Okto Research dac8 PRO D/A processor Measurements

Sidebar 2: Measurements (from February 2021, Vol.44 No.2):

Kal Rubinson loaned me his 8-channel Okto dac8 PRO so I could compare its measured performance with that of the dac8 Stereo. From the front, the PRO looks identical to the Stereo. On the back, there are four AES/EBU inputs and one USB Type B port. Apple's USB Prober utility identified the multichannel DAC as "dac8PRO" from "Okto Research" with the serial number string "000032." The firmware version was 1.3. Like the dac8 Stereo, the PRO's USB port operated in the optimal isochronous asynchronous mode, and Apple's AudioMIDI utility revealed that the dac8 Stereo accepted 16- and 24-bit integer data sampled at all rates from 44.1kHz to 192kHz. The AES/EBU inputs accepted data sampled at rates up to 192kHz.

The dac8 PRO's maximum level at 1kHz was 4.24V at both the balanced and headphone outputs, and the processor preserved absolute polarity (ie, was noninverting) from both outputs. The balanced output impedance was higher than the Stereo's 50 ohms, at 200 ohms from 20Hz to 20kHz. The headphone output impedance was lower, however, at 0.5 ohm across the audioband. The dac8 PRO offers the same seven reconstruction filters for PCM data. These filters all behaved identically to the two-channel processor's regarding impulse response, ultrasonic rolloff, and frequency response, so I haven't shown them. Channel separation was also the same. The PRO's rejection of jitter via its USB and AES/EBU inputs was identical to the Stereo's, with USB clean but AES/ EBU introducing some very low-level, supply-related sidebands.

Fig.1 Okto dac8 Stereo (left channel blue, right red) and dac8 Pro (left cyan, right magenta), spectrum with noise and spuriae of dithered, 24-bit, 1kHz tone at –90dBFS (5dB/vertical div.).

I did find a small difference between the processors when I examined the multichannel DAC's resolution. An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the PRO's noise floor by 25dB compared with the Stereo's 30dB. This is shown in fig.1, which shows spectral analyses with the 24-bit tone reproduced by the dac8 Stereo (blue and red traces) and the dac8 PRO (cyan, magenta). The PRO offers just over 20 bits of effective resolution compared with the Stereo's 21 bits. The difference is very likely inconsequential.

Fig.2 Okto dac8 Pro, left channel, 1kHz output level vs data level in dBFS (blue, 20dB/vertical div.); linearity error (red, 0.1dB/small vertical div.).

Fig.2 shows the dac8 PRO's linearity error plotted against absolute level. Like the two-channel processor, the error is superbly low, but compared with fig.10 in the dac8 Stereo's measurements, the higher level of the multi-channel DAC's noise floor introduces slightly greater error below –130dBFS.

Fig.3 Okto dac8 Pro, 24-bit data, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 200k ohms (left channel blue, right red; linear frequency scale).

To examine the dac8 PRO's harmonic distortion signature, I used Audio Precision's high-performance APx555. The result with a 50Hz signal at 0dBFS is shown in fig.3. The total THD+N was 0.000189% compared with the Stereo's 0.00016%. The spectrum looks very similar to fig.12 in the dac8 Stereo's measurements, with the third harmonic highest in level at –120dB (0.0001%). The second harmonic is about 6dB higher in level than with the two-channel DAC, but as this is still at –124dB (0.00006%), the difference is inconsequential. Intermodulation distortion was also extremely low in level.

The dac8 PRO's measured performance indicates that almost no compromises were made to squeeze eight D/A channels into its slim chassis.—John Atkinson

tonykaz's picture

Thank you for helping to educate me. Immediately I searched for useful definition for "ad seriatim". hmm Dam, it seems legal but useful for describing the endless stream of Audio Gear making their way thru my curious ownership.

and Magritte, who I presume is the surrealistic painter.

Cheap, cheap, cheap!!!

Isn't this piece of gear a 'give-away' considering today's state of affairs in High-End? Perhaps this quoted price is about as logical as a Magritte piece of art. hmm.

Thank you for including us in your fascinating journey into the world immersive sound. We have an IMAX theatre nearby, I wonder what gear they use.

Someone referred to you as Dr.Kal, are you a medical man?

Tony in Venice

JRT's picture

I have been looking forward to Kal's review (report?) of the Okto Research dac8 PRO multichannel D/A processor since it was briefly discussed here many months ago.

Also happy to see Jim Austin relaxing rules in this case that would have otherwise barred such review (report?).

I am a subscriber, and will look forward to John's measurements.

Stay well.

JRT's picture

I did not see any mention of that on the product webpage at Okto's website, or in the manual downloaded from the link provided there.

Could you please explain a litle about how a second unit would be slaved to the first, the interconnection, etc.?

Kal Rubinson's picture

Sure. Feed the AES/EBU output of the first DAC (in pure USB mode) to the AES/EBU Input 1 (Channels 1-2) of the second DAC and put the second DAC into "USB/AES" mode. You can daisy-chain additional DAC8s from the second one, if needed. In this arrangement, the link conveys the synch but you may have to sacrifice on pair of channels on each of the DAC8s except for the first one. That means the following DAC8s are, effectively, DAC6s.

The remaining job is to have your source component recognize the individual DAC8s and send the proper signals to each. You are most likely to achieve this with a Mac and JRiver since MacOS allows you to aggregate multiple USB devices into a single "virtual" multichannel device which JRiver recognizes as such.

There is a thread about this on ASR and I will append a link to it when I can. It is not simple "plug-and-play."

JRT's picture


dualazmak's picture

Kal, very happy to find your nice introduction of DAC8PRO here in Stereophile after you helped me a lot on DAC8PRO at the ASR thread!

Just for Stereophile audio enthu friends' reference, I am intensively building my multichannel multi-amplifier audio system using DAC8PRO and software crossover EKIO on Windows 10 PC.


If this kind of citation would not be suitable/feasible here, please delete the above link without hesitation.

MhtLion's picture

I believe this processor can be used as 2 channel only DAC, right?