same battery power 2 units: audio interference - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 10:06:15 am
content: I have an audio preamp Zoom F4 that I connect to a camera Panasonic GH5 with the EDIT 3.5mm jack audio out to the GH5 jack mic In, setting it at mic level.The problem is:when powering BOTH F4 and GH5 with a single V-mount battery adapter (out 8.8V to GH5 and 12.5V to F4) I get strong audio noise on the GH5! Even if the F4 is powered off!If I power one of the two with the v-mount battery and the other one with its 220V ali or its own battery, all is fine: no noise.What's the problem and how to solve this?TIAEdit:I did also this thread to some F4 related gnd problems:https://www.eevblog.com/forum/beginners/horrible-'hum'-with-lot-of-harmonics-in-audio-recording/I don't know if it's related with this new problem...EDIT2:I tried to power both units also with a 12V ali bench + stepdown converter = same result.

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2017, 10:22:54 am
content: your going to have to draw a mock up of how you have things connected, as I am interpriting it, you have a 3S lipo battery, and are powering 1 device off the 3S and one off the 2S tap at the same time?Or if its via a switcing converter to step one up or down, some idea which would be better.All high impedance nodes act as antennas, its only via sheilding the wiring these nodes run through to the lowest impedance reference plane of your input device that you stand a chance.If your sheilding just goes to an isolated case ground that never actually links up with the system ground it wont help you out there,A way to test if external interference is your issue would be to disconnect the output of the F4, and hard short out the signal to the GH5 there, see if the noise is gone, next leave it open and see if it gets worse, this will help you narrow down what role the F4 is playing in this problem.

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 10:42:17 am
content: I'm using this adapter:https://www.amazon.com/DP500III-V-Mount-Uninterrupted-Supply-Battery/dp/B01B6011GOthat has 12.5V and 8.8V out. Inside it there are several stepdown converters for different voltages.From this I have 2 cables powering the F4 and GH5.Then the only cable connecting the 2 units is the EDIT 3.5mm to 3.5mm stereo audio cable from F4 audio out to GH5 mic in.I disconnected the cable from F4 and there is no noise, BUT if I short the cable attached to GH5 the noise comes again and it's a lot.So, it's from GH5?

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 11:27:49 am
content: I'm sorry: when hard connecting the 3.5mm cable I touched it, so it was also my body making antenna and noise...If instead I hardwire the cable connected to GH5 alone there is no noise. Even as I said with no hardwiring: no noise.So it seems the noise is coming from F4. Or some sort of ground loop (but without the earth of an AC connection?).

author: capt bullshot
author_rank: Super Contributor
timestamp: October 23, 2017, 11:34:59 am
content: From your description in the OP, I'd say this is a typical ground loop problem:The power outputs from the supply unit are common ground, this is the first path between the other two devices, the audio connection is the second. So the noisy supply current of one of the audio devices diverts between its supposed path (the power connection) and the loop through the audio cable and the other devices power connection. Noisy power supply current flow through the audio cable causes the noise on the audio signal.No way out of here but breaking the loop - as you do when you supply the units from separate sources. If you want to supply the units from a common supply with common ground, you'll have to isolate the audio path by using a suitable audio transformer.

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 11:42:42 am
content: I supposed ground loop was only with AC transofrmers connections with earth... good to know it's also from batteries!Ok, so a DI device to put inbetween the F4 and the GH5 audio connection?Do you suggest passive or active one? Some brand advice, cheap but good?

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2017, 11:54:50 am
content: If its ground loops, the easiest approach would be, if your power consumption is low enough, use an isolated power converter, and isolate 1 of the 2 devices,

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 12:23:23 pm
content: Units consume8.7V=0.8Aand12.5V=0.3AAny link on isolated unit to buy?

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 01:00:57 pm
content: Do you mean something like these?http://it.farnell.com/tracopower/ten-6-1212n/convertitore-dc-dc-12v-0-5a/dp/2579414https://www.mouser.it/ProductDetail/Mean-Well/SCW05A-12/?qs=sGAEpiMZZMvGsmoEFRKS8OJZAKgiQktX4nHdrar6TV4%3dI don't find anything with 8.8V, so the only solution is to isolate the Zoom F4 with 12V output...

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2017, 01:15:42 pm
content: Ok, dug into the actual manuals for both devices, as the XLR outputs of the F4 are already differencial (Hot / Cold), you may be able to get away with just changing how you have it wired to the camera,F4 Hot connects to Panasonic Signal TipF4 Cold connects to Panasonic SleeveF4 Ground Connects Only to Signal Sheild.

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 01:19:36 pm
content: It's not possible: there are 2xXLR balanced out L and R, so there are 1xHot and cold for L and 1xHot and cold for R...Instead I only have one Tip and one Sleeve on my GH5 input because it's unbalanced.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 23, 2017, 01:30:34 pm
content: Quote from: frank10 on October 23, 2017, 01:00:57 pmYes, I think these should do the job

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 06:40:57 pm
content: Beware, those DC to DC converter modules can be very noisy and may require filtering.I was going to suggest a differential amplifier, powered from a splice, as close to the camera's power connector as possible, with as shorter cable, as possible, but if you're going to do that, you might as well build your own pre-amplifier.

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 07:10:40 pm
content: Thank you Hero999, but the thing becomes too complicated I think. I'm searching some simpler solution.I found also these ones, even cheaper than DC-DC:http://www.ebay.it/itm/Filtro-antidisturbo-isolatore-filtro-di-massa-su-segnali-rca/351174690463?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649So I have 2 ways to test: first I think I'll try with audio with those gnd isolation and next eventually acting on power with DC-DC isolation. Then... we'll see.I mark also I'm not interested in the best quality on this connection: the audio will be recorded with best quality on the F4.I'm trying to have a backup emergency copy of the audio also on GH5 (but I hope I'll never have to use it). And also to put LTC audio track on one GH5 channel to easy sync all the units (F4 and Atomos recorder for video are already synced with TC out). Even this for emergency purposes.So, I don't need perfection, but in the meantime neither this crappy actual sound...On the TC test, over the base noise I also get the TC on the channel on which should not be there!! So, actually this connection is unusable at all!!I hope to fix this with those isolations.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 07:45:17 pm
content: Quote from: frank10 on October 23, 2017, 07:10:40 pmI agree about complexity.I was thinking an audio isolation transformer.https://www.mouser.co.uk/ProductDetail/Murata-Power-Solutions/DA102MC-R/?qs=sGAEpiMZZMv0IfuNuy2LUZZx%2f%252bgXdowwv6mtE9xanYU%3d

author: Unknown
author_rank: Unknown
timestamp: October 23, 2017, 08:28:56 pm
content: Ok, interesting!But how do you see the frequency range? I looked at the datasheet, they only say it isolates from 50-60Hz... but no response. And they say for digital audio.For comparison I found one here:http://www.ebay.it/itm/Qty-2-600-ohm-1-1-audio-isolation-transformers-for-datamodes-PSK31-RTTY-SSTV-/253203355454?hash=item3af418973e:g:IusAAOSw2XFUbxiait says:1:1 ratio and 600 ohm impedance, Frequency response: 300Hz to 3kHz minimumso very bad response...and another one, good, here:http://audac.eu/products/d/tr106---isolation-transformer-line-level1:1 ratio and 600 ohm impedance, with 0.5 Hz - 100 kHz and low distortion.What should I look at?I aim for a good, normal, but cheap one.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 08:55:36 pm
content: I agree, the specifications aren't very detailed. The isolation frequency depends on the interwinding capacitance and the impedances. The one which I suggested, specifies 50 to 60Hz will actually have better isolation at lower frequencies and poorer at higher frequencies. On reflection, it's not suited to your application. The data sheet gives specifications from 100kHz to 3MHz.The minimum operating frequency depends on the voltage. If the voltage on the one rated down to 300Hz, were reduced to a tenth of the maximum rating, the frequency could also be reduced down to 30Hz.The maximum operating frequency depends on the core material. At higher frequencies, whe hysteresis and eddy losses will be greater.Go for the third one you linked to. It has the best specification.

author: Unknown
author_rank: Unknown
timestamp: October 27, 2017, 08:42:43 pm
content: Ok, I bought tha audio mass isolator and... it works! Solved.Now I have the other related problem with TC: from F4 it is VERY loud even with -12dB attenuation inside GH5.I made a 2 resistor pad of 63dB (!) and I put on the input of the mass isolator but at the output I got the original clipped levels! What's this? Shouldn't it mantain the levels of the input??Anyway I put the EDIT pad on output of mass isolator and finally I got the levels correct at about -18dB without noise!BUT I get a bit of the TC ugly sound (-44dB) also on the other channel, of course without anything connected! (This was also without the mass isolator)What could I do? Continuing to lower the levels with pad, hoping it'll become inaudible, or what?Do you think it's a GH5 limit on the channel separation (it seems strange...)?

author: Unknown
author_rank: Unknown
timestamp: October 28, 2017, 09:27:02 am
content: I tested with a 1kHz slate tone to see if are the levels to make this separation error, and YES, for example @-10dB level sound, you get also in the other channel a bit of the 1kHz sound! About -60dB: you can see on green graph.BUT if you lower the signal level to, say -28dB, the other channel is OK (there is noise level, but no peak 1kHz on the blue graph):INSTEAD with TC this does NOT work!:TC with a pad of -77dB (!), you get a signal level of -30dB (so even lower than the slate tone example) BUT there STILL is the TC signal on the other channel! and it can be heared: