interface module:

This asynchronous USB module is the same device that we use in our popular
Off-Ramp 5 USB converter. This is a 5th generation USB converter and rates #1
against all other converters on the market. The USB interface enables bit-perfect
playback from Mac and PC, supporting sample-rates up to 24/192.

1.1.2. Ultra-Simple Analog Path:

The analog path consists of an I/V stage (ultra-low noise op-amp) driving a single
output transistor through a coupling cap. All stages are run Class-A, so it gets a
bit warm. There are no resistors at all in the analog signal paths, except for very
small necessary damping resistors on the output. Resistors add noise, so they
are best avoided.

1.1.3. D/A Volume Control:

The revolutionary volume control is accomplished without any added parts to
the line-output signal path. It is not an entirely digital or entirely analog volume
control. It is a little of each actually. The volume is adjusted by changing the
reference voltage of the D/A conversion. It does NOT adjust attenuation of a
resistor divider, change the gain of an amplifier stage or truncate bits in the digital
data. All of these would add noise and/or distortion to the signal. This volume
technology actually reduces S/N ratio with lower volume.

1.1.4. Ultra-Linear Output Stage:

The output stage is a single transistor in Class-A mode. It is well known that this
simple configuration results in some compression distortion, but also sounds
most natural. In order to effectively eliminate this compression distortion, we
incorporated a compensation circuit in the Overdrive. The output stage
impedance is low enough and the power output high enough to drive most high­impedance headphones easily, so we provide an adapter. It can drive either
normal or balanced headphones such as Sennheiser.

1.1.5. Fully Balanced Analog Path:

There are 4 analog paths, all identical. They connect from the D/A converter chip
to the outputs. All 4 are used for the balanced outputs, 2 of the 4 are used for the
Single-Ended outputs. Resistors provide some isolation, so all outputs may be
used. We recommend low-capacitance interconnects if all 4 outputs are used,
and shorter is better. The output drive is sufficient for all preamps and amps, and
even high-impedance headphones like Sennheiser.

1.1.6. Selectable Digital Filters:

A front panel switch selects 3 digital filter responses, low, medium and high. For
most DACs these are automatically selected, low for 44.1, medium for 96 and
high for 192. However, the Overdrive allows these to be selected manually in
order to improve audio quality. This makes the Overdrive closer to a NOS DAC.

For sample-rates of 176.4 and 192, the high position MUST BE SELECTED.

It is recommended that the high position be selected for all sample-rates,
however some users may find that some ultrasonics are audible at 44.1. For
these, the medium or low settings can be used. The settings do not engage until
the track is stopped and restarted.

1.2. Design Choices:

A number of design decisions were made in order to optimize performance and sound
quality over all other criteria:

1.2.1. Small Chassis size

This was critical to reducing both digital and analog signal path lengths. Long
signal paths add distortion due to circuit-board dielectrics and also introduce
more noise due to ground-plane coupling and crosstalk.

1.2.2. Op-Amp I/V conversion

This was necessary in order to optimize the loading of the D/A converter. Some
D/A chips require certain loading and voltage on their outputs in order to
guarantee best linearity and low-distortion.

1.2.3. AC-coupled output

This was chosen because the alternative of DC-coupling would not allow a pure
Class-A operation throughout, which delivers lower distortion. The coupling
capacitors available now are very close to a copper wire in performance.

1.2.4. Separate Substation AC chassis

This is a key element contributing to the performance of the Overdrive SE. A
separate Substation enables more densely packed energy storage for each
voltage, more optimized layout and creates space between the magnetics in the
supplies and the Overdrive Chassis to eliminate coupling.

2. Break-in

All electronics needs a period of break-in. New capacitors and dielectric insulators must
stabilize with voltage and current applied. The Overdrive SE is broken-in by Empirical Audio
for approximately 5 days before shipping. An additional 25 days of 24/7 break-in is required
for full break-in, with all inputs driven. Each input breaks-in independently, so all should be
driven with a signal. Outputs do not have to be connected in order to break-in.

3. Volume settings

3.1. Volume Control

The volume control mode can be selected by using the right-most recessed toggle
switch on the front panel. Up position selects volume control. The switch selects
between volume or line-out on power-up only, not on-the-fly. To make a change, you
must power off the Substation and back on using the front-panel power switch. As with
all systems, at initial power-on, the volume should be rotated to its lowest setting before
playing music.

3.2. Optimum Manual Control

Manual control can be optimized for performance if the volume knob is near maximum.
The internal gain jumpers behind the front panel should be selected so that the volume
knob is rotated 50% at normal listening levels. The default settings for these jumpers is
medium.

3.3. Optimum Remote Control

Remote volume control capability can be achieved quite easily by using a combination of
the manual volume control, a digital volume control (such as iTunes, Amarra or Pure
Music volume) and the USB interface. If one adjusts the volume to a comfortable
listening level with the manual control with the software digital control at max, then only
small reductions of the digital control from maximum are necessary to adjust the track­to-track amplitude variation. Up to –9dB of reduction is possible without degradation in
sound quality with a high-quality digital volume. This is even better than having yet
another remote control. Most of us already have too many of those. It allows you to use
an iPod Touch or iPad for everything: song selection, muting and volume control,
without any loss of resolution. This is the best possible scenario.

3.4. Gain Switch

A two-position gain switch is recessed into the front-panel. It is the left-most of the two
switches. The upper position is low-gain, which is typically used for volume control
direct to amps. The lower-position is high-gain, typically used with volume driving amps
directly to low-efficiency speakers, or line-level to a preamp. This can be switched at
any time, but will only take effect after powering down the Substation and back up again
using the front-panel power switch. For driving most amplifiers directly with 1V
sensitivity for full power, the switch should be in the “L” position. If you have low­sensitivity speakers, you may need the “H” setting.

3.5. Range Jumpers

Two jumpers are provided to set the volume range. The range jumpers are located just
inside the front panel to the right of the volume control as you look at the front panel.
The settings are illustrated on the circuit board. Three volume ranges are possible. The
range setting provides a lower volume extreme. It does not affect the maximum
volume, only the minimum volume. If the range jumpers are set for “Min”, the volume