Rotation-related noise from turntable

cgallery

Member
2015-03-14 3:27 am
I used the motor/platter from a Technics SL-6 turntable to build my own turntable with my own tonearm.

This is a direct-drive motor where the rotor is a ring of magnets attached to the bottom of the platter.

I traced the wiring to figure out how to make a 12vdc wall wart power the motor.

Seems to work well.

At higher gain on my preamp (higher than I'd typically listen with my current 5v cartridge but maybe not for a cartridge with a lower output), I can hear a faint sort of static sound.

It takes about thirty seconds for the sound to start, after I start the motor. If I stop the platter with my hand, the sound stops. When I let go of the platter so it starts to spin again, the sound starts right away. If I push on the spindle or rim, the sound intensifies a little.

I have an inline switch wired after the wart, and if I switch this off, the noise continues as the platter coasts to a stop, and then the noise stops. If I then move/spin the platter by hand (with power turned off but the cable still connected), the noise resumes. If I disconnect the power supply entirely from the motor and spin the platter by hand, there is no noise.

As I said, I'm using a 12vdc wall wart to power the turntable motor. I've tried two different makes/model of wall warts, with the same result.

Something tells me this is some sort of interaction with my wall warts, but I'm unsure of how to troubleshoot it.

Any ideas of simple things I could try?

Thanks in advance.
 

cgallery

Member
2015-03-14 3:27 am
Thank you for all the help, everyone.

I'll add another datapoint that might help everyone, help me.

Last night I had the bright idea to run the unit on one of those eight AA battery packs. The noise doesn't occur under this source of power. But if I swap back to the wall wart, the noise starts again.

I had settled on this Technics wall wart because the previous wart introduced a lot of noise. The Technics looked larged and heavy (transformer) and I don't hear any noise until very high gain on the preamp.

When I'm on battery power, the total cable length is approx. 14-inches. When I'm using the wart, my total cable length for DC is probably approx. 6-feet, plus then the wart and then approx. 5-feet or so of AC cord. The extra lenght of DC and AC cord are cable tied to make them fit into my cabinet.

I will experiment with choking the power cord. Would adding some capacitors across the DC at the input of the motor help? At that point I have one of those European style terminal blocks where I make my connection between the wall wart, and the turntable's input connector.


Edit to add: I have a fairly decent assortment of capacitors I've used to fix motherboards with bad caps. They're typically decent lower-ESR Panasonics and such, if any of those might help.
 
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cgallery

Member
2015-03-14 3:27 am
I winged adding capacitors at the input of the motor, across the power input. I tried a 470uf 16v, and I also tried something like a 2200uf 50v. I don't think either helped, but perhaps the values were inappropriate.

I also tried a choke on both the AC and DC side of the cord wart, with no change.

On thinking about it, realizing the problem doesn't occur under battery power, I think my problem is that my wall wart just isn't appropriate. So I've ordered one of those Pyramid linear regulated power supplies, and I'll see if that works any better.
 

cgallery

Member
2015-03-14 3:27 am
Static electricity is generated by the platter/bearing as it revolves.
Grounding the platter bearing/motor housing/tonearm base is required.

That was my first thought.

On these units, there seems to be no continuity between the spindle/platter, and the motor's bracket, they appear to be intentionally isolated.

I have tried touching the ground wire to the spindle, and I pickup hum and I can't remember if I eliminate the noise. It is possible that I can't hear the noise over the hum I'm getting. It is also possible the hum is because I already have a connection to the motor bracket, maybe I should remove the motor bracket grounding wire and try the spindle only. OTOH, I don't think I have a way of keeping a wire ON the spindle, due to the design of the unit.

And I'd think if my problem was static, that running on battery power wouldn't eliminate the noise?

I wasn't aware or at least sensitive of this noise until last Friday evening. I had made some adjustments to my tonearm and when I went to listen, I was getting quite a bit more of this static-like noise, mixed in with some hum. I made all sorts of adjustments to my grounding scheme but couldn't get rid of the noise.

Then I realized that my strap at the tonearm wasn't making a good connection due to the way I had moved some fasteners around. Once I had that corrected, the noise floor dropped again. But at higher gain, I could still faintly hear this static-like noise.

I should be careful in calling it a static-like noise, too, because it might be a switching noise. The motor fires coils that spin the platter, the motor has an elaborate control PCB attached to it.

I'm sure if I demonstrated the issue to you guys in person, you'd say, "oh Phil that is easy, it is this."

I'm hoping the better power supply fixes it. If not, I'll play some more with the grounding I guess.
 
When you tried the choke and it didn't help, was it the type that might be found in a tube power supply? If so, a common-mode choke might still be the answer.

The 'regular' choke would not prevent the ground wire from behaving like an antenna.

And the 470 uF electrolytic likely would not have good enough high frequency performance (if it IS just the noise on the 12V conductor -- or reflected to the wart).

Check out the line input side of a typical switching supply. Usually a couple of fairly expensive (partly 'cause they have to tolerate full line voltage, day in and day out) film capacitors with a two-winding toroid in between.

You could even try it just for a test. Leave it unplugged of course! Pick one side (of the toroid, parallel with the cap) and jumper to your wall wart output; then jumper from the other side to your spindle motor.

What fun, eh?
Cheers, Rick
 

cgallery

Member
2015-03-14 3:27 am
When you tried the choke and it didn't help, was it the type that might be found in a tube power supply? If so, a common-mode choke might still be the answer.

My apologies, I used a clamp-on ferrite core. Not what you were talking about, I assume.

The 'regular' choke would not prevent the ground wire from behaving like an antenna.

And the 470 uF electrolytic likely would not have good enough high frequency performance (if it IS just the noise on the 12V conductor -- or reflected to the wart).

Check out the line input side of a typical switching supply. Usually a couple of fairly expensive (partly 'cause they have to tolerate full line voltage, day in and day out) film capacitors with a two-winding toroid in between.

You could even try it just for a test. Leave it unplugged of course! Pick one side (of the toroid, parallel with the cap) and jumper to your wall wart output; then jumper from the other side to your spindle motor.

What fun, eh?
Cheers, Rick

Well it is fun for me, probably not so much for you guys.

I don't fully understand your test, but I have the linear regulated supply being delivered by Amazon shortly. So I think I'll give that a shot and if I'm still having problems, then I'll ask some more questions about your test.

I'm trying not to waste everyone's time.
 

cgallery

Member
2015-03-14 3:27 am
Due to this being a "home brew" machine, all we can do is speculate, because there's too many unknown variables to deal with.

Well that is fine, your speculation is certainly guided by more experience (compared to mine).

ANYWAY... I got the regulated/linear supply and drum roll please... The noise was still there.

BUT, I found that now I'm able to remove the section of the preamp ground strap connected to the motor's steel board, which eliminated the crackling noise while maybe increasing other background a very nominal amount. I need to do more testing there.

With the other power supplies I had tried, I was unable to disconnect that part of my ground strap without getting enough noise that it was unlistenable, though.

With the new supply and the ground strap lifted from the motor, I'm also able to touch/push the turning spindle/platter without any resulting crackling noise.

I need to do more listening, I'm also curious if I was somehow creating a condition where more static was somehow being introduced during record play.

I think the optimum configuration might be replacing that ground strap while somehow arresting the noise. Don't know if that will ever be doable, as Technics didn't have such a cable in place (at least according to my recollection), and I'm probably squarely in nit-picking territory now.
 

cgallery

Member
2015-03-14 3:27 am
How is your tonearm constructed and wired? Having to deal with electrostatic pickup indicates a potential lack of shielding. Make sure the tonearm tube is connected to either a dedicated ground lead or at least audio ground.


It is in a shielded jacket, and the jacket is connected to the ground strap that is connected to the preamp.


I'm not sure the noise is static. If it was static, I don't think the replacement supply or the battery-based power would correct it.


It just sounds like static.


But more likely, it is something being introduced my the motor controller?



Or am I completely nuts on that?
 

cgallery

Member
2015-03-14 3:27 am
Well I guess final follow-up, I don't think I can hear a difference between the motor running and the motor not only not running, but completely disconnected. There is no sense in investigating the strap between the motor's bracket, and the rest of the turntable ground, as I don't think I can do any better than silent.



So I'm going to call the linear regulated supply a win.
 

cgallery

Member
2015-03-14 3:27 am
it's the magnets embedded to the platter that are inducing the noise when the platter is in motion, me thinks.

I'm pretty sure it was a combination of the power supply + my grounding scheme, as it is fixed.

I've included a pic. The new power supply is on the bottom shelf, I should have tried to get that in the photo.

I'm currently running an Audio Technica VS215E in a 1/2" adapter. It came with the original SL-6 and is a good match for my lower-mass tonearm. I had tried AT-VS95* 1/2" cartridges and they were good, but I had some trouble tracking vibraphone. I have a sampler album with some solo vibraphone, turns out it is a bit of a torture track, probably akin to the Shure album with the bells. Anyway, the old p-mounts with their higher compliance values seem to be a better match, as the problem went away.

The p-mount also does a better job tracking the HiFi News test album skating force tracks. I can track them all. I get distortion on the highest amplitude track, but the needle stays in the groove. All the other skating force test tracks have no distortion.

I had built a wall shelf for the unit but found I was getting street noise (I live on a busy street). I think the combination of the cooler temperatures and harder tires/ground was causing the stiffer suspension I was using previously some difficulty. With the adjustable spring feet, I can rap the shelf and get no sound.

Also added a better/easier azimuth adjustment for the tonearm. My counterweight is a stack of stainless washers between nuts, and I've added an asymmetrical piece at the end. Once I set the tracking force, I dial the azimuth in by spinning that piece.

It is sounding pretty good. This was V1, I'll likely do a V2 with a few improvements learned from V1. I already purchased another motor/spindle/platter via eBay so I can be working on V2 while still "driving" V1.
 

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