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Superchunk
2009-05-17, 12:32
Hi all, I'm having an issue with my SB3 and I'm at my whit's end, so just looking for any ideas.

On Friday I got a new power amplifier (tube). I moved my stereo to a new stand and hooked up the power amplifier. Up to this point everything was working fine.

Now when I have my SB3 hooked up to my preamp I get a fart-ie type noise (motorboating?). Not a steady buzz or a ground-loop type humm, a variable spitting type noise.

I plugged the SB3 directly to the power amp, and the noise was gone.
I plugged a CDP into the preamp, no noise.

So basically, the noise only shows up when the SB3 is plugged into the preamp, while on their own both the preamp and the SB3 seem fine.

I've tried different interconnects, plugging into different sockets - moving things away from each other...

I can't think of a damn thing left to try, and I don't even know what piece is causing the problem (but I think it's the SB3).

Can anyone else think of anything else I can try? I was thinking a different power supply - is there some sort of cheapo I can get at radiohack to try it (ie not an upgrade, just something different)?

Thanks

Wirrunna
2009-05-17, 15:39
I assume that there was no noise when the old amp/preamp was used?
Does it happen if the SB3 is plugged in but not selected as the source?
This sounds to me like some sort of earthing problem (when I used to build my own amps and preamps it was a regular occurrence on new projects).
This thread http://forums.slimdevices.com/showthread.php?t=60122 may be of some help.

Superchunk
2009-05-17, 15:44
yes, I forgot to mention - it makes the noise regardless of what input is selected, and even when the preamp is off the noise continues although much quieter.

The power amp is new, the preamp is not but the SB3 works into just the power amps...so maybe it's a combination of the two?

Thanks for the avenue, I'll read up on it.

Superchunk
2009-05-17, 16:41
Another point - I unplugged my SB at the unit and the noise stopped. So the p/s being plugged in does not cause noise, but the unit being plugged in does. But again, only when hooked up to the preamp...

Sigh.

Wirrunna
2009-05-17, 17:33
What are the details (Make, model etc) of the amp and preamp ?
Sean Adams, any ideas ?

Superchunk
2009-05-17, 18:21
The preamp is an Anthem 1L Pre. It's a tube preamp using 6922 tubes.

The power amp is a El34 based locally made push-pull.

Does it not seem strange that the SB+preamp+different amp worked fine, and the SB+amp works fine, but the combo of those three specific elements doesn't work? Could it be that somehow the power amp plus the preamp are creating some conditions that cause some sort of interference or something in the SB? I don't know enough about electronics to really guess.

Wirrunna
2009-05-17, 19:01
If the EL34 based power amp is locally made, can you get the constructor to come and have a look or else take the amp/preamp and SB3 to him ?

Wirrunna
2009-05-17, 19:05
Does it not seem strange that the SB+preamp+different amp worked fine, and the SB+amp works fine, but the combo of those three specific elements doesn't work? Could it be that somehow the power amp plus the preamp are creating some conditions that cause some sort of interference or something in the SB?

I think that is exactly what is happening.

Superchunk
2009-05-17, 19:27
Yeah I think I can get the builder to come by and have a look - and yeah I guess there isn't really any other explanation.

Sucks, because I suspect this will be a case of ditching the SB as the most straightforward solution.

Superchunk
2009-05-17, 19:41
Ahaha, yay, I found out what is causing the issue. Where I moved my audio stand is now near my modem/router. The internet feed is causing the interference. Not really sure how/why - but I unplugged everything and if I unplug the router and modem, then unplug the SB, then plug the SB back in and then the router/modem, the noise is gone until the router/modem (don't know which is the offender) boots back up and gets a signal - as soon as the SB gets a signal again the noise starts back.

This is good, because it means that there isn't a fundamental issue in my audio chain. I'm also switching ISPs on Tuesday, so won't take long to see if a new modem (and probably moving the router etc...) fixes it.

Thanks a lot for the help advise - hopefully it goes slick from here. I'm gonna poke around here and see if others have received similar interference via their router or modem.

JadeMonkee
2009-05-17, 21:38
I remember if I used my laptop while sitting close to my speakers (they have a built-in amp), I could hear the wifi connection through them. It was a "brr" kind of noise.
I guess that was what was happening with you, maybe?
Was the SB3 wired or wireless?
The solution (as you said) would appear to be to find a new place for your router and modem.

Phil Leigh
2009-05-17, 22:46
It's not uncommon for valve amps to be susceptible to RFI problems that are often exacerbated by grounding (earthing) arrangements. It is likely that both your pre-amp and power amp have a mains earth connection and that the screen on the lead between pre and power completes a triangular pathway between them. Sometimes this can cause a "ground loop" problem, sometimes it creates a borderline instability which can give rise to RFI problems. Does the power amp have balanced inputs?
You might need to lift the earth on the interconnect at the pre-amp end if moving the (new) router/modem doesn't work.

If you have access to a multimeter, check that there is no voltage differential between the caseworks of the pre and power. Also check that the resistance is zero.

If your setup is susceptible to RFI you might want to try replacing the switched PSU of the SB with a linear one.

Superchunk
2009-05-18, 15:14
No, no balanced outputs.

"Lift the main on the interconnect at the preamp end" eh? I'll try that if I don't get victory with the new net setup.

Wirrunna
2009-05-18, 21:10
No, no balanced outputs.

"Lift the earth on the interconnect at the preamp end" eh? I'll try that if I don't get victory with the new net setup.

Another way is to put a 10 ohm resistor between the interconnect screen and earth AFTER lifting the earth on the interconnect at the preamp end. This effectively grounds the cable shields while breaking the earth loop.
I would also discuss the problem with the EL34 amp constructor.

Other things I would try -
1. Move the router/modem as far away from the amp as possible.
2. Move the SB3 as far away from the preamp as the interconnects will allow.
3. Put one of those clunky ferrite clampon rf blockers on the cables to the SB3, first try the power cable, then the interconnects.
4. Cable connect the SB3 and turn off wireless in the router.

Superchunk
2009-05-19, 15:17
Another way is to put a 10 ohm resistor between the interconnect screen and earth AFTER lifting the earth on the interconnect at the preamp end. This effectively grounds the cable shields while breaking the earth loop.

It will take me a while to figure out what all that means, not being electronically inclined - but I will try.



I would also discuss the problem with the EL34 amp constructor.

Initiated conversation.




Other things I would try -
1. Move the router/modem as far away from the amp as possible.

Tried it.



2. Move the SB3 as far away from the preamp as the interconnects will allow.

Tried it.


3. Put one of those clunky ferrite clampon rf blockers on the cables to the SB3, first try the power cable, then the interconnects.

Looking into that - didn't have them at radio shack but I'll try calling some places tomorrow.



4. Cable connect the SB3 and turn off wireless in the router.

Tried that. Seemed to be slightly less interference.

I tried a cable connection direct from modem and router - neither made a big difference.

Superchunk
2009-05-19, 15:30
Anyone know of good directions for lifting the ground on the web somewhere?

Jeff52
2009-05-20, 05:57
Anyone know of good directions for lifting the ground on the web somewhere?

You may want to read this guide http://www.audiosystemsgroup.com/RFI-Ham.pdf about RFI issues.

Superchunk
2009-05-21, 16:46
Thanks Jeff, I'll take a look at that.

Tried a few more things today.

Tested the preamp tubes - all good.

Put a tube shield over the preamp tubes...no difference.

But I did notice that there is a cable box directly on the other side of the wall where I recently moved my setup. Tomorrow I'm going to move the whole stand to the other side of the room and cross my fingers that that does it. The only other thing I haven't tried is ferrite clamps and after that I might be done and have to try a different source (thinking maybe modded xbox with a dac).

Superchunk
2009-05-27, 12:28
What does this mean?

I just got a demo dac for a week - when I use the digital out to the dac it works fine...

???

Wirrunna
2009-05-27, 14:30
So SB3 (Digital out)-> DAC -> PreAmp -> Amp = no buzzing noise,
but SB3 (Analog out)-> PreAmp -> Amp = Buzzing noise,
and SB3 (Analog out)-> Amp = no buzzing noise,

Correct ?

It's got me stumped.

Superchunk
2009-05-27, 14:39
You got it.

It's very stumpy, no?

Also frustrating as I just spent almost $2k on my stereo and simply can't afford to replace the SB, and yet it's not ready to use! Sigh.

Wirrunna
2009-05-27, 16:13
I still think it is some kind of earthing problem.
Have you tried different interconnect cables ?
Also have you tried plugging the SB3 into a different input ?
For example, take the cables out of your CD player and plug them into the SB3 and see if that makes a difference.

Superchunk
2009-05-27, 16:34
Yeah, I've tried several different cables - 3 different ones, all of which work for other stuff - and I've tried all the inputs.

But I think you are right about the earthing thing - I suspect the DAC is creating another stage which interrupts the 'bad' connection being made between the SB, preamp and the amp (which is letting in the rf in this particular instance). That's my guess.

I'm going to try a cheater plug tomorrow, just to try it.

Wirrunna
2009-05-27, 22:13
By the look of the preamp (http://www.anthemav.com/OldSitev1/pdf/pre1l.pdf) it is well shielded - looks like a steel chassis weighing 20 pounds. However, from the picture it appears that the RCA input plugs are mounted on a circuit board and the ground hookup may be loose or a "dry" joint. The preamp must be at least 10 years old
From this thread http://forums.slimdevices.com/showthread.php?t=60122 it appears that the SB3 expects to be grounded through the interconnect shield and if that was not grounded at the preamp then it could make the noise you describe - like the noise you got when you plugged in a magnetic P/U cartridge in a turntable and didn't have the RCA plug to your preamp plugged in all the way.
Can you get hold of a multimeter to check if the outside of the RCA input sockets on the preamp are grounded ? Or better still, take the SB3 and preamp to the amp builder and show him the problem.

Edit:
OK, I tried an experiment that induced a lot of noise, I unplugged the interconnects from my SB3, then plugged a second set of interconnects in to the SB3 (the RCA sockets on the SB3 are designed so that the shield always makes contact before the active center part of the plug) and made the centers of the new plugs touch the center of the interconnect leads to the amplifier without the shield (outer part of the plug) touching. The result - lower volume, distorted signal and a hell of a lot of background noise.

Superchunk
2009-05-30, 05:52
Okay, because it was working via the DAC I felt compelled to try again without the DAC. Using an RCA I had used before, I plugged it into an input I had used before...and it worked. The DAC is still hooked up, but not turned on. I dunno, but it's working. I can't guarantee I had used that interconnect on that input specifically, so maybe it is some magical alignment. I'm just hoping it keeps working, but I'm very happy now.

Thanks so much Wirrunna for all of your help - ur nice :)