Purifi 3-Channel Amp has introduced radio Interference

[Acerun]

I just bought a three-channel Purifi amplifier from Audiophonics... But it's introduced FM interference that was not there with my Hypex NC252MPs. Both the Hypex and the Purifi are/were hooked up to my Denon X8500H. To use the Purifi amp I had to buy RCA to XLR cables, which I did and I purchased Mogami Gold.

Any suggestions on how to get rid of this FM interference? There is a distinct hiss/spit and I can hear a radio station. They Hypex were dead silent.

My current plan is to try different RCA to XLR cables

 

[restorer-john]

Are you sure it’s a FM station, not a local AM station?

I’d be asking audiophonics about their front end RF filtering…

 

[Acerun]

You are right...maybe AM, I don't know why I said FM, I adjusted the thread title. I did write to Audiophonics!

 

[Acerun]

Oddly I believe the problem is in the Mogami Gold cable. I have some other 2549 mogami RCA's and I had a couple XLR adapters and it slapped down the RF interference dramatically. No more radio station. But it's still not as quiet as the Hypex. I can hear a hiss up to about a foot away.

 

[MaxwellsEq]

Sounds faulty to me. I can barely hear any hiss with my ear to the tweeter with my 1ET400A amp.

 

[restorer-john]

Sounds faulty to me. I can barely hear any hiss with my ear to the tweeter with my 1ET400A amp.

Is it turned on? Turn on your hearing aids.

 

[MaxwellsEq]

Is it turned on? Turn on your hearing aids.

Yeah, ok, so the hiss is no louder than the tinnitus! But in terms of relativity, the hiss is slightly less than the Class A/B amp the Purifi replaced. Both are inaudible at 30cm.

 

[Acerun]

Audiophonics asked me if the same thing was happening with the XLR removed. I removed the XLR and the radio station is gone, but there is a soft hiss/crackling sound from the speakers from some distance...with the volume up I can hear it at over a meter, and the left speaker has a repeating tick, tick, tick sound. When the music is playing it is otherworldly, but I'm starting to think this unit is not right.

Update: I have switched the cables from Mogami Gold to Mogami 2549 RCA with an XLR adapter and no more radio and the hiss/ticks are down significantly. I would say that I can't hear any hiss from half a meter now...Is that normal? If I put my ear close to the tweeters, I can hear hiss and some light static crackles. My Audiophonics NC252MP were dead silent all the way up to the tweeter, so this is a change. Is some light hiss/crackle normal inside of half a meter?

 

[MaxwellsEq]

speakers from some distance...with the volume up I can hear it at over a meter

Ahhh, you are not really checking the right thing. The power amplifier self-hiss can only be checked when all sources are silent or better still disconnected.

1. So power off everything, disconnect the cables to the power amp, turn it back on and listen to the hiss.

2. Then power off, reconnect ONLY the Denon, but have no cables connected other than the output to the power amplifier and leave it turned off. Switch the power amplifier back on and listen to the hiss.

3. Then switch off the power amplifier, turn on the Denon (but have no cables connected other than the output to the power amplifier), and leave the volume right down, switch on the power amplifier and listen to the hiss. You are now listening to the combined output stage of the Denon, the cabling and the power amplifier. This should be close to silent.

 

[restorer-john]

Is some light hiss/crackle normal inside of half a meter?

Crackle is not normal and signifies a problem. Hiss is always present, but is pretty low in the latest amplifiers of that type.

You can’t really determine hiss with an unterminated input, it should be shorted, but you probably aren’t into making up shorting plug/leads for this.

I’d move the amplifier to a different area/power point, shut off everything in the immediate area, connect the speakers and listen. It should be quiet. If not, contact the seller.

 

[Acerun]

Ahhh, you are not really checking the right thing. The power amplifier self-hiss can only be checked when all sources are silent or better still disconnected.

1. So power off everything, disconnect the cables to the power amp, turn it back on and listen to the hiss.

2. Then power off, reconnect ONLY the Denon, but have no cables connected other than the output to the power amplifier and leave it turned off. Switch the power amplifier back on and listen to the hiss.

3. Then switch off the power amplifier, turn on the Denon (but have no cables connected other than the output to the power amplifier), and leave the volume right down, switch on the power amplifier and listen to the hiss. You are now listening to the combined output stage of the Denon, the cabling and the power amplifier. This should be close to silent.

1. The most hiss/crackle
2. Significant reduction in hiss/crackle
3. Same as #2

For whatever reason, the noise has seemed to calm down. I need to get very close to the tweeter to hear the hiss. and slight bits of crackle. Not sure if crackle is the right word but it's not uniform hiss from the tweeter.

 

[Acerun]

I think it's the tiniest bit of RF interference remaining.

 

[MaxwellsEq]

1. The most hiss/crackle
2. Significant reduction in hiss/crackle
3. Same as #2

For whatever reason, the noise has seemed to calm down. I need to get very close to the tweeter to hear the hiss. and slight bits of crackle. Not sure if crackle is the right word but it's not uniform hiss from the tweeter.

If you have hiss and especially crackle when the only wires connected to the power amplifier are the speaker cables and the mains plug, it sounds like a fault. As @restorer-john says, you need to short the input, to be certain, but if I was you I would contact the vendor and seek their advice. I'd also try a different location and mains cable just to be sure that something isn't wrong with the mains earth.

 

[Acerun]

Got it. Thank you both for your help. Audiophonics wrote back that there is no RF filter on this unit.

I do think that the faint crackle is a tiny bit of RF interference remaining... But maybe not because there's actually more crackle without the XLRs connected. My ear has to be within a few inches of the tweeters now to hear it, so not a dealbreaker. I have been using a shared power strip so will experiment there with isolation and keep in touch with Audiophonics to figure out whether it needs to go back.

 

[Acerun]

From Audiophonics:
"It's normal without XLR connected you have a little noise. You hear this sound because the input are in high impedance. You dont have this problem with Ncore because Ncore have 10k resistor grounded on the input, not the purifi amp."

I find the performance of this amp gripping, and I love that it covers L/R/C in one sleek package...So I think I can forgive a little noise from 3 inches.

 

[MaxwellsEq]

From Audiophonics:
"It's normal without XLR connected you have a little noise. You hear this sound because the input are in high impedance. You dont have this problem with Ncore because Ncore have 10k resistor grounded on the input, not the purifi amp."

I find the performance of this amp gripping, and I love that it covers L/R/C in one sleek package...So I think I can forgive a little noise from 3 inches.

OK, that's interesting. Many amps put a resistor on the input to cover for no connectors. But as soon as you plug the Denon in, it's output impedance is relatively low and with no signals (and the Denon volume turned to zero) you should have the lowest noise you will get in your setup.

 

[Rick Sykora]

Could try one of these...

Amazon.com

If it does not help, at least is easy Amazon return.

 

[Acerun]

OK, that's interesting. Many amps put a resistor on the input to cover for no connectors. But as soon as you plug the Denon in, it's output impedance is relatively low and with no signals (and the Denon volume turned to zero) you should have the lowest noise you will get in your setup.

Yes...Which tracks with my experience. The most noise was with no connection, but with XLRs connected to the Denon, the noise level dropped dramatically. I'll keep experimenting with XLR cables and power cords and direct connections to the power outlet to see if anything changes. I think it's acceptable now, but would love to find a way to improve it even more. But when it plays, this amp plays. I love it. Thanks again for your ideation.

 

[Acerun]

Could try one of these...

Amazon.com

If it does not help, at least is easy Amazon return.

I'm trying to wrap my head around this and the picture doesn't show the other end. I suppose one end is a male XLR and the other female and it filters RF?

 

[antcollinet]

How are your RCA to XLR cables wired? Is it possible they are leaving one end of the balanced input open circuit (not connected)?