Okto Research DAC8 PRO User manual

dac8 PRO
A versatile, high-performance 8-channel D/A converter
Owner’s Manual

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Contents
Specifications ..............................................................................................................................................................................................................3
Mechanical drawings................................................................................................................................................................................................5
Back panel overview.................................................................................................................................................................................................6
Package contents ......................................................................................................................................................................................................6
Digital connections....................................................................................................................................................................................................7
USB...........................................................................................................................................................................................................................7
Drivers.............................................................................................................................................................................................................7
AES/EBU inputs.....................................................................................................................................................................................................9
Signal integrity.............................................................................................................................................................................................9
SPDIF compatibility................................................................................................................................................................................... 10
AES/EBU output ................................................................................................................................................................................................. 10
Analog Connections............................................................................................................................................................................................... 10
Main outputs....................................................................................................................................................................................................... 10
Driving power amplifiers directly......................................................................................................................................................... 10
Volume operation......................................................................................................................................................................................11
Interfacing with legacy devices with single-ended inputs only...................................................................................................11
Headphone output ........................................................................................................................................................................................... 12
Input modes.............................................................................................................................................................................................................. 13
Pure USB............................................................................................................................................................................................................... 13
USB / AES............................................................................................................................................................................................................. 13
Pure AES............................................................................................................................................................................................................... 14
Controls ...................................................................................................................................................................................................................... 15
Rotary control ..................................................................................................................................................................................................... 15
Remote control................................................................................................................................................................................................... 15
User interface and menu system........................................................................................................................................................................ 16
Main screen, left................................................................................................................................................................................................. 16
Main screen, right.............................................................................................................................................................................................. 17
Main menu........................................................................................................................................................................................................... 17
Volume submenu.............................................................................................................................................................................................. 18
Routing submenu.............................................................................................................................................................................................. 18
System submenu................................................................................................................................................................................................ 19
Displays submenu..............................................................................................................................................................................................20
Filters submenu..................................................................................................................................................................................................20
Status submenu..................................................................................................................................................................................................23
Advanced submenu..........................................................................................................................................................................................24
Mains voltage selection.........................................................................................................................................................................................24
Troubleshooting.......................................................................................................................................................................................................25
Issue 1: USB input not recognized................................................................................................................................................................25
Issue 2: Dropouts during AES/EBU playback............................................................................................................................................25
Service and warranty..............................................................................................................................................................................................26
Manual revisions......................................................................................................................................................................................................27

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Specifications
Digital inputs / outputs:
-USB
-8 output and 8 input channels
-asynchronous, UAC 2.0 compliant
-playback data formats
-PCM, up to 192kHz / 32-bit
-native DSD, 64x / 128x
-DSD over PCM (DoP), 64x
-recording data format
-PCM, up to 192kHz / 32-bit
-AES/EBU
-4 AES/EBU inputs (SPDIF compatible)
-up to 192 kHz / 24-bit
-resampling-free (i.e. requiring a single signal source)
-unique USB/AES input mode ensures a bit-perfect data flow from
AES/EBU to USB, allowing for a real-time signal processing or
simultaneous recording & playback with no resampling applied
-1 AES/EBU output
-fixed to playback channels 1 and 2 (of USB or AES/EBU, depending on
the selected input)
Analog outputs:
-Main outputs (back side)
-8 x Neutrik gold-plated XLR connector
-output level: 4.2 V RMS differential full-scale
-output impedance: 200 Ohm (differential) or 100 Ohm (SE)
-capable of directly driving power amplifiers
-Headphone output (front side)
-6.3 mm (1/4”) TRS connector
-Output level: 4.2 V RMS full-scale
-120 mA linear output current
-fixed to output channels 1 and 2
-output impedance <100 mOhm

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Analog performance
-0.000032 % / -130 dB THD (Total Harmonic Distortion) full-scale, 1kHz
-0.00011 % / -119 dB THD+N (Total Harmonic Distortion + Noise) or 119 dB SINAD
(full-scale, 1kHz)
-125 dB of dynamic range
-0.000224 % / -113 dB THD+N @ 100 mW into 32 Ohm headphone output
Menu functions
-Individual volume for each of the 8 output channels (relative to the master volume)
-Routing matrix (any input channel to a specific output channel)
-Upper volume limit
-Remote control learning
-Choice between 7 PCM reconstruction FIR filters
-Configurable display timer to dim or turn off the displays
-Configurable Auto on / Auto off with non-zero audio data detection
-Adjustable 2nd and 3rd order THD compensation
Other features
-Trigger output
-5 V voltage level (works with most 12 V trigger inputs)
-short-circuit proof with 15 mA maximum current
-pulse or continuous
-Full control with either rotary or remote control
-Rack-mountable with optional 2U, 19” rack mounting brackets
Dimensions
-446 x 183 x 50 mm, (17.56 x 7.2 x 1.97 in), excluding removable feet
-Weight: 4.5 kg (10 lbs)
Power
-Mains voltage: 120 V AC or 230 V AC, according to the mains voltage of the country
from which the order was placed, unless requested otherwise
-Standard 3-pin IEC 60320 power connector
-Power rating: 15 W
-Fuse: F400mA

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Mechanical drawings

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Back panel overview
Package contents
-dac8 PRO (silver or black depending on your choice, 120 V or 230 V according to the
mains voltage of the country from which the order was placed, unless requested
otherwise
-rack mounting brackets (optional)
In an effort to reduce electronic waste, we've decided not to include the standard disposable
USB and power cables in the package. We understand that audio enthusiasts often already
own their preferred cables.

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Digital connections
USB
The dac8 PRO is compliant with USB Audio Class 2 (UAC2), a widely used digital audio
interface standard that enables asynchronous operation. This feature allows the DAC to
control data flow according to its own high-precision oscillator, bypassing any requirements
for the precision of the clock in the source device or cables.
Drivers
The UAC2 standard also enables operation with Mac OS and Linux without the need for
installing additional drivers. The dac8 PRO will be recognized as an 8-channel playback
device and an 8-channel recording device. The only action required by the user is to select
the dac8 PRO in your operating system's audio control panel.
This also applies to Windows 10 and newer; however, we strongly recommend installing the
Okto Research ASIO driver to bypass the Windows sound mixer and enable exclusive access
to the device for bit-perfect, low-latency operation. The driver can be downloaded from the
following link:
https://oktoresearch.com/assets/OktoResearchUsbDriver.zip
To install it, extract the executable file from the archive, run it and go through the steps.

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After a successful installation, the Okto Research icon will appear in your system tray. If the
unit is connected to the computer, it will be recognized by the driver, as shown in the
following image.
Please note: this driver will only work with units running firmware 1.5 and newer. Units with
older firmware have different USB identifiers and will not be recognized. Please contact us
to receive an ASIO driver compatible with older firmware versions.
Once the installation is complete, the "Okto Research USB device" will become available in
any ASIO-capable playback software. The following image shows the output device setting
in JRiver:
To reduce latency, navigate to the USB driver's control panel, select the "Buffer Settings"
tab, and change the buffer size in the dropdown menu. The dac8 PRO is tested to work with
1.5 ms output latency for playback and 1.67 ms for recording; however, the host computer
must be able to provide data in a timely manner to avoid buffer underruns and dropouts
during playback.

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Warning: The USB input of the dac8 PRO may not work correctly when connected to a USB
hub or a legacy USB 1.0 port. Be sure to use a USB 2.0 (or newer) port when connecting the
unit.
AES/EBU inputs
The dac8 PRO offers four AES/EBU inputs that are free of resampling. This allows for bit-
perfect playback, as well as recording at the USB host side. However, it also requires the
inputs to be mutually synchronized and carry signals with matching sample rates.
Due to the device's design, there is a specific order in which individual AES/EBU inputs must
be connected. If only one input is used, it must be the first input; if two are used, they must
be inputs 1 and 2. There are no further requirements for inputs 3 and 4.
Signal integrity
AES/EBU (as well as SPDIF and TOSLINK) is a synchronous connection, meaning that the
signal source device also serves as a clock source. Moreover, the clock signal is encoded
into a single bitstream with audio data using biphase mark code. This places the
responsibility of reconstructing the clock signal on the receiving device, and in the case of a
D/A converter, deviations can lead to undesirable noise or spurious signals at the output.
Although the dac8 PRO employs the highly effective Sabre jitter elimination algorithm, it is
recommended to ensure the best signal integrity possible. One important and commonly
overlooked factor is cable impedance. The source and termination impedance for AES/EBU
is 110 Ohms, and while reception will likely work with a line cable, using a dedicated cable
with 110 Ohm impedance is highly recommended.

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SPDIF compatibility
The AES/EBU inputs of the dac8 PRO will also accept a signal with the SPDIF voltage level
of 0.5V peak-to-peak. The termination impedance requirements differ between the two
standards, and while the SPDIF signal will generally be received correctly, it is recommended
to either employ an impedance-matching transformer or use a 75 Ohm RCA –XLR cable
and solder a 240 Ohm resistor to the XLR connector on the receiving side, effectively
decreasing the termination impedance from 110 Ohms to 75 Ohms.
AES/EBU output
The AES/EBU output is always active, transmitting channels 1 and 2 of the audio data
currently being played by the dac8 PRO, with the master volume applied. The per-channel
volume value is not taken into account.
Analog Connections
Main outputs
The main analog outputs provide 8 channels that represent 8 signals from the selected
digital input, converted to the analog domain with extremely high precision.
The main outputs are not muted when the headphone jack is plugged in. The reason for
this is that the operational amplifiers driving the main outputs are also responsible for driving
the headphone amplifier.
Driving power amplifiers directly
Low output impedance allows the unit to directly drive inputs of power amplifiers without
the need for a pre-amplifier. In such a setup, the full-scale output voltage of the DAC, which
is 4.2 V RMS in the case of the dac8, and the input sensitivity (voltage required for full power)
of the power amplifier should ideally match to optimally utilize the dynamic range of the
devices. Below are two examples of how the dynamic range can be negatively impacted if
there is a significant difference between the two values:

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Scenario 1: Consider a power amplifier with 1 V RMS input sensitivity. The limitation might
occur in the power amplifier if the dac8 PRO volume is set to -12 dB or higher. Consequently,
the upper 12 dB of the dac8 PRO's output would remain unused. By lowering the output
voltage of the dac8 PRO to 1 V, its noise floor could be reduced by 12 dB, preserving the
dynamic range.
Scenario 2: Consider a power amplifier with 8 V RMS input sensitivity. In this case, the dac8
PRO wouldn't be able to utilize the full power of the amplifier, resulting in a loss of 6 dB in
attainable sound pressure level. Elevating the dac8 PRO's output voltage to 8 V RMS would
enable it to harness the full power of the amplifier, optimizing the dynamic range of the
setup.
The custom output voltage is available as an optional feature at an additional cost. This
value, determined by a significant number of small surface-mount resistors, cannot be
adjusted by users. The selectable range lies between 1 V RMS and 10 V RMS.
The outputs are protected against long-term short circuit for the default or lower output
voltage. However, we advise against stressing the outputs with a long-term short circuit.
Volume operation
The volume control consists of a master volume (displayed on the right screen) and per-
channel volume, accessible in Menu > Volume. The resulting value for each channel can be
calculated by adding the two values. To bypass the volume control completely, simply set
all the values to 0 dB.
Interfacing with legacy devices with single-ended inputs only
We highly recommend using balanced cabling between analog audio components
whenever possible. The main reason is that, due to the different reference, ground is not a
part of the balanced signal path. This makes equalizing currents created by ground loops
and electromagnetic interference, which would otherwise be injected into the signal,
harmless.
If you need to interface the dac8 with a legacy pre-amplifier or a power amplifier that has
single-ended inputs only, there are three methods to do so:
1) A solid-state conversion circuit, for example the Neurochrome Universal Buffer
2) Audio transformers like Lundahl LL1584-3FXPHM

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3) A special adapter cable with floating pin 3, like “Benchmark XLRF to RCA cable for analog
audio with pin 3 floating”. The cable does not perform actual signal conversion; instead,
it only uses one half of the balanced output from the dac8 without shorting the other
half to ground. This method will result in a loss of performance, but it is recommended
as a cost-effective way to connect subwoofers.
Warning: We strongly discourage the use of balanced-to-unbalanced cables and
converters. Doing so will result in significantly decreased analog performance due to the
short they present to the output of the dac8 PRO's analog stage, potentially overloading it
and risking damage after long-term use with high signal levels. Warranty does not apply to
units with analog circuitry damaged by using XLR to RCA cables and converters.
Headphone output
The headphone analog output, available on the front panel via a 6.3 mm (1/4") TRS
connector, provides a 4.2 V RMS full-scale voltage level, up to 120 mA linear current, and
less than 100 mOhm output impedance.
It is suitable for a majority of headphones and earphones, ranging from 16 Ohm IEMs to
large 600 Ohm open-back headphones. As mentioned in a previous chapter, plugging in
the headphone jack does not mute the main outputs.

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Input modes
The dac8 PRO supports two digital interfaces: USB Audio Class 2 and AES/EBU. There are
three available input modes: Pure USB for USB playback, Pure AES for AES/EBU playback,
and a combined mode called USB/AES. The USB/AES mode takes advantage of the bi-
directional operation of USB Audio Class 2, allowing for simultaneous playback from USB
and recording from AES/EBU. The diagram below illustrates the data flow in each mode,
with the clock symbol signifying the source of the clock signal in each scenario.
Pure USB
The Pure USB input mode allows 8-channel playback from the USB host using the dac8
PRO's internal low phase noise oscillator as the clock source. This means that the dac8 PRO
is in control of the playback speed by providing feedback to the host.
USB / AES
USB/AES is a unique mode of operation that utilizes the clock recovered from the 1st
AES/EBU input as feedback for the USB host to set the pace of the playback. This ensures
that the data retrieved by the AES/EBU inputs are transmitted in a bit-perfect manner to the
USB host through the eight USB input channels to be either recorded or processed and then
sent back to the dac8 PRO through 8 playback channels.
Because the UAC2 audio device is unable to change the USB host current sampling
frequency, the dac8 PRO will display a warning message, "fs mismatch," if the received

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AES/EBU signal's sampling frequency does not correspond to the USB host driver's current
frequency. User may be required to take action to ensure that the two sample rates match.
Pure AES
Pure AES mode enables playback from the eight digital audio channels available on the four
AES/EBU input connectors. It utilizes the clock recovered from the first AES/EBU input for
processing the received digital audio data, and the dac8 PRO's internal low-phase noise
oscillators for the actual D/A conversion. The dac8 PRO's jitter eliminator is active in Pure
AES mode, ensuring the highest possible jitter rejection and performance.
A USB connection is not required for operation. However, if the sample rate of the signal
coming in through the AES/EBU inputs matches the USB driver's sample rate, recording
from the AES/EBU inputs will also be available, similar to the USB/AES mode. Since this is
not the primary purpose of this input mode, an error message will not be displayed if the
sample rate condition is not met.

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Controls
The dac8 PRO can be operated using either an integrated push-on switch rotary control or
an IR remote. Thanks to the 'Learn Remote' menu function, it can pair with any IR remote
with a minimum of seven buttons. For the best user experience, we recommend using the
Apple Remote or a similar substitute.
Rotary control
Device state
Off
Main screen
Menu
Menu,
selected
Input
Left/right turn
-
Volume
control
Navigation
Modify
Push
Turn on
Enter menu
Select
Confirm
Long push
Turn on
Turn off
Back
Confirm
The navigation direction is intentionally different for the left and right screens with respect
to the movement direction of the cursor.
Remote control
The dac8 PRO can be paired with any remote that has a minimum of seven buttons. For a
detailed description of the pairing process, please refer to the “System submenu” section.
The image below illustrates button functionality as demonstrated with the Apple Remote

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Short press: turn on, mute
Long press: turn off
(or three subsequent presses in case of
some remotes that don’t use the NEC
protocol)
Enter menu, exit
menu
Select, confirm
User interface and menu system
Warning: All changes made in the device menu will be saved at the moment the device is
turned off using the central control knob or infrared remote control. The changes will not
be saved if the device is turned off by disconnecting from the power supply. This is to protect
the flash memory from unnecessary writes, which increases its lifespan.
Main screen, left
-Input mode indicator: displays the selected input mode
-Audio data information: displays sample rate, bit depth and data format (relevant to
the input mode)
Volume- / Menu
down
Volume+ / Menu
up
Input change / Menu
back
Pure USB
96 kHz 32 bit PCM
1 2 3 4 5 6 7 8

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dB
max
-Per-channel playback indicator: displays the presence of digital data on the input of
each of the 8 channels
Main screen, right
-Master volume: controls volume of all analog outputs as well as AES/EBU digital output.
The displayed value is in dB, relative to the nominal full-scale output.
-range: 0 to -99 dB (default: -50 dB)
-Mute icon: appears when mute is enabled
-Min / max indicator: only appears if the volume reaches upper limit (set in the Menu >
Volume > Lim) or minimum value (-99 dB)
Main menu
The main menu will appear on the left screen upon pressing the knob, or by pressing the
Menu button on the remote control.
-32
Volume
Routing
System
Displays
Filters
Status
Advanced

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Volume submenu
-Mute: mutes all the analog outputs (default: off)
-Limit: sets the maximum allowed volume level
-range: 0 to -50 dB (default: 0 dB)
-CH1 to CH8 volume: per-channel volume for individual channels
-range: 0 to -50 dB (default: 0 dB)
Routing submenu
-Out 1-to-8 source selects the source channel for the respective analog output
Mute
Limit 0 dB
CH1 volume 0 dB
...
CH8 volume 0 dB
Out1 source: CH1
Out2 source: CH2
...
Out8 source: CH8

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System submenu
-Input: selects signal input. Please refer to chapter “Input modes” for details
-Pure USB (default)
-USB/AES
-Pure AES
-Trigger: sets trigger output as continuous or 10 ms pulse during turn on and off
-cont. (default)
-pulse
-Auto ON: turns on the unit when incoming signal of non-zero value is detected
-Auto OFF: turns off the unit after a time period without input signal
-never
-10 min
-30 min
-2 hrs (default)
-Learn remote: pairs the unit with any IR remote. After activating the function, you will
be asked to press 7 buttons in a sequence: Up, Down, Left, Right, Enter, Menu and
On/Off/Mute. To cancel the procedure, simply press and hold the rotary control.
-Load defaults: loads factory defaults
Input Pure USB
Auto ON
Auto OFF 2hrs
Trigger cont.
Learn remote
Load defaults

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Displays submenu
-Brightness: sets the display brightness in 6 steps (default: 6)
-Timer: sets a timer to dim or turn off the displays
-off (default)
-10 s
-60 s
-5 min
-After timer: sets display action after the selected timer value expires
-off
-dim (default)
-Menu return: sets the time for an automatic return from the menu to the main screen
-off (default)
-10 s
-60 s
-5 min
Filters submenu
-PCM filter: selects FIR filter type for interpolation of PCM-encoded signal
-FRLP (fast roll-off, linear phase):
PCM filter FRMP
DSD filter auto
Brightness 6
Timer off
After timer dim
Menu return 60s