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The Well tempered Light Bulb

pete_budgethifi

pete_budgethifi

New Member
:tresbon:

This tweak is so amazing that I think it deserves a thread of its own. Please please pleeeeeaaase try it and post how you get on!

I've just done the most fun tweak yet on my PL12D project. If you have a turntable with an AC motor, that motor vibrates with the mains frequency and hums along with the music. Reducing the AC voltage that the motor gets reduces this vibration and hum dramatically! The poor tweaker's Linn Lingo - all you need is a light bulb holder ( a ceiling fitting is best because it stands up upside down) a plug and a flying socket. Wire up Socket - holder - plug such that the light bulb is in SERIES. The bulb is an electrical resistor which will drop the voltage to the motor. Plug the deck into the flying socket and put a low wattage bulb in the holder.

The lower the wattage of bulb, the greater the resistance and the more will be the voltage drop. The idea is to drop the voltage as much as possible without stopping the platter turning properly. I am using a 40 watt bulb but will try a nightlight 10 watt when I can find one. The effect is truly AMAZING!

The bulb does not light up, it just gets a bit warm.

Sanity check... If the bulb lights up normally, you have wired it in parallel... If all the other bulbs in the house go out, you have wired a short circuit.

Pete
 
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Sounds interesting, but I would think adding better isolation would be better in the long run.

Running a motor at a lower voltage makes it draw more current to keep going. This can heat up the motor and reduce its lifespan.

Just a thought. Not trying to rain on your idea.
 
Laws of Physics

Hi,

I think you are a bit wrong. You are quoting valid electrical theory, but based on a false premise, which is constant power. If the motor was giving the same power, yes, then at reduced voltage, it would have to get more amps. That is the difference between car electrics and domestic electrics. But the point is that we are reducing the power. It runs cooler, lasts longer, consumes less current, and does not hum so much.

Regards,

Pete
 
AC tt motors are 'synchronous'. That synchronicity is cps dependent. As long as they get enough power supply to do their job, don't get pulled down by wide bass excursions etc., Pete's method ought to do no harm.

The above is pure suposition in the style of Aristotle.

Bob
 
I heard about this tweak a few years ago.

Perhaps this is why a system "sounds" better with a lava lamp plugged into the same circuit?

:dunno:
 
A bit worried

Well Jordan got me a tad worried there ! ... So I have been checking around on google. There is mention of overheating in AC motors when the supply voltage drops, but that was in heavy engineering applications and only as a result of the motor running slow, below synchronous speed. As Bob says, turntable motors are AC synchronous and their speed depends on the mains freqeuncy. In a lot of decks the motor is more powerful than it needs to be. This is especially true of the pl12d monster! So reducing the voltage (sensibly) will not effect the speed, so everything will be OK.

Here is an extract from the description of the VPI - SDS Audiophile Power conditioner...

During motor startup, the SDS increases its output voltage in order to bring the platter up to speed quickly. Once the desired platter speed has been reached, the SDS ramps the output voltage down in order to reduce motor vibration and, therefore, the system's noise floor.

If you are unhappy with the light bulb tweak, buy one of these - only $999.99


Pete
 
Just wondering; would a variac accomplish the same thing? I doubt if I'll try it but just say you reduce the voltage down just a bit. Can't see how reducing the voltage would be benefical but then again what do I know.
 
Sometimes it helps the non-specifically educated (me anyway) to think of electrical things in analogous hydraulic terms. So... voltage is pressure. Contemplate a hydraulic circuit in which the fluid-under-pressure is constantly alternating direction (which means that some actuator is alternating direction). Try to visualize how a significant change of pressure could affect the system.

This "understanding through analogy" works fairly well in dynamic systems, except in the details; the above case suffers from 'flow' differences between a fluid and electricity. The stiffness element may be valid though.

Bob
 
motorstereo said:
Just wondering; would a variac accomplish the same thing? I doubt if I'll try it but just say you reduce the voltage down just a bit. Can't see how reducing the voltage would be benefical but then again what do I know.
Click to expand...
I wondered the same thing. Coincidentally, when I was first building tube amps and did not have a variac to slow start them up the first time, I was shown a similar rig where in you similarly wired up a light bulb for the same reason, it slowed the juice to the amp, and ideally would go bright at first, then dim down if everything was ok, if it stayed bright, shut down and see whats wrong. Interesting, seems to me a variac would work fine, and sure is cheaper (used anyway) than $999.99. I am or course assuming that it wouldnt alter frequency.

Russellc
 
Seems okay to me. The main thing that affects current is load. The heavier the load, the bigger the current.

One good test of whether a motor is being overloaded is this. Many inexpensive DVMs (under $50) have a thermometer function. You take the thermal probe and touch it to what you're measuring (here the coil windings or stator laminations) and take its temperature. Say that after running for thirty minutes at a full 120vac the motor -- measured on the stator laminations -- stabalizes at about 43 degrees Celsius (that's a normal value based on actual experience.) Now, stick the bulb in series with the power switch running to the motor. Run this for half an hour. Say, now the temperature stabalizes at 38 degrees Celsius. Your AC voltmeter may tell you that the voltage across the motor is now 80vac and across the bulb 40vac. Clearly, the reduction in temperature is not proportional to the reduction in voltage, suggesting that efficiency has dropped slightly as the drive voltage has declined. However, so what? The operating temperature of the motor is still 5 degrees less, and it's the operating temperature that's the real factor in motor wear, as this defines the degree of expansion and contraction of stator laminations, rotor and coil windings (which, other than bearing wear, is the REAL wear-and-tear on a synchronous motor.)

Next time I have a turntable like yours apart, I think I'll run an actual experiment and come up with some true, rather than hypothetical, numbers.

Best,

Fred Longworth
StereoTech Classic Audio Repair
http://www.repairaudio.com
 
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Urizen said:
I heard about this tweak a few years ago.

Perhaps this is why a system "sounds" better with a lava lamp plugged into the same circuit?

:dunno:
Click to expand...
No.
Pete's tweak uses an additional load in series with the motor. The lava lamp is a paralell load, it works only on your brain, and has no effect on the turntable.
 
I have been watching this thread, and am confused about load and heat. These motors are brush-less. They have no clue as to the load being put upon them. Lowering the voltage will simply reduce torque, until the motor cannot cope with the load it needs to carry. The field winding is simply the driving force for armature rotation.
If they had brushes, the reduced voltage would certainly make them run slower and warmer, because of that technology.

Do I make sense, or should I stick with the repair end of the turntable game?

Seth
Forever Analog
 
Thanks Fred

Fred, that would be great! Nothing like a bit of hands on measurement. Also a question... I seem to remember that years ago when I was using the PL12D as my main source, it suddenly developed motor hum, as if overnight. Now when I dismantled the motor recently and cleaned it up, oiled the bearings etc, everything seemed Ok with it. I have read that winding shorts can cause vibration. What about other causes? Can a wonky phase shift capacitor cause problems too? The motor turns powerfully enough, but I'm sure it used to hum much less. What is the right type of phase shift cap to get if I want to replace it?

Pete
 
The light bulb will surely present a varying load, possibly smoothing current surges or something similar.

Notice how Nelson Pass uses them as a load on his Zenlightenment amp. He says they act a little like a constant current source.
 
:scratch2:

Well, I would have thought that the bulb is primarily a fixed resistor used in this application, but having said that, resistance goes up with temperature, and unlike a normal circuit resistor component, the bulb will vary in temperature hugely with the voltage across it. I don't think this variation will occur fast enough to smooth mains spikes etc, but the spikes will be reduced by the mere fact of having extra resistance in the circuit.

In the case of a valve amp, it seems that the valve (tube) can behave such that it varies in resistance widely as it is warming up. This is a slower change and might bring the current stabilisation into effect.
 
I think that this should be just fine. And the Variac would accomplish the same basic goal, but would have far superior voltaged regulation than would the light bulb.

The key is that the motor needs to get enough voltage to be able to maintain its sinc. If it does not get enough current to maintain it sinc, it will vibrate violently. So I would use a variac to see how far down you can take the voltage, before the motor starts to loose sinc, and then up your voltage by roughly 15% for a good safety measure. That should be about the perfect voltage to both minimize vibration, and yet still maintain sync.
 
Safety Note

:sigh:

If using a variac, make sure it is a "true" variac designed for mains voltage and the correct current rating. DON'T mess around with a variable resistor or "pot" you might find lying around, like an old transistor radio volume control or something. :no:
 
I think the filament will vary quickly - as quickly as the voltage changes. After all, the temperature coefficient of carbon comp resistors gives them a quality whereby they change resistance with instantaneous voltage creating the assymetrical distortion some of us like.
 
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