I am putting this post up here because have I had a very unfortunate situation where a builder in France has put together an X-Altra MC/MM for use with the above turntable and it hums on MM input. We have determined it is not the preamp. It works ok on MC but not as quiet as it should be.
I scoured around on the web and found a number of places where users have experienced exactly the same problem with this turntable when attempting to use it with an external phono preamp that has an earth (safety ground) connection. It appears the Rega phono preamp works with it. This has a 24 VAC wall wart PSU and no earth connections.
This is the first issue of its kind out of the 80 or so X-Altra MC/MM phono preamps that have been built, but it clearly is an issue with other preamps as well.
Loud hum from technics 1500c = broken unit? | Audio Science Review (ASR) Forum
Turntable hum with Technics SL-1500C pre-amp : r/turntables
Please see this long thread with the same problem:-
Technics SL1500C TT hum with external phono | Steve Hoffman Music Forums
I scoured around on the web and found a number of places where users have experienced exactly the same problem with this turntable when attempting to use it with an external phono preamp that has an earth (safety ground) connection. It appears the Rega phono preamp works with it. This has a 24 VAC wall wart PSU and no earth connections.
This is the first issue of its kind out of the 80 or so X-Altra MC/MM phono preamps that have been built, but it clearly is an issue with other preamps as well.
Loud hum from technics 1500c = broken unit? | Audio Science Review (ASR) Forum
Turntable hum with Technics SL-1500C pre-amp : r/turntables
Please see this long thread with the same problem:-
Technics SL1500C TT hum with external phono | Steve Hoffman Music Forums
The OP of that thread posted an update. The problem was with the phono stage, not the turntable.
jeff
First check the interconnection cable (aka RCA cable). Some cables will come with shielding and the shielding will be ground on the signal output side (ie. your turntable). You can reverse the cable direction and see if any changes were happened
The expensive cables may have this configuration. This kind of cable is good for general purpose but not for turntable.
The expensive cables may have this configuration. This kind of cable is good for general purpose but not for turntable.
Floating (all class II) system, or just one component in class I, or more components in class I ?
There are quite a few mentions of the noise problem on the web with this T/T. I have a feeling the way the built in phono stage is bypassed is where the problem lies. Anyway, I’ve suggested to the builder he returns the T/T since he has only had it a month.
He tells me the earth on the power lead measures 0 ohms to the RCA ground. Straight away I know that’s not a good idea in a T/T. You earth the metalwork to the plug if you have to, but the cart should not be connected to safety ground in any way. It must float and be referenced to the amplifier signal ground. I really don’t understand why Technics have earthed anything on their T/T since double insulated is really easy for big manufacturers nowadays. The builder is as you can imagine mightily pissed off.
I would trade it in for a 1200GR2. End of problem, hopefully. Trying to get a warranty repair is probably going to be an exercise in frustration.
jeff
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I see that the Technics SL1500C has a built-in pre-amp with a bypass switch. Apparently, Technics was too cheap to switch the ground.
I read about an Audio-Technica turntable with a related problem: the built-in RFI filter could not be switched out.
People should be wary of turntables with built-in pre-amps.
Ed
I read about an Audio-Technica turntable with a related problem: the built-in RFI filter could not be switched out.
People should be wary of turntables with built-in pre-amps.
Ed
I do not have any experience with SL1500C. I have SL-1200MK2 and it is absolutely quiet, at least with my phono pre designs. Not only by ear, but also by measurements.
The problem is with the way the built in phono stage is bypassed on the SL1500. I don’t think the SL1200 has a built in phono stage. Technics make good turntables (I owned one years ago) but the SL1500C it seems is a bit of a dog.
I agree with Ed.
I agree with Ed.
I don’t know. I will talk to him again tomorrow to see what he wants to do. I think he should take it back and change it for a non built-in preamp version.
I supplied the SL-1500C and internal photographs to the newsstand magazine HiFi World for their review. It did not hum here or there with external phonostages. Maybe try the "ground lift switch" if fitted to the phonosage in question ? YMMV
However it's frankly not a good turntable........................
However it's frankly not a good turntable........................
It might be possible to massage this problem away by some combination of exotic (read: probably non-code) PE and/or close proximity of mains (including PE) and signal, but really, why? The turntable design is fatally flawed - you just can't accept a PE ground loop at phono levels; the noise effects will be too large. Also, there's no technical reason why it's done that way, just laziness.
All good fortune,
Chris
All good fortune,
Chris
In America a mains isolation transformer, including the medical-rated ones, have continuous PE, so don't break a PE "ground" loop. Any "ground lift" is outside of code. So, unfortunately, the only within-code solution is the old fashioned one of keeping signal returns isolated from PE.
All good fortune,
Chris
All good fortune,
Chris