Active speaker ground loop? Need detective help.

[BentMike]

Please bear with this non-Klipsch query...I trust you all more than most.

I happened upon some Tannoy Reveal Active nearfield monitors. Awesome little cabinets. They will be nice of at one end of the lab. They are pretty pleasant to me except...

Just to try them out I soldered up a splitter to a PC audio output cord (that is the easiest and most ready source of tunes here) - the shield on each side goes to the ground of each 1/4" jack. (If I swap shield and signal - no sound.)

I plug one in and it works, the second does not. So I start messing around and pull the power cord (instead of switching) I get a blurt of music. Tells me the amps CAN work, and that it is probably not the PC sound card -as it sounds like, when lifting the ground, it lets the caps run the amp for just a moment.

After some messing around I realized I got music when I lifted the ground on the back of the IEC socket.

But I get a hum now that is qui te'annoying.

I moves to the other speaker if I switch the cord.

These speakers each have their own small ±28V PS and I think they are bi amp'ed - looks like one SS amp for each driver - no crossover seen.

I put a meter (set to Amps) between the speaker (with the lifted ground) back plate and a conduit (grounded)..sound drops right off, but there almost nothing flowing - 6microA I saw. Voltage as about .02 AC, and .02 DC. PC and both speakers are plugged into the ame power strip.

About all else I can do is smell it and look around. Nothing seems burnt.

Is it just not possible to use a PC with tow active speakers for some reason?

I haven't ferreted out any schematics yet. I am not very productive with such things, though I have a nice scope I can bring into play.

Help?

BentMike

[CECAA850]

have you tried something like this?

[CECAA850]

You may not be to split it to 2 speakers that way though. Nope, that one sums L and R into a single signal.

See below

[CECAA850]

2 of these

And one of these

Should get you there.

[BentMike]

I have not, yet. This is a found project. Going cheap for now.

I am pretty sure it a ground loop since I have to lift a ground to get any function, but I am willing to hear other opinions. I would like to actually understand whatever solves it...not always achievable with noise and hums.

When I run it on a battery powered music player the speaks are quiet as can be. I should reattach the ground while running off the Stone. But gotta work right now.

Thanks for your thoughts.

BM

[BentMike]

These are equivalent to what I soldered up. So I am not sure why they will help.

Like I said, it works great with a not connected to AC music player.

An edit:

My wiring is equivalent to the little one piece 3.5mm stereo to 2 RCA adapter.

I don't know what those big honking 3 pins and a shield jobbies are doing. But, I still have to bring the com and signal from the PC to each of the speaks. Is there some sort of magic in the extra stuff that buffers the com?

[CECAA850]

I moves to the other speaker if I switch the cord.

If your sound issue moves from speaker to speaker when you switch cables, that indicates a cable issue (unless I'm misreading). That's why I suggested the above solution. They're very cheap.

[BentMike]

Sorry, that is ruled out now.

Truth is, I get hums out of all sorts of stuff that is plugged in (including my regular preamp to Crown 75A to passive speaks),

but in this case the signal gets completely suppressed if I don't lift the ground.

I am unfamiliar with bi amping, and powered speakers, so it is perplexing.

[CECAA850]

What exactly is your signal chain and connector method?

[BentMike]

120VAC--> power stripA-->PC integral sound card-3.5mm recept on back of PC-->3.5mm 2ch jack-->1 com & 2 sig --split -->

split (cont'd)--> 2, each com and sig--> each, 1/4in jack --> each, Tannoy powered speaker input receptacle

120VAC--> power stripA--> power stripB-->L Tannoy powered speaker IEC Receptacle

120VAC--> power stripA--> power stripB-->R Tannoy powered speaker IEC Receptacle,w/gnd to chassis lifted

In theory, they all have the same power and ground coming in.

[babadono]

BentMike,

Hmmm..... Do you have a volt/ohm meter to go along with your scope? When you say 1/4in jack you mean a phone jack correct? Is the input to the speaker a 1/4 jack? And you are plugging in a 1/4 plug correct? Is the speaker input jack tip/sleeve or tip/ring/sleeve?

babadono

[Mongo171]

I think pics are worth thousands of words in this case. Both of the connectors, cables and connections to the equipment.

[babadono]

BentMike,

Duh, nevermind about if you have a meter, I re read your post. I am trying to gain a little info about the speakers. What model are they? Looks like some have seperate balanced and unbalanced inputs and on others they tried to do both with the same input jack.

babadono

[BentMike]

Yes, the model is called Reveal Active, there is a Reveal Passive using the same cabinet, but with no amplifiers.

Yes, there is a receptacle package - exactly as on a Crown 75A that has both a 1/4" headphone jack OR a balanced jack - XLR? it is called? I am using 1/4" jacks, the PC com is on the shield, the signals are in the core.

The passive model obviously doesn't have to go that far. FWIW, these are the older model that is charcoal gray and teal blue, dark turquoise, whatever it is.

Each speaker has two amps one for each driver (or at least I can see two chips that look like SS amp chips clamped up against a heat sink. There is a nice little toroidal transformer makien ±28VCD. Some pretty small 4300mF caps and little inline bridge package. it is a nice clean package all around. I like them. Kind of handsome, not big. And seemingly great with a music player - though it may predate such things.

Something like this would make a very nice DIY to copy.

I tried grounding the chassis while running off of a music player - no signal drop out. SO it happens when running off of the PC or off of my Toshiba pre.

This is diverging:

I normally run a Crown 75A off the Toshiba - and with humming noise. I tried this setup getting power from an isolation transformer (Faraday shielded) with a dedicated ground 10 feet in wet soil with 00 wire brazed to it. (I have been having other noise trouble.) I have a rats nest of wires. Could be some coupling - hard to solve that.

BM

[babadono]

BentMike,

OK good so you are using the unbalanced inputs. Do you have a 1/4 phone plug that you can short the tip to the sleeve on? Plug this into the input of each speaker, this effectively makes the input to the speaker it own ground(No signal). Is the hum gone?(Or at an acceptable level?) On both speakers? If yes both speakers are good. If one of the speakers is quiet but the other is not the problem is in that speaker(or amp or power supply since this is a self powered unit). This is the first step. You must determine if the two speakers are good. Just curious does the IEC power input connector have 2 or 3 contacts? If 3 does the ground contact tie to the 1/4 phone jack sleeve?

babadono

[DrWho]

If your signal goes away when the amplifier is earth grounded, then you have your signal connected to ground somewhere. When you lift the ground, you're basically modulating the ground reference of your power supply with the signal... and the reason you get buzz is all your rectifier currents are flowing across the signal cable. This tells me something is wrong with your signal cabling. Your goal should be to find a way for all the earth grounds to be connected on the AC cables and have audio pass through no problem. If you get audio and buzz, then we need transformer isolation and we can cross that bridge later. Also, I would make sure you have the correct pinout for the Tannoy input - British connections were backwards in the past and I believe Tannoy is British?

[babadono]

Dr Who,

Where did the OP go? Did we scare him off?

Bent Mike,

Is your problem fixed?

babadono

[djk]

http://www.edcorusa.com/p/46/s2s10k-10kt

[BentMike]

The speakers are at work, and I am not there this weekend. I'll get back later after I can try some of the suggestions.

To reiterate - there is no noise at all, and no hum, no drop outs when I use a battery powered music player. The problem is when I connect to a PC. The PC requires a 3.5mm jack, and there is 2 signals and a common. The same common and one signal goes eacf the active speakers, and the connection is a 1/4" headphone jack, the signal is wired to the tip contact.

I haven't been able to locate a schematic of the amps. The owners manual says very little that helps with troubleshooting, I know the output is by MOSFET, that is about all. The chips look like an LM3886 or similar.

Spec sheet.

Input 32 kW balanced on combined XLR/jack

Sensitivity 0.775 Vrms (0 dBu)

Crossover frequency 3000 Hz

Amplifier output power LF 50 W rms (4 W)

HF 50 W rms (4 W)

Output noise - 80 dBV

Power supply Fixed mains voltagev

IEC inlet with detachable power cord

Power consumption 10 to 160 VA

Drive unit Tannoy shielded 6.5 bass driver

Tannoy shielded 1 soft dome tweeter

Low frequency design Optimised bass-reflex loaded, in 12 litres

Cabinet construction 40 mm MDF front baffle,

16 mm high density particle board.

Cabinet finish Azure blue satin finish front

Grey suedette vinyl sides and back

Cabinet dimensions (HxWxD) 340 mm x 210 mm x 260 mm

Cabinet weight 8.5 kg

Shipping dimensions (HxWxD) 440 mm x 620 mm x 290 mm

Shipping weight 18.5 kg

NOTES : (1) +/- 3 dB , measured at 1m in an anechoic chamber.

(2) Peak SPL at mix position for 1 pair driven

[BentMike]

The Edcor buffer, why is this pertinent? Or, how do we know it is pertinent? No to be impertinent.

I would want to have a better understanding before plunking down $60.