Because quite a few of us 0404 USB owners have wondered about the adequacy of the 5 VDC switching power supply for the unit and what improvements might be possible, I did a little experiment (so that's why I am posting in the DIY forum, sorry if that't not 100% right!)
I connected an old pair of Signet TK22's to the 0404's headphone output jack, after unscrewing the barrel to expose the connections. I clipped one DMM lead to a signal tap, the other to the common. Put a test CD in my PC's drive and played it via Foobar's "play audio CD" option, using the ASIO driver and no other Foobar options enabled (meaning, I think, that there should have been no gain applied in any way.) Each test tone is clearly announced, and when the 1 kHz test tone played at 0 dBFS, the meter reading was "off scale high" when set for 2 VAC; it measured 2.09 VAC on the 20 VAC range setting. The -14 dBFS tone read 0.58 VAC.
I thought that a (nominal) 5 VDC supply would have been split to provide a +2.5 VDC rail and a -2.5 VDC rail. From what I understand of VAC measurements, a DMM would normally read out the rms value of the AC voltage.......which is the peak voltage divided by 1.414 for a sine wave AC waveform (right?)
If that is the case, I presumed that the highest Vrms output that could be supplied from a split 5 VDC source would have been (2.5/1.414) or 1.77 Vrms if the amp section ran rail-to-rail.....which I wouldn't expect.
I do know that my cheap-o meter reads exactly the same as a friend's Fluke on ~120VAC house current when we compared them a couple of years ago, too--but whether that comparison remains valid for all VAC ranges, I have no idea.
Side note: I had assumed that the 0404 USB supplied the 48V phantom voltage on the mic inputs via a tiny transformer to step up the 5 V input.....but at such a low current requirement, I presumed that would be simple, whereas the headamp section would (I presume again?) simply run on the supply voltage as is.
Any explanations for what I am seeing? Is my understanding of split voltages versus VAC/Vrms way off?
I connected an old pair of Signet TK22's to the 0404's headphone output jack, after unscrewing the barrel to expose the connections. I clipped one DMM lead to a signal tap, the other to the common. Put a test CD in my PC's drive and played it via Foobar's "play audio CD" option, using the ASIO driver and no other Foobar options enabled (meaning, I think, that there should have been no gain applied in any way.) Each test tone is clearly announced, and when the 1 kHz test tone played at 0 dBFS, the meter reading was "off scale high" when set for 2 VAC; it measured 2.09 VAC on the 20 VAC range setting. The -14 dBFS tone read 0.58 VAC.
I thought that a (nominal) 5 VDC supply would have been split to provide a +2.5 VDC rail and a -2.5 VDC rail. From what I understand of VAC measurements, a DMM would normally read out the rms value of the AC voltage.......which is the peak voltage divided by 1.414 for a sine wave AC waveform (right?)
If that is the case, I presumed that the highest Vrms output that could be supplied from a split 5 VDC source would have been (2.5/1.414) or 1.77 Vrms if the amp section ran rail-to-rail.....which I wouldn't expect.
I do know that my cheap-o meter reads exactly the same as a friend's Fluke on ~120VAC house current when we compared them a couple of years ago, too--but whether that comparison remains valid for all VAC ranges, I have no idea.
Side note: I had assumed that the 0404 USB supplied the 48V phantom voltage on the mic inputs via a tiny transformer to step up the 5 V input.....but at such a low current requirement, I presumed that would be simple, whereas the headamp section would (I presume again?) simply run on the supply voltage as is.
Any explanations for what I am seeing? Is my understanding of split voltages versus VAC/Vrms way off?
