Noisy ART Tube MP pre-amp

[keith.rogers]

Originally Posted by RJVB

...
I already have one of those tiny µUSB-Lightning dongles, are you telling me that a simple USB-A(f)-µUSB adapter cable will allow me to connect any audio interface to an iDevice? Or indeed the standard lightning cable with a fem/fem USB-A cable?

It's a special adapter for USB to lightning, far as I know.
https://www.apple.com/shop/product/M...camera-adapter

I think the straight (small) adapter gizmo is only going to work if you have an interface that has a jack specifically labeled for iOS, or what I see on Presonus iTwo or Focusrite iTrack Solo, where it is a USB-A connector labeled DEVICE.

Here's an interesting device (mixed reviews, but the negative ones are all 3-4 years ago).
http://sweetwater.com/store/detail/i...os-and-android

I have an old iRig HD that was designed for a guitar that I've used with an instrument cable out of a mixer or preamp. Worked in a pinch.

[RJVB]

I just remembered I have a splitter cable coming in that converts a 3.5TRRS to either a mono mic 6.35 jack or a stereo 6.35 TRS. I ordered it to use a clip-on piezo transducer (with 6.35 jack plug) with the tuner app on my phone. If that works then at least I'll know that a guitar cable from the ART's jack output will put a signal on my iPhone's mic in.

PS: either way, as long as Apple haven't fixed the battery drainage issue with iOS 14 I need to keep my iPhone on the charger during video conferencing. There's also the issue of lag: I'm pretty certain that Apple have set the correct delay to get optimal sync between the sound from an external analog mic and the video from the internal cameras.

[RJVB]

OK, I have to correct my 20Hz estimate: both my tuner app and an analysis app I just downloaded now say 50Hz. Also strange, because I don't hear the well-known "mains hum".



Looking at the waveform with an oscilloscope app with trigger function I see this:


The signal at 2.1ms must be responsible for the "prrrffft" I'm hearing, but I can't really believe that something "flapping" at a 1ms interval like this would sound like what I'm hearing. In fact, the empirical "flutter fusion frequency" estimates I've just seen vary between around 45Hz to 60Hz in normal subjects (probably around 70Hz for blind people). I may thus be hearing some kind of beating frequency(?!)

[keith.rogers]

Are you still capturing this while the phone is being powered from a wall plug, or is the noise there even when it's running on internal battery power?

If it goes away when on battery, then I'd get one of those battery packs to operate the phone when in your video call/stream to prevent the "ground loop" noise.

If it's always there, then it's entering either via cabling or noisy power supply to the ART, would be my guess, and if you've exhausted cabling options, you'd need to find a cleaner kind of preamp or power supply for it. Or, skip the preamp and just get an inline, battery powered phantom power source - something like this, maybe:

https://www.amazon.com/Xvive-P1-Rech..._encoding=UTF8

[RJVB]

Those screenshots were made with the phone running off battery, the sound samples probably too though I didn't pay attention to that.

I've ordered a 2nd XLR-> 3.5 jack cable, this time with a mono jack (TS). We'll see if that one gives the same kind of noise, but as I said, I don't see how a cable could produce noise that increases in intensity with the pre-amp's output gain (but not its input gain, which will also increase the signal). Maybe something internal to the cable can have that effect, but outside interference should become proportionally less prominent when the signal increases.

There's also the fact that the ART's jack output is *much* cleaner and lacks the "prrrffftt" component completely. For me that speaks against the hypothesis that the noise comes from the mains.

I've read up about balanced connections. I probably don't understand everything but one take-home message was how to connect a balanced 3-pin XLR to an unbalanced 2-pin connection. Usually the low/negative signal should be connected to the ground (as is the case in my cable) but apparently there are cases where this should not be done. I have yet to figure out what cases those are, and whether they apply to the ART.

[MarkF_48]

Originally Posted by RJVB

I've ordered a 2nd XLR-> 3.5 jack cable, this time with a mono jack (TS).

I've read up about balanced connections. I probably don't understand everything but one take-home message was how to connect a balanced 3-pin XLR to an unbalanced 2-pin connection. Usually the low/negative signal should be connected to the ground (as is the case in my cable) but apparently there are cases where this should not be done. I have yet to figure out what cases those are, and whether they apply to the ART.

The cable you ordered may work and this is the way I've done it when I've connected an XLR fitted mic to a 3.5mm input jack on a consumer camcorder or other similar devices. Not the best way to do it, but in a pinch should get audio into a device. If your phone has a TRRS jack (combination mic and stereo headphone), it may present a problem depending what connections the plug makes to the jacks contacts. Most laptops now use TRRS jacks like this and can be a problem if you aren't aware of it when connecting audio devices. I think you may have mentioned having an adapter cable for a TRRS jack(?)
Good luck......

A good reference site for making audio connections.....
https://www.ranecommercial.com/kb_ar...p?article=2107