Ah, but objective scientific reviews?
Amir can throw a sample in a mass spec to give us all the pretty graphs while everyone complains about the last two subjective paragraphs at the end.
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Peavey USB-P Review (Balanced DAC)
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Amir can throw a sample in a mass spec to give us all the pretty graphs while everyone complains about the last two subjective paragraphs at the end.
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A mass spec would be a wonderful toy to have!Amir can throw a sample in a mass spec to give us all the pretty graphs while everyone complains about the last two subjective paragraphs at the end.
Rock Rabbit
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Balanced output -9 dBu line or -13 dBu mic for PA that's not low output. Insert in any balanced input channel of a console (typical max gain +10 ref 4dBu) then the master volume could bring another +12 dB. Or insert in a mic input and get up to 60 dB gain, that's the intended use
bump: I was just given one of these things for free, out of curiosity I kind of want to remove/bypass the isolation transformers and get rid of the ground lift, then re-test. If they're through-hole mount it would probably take less than 5 minutes. Not because it would be useful to me, but I'm mostly just curious how much of this horrible result Amir got was strictly due to the cheap isolation transformers. I can at least test doing a before & after loopback into my Motu AVB, its noise floor as tested here is a good ~60db lower than this peavey thing so it would at least show any *relative* improvements
went ahead and reverse engineered the board (the output side for the right channel anyway, left channel is identical) and drew a schematic just out of curiosity. I remember @pma wondering in the teardown thread if the poor transformer performance was due to DC offset/voltage being present on the transformer input - looks like that shouldn't be possible as the transformer input is AC coupled via 100 uF capacitor
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restorer-john
Grand Contributor
The RC comprised of the 100uF and 15R resistor has a corner (F3) of 106Hz....
So they run the PCM-2705 in headphone mode to reduce the output impedance and hence the ability to drive the transformer?
So they run the PCM-2705 in headphone mode to reduce the output impedance and hence the ability to drive the transformer?
I thought the capacitance of that component seemed odd, and like the note says it was unlabeled so I measured in situ and guessed - I just removed it and measured it and they're 0.022uf caps so that'd be a -3db freq for that RC of ~48khz? Makes sense that it's a 0.022uf, as now *all* those components match TI's example schematic exactly, everything up to the transformer input instead of a line out jack in TI's example. I've updated the image with the proper cap value
As for headphone mode, I'm not sure, this isn't my area of expertise - but going through the datasheet, I don't see any way of switching output modes - there's just the one DAC output of Vout Right and Vout Left - it seems for headphone applications you're supposed to tie in to the built in headphone amplifier Vcc and AGNDs, which are unused on this peavey board as far as I can tell
Getting 655mV full scale into the transformer primary, with 635mV out of the secondary, and 460mV out of the final XLR output jack after the resistor network
Using my Motu as an ADC into Arta (0.00019% THD+N analog inputs as tested here on ASR), I ran a couple THD+N tests from the Peavey's output. I was surprised how well my measurements matched Amir's Audio Precision measurements (1khz 0.0190% vs 0.0188%):
In Amir's teardown thread of this unit, he ran a more extensive frequency vs THD+N graph, which revealed that distortion got significantly worse the lower you go. I was also able to reproduce that very closely, it jumps to 0.11% at 100hz:
Then down to 50hz, 0.27%!
for the finale, an ear splitting 0.66% THD+N at 25hz:
I think if anything improves by removing the transformers, it would be this (no longer so frequency dependent for THD+N). I'll retest after I rip them out
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100uF makes a high pass filter with the 3kohm resistor .... F(-3dB) = 0.5Hz, this should be no problem.
There is another question, output impedance of the PCM2705. It may be unhappy driving the transformer.
Well, went ahead and removed the transformer from one channel so I could easily A/B:
My uneducated conclusion is: massive improvements - THD+N went from 0.019% AT 1khz, to 0.014% :
That's an ok improvement, but as you can imagine, it gets better the lower you go in frequency. At 100hz, it went from 0.11% THD+N to 0.014% :
At 50hz, from 0.27% down to 0.014% :
And for the kicker, at 25hz - from 0.66% THD+N to 0.016% :
So, I turned a balanced isolated DAC into an unbalanced unisolated DAC, which kind of defeats the live gig purpose of this particular model, but I was more curious than anything. And for the spare stereo home use that it's going to be relegated to, I'll take the huge distortion improvements over galvanic isolation (that, and you can still get better results than the above with a $30 USB-C DAC off of amazon)
My uneducated conclusion is: massive improvements - THD+N went from 0.019% AT 1khz, to 0.014% :
That's an ok improvement, but as you can imagine, it gets better the lower you go in frequency. At 100hz, it went from 0.11% THD+N to 0.014% :
At 50hz, from 0.27% down to 0.014% :
And for the kicker, at 25hz - from 0.66% THD+N to 0.016% :
So, I turned a balanced isolated DAC into an unbalanced unisolated DAC, which kind of defeats the live gig purpose of this particular model, but I was more curious than anything. And for the spare stereo home use that it's going to be relegated to, I'll take the huge distortion improvements over galvanic isolation (that, and you can still get better results than the above with a $30 USB-C DAC off of amazon)
Hello, I see this is a somewhat older thread and maybe nobody will see this post, but I joined to comment. I just pulled the trigger on this piece, then found this thread! Well, I'm still gonna give it a shot because my use case is probably different from others' here and this Peavey box might work fine for me.
I really wanted to say one thing about the overarching theme of these comments regarding this box's low output: in the world of professional live sound, i.e., theatres, clubs, etc. there are many connections made through passive DI boxes. These are designed to convert and isolate high-impedance sources to low impedance balanced connections that can be run for hundreds of feet to a front-of-house console (and also split to a monitor mixing board as well). In the course of this high-to-low Z conversion via transformers, the signal level is reduced to "mic level", i.e. a very low level; the kind of voltages you might see output from a dynamic microphone before it gets to any preamp stage. Consoles designed for live sound are more than up to the task of amplifying these signals. I've been doing it like this for years with my keyboards (I tour with a band) - using the headphone output of my computer to feed passive DI boxes that send my sound to the house PA system.
I have no illusions that the transformers in this Peavey box are equal to Jensens, or that the DAC will satisfy any true audiophile - but I'm not sure it's fair to knock this piece because it outputs a signal that's similar in level to almost any passive DI box on the market. That's all I wanted to say. I have seen other USB DIs - Radial, Mackie, and ART for example - and they all output a line-level signal, so I assume they have active electronics to do that. This Peavey box does not – but for my single intended use, I don't need it to. In my case, I'm playing live shows with a band and needed a separate audio stream from my computer to send to the house. I do appreciate the very useful info here, which tells me I may need to be careful with gain staging to avoid audible distortion. I'm hopeful the performance of this box will be "good enough" for my intended use – but I bought from Amazon just in case, as they make returns pretty easy.
I really wanted to say one thing about the overarching theme of these comments regarding this box's low output: in the world of professional live sound, i.e., theatres, clubs, etc. there are many connections made through passive DI boxes. These are designed to convert and isolate high-impedance sources to low impedance balanced connections that can be run for hundreds of feet to a front-of-house console (and also split to a monitor mixing board as well). In the course of this high-to-low Z conversion via transformers, the signal level is reduced to "mic level", i.e. a very low level; the kind of voltages you might see output from a dynamic microphone before it gets to any preamp stage. Consoles designed for live sound are more than up to the task of amplifying these signals. I've been doing it like this for years with my keyboards (I tour with a band) - using the headphone output of my computer to feed passive DI boxes that send my sound to the house PA system.
I have no illusions that the transformers in this Peavey box are equal to Jensens, or that the DAC will satisfy any true audiophile - but I'm not sure it's fair to knock this piece because it outputs a signal that's similar in level to almost any passive DI box on the market. That's all I wanted to say. I have seen other USB DIs - Radial, Mackie, and ART for example - and they all output a line-level signal, so I assume they have active electronics to do that. This Peavey box does not – but for my single intended use, I don't need it to. In my case, I'm playing live shows with a band and needed a separate audio stream from my computer to send to the house. I do appreciate the very useful info here, which tells me I may need to be careful with gain staging to avoid audible distortion. I'm hopeful the performance of this box will be "good enough" for my intended use – but I bought from Amazon just in case, as they make returns pretty easy.
Just to tie a little bow here, I used this box on a rehearsal today for our tour coming up. Yesterday I got it and spent some time at home checking it out in a decidedly unscientific way. I a/b'd it with the headphone out of my MacBook Pro going into my QSC K8 powered speakers (the normal signal chain for my gigging rig). The headphone output sounds a tiny bit "crisper" or "cleaner", which I'll guess are due to differences introduced by the cheap transformers in the Peavey box. Nevertheless, the differences – for the program material I am dealing with and considering everything in the signal chain – are minor, and only apparent when doing an immediate a/b comparison. I know this may sound like a joke to you guys with your test equipment measuring every spec down to the hundreths of a percentage (I also know the headphone output of a MacBook Pro isn't exactly an audiophile's dream either)! After reading about the low output I was expecting to have to switch my QSC's input from "line" to "mic" level but that was not the case – there was enough signal to drive my QSCs to a reasonably loud level when using the line level position.
At the rehearsal, the Peavey box's XLR outs were fed to a mixing console which then sent its output to individual monitor wedges, and it sounded fine for what we needed - playing percussion loops I trigger from my midi keyboard. So, I'm taking the chance that this box will survive the rigors of the road as we do a 3-week tour of England and Scotland starting in two weeks.
One thing to mention is that you don't want these connected to a sound system with the volume turned up when connecting it to a USB port on your computer – a very loud pop ensues which might do damage! This also happened when I restarted my MacBook Pro with the Peavey box plugged in and connected to my speakers. I wonder if this happens with the more expensive USB direct boxes like the Radial? I assume this is part of the process of the audio driver loading.
Cool that I found this place; I was very impressed to find such a comprehensive examination of this box. You don't usually see such detailed specs on any manufacturer's web site; thanks for that. I also hope my few posts about it help anyone who might be looking at this type of device, where hi-fi perfection is not a prerequisite!
At the rehearsal, the Peavey box's XLR outs were fed to a mixing console which then sent its output to individual monitor wedges, and it sounded fine for what we needed - playing percussion loops I trigger from my midi keyboard. So, I'm taking the chance that this box will survive the rigors of the road as we do a 3-week tour of England and Scotland starting in two weeks.
One thing to mention is that you don't want these connected to a sound system with the volume turned up when connecting it to a USB port on your computer – a very loud pop ensues which might do damage! This also happened when I restarted my MacBook Pro with the Peavey box plugged in and connected to my speakers. I wonder if this happens with the more expensive USB direct boxes like the Radial? I assume this is part of the process of the audio driver loading.
Cool that I found this place; I was very impressed to find such a comprehensive examination of this box. You don't usually see such detailed specs on any manufacturer's web site; thanks for that. I also hope my few posts about it help anyone who might be looking at this type of device, where hi-fi perfection is not a prerequisite!
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