Technics SL1500C hum through external phono??

[Entrigo]

Hello wise forum-dwellers!

Going mad and would really really appreciate some expert tech advice!

Posted a week or so ago about this issue, thinking the problem was either with my amp or the phono stage but now I think it might be the turntable.

The lowdown:
Over the last couple of months, I ran my Technic TT through different phono options. All of this bar using the Technics own preamp produce a hum. This at full volume is roughly 40dB. As in, measuring around 75dB in total, with a background noise level of 35dB - measured in front of the speakers with a meter app on my phone (it's not scientific, I am a heathen etc so please bear with me).

Combinations tried follows:

• TT through Rega Fono MM into line in of my Marantz PM7200 amp - hum
• TT through Simaudio Moon 110LP into line in of Marantz - hum
• TT through Marantz phono in - hum (though quieter - but then the Marantz sounds MUCH quieter than any of the two external phono - whatever the reason, at the same volume knob position/distance/record playing, it reads nearly 30dB quieter than the Moon)
• TT through Moon into line in of a brand new Fosi mini amp - hum
• TT using own preamp into line in of Marantz - SILENT!

I also tried:
switching off all electrics, double grounding (extra metal wire from ground of Moon to ground pin of next electrical socket), moving away from amp, sitting on foam isolation pads, plugging Moon into extension from next door's socket, swapping RCAs around. Nothing. Also no hum from any of the other channels of the amp.

So:

1. Could it be the turntable phono out signal path or connections that is the problem?
2. Or some other issue in the connection between the TT and the external phonos?
3. Has anyone with a Technics SL1500C which has a similar issue?

I appreciate a proper answer is hard just on paper, but would really appreciate some insights before I take my 'baby' to my local shop - as they have a 4 weeks lead time - and I would rather avoid being without for 4 weeks and them saying "nothing wrong with this"...

Thanks!

 

[Gray]

Could it be the turntable phono out signal path or connections that is the problem?

Without doubt, that's the most likely explanation.
Based on the tests detailed in your 5 bullet points above, it can't be anything else.

You can be very specific to your local shop - the hum is only present from the phono out and not the line out RCA socket pair.
It will take them minutes to confirm that into a phono preamp of their own

 

[twinkletoes]

@Entrigo Just out of curiosity, have you connected a ground cable from the turnable to the phono stage.
just you didn't mentioning it specifically. Im guessing the 1500c has provisions for this like my 1200gr

 

[nopiano]

Just to recap then, you experienced hum from your TT only once you by-passed the builtin phono stage and began using an external stage?

Did you ground the TT to the phono stage, or leave it connected to the amp?

 

[daytona600]

Can you take the technics back to your dealer for set up advice

 

[Gray]

Can you take the technics back to your dealer for set up advice

I think that what he's said in a previous thread shows that he's ok with setting up (including grounding to external phono stages and amps).

Going by what he's said in post #1 here, the dealer should be able to immediately confirm that there's an issue with the phono outputs (and no problem from line outs).

 

[Entrigo]

@Entrigo Just out of curiosity, have you connected a ground cable from the turnable to the phono stage.
just you didn't mentioning it specifically. Im guessing the 1500c has provisions for this like my 1200gr

Yes I did. Didn't mention as assumed it was obvious - though of course not... I think I did mention I tried double grounding as well however.

Anyway yes, earth wire from TT to preamp, then tried grounding to amp instead, to amp body... Double grounding with an extra metal wire (quite thick garden /DIY wire as had nothing else) from already grounded preamp post to earth pin in empty wall socket...

Funny thing - recently took the earth wire out completely as using the internal phono. Someone suggested taping the earth wire to an empty line-in on the amp instead, so I tried.

It worked!

Much, much quieter hum - almost nonexistent at full volume.

For a bit. Then it increased. And stayed the same again wherever I grounded: preamp, amp, amp line-in, RCAs metal casing going into the preamp...

It is quieter than before though.

Ah. And it's there whether the turntable is on or not. So possibly not the turntable?

Driving me nuts! I have listed the turntable for sale (got a Rega amp so thinking of going for a P3 or P6 to match) but:
1. Principle - I HAVE TO get to the bottom of this
2. If it's not the TT I can remove mention of any hum in the listing and hopefully get some more money for it
3. If it's an electrical circuit or whatever issue, I will have it still with my new setup...

 

[Entrigo]

I think that what he's said in a previous thread shows that he's ok with setting up (including grounding to external phono stages and amps).

Going by what he's said in post #1 here, the dealer should be able to immediately confirm that there's an issue with the phono outputs (and no problem from line outs).

The TT was bought second hand on eBay.

Someone elsewhere suggested that maybe the previous owner imterfered with it, but it genuinely looked unused as advertised (strangely enough for eBay). And I would assume they would have gone back to the dealer if there was an issue.

 

[Entrigo]

Just to recap then, you experienced hum from your TT only once you by-passed the builtin phono stage and began using an external stage?

Did you ground the TT to the phono stage, or leave it connected to the amp?

Yes. Same hum through amp's phono, Rega Fono MM and Simaudio Moon.

All properly connected and grounded.

 

[Entrigo]

@Entrigo Just out of curiosity, have you connected a ground cable from the turnable to the phono stage.
just you didn't mentioning it specifically. Im guessing the 1500c has provisions for this like my 1200gr

Yes. Sorry forgot to mention in the OP as thought it was obvious, as I mentioned double grounding. But yes, nothing is obvious and missing step 1 is often the case of most issues haha..

Anyway, explained a bit better above all I tried.

Anyway. As after reconnecting the ground wire today the hum was initially very subdued to nonexistent, I have bought a new earth wire, just in case the wire is at fault.

But then, as I was saying above, the hum is there even if the turntable is switched off... 🤯

This is seriously driving me up the wall.

I have been discussing this with a couple dozen people on two forums by now, plus with my brother who did a bit of sparky work back in the day and is a sound tech. And another sound tech friend.

Still none the wiser...

 

[Gray]

Someone elsewhere suggested that maybe the previous owner imterfered with it

Aah, I hadn't realised it was secondhand.

The cartridge must be correctly wired (to have worked silently on line out), but I reckon you're right to suspect the connection(s) to the phono socket(s)....specifically.
Certainly not something we'd expect on a new deck, but where previous owners are involved, anything's possible.

Get one of your tech friends to open it up and investigate - not least to confirm solid internal grounds to both sockets.