audio noise - long phono cable

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Alan Alan
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audio noise - long phono cable

Not quite Home Built Boat but I know there are some clever people here so....

I'm trying to connect an old stereo (Kenwood AV amp plus Bose speakers) to an Apple Mac via a 5m phono extension plus 1.5m phono to 3.5mm jack and am plagued with interference from the computer. The noise comes from the bluetooth mouse and keyboard and is also audible when certain moving images (i.e. changes on a web page) occur on the screen.

The complication is, when I connect an MP3 player to the long lead I get perfect sound, no noise, so I don't think there is a problem with the amp, or the lead. And when I plug headphones into the Mac I also get excellent sound so I am not sure there is anything wrong with the Mac. This lead is an expensive replacement for a cheaper one which I thought was the cause of the original problem but the interference is exactly the same. I have also tried all amp inputs (except phono) with no change.

Any ideas?
Alan
Port-Na-Storm Port-Na-Storm
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Re: audio noise - long phono cable

Hi Alan,
I have no idea what i am talking about here, but it's never stopped me before.........................

What happens when you switch off the bluetooth, and maybe the WiFi?  
Is the cable acting as an antenna and picking up the radio signals?

If switching these off cures the problem then I'd wonder if the cable you are using is sufficiently shielded, I'm thinking of ye olde coaxial cable which had bullet-proof shielding to prevent nasty hum and interference.
Or would it be possible to use one of these
https://www.amazon.co.uk/d/c0d/Golvery-Bluetooth-Receiver-Wireless-Reveiver-Adapter-Streaming/B01KZ1S46A/ref=sr_1_11?ie=UTF8&qid=1482764361&sr=8-11&keywords=bluetooth+adapter+bose

to connect your MAC to your Amp utilising the bluetooth?
Of course a purist would ask where your turntable is.

Only this morning I plugged my Telecaster into my Marshall Amp to be greeted by more crackling than a roast pig. I'm now trying to decide whether its the guitar, amp or cable or most likely a combination of all three.  Of course a purist would tell me to pick up my acoustic, and a friend would tell me to give up.

Good Luck.  
Paul (admin) Paul (admin)
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Re: audio noise - long phono cable

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Alan
Is the phono lead mono or stereo?

Post a picture of the phono to Jack plug convertor. And why do you need a jack plug?

My NAD amplifier circa 1985 has RCA phono sockets on every channel, which work fine with my Samsung smart TV. I connect to the 3.5mm headphone socket and split to 2 phono sockets on the NAD 3020; a classic hifi amp.

The noise could be a ground fault from the MAC...

Paul

Chris Waite Chris Waite
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Re: audio noise - long phono cable

In reply to this post by Port-Na-Storm
I hum and interfere

CW
Alan Alan
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Re: audio noise - long phono cable

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Paul
It's a stereo (3 segment) 3.5mm jack (from the Mac) to 2 phono, then a 5m stereo phono extension (female one end, male the other) into the back of the amp. I did wonder if an old amp like this might not be screened against Ghz frequencies, but there is no interference if playing a record - or MP3 through the long cable.

Graham
I have a Pioneer PL512 plugged into the phono! And potentiometers on old music amps are often a source of noise. And now you have reminded me I have to hijack my own thread by showing this hi-fi quality instrument amp I made decades ago, can't remember the name of the ready-made circuitry but the sound was exceptional:
Paul (admin) Paul (admin)
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Re: audio noise - long phono cable

Alan
There is a chance the output of the MAC is unbalanced I.e. the common ground pin on the 3.5mm output connects to mains earth. Likewise if your amp is grounded to mains earth (because of the aluminium panel)...creating a hum loop (Google it).

A portable mp3 player would not have that problem, neither would the headphones.

Yes the inputs may not be filtered for HF frequencies.

Try a much shorter lead; also rotate the amp through 90 degrees like the old AM radios to see if the interference drops.

Is the amp inside a fully screened metal case? If not it could be picking up radiation from the 5m long cable or the MAC itself.

Cheers
Paul
Alan Alan
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Re: audio noise - long phono cable

I don't think there is a hum loop as you described Paul, there is no audible hum at all. I did manage to remove the long extension lead entirely by balancing the Mac on the edge of the desk and pulling the amp off the shelf and sitting it on a stool: that got the two things close enough together to use a conventional 3.5mm stereo jack to two phono lead; no change in the noise from the mouse and Mac so that eliminates the long lead.
For now I can work around by keeping the amp volume low and the Mac volume high; that might even be a clue.

edit: rotated the amp 90 degrees on the stool, no change.
Paul (admin) Paul (admin)
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Re: audio noise - long phono cable

A bit of festive diagnosis....did you keep the aluminium foil you wrapped the turkey with? it would make a super Faraday cage around the Mac or the amp or the mouse.

These computers chuck out a lot of digital signals that produce harmonics all over the radio spectrum.

Likewise older amplifiers (back to the 70s, 80s) never needed much shielding because they only had to deal with interference from radio and TV signals, which are far weaker than the (modern) signals a few feet away from a PC.

Borrow a USB mouse from somebody and try that, after removing or switching off the Bluetooth mouse and disabling Bluetooth within the Mac. Also switch off both Bluetooth and WiFi, aka "plane mode". I suspect the noise will stop then.

Bluetooth operates on 2.4Ghz like WiFi does, but in a different "language". WiFi uses 13 fixed frequency channels but Bluetooth does frequency hopping 1600 times per second, following one of 79 low power frequency patterns. It adds up to a lot of noise.

Is your amp shielded within that wooden box?

-Paul
Alan Alan
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Re: audio noise - long phono cable

Paul, the amp in the picture is a bit of a red herring, I was reminded of it by a comment from Graham a few posts back; the problem amp is an old AV amp. Anyway, I went out and spent £20 on a ground loop isolator from Maplin, complete with phono ins and outs - and the problem is solved!

So you were right, ground loop it was, thanks!