Mains Hum Pick-up

[Tannoyed]

I have just bought a Pro-Ject Debut Carbon turntable having retired my Dual 505 after many years of stirling service. I am not looking for perfection, just something that sounds reasonable and this new one is very good however...

If you turn the volume control up past half way with no input there is a fair bit of mains hum to be heard. Not sufficiently loud to be noticeable when playing most music-it sits at a level comparable with the surface noise from any record, however with every turntable I have owned previously, even with the volume control flat out all I hear is a low level of white noise generated by the input stages of the amplifier. This new deck has a carbon tone-arm. DO they use a screened cable up the middle or do they just hope for the best? I imagine carbon fibre cannot be earthed being a suspension of carbon within a non-conducting resin. Could this be the reason?

Have swapped leads/checked earthing etc all to no avail. My old turntable is perfectly quiet at 50Hz!

 

[MajorFubar]

I don't have an answer but googling 'project debut carbon mains hum' gives a page full of hits (ie you are not alone), some of which list possible solutions.

EDIT: I see you have already posted here:

https://www.whathifi.com/forum/turntables-and-lps/project-debut-carbon-dc-ground-hum

 

[Al ears]

I don't have an answer but googling 'project debut carbon mains hum' gives a page full of hits (ie you are not alone), some of which list possible solutions.

EDIT: I see you have already posted here:

https://www.whathifi.com/forum/turntables-and-lps/project-debut-carbon-dc-ground-hum

+1

Not all of the arm assembly is carbon fibre. It should still be possible to earth deck but may be a case of too many earths.

 

[Tannoyed]

I did consider hum loops but really it is set up exactly the same as any amplifier I have owned in the last 40 years! I tried taking a connection from Earth potential (the mains plug) and used that to earth the turntable-result exactly the same! I have tried new phono leads-admittedly not very good ones-no better or worse. The ones that come with the deck look quite good actually.

Common mode rejection ratio of the amplifier perhaps? It is a Roksan Kandy so not a bad one. It does not explain why a Dual 505 is perfectly ok in this respect though. Baffled!

I think it must be down to poor screening within the turntable.

 

[Tannoyed]

I have already posted something but it got blocked because the spam filter thought it was spam (it was that long sorry) so here is the very expurgated version.

Take a 10nF disc ceramic capacitor and connect it from one side of the motor power supply (either side will do) to the earth terminal on your amplifier. This cures the problem completely- I have just tried it.

It must have a voltage rating in excess of 15V and it must be a disc ceramic component (low series inductance). This takes the awful electrical racket to earth once and for all.

 

[MajorFubar]

Would bridging the two contacts of the motor with the capacitor not also give the same cure? Reminds me of when my dad used to build model trains in the 70s, he always used to choke the motor with a disc capacitor to reduce interference.

 

[emcc_3]

Had exactly the same problem with two debuts.

First started to intermittingly hum out of one channel, the hum replaced all music.

The warranty replacement was even worse had exactly the same issue as your having. Hum was noticeable at listening levels in between tracks.

If you Google on the internet there is lots of people who have had this problem. It appears to be the result of poor QC/design fault.

It isn't your amp either as this same problem occurred on both my Roksan and Yamaha. As soon as I got the rega no problems and no ground hum.

If still within return period I would do so.

I started a thread on this forum about the same issue.

 

[chebby]

You’d think that experienced and reputable turntable manufacturers would have cracked this problem by now. But no, it seems that everytime I look at the forum there are hum problems with record players.

 

[BigH]

Yes lots of problems with hum on this TT, if you look on google you should find some fixes. There are some hum filters you can buy, not sure if they will work but maybe worth a try if you can return for a refund.

 

[MajorFubar]

You’d think that experienced and reputable turntable manufacturers would have cracked this problem by now. But no, it seems that everytime I look at the forum there are hum problems with record players.

They did, over half a century ago if not more. But since then the build quality of the budget models has plummeted. They keep throwing exotic materials at them like carbon tonearms and translucent glass platters to make them look good and sell to the easily deceived, but underneath, they're poorly made and poorly designed. Even a Garrard SP25 didn't suffer from mains hum, nor electromagnetically induced hum. And they were more or less regarded by hifi purists as cheap rubbish, of interest only to mobile DJs or as an upgrade to the BSR AU8 in your teenager's Dansette Major.

 

[Tannoyed]

yes with a Goldring cartridge if I remember rightly!

The good old garrard SP25!

 

[Tannoyed]

My dad had a Ferranti FM receiver with an input for ceramic cartridges (a prolific output and a sapphire stylus!) He used a Collaro Conquest autochanger with it. The radio I still have and use every day having refurbished it last year. It still works perfectly and it was made in 1957!!!

 

[Tannoyed]

Hi

Putting a cap across a motor does chop the transients down on a motor but here we have capacitive coupling between the supply and the sensitive input stage of a phono amplifier. Only a few micro volts would be sufficient to cause a problem that would be noticeable. After all full output is achieved with an input of only 5mV rms!

This will work on any turntable. I have completed the modification and there is no hum even with the volume control turned fully up.

The problem is twofold. First there is inadequate screening within the turntable. Around the cartridge connections this is unavoidable with any turntable so it is vital that the cartridge lives in an electrically quiet environment. Using a switch mode power supply connected to the mains is a disaster waiting to happen. The power lead is screened but the radiation is from the negative and positive supply lines ie the screen itself acts as a transmitting aerial.

I cut open the plastic insulation near the power connector plug, eased the outer screen wire (negative supply) away from the inner wire, tinned a small area of the screening wire and soldered one side of a 40nF (0.04uF) disc ceramic capacitor to that. I soldered a wire to the other side of the capacitor and connected this to the earth terminal on the turntable. I covered the whole thing in heatshrink sleeving to add strength. Result no hum whatsoever! The low reactance of the capacitor allows the transients and high frequency harmonics to pass gratifyingly to earth where they are buried forever-and good riddance. Use a disc ceramic because these are ideal for decoupling noisy supplies by virtue of their low series inductance. The hum is not just good old 50Hz although the fundamental is certainly 50Hz. You can hear that there are higher frequencies involved because the hum has a rasping buzzy nature,m not just a smooth sinewave type hum. This can only come from the cheap and nasty power supplies used by these cheapskate manufacturers.

A wire link might do it even better but I didn't feel brave enough, not knowing exactly what is inside the power supply so went for a safer option!

 

[MajorFubar]

yes with a Goldring cartridge if I remember rightly!

The good old garrard SP25!

Would depend what amplification you used it with. Most were used with ceramic cartridges with a flipover stylus. But yeah I've seen them hosting such luxuries as a G800.