Powered monitors line popping

6V6dude

Member
2014-09-09 11:07 am
I have JBL SLR305 monitors which always pickup pops when AC, fridge or dishwasher turns on/off. A loud pop. Nothing else or any other transformer based amps I have do this so I put it down to a cheaply made switching power supply in those JBLs which picks up noise from power line. I tried different sockets around the room, extended cord to other room but nothing has any effect.
Is there anything I can do apart from getting isolation transformer? Some years ego I had one which I used to eliminate ground loops but it was always buzzing when it was on which was really annoying in the room.
I read people talking about ferrite ring on power cord but I'm not too sure if that could do anything at all for mains power, possibly just a snake oil.
 
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Sounds like a typical ground loop issue. Please describe the audio cabling to these in detail - I strongly suspect it's effectively unbalanced. (In the worst case you'd have to solder something up, maybe what you have can also be modified.) The source is something like a PC?

Appliance manufacturers occasionally like to cheap out on suppressor capacitors (intended to thwart switch arcing, basically a snubber) and omit them, but I doubt all 3 of the ones mentioned would be either cheap or very old, although it is possible of course.
 
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6V6dude

Member
2014-09-09 11:07 am
The AC (portable one) and dishwasher are old. But the fridge, actually a freezer is new. I can live with the dishwasher that's only used in the evening, AC I can turn off, but this new freezer goes on/off every 15 minutes. I can not hear the popping when recording or through mixer, only the JBLs pick it up, nothing else. When I'm using normal separate amp and speakers, it's quiet. Just the JBLs do it.


I'm using RCA cables from mixer to the JBLs, not balanced cables. But I use the same for my other stand alone amp with speakers and there is no popping on that one. Both speaker systems are coming from a same line output on the mixer, I just have a switch box to select one speaker system or the other.
Other than the popping, there is no hum, PC noise or anything else so I can't see it being a ground loop.
 
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I'm using RCA cables from mixer to the JBLs, not balanced cables. But I use the same for my other stand alone amp with speakers and there is no popping on that one. Both speaker systems are coming from a same line output on the mixer, I just have a switch box to select one speaker system or the other.
Other than the popping, there is no hum, PC noise or anything else so I can't see it being a ground loop.
Mixer L-out --> Speaker L --> PE --> Speaker R --> Mixer R-out

How's that for a ground loop? You'll have a very hard time getting the loop area on this to a minimum.

Your "other amp" probably is a standard hi-fi job, IEC Class II (double insulated), with both L and R inputs right next to each other to boot.

Most studio gear, LSR305s included, is IEC Class I (earthed), tying its signal ground to PE. Unless the designer was particularly clever, unbalanced input on this is generally a lost cause. There have certainly been lots of reports about excessive noise caused by unsuited connections on these JBLs alone. I've been using studio monitors for close to 13 years myself now, several of these with a Behringer HD400, and helped out a good amount of people with issues over the years.

Assuming the mixer is getting a nice solid ground from somewhere (either a PE connection or the source), you can make adapter cables that connect signal to XLR pin 2 and ground to XLR pin 3. Chances are your issues will be fixed.

If the mixer even provides balanced outputs already, I would think about modifying the switch box to accomplish that, so that the other amp gets its unbalanced signal as before but the JBLs can be fed balanced. I might think about how to accomplish that later, right now its like 5 am here and I really have to catch another nap.
 
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6V6dude

Member
2014-09-09 11:07 am
Yes the mixer has balanced outputs, I'm just using RCA adapters on it. I always thought that balance cables were only needed for long distances or RF interference like hum etc. Pops I get are from sparks switches produce. I get it from light switch too but it's very quiet due to low load so that doesn't bother me - or I'm used to it I should say. But if you thing balanced leads can cure that, I'll try it. Thanks!
 

6V6dude

Member
2014-09-09 11:07 am
Sorry for the late reply. I've done a test just using one mic lead on one speaker and to my amazement the popping all stopped. So now I'm waiting for some leads to arrive. I am still baffled why it picks up line pops with RCA plugs even though I only use 1m long leads but the balanced leads appear to be the only solution to fix this.
Only a problem I see is that if I was to use the JBL for home HiFi and connecting to home RCA based preamp, there would be no way that I know of to avoid the interference unless I use a pro mixer with balanced outs.
That was my plan, that eventually I might get the 8" JBL pair for home but might have to re-think that now unless there is another way. Thanks for the help!!!
 

6V6dude

Member
2014-09-09 11:07 am
Exactly what I'm thinking, JBL done the cheapest job on the switching power supply they could, probably thinking everyone will use it in studio only with balanced cables which is the only thing that fixes the problem.
How close? Those appliances are on the other side of the house on 2th floor. That's far! I've got mixers that have switching power supplies, both external and internal, one of them is dirt cheap but they're all are quiet as a mouse. Only the JBL do it.
But it'll also pickup pops from light switches in the room if I turn the switch on slowly which can cause a tiny spark in the switch.
So the problem that remains is that if I ever want to use the JBLs only for home hifi, I don't see any way avoiding the interference because none of my hifi amps/preamps have balanced outputs. It's just not something consumer gear normally has or even needs.
 
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For fear of sounding like a broken record...
Assuming the mixer is getting a nice solid ground from somewhere (either a PE connection or the source), you can make adapter cables that connect signal to XLR pin 2 and ground to XLR pin 3. Chances are your issues will be fixed.
If you know what's lurking inside you could even make the fancier version that attempts to replicate the signal output impedance between ground and pin 3 (position depending on type of cable - ahead of twisted pair but after coax). So if the signal has, say, 220 ohms and 10 µF, you would give the ground side the same. Tons of Behringer and other inexpensive (home) studio equipment is connecting its balanced outputs this way. It's not ideal for distortion but noise wise does not matter.

In an IEC Class I appliance like most studio monitors (this one included), the choice of SMPS vs. linear power supply actually is not particularly critical, as leakage currents are going straight to protective earth. You would have gotten your ground loop either way.

SMPS have one non-negligible advantage in this application: No transformer hum! That can be really annoying in an application that basically bolts a power supply right to a speaker enclosure. Come to think of it, the one of my O110s that likes to produce audible buzz once in a while has not done so in a good while now... I was suspecting less than pretty-looking mains voltage due to some sort of industrial process in the area (as it was only doing it during daytime and starting and stopping abruptly), and that may be affected by the current shutdown.

What these JBLs do suffer from is the curse of large tweeter waveguides paired with Class D amplifiers, high output noise. The additional expense of some series resistance (or an L-pad) for the tweeter while upping internal levels would have prevented that - measurements indicate that the tweeter is topping out at about 7-10 dBW, and has substantially better level handling than the midwoofer in any case. A reshuffling of 6 dB should have been doable in any case, perhaps more. Oh well, hindsight is 20/20 I guess, not much you and me can do about it now.
 
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