Peavey USB-P DAC Teardown

[Angsty]

At 20 Hz and full digital level input, distortion shoots up to nearly 2% for a SINAD of around 35 dB! Most of our measurements are at 1 kHz which gives these a ton of benefit of doubt.

Does this suggest that there is merit at looking at the SINAD at multiple frequencies in DAC testing, in general?

 

[Francis Vaughan]

Does this suggest that there is merit at looking at the SINAD at multiple frequencies in DAC testing, in general?

Not for this reason. Transformers have a terrible time at low frequencies. They have very different distortion mechanisms to pretty much anything else in audio, and looking at low frequencies is going after where we know they are weak, and where we can separate the good from the bad. Unlike other audio electronic components the integral of the signal matters, as this gives us the level of magnetisation of the core. So as the frequency drops the absolute magnetisation of the core rises and rises. Pretty quickly we get into the highly non-linear part of the core's BH curve, and see not only significant non-linearity, but also hysteresis - so path dependant non-linearity. You can get into arguments about things like thermal lag in power transistors, or retained charge in capacitors, but for the most part there are no other components in audio systems that display significant path dependencies.*
It would have been fun (in a grim way) to see an IMD plot for these transformers. The grass would have been very tall.

Transformers are very linear at low levels, so we don't see the typical higher percentage distortion at low levels and dropping as level increases. In this way they are backwards from typical components. Noise is of course constant, and they can pick up interference easily.

*There are speaker drivers that you can drive into different breakup modes, and this does count as a path dependency. You can drive them into a breakup mode at a given level, and it won't come out of it as the level drops, not until well below the level it first broke up. Yuk.
ETA - voice coil heating is the other obvious path dependency in speakers.

 

[amirm]

Does this suggest that there is merit at looking at the SINAD at multiple frequencies in DAC testing, in general?

I run the SINAD versus frequency for DACs. Here is a sample:

 

[trl]

Part of me can't help but feel that PS Audio is nothing more than "pretend" audiophile stuff.... It does't sound bad per say, it just doesn't sound as good as the price, and worse yet the build quality is bordering on crappy.

A bit off-topic: I'd say that most audiophiles are more inclined to own high-end audio devices (that are expensive, but not so well measuring), while high-fidelity audio equipment is just for a small part of audiophiles only. So, we should probably not link the audiophile term with the high-fidelity.

The beauty on the outside and on the inside of an audio device, its heavy weight, its oversize, its build quality and the high rating of a brand name might influence an audiophile more than a plastic look with a good SINAD and great measurements. It's probably something that we should get used with, in the name of the "audiophile diversity". If someone wants to pay 5K or more on beautiful and heavy audio equipment with Lundahl transformers on the outputs, but with a less than 90 dB SINAD, then well...it's not my money. If there's something that intrigues me it's the fact that some might say that specific expensive device is better than the others just because it "sounds" better, without real proofs (well, maybe "you can't measure what I hear" proof).

 

[pma]

I run the SINAD versus frequency for DACs. Here is a

I think the problem is that many people do not understand that SINAD and THD+N are the same thing, once expressed in dB and then in -dB or %. Maybe that’s because the SINAD is so much promoted here, as the THD+N at 1kHz has not been so popular.

 

[amirm]

Yeh, I realized that as I was posting that graph.

 

[The Capstan]

As always, questions, comments, recommendations, etc. are welcome

Thanks Amirm. Always interesting to see the “guts” of the hardware and see how these gadgets have been engineered.
Do you think it will be possible to dissect the Loxjie A30? There are some interesting debates that could benefit from it (namely: the type of usb interface and sample rate/format support).

Best and keep up with the good work!

 

[amirm]

Do you think it will be possible to dissect the Loxjie A30?

Unfortunately that was a loaner and I have packed it to send it back to its owner.

 

[Sharpi31]

I’ve worked in commercial AV for years - connecting laptops to office meeting room AV, or pub/club sound systems, can be very problematic. The classic scenario is the laptop that sounds fine until the charger is connected, and then there is tons of hum on the audio signal. It’s not uncommon for the installed AV to be powered from a different mains phase than the sockets provided for user devices... I see this device as something an amateur DJ would have in his/her bag in case problems arose in a pub/club gig. It’s an audio sticking plaster, not a performance orientated design. But I agree with @Francis Vaughan that the laptop line out (or better USB DAC line out) coupled with a decent Jensen (or other) DI/isolation box would be a better solution.

 

[PeteL]

Given Peavey's pedigree, I think this is mainly aimed at live gigs performance, on stage for SFX, clips, loops, sound bites, applications like that, not for actual music listening in hifi settings. Nobody would know if that drum loops as different harmonics than what the actual file contain... It's cheap enough, and looks rugged enough, that any FOH or monitor engineer can have a couple in it's gigs bag. It get's used.
Personally (In an other life), I used to carry this one: https://www.radialeng.com/product/usb-pro . It's 5 times the price. It's probably better, but in all honesty, the number of times that fidelity actually mattered is zero... I would not know if it has a good SINAD. The Peavey would have been a much better choice, because those things get lost and stolen when touring. No one in the audience would notice.

 

[pma]

It might be nice to have a look if there is not a DC voltage at the transformer input, coming from the DAC. Even a very small amount of DC voltage is a killer of transformer distortion parameters.

 

[trl]

I used to carry this one: https://www.radialeng.com/product/usb-pro . It's 5 times the price. It's probably better, but in all honesty, the number of times that fidelity actually mattered is zero... I would not know if it has a good SINAD.

Source: https://www.radialeng.com/wp-content/uploads/2018/04/Radial-SmartSheetBook-USB-Pro-update-1.pdf

Source: https://www.radialeng.com/wp-content/uploads/2018/04/USBpro-UserGuide.pdf​

Seems that the two transformers are placed in a similar way like the Peavey USB-P DAC. The difference with the Radial DAC is that they can be bypassed (it's default settings actually).

Interesting way to isolate the balanced outputs, although these days this can be easily and better implemented inside the USB transport module.

 

[PeteL]

View attachment 97117
Source: https://www.radialeng.com/wp-content/uploads/2018/04/Radial-SmartSheetBook-USB-Pro-update-1.pdf


View attachment 97118
Source: https://www.radialeng.com/wp-content/uploads/2018/04/USBpro-UserGuide.pdf​

Seems that the two transformers are placed in a similar way like the Peavey USB-P DAC. The difference with the Radial DAC is that they can be bypassed (it's default settings actually).

Interesting way to isolate the balanced outputs, although these days this can be easily and better implemented inside the USB transport module.

Well, the THD spec is obviously better, the band limited Freq response "With Isolation" would help with the atrocious low end THD that we see with the Peavey, but interestingly, the flow diagram don't show that, the "bypass switches" don't bypass the filters, so one of the two seams erroneous... And when we see this, what else is erroneous... Now: "these days this can be easily and better implemented inside the USB transport module", well yes and no. Direct injection boxes have been working like that for ages, and criticizing this, is criticizing their very existence. Still you'll find a few of them on every stages of the world, granted, normally not including a DAC. Let's not forget that passive D.I. boxes are made to have level and impedance appropriate for sound consoles microphone inputs. Sure it's not optimal but it is what it is, it makes less sense with digital consoles, but sound reinforcement technological choices with the very high cost of a full concert production has to be incremental, and DI Boxes are not disappearing any time soon. So even tough @amirm don't like the low output from this DAC/DI. It is what it is and it's a standard, still. It doesn't excuse all the other flaws tough, but let's not think that we'll stop seeing transformer based isolation on stages in a near future, a huge industry depend on them and they are a widespread necessity.

 

[Francis Vaughan]

As I noted earlier, you can isolate a USB connection. It is technically a lot more effort than a transformer output, but still a lot cheaper than a good transformer. The big difference is that the two output channels are not isolated from one another. Most of the time that won't matter, but when it does, it will really matter.
Another factor is how safe the isolation provided is. Mostly people don't worry about that, and the presence of ground/ground-lift switch nullifies a lot of this. But in big setups I would feel happy being sure that significant voltages imposed on one part of the system can't arc through the box if it is claiming to provide isolation. Transformers that provide enough isolation are ubiquitous, you only need to look inside any modem to see how common the problem is. They are designed with the specific intent that a total failure inside the modem, one that connects the mains (and by mains we mean mains with a serious fault, such as a high tension short to the feeder) to the inside of the device, won't get out into the phone network. (Phone companies are very resistant to the idea of customer's equipment electrocuting their linesmen.) Whether your isolated USB circuit meets the same safety standards as the transformer is an open question. I'll bet many don't meet any standards at all.

Eventually the world can migrate to digital over fibre for all connections. That day is decades away. USB over a fibre repeater is another (more costly) option to the USB DAC question. However the continued need for phantom power means that wire is never going to be fully replaced. Powering the DAC over USB is of course a similar highly convenient aspect of the system.

 

[trl]

Eventually the world can migrate to digital over fibre for all connections. That day is decades away.

New threads will come up quickly: "Soundstage on single strand vs. dual strand fibres", "PRAT changes on LC vs. ST connectors" etc. Then I will probably miss the copper vs. silver threads.

Meanwhile, we can still use the 5 to 10m TOSLINK optical fibre to isolate the USB-transport from the receiver.