I have been performing some measurements on my Rega 25 turntable, using ARTA + soundcard + interface, calibrated in dBV. The tonearm and wiring is all original; I have installed the 24v AC 2-phase synchronous motor conversion, driven by my own synthesisted 2-phase supply.
To me, the EM pickup seems excessive, so I would very much welcome any comments on this.
The layout of the turntable is shown:
(All graphs below are on the same scale to allow easy comparison of the image set)
Firstly, here is the noise floor of my measurement system, with the input probe grounded:
Next, the output of the phono stage, with it's input shorted at the phono socket. All values above measurement system noise floor:
Now with the turntable connected to the phono stage (motor not running, tonearm parked). There is already some environmental pickup of 50Hz mains, plus odd harmonics:
With the motor running at 50Hz (for 33rpm), and the tonearm in the parked position, a slight increase in the 50Hz component:
Changing the motor frequency to 67Hz (for 45rpm), the additional component from the turntable motor can more easily be distinguished:
Moving the tonearm to the centre of the record position, the EM pickup from the motor is greatly increased, at both 50Hz and 67Hz, such that it can easily be heard at normal listening volumes when the tonearm is raised:
To me, the EM pickup seems excessive, so I would very much welcome any comments on this.
The layout of the turntable is shown:
(All graphs below are on the same scale to allow easy comparison of the image set)
Firstly, here is the noise floor of my measurement system, with the input probe grounded:
Next, the output of the phono stage, with it's input shorted at the phono socket. All values above measurement system noise floor:
Now with the turntable connected to the phono stage (motor not running, tonearm parked). There is already some environmental pickup of 50Hz mains, plus odd harmonics:
With the motor running at 50Hz (for 33rpm), and the tonearm in the parked position, a slight increase in the 50Hz component:
Changing the motor frequency to 67Hz (for 45rpm), the additional component from the turntable motor can more easily be distinguished:
Moving the tonearm to the centre of the record position, the EM pickup from the motor is greatly increased, at both 50Hz and 67Hz, such that it can easily be heard at normal listening volumes when the tonearm is raised:
I would very much like to know whether the EM pickup from the motor is typical or excessive (as this is what I feel, but without any experience to compare it with), and what should be looked at to potentially address this, thanks
This looks like coupling between coils. Your motor coils radiate a magnetic field during motor operation and your cartridge operates as it should so that when its coils intersect the fields produced by the motor it passes them on to the pre-amp.
Maybe the motor needs more separation from the cartridge or another steel enclosure.
Maybe the motor needs more separation from the cartridge or another steel enclosure.
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Yes that was my premise. The Rega motor placement is pretty close to the spindle unfortunately, so wondering if this much pickup towards the centre is par for the course. An alternative motor placement would be quite invasive!
Two ways to overcome this is.
Cheers
- rewire the arm, adding a separate earth wire, platting the 5 wire together. R/H+/- : earth wire : L/H +/-.
- Dump the AC motor & convert the unit to a DC unit.
Cheers
What is your cartridge?
Some brands do not have an internal shield (example: Grado). This would exacerbate motor hum pickup.
Some brands do not have an internal shield (example: Grado). This would exacerbate motor hum pickup.
There might be some cartridges that offer better immunity either by virtue of coil orientation or possibly shielding.
After thinking a bit more it seems to me that the fields should nearly cancel and this shouldn't be a problem at all since all of coils see the (very nearly) same field and would therefore output the same voltage.
Maybe what you're seeing is "nearly cancelling" but -50dBV is still 3mV and that does sound high. EDIT: Never mind; this is after the phono stage.
Do you have another cartridge you could swap in?
After thinking a bit more it seems to me that the fields should nearly cancel and this shouldn't be a problem at all since all of coils see the (very nearly) same field and would therefore output the same voltage.
Maybe what you're seeing is "nearly cancelling" but -50dBV is still 3mV and that does sound high. EDIT: Never mind; this is after the phono stage.
Do you have another cartridge you could swap in?
Thanks for the interesting replies. I'm a turntable complete novice, and have so far been using it 'as is'.
Looking at the Rega owners manual, it has these comments on the wiring:
Of course, this is DIY where anything goes, but it does suggest to me that it should at least be of passable quality as supplied.
The wiring emerges from the tonearm and terminates in a pair of serious-looking Neutrik phono plugs:
When performing the spectra measurements, I did notice that both phono plug shields had to be connected to the phono preamp ground in order to get the lowest interfera, whichever channel was being measured. Investigating with the DMM, the shields for Left and Right are isolated from one another, and the Left channel shield is ~zero Ohm to the metal of the tonearm itself. So it appears to look something like this, is this in any way possible/correct?
I cannot quite reconcile these spectra and DMM measurements, why the shield for the Right channel not being grounded should affect measurements on the Left channel, which is providing the ground for the tonearm, does anyone have any insights on this?
Cartridge wise, it has whatever it originally came with, it appears unmarked, so maybe an OEM part for Rega, but I have a very vague memory of an Audio Technica box that originally came with it, could that be the case?
Each pickup resistance is ~190 Ohm. Here is a close-up:
The mounting screws are electrically connected to the ground, but I've no idea whether there is an internal shield in the cartridge.
Sorry, no.
I wanted to do the best I can firstly as is, but of course removing the source is the ultimate solution.
I still have the original line voltage motor (110v premotec, which has a passive network to allow it to run on UK 230v AC), but as the pulley was missing I didn't get to use it or do any measurements before swapping for the low voltage AC motor, which then allowed me to use a low voltage synthesised generator. Swapping back to this I'd ultimately have to make a high voltage amplifier for the synthesised generator, so would be more work!
Looking at the Rega owners manual, it has these comments on the wiring:
Of course, this is DIY where anything goes, but it does suggest to me that it should at least be of passable quality as supplied.
The wiring emerges from the tonearm and terminates in a pair of serious-looking Neutrik phono plugs:
When performing the spectra measurements, I did notice that both phono plug shields had to be connected to the phono preamp ground in order to get the lowest interfera, whichever channel was being measured. Investigating with the DMM, the shields for Left and Right are isolated from one another, and the Left channel shield is ~zero Ohm to the metal of the tonearm itself. So it appears to look something like this, is this in any way possible/correct?
I cannot quite reconcile these spectra and DMM measurements, why the shield for the Right channel not being grounded should affect measurements on the Left channel, which is providing the ground for the tonearm, does anyone have any insights on this?
Cartridge wise, it has whatever it originally came with, it appears unmarked, so maybe an OEM part for Rega, but I have a very vague memory of an Audio Technica box that originally came with it, could that be the case?
Each pickup resistance is ~190 Ohm. Here is a close-up:
The mounting screws are electrically connected to the ground, but I've no idea whether there is an internal shield in the cartridge.
Sorry, no.
I wanted to do the best I can firstly as is, but of course removing the source is the ultimate solution.
I still have the original line voltage motor (110v premotec, which has a passive network to allow it to run on UK 230v AC), but as the pulley was missing I didn't get to use it or do any measurements before swapping for the low voltage AC motor, which then allowed me to use a low voltage synthesised generator. Swapping back to this I'd ultimately have to make a high voltage amplifier for the synthesised generator, so would be more work!