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Dual 505-2 humming

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shaunwyatt

New Member
Hi,i have a dual 505 which i have been given,I have put a new stylus and belts on changed the RCA leads cleaned the earth contacts and the cartridge contacts but i still get a loud humming.I have tried the turntable on two amps with the same problem.When i touch the wires on the contact plate the humming gets louder.:tears:

Thanks Shaun
 
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Try disconnecting the ground wire. For some reason they just don't like being grounded sometimes. Is it close to an amp or speaker? More info on equipment and set-up will help us help you.
 
I have the same model still in it's box.
It does the same thing, this may solve both our problems.
Fingers crossed
 
Thanks for the replies

I have a Quad 33 and a 303 power amp and i have also tried it on a Cyrus amp

I will try it without the earth on and move it

Thanks again Shaun
 
Still the same problem

I have power supply disconnected and all the wires from the contact plate removed as soon as i connect the rca plugs the hum starts.

All the wires check out fine from the cartridge back to the contact plate
 
What brand and type of cartridge is on it? Moving magnet, moving iron, or moving coil?

Is the turntable sited anywhere near the amplifiers (either sitting next to them, or on top of them)? That can cause hum, so it's best to keep them seperated if at all possible. Power amps (or integrated amps) are the worst, as the large transformers in them are surrounded by large hum fields. Preamplifiers are better, as they use much less current, so if you have a choice, put it near your preamp. TV's and video screens are another culprit for inducing noise.

Are the turntable output leads running alongside any power leads for any distance?

What sort of equipment rack do you have? If it's one that has legs that hide the leads, and you've run the leads inside the legs, alongside power leads, that can induce hum into the leads - it's best to keep signal leads and power leads away from each other.
 
Thanks for the replies

It is a moving magnet coil

I have tried it in the spare room away from any interference

I have it all on the floor of the spare room at the moment

I have a Rotel amp a Maplins preamp
 
shaunwyatt said:
Still the same problem

I have power supply disconnected and all the wires from the contact plate removed as soon as i connect the rca plugs the hum starts.

All the wires check out fine from the cartridge back to the contact plate
Click to expand...

I'm still betting on a bad wire. Could be in the cartridge itself.
 
I think the crucial statement you made was “I….changed the RCA leads”! (My reaction was “Oh no!”) So I’ll go through what wires should be there.

The output leads should be terminated on the RCA plugs (if you connected new plugs) so that the outside shield wire is connected to the outer ring of the plug, and the inside wire should be connected to the inner pin on the plug. You should (preferably) use colour coded plugs – red for the right channel, white for the left channel, although it’s only convention – as long as the left and right channels are clearly marked as such, it will be OK.

They should be terminated inside the turntable, so that the outside (shield) wire of the left cable is connected to the blue wire from the tonearm, and the inside left cable wire is connected to the white wire from the tonearm. Now, the right cable outside shield wire is connected to the green wire from the tonearm, and the left inside wire is connected to the red wire from the tonearm. Finally, there should be a single black wire from the tonearm, and that should be connected to a single black ground wire, which should (only) be terminated at one ground point, on the preamplifier (or integrated amplifier/receiver).

No other wires should be connected anywhere to those wires, and with the headshell removed, there should be no contact between any of the wires, testing with a multimeter on the resistance (ohms, or Ω symbol) scale. In particular, the earth wires for each channel (green and blue tonearm wires) must not be connected to each other inside the turntable. They will be connected together inside the amplifier to signal earth, but connecting them together in the turntable will cause an earth loop, as will connecting either of them to the black turntable earth wire – that should be by itself, right to the phono earth terminal on the amplifier.
 
Thanks ETI_5000

I soldered them onto the the contact board in the correct postion

I have continuity from the clips on the back of the cartridge to the RCA'S

The hum gets louder when i touch the arm.The music seems louder than before but the hum is still there.

Should it still hum with the cartridge unscrewed and removed?

When i play a record and disconnect the RCA'S from the amp should the + on the two RCA'S have continuity through the multimeter.The - has continuity with this test

Thanks Shaun
 
Ground loops are definitely a PITA. It seems that there is no end to what might cause one. It could be something like a dimmer switch in the circuit or a computer. Sometimes it's the TT sharing an outlet with another component. I can only suggest searching AK for "ground loop". Sorry I can't be more help.
 
shaunwyatt said:
I have continuity from the clips on the back of the cartridge to the RCA'S

The hum gets louder when i touch the arm.The music seems louder than before but the hum is still there.

Should it still hum with the cartridge unscrewed and removed?

When i play a record and disconnect the RCA'S from the amp should the + on the two RCA'S have continuity through the multimeter.The - has continuity with this test
Click to expand...


Well, you certainly should have continuity (very low resistance) from the hot pins on the tonearm, to the centre (hot) pins on the RCA plugs. You can’t call them “+”, because the audio signals are alternating current – they change sign, from positive to negative with respect to earth, at audio frequency. All four pins on the tonearm (that the headshell plugs into) should get continuity to the respective RCA plug connection, and obviously they do, as you've tested them to the cartridge pins.

However, you shouldn't have continuity between the earth (outside) RCA pins of both channels, when they are removed from the amp. if you have, you'll have an earth loop, which, because it will be within the electro-magnetic field from the amp's transformer, will pickup hum at mains frequency - a small alternating current, at mains frequency, will then circulate around the loop. The only place they could be connected together is inside the turntable, and they definitely shouldn't be connected together at that end. That is, there should be no wire to connect the blue wire from the tonearm, to the green wire. They may be signal earths, but the only place they should be connected together is inside the amp circuitry.

Have you tested the turntable earth lead, to see if there’s continuity between the amp end of it, and the metal of the tonearm? It makes me think there's something wrong, if you hear more hum when you touch the tonearm, if the earth is properly terminated at the amp's phono earth terminal.

I don't think it should hum when you remove the headshell and cartridge off the tonearm, unless there's an earth loop - there won't be any path for any signal if there's no loop, as it should be open circuit at the headshell connection pins on the tonearm.

Are the signal leads definitely shielded (a woven wire shield, and perhaps other shields, like some foil, inside that, and around the centre hot wire - similar to this http://www.altronics.com.au/index.asp?area=item&id=W3000 )? If, for instance, the wires are twisted pairs, they won't be properly shielded against hum pickup from house wiring and things like that.
 
G'day all, I believe that this has already been checked, but if the interconect's inner and outer wires were accidentally transposed (reversed) then the interconnects would be very sensitive to general hum and noise pickup. Just a possibility anyway. Regards, Felix aka catman.

Addition: From the photo, try putting a lug with the turntable earth wire under one of the mounting screws on the case as the RCA sockets look to be isolated from the case. I presume that the case is metal?
 
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Thanks Catman

I do have new RCA'S which i have fitted,i have checked again the connections in case they are reversed

Earthing the preamp makes no difference and a earth straight to the contact plate makes no difference

The case is metal
 
G'day mate, interesting! Are you able to 'short out' the interconnects at some point in the turntable?


All hum should be completely silenced when this done, if all is wired correctly (interconnectwise). Please let us know how you go. Regards, Felix aka catman.
 
Thanks catman

I have fitted new RCA'S and checked the wiring again

I have tried different mounting positions including the contact plate on the metal case

If i take a earth from the preamp to the red wire on the back of the cartridge the hum stops,but if i do the same from the main amp a very strange buzzing occurs
 
shaunwyatt said:
I know this is a piece of crap but is it worth buying to find out if the cartridge is at fault and then replace afterwards, it looks like it will fit

http://www.audiomate.co.uk/proddetail.php?prod=G050ZM
Click to expand...

I don’t know much about Dual turntables, so don’t know if that headshell will fit. It seems like a universal headshell, and if I’m not mistaken, because your Dual tonearm is straight, you need a special offset headshell, not a straight universal one – that one’s designed for s-shaped tonearms. I think only Dual ones fit, and they aren’t anywhere near as common. You can buy other cheap cartridges to fit to your headshell – Ed Saunders’ Red Ed http://www.edsaunders.com/reded.htm , which is a quality Nagaoka hi-fi one, is good value. However, I’m not entirely convinced it is the fault of your cartridge - we can probably isolate other things before you spend more money.

Does your turntable have an earth wire?
What happens when you terminate that earth wire either back to the main amp, or the phono preamp? Does it still hum then? Or is the tonearm earthed via one of the signal leads?
Do you have any other phono preamps (or amps with a phono input) you can try? (I’m starting to suspect that may be the problem.)
If you can get access to another amplifier, perhaps from the person you got the turntable from, you may be able to see whether the turntable/cartridge/wiring is the problem. A good quality older Japanese amplifier from the 80’s or 90’s (with phono input and earth terminal) shouldn’t hum if there’s not a problem with the turntable.
 
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