speed inaccuracy Garrard 401

Dear all,

Several years ago I bought a second-hand Garrard 401 in immaculate condition. Altough the unit operates smooth and very silent, there is no audible rumble, something is wrong its speed accuracy.

After a warm-up period of approximately 30 minutes, the motor starts to drift from the nominal speed. The stroboscope pattern slightly revolves in counter-clockwise direction. Sometimes, after several hours of operating, the motor stabilizes to its nominal speed.

I already contacted Loricraft (Terry O'Sullivan) about this. Terry replied that this form of fluctuation is normal for a Garrard 401.

Exactly the same fluctuations I observed on my Thorens TD 124 MkII. Both turntables are equiped with an AC 4-pole asynchronous induction motor (of shaded-pole type).

Does anyone know if an external Power Supply (i.e. a Wienbridge Oscillator) will solve the problem, or is this phenomenon typical for this kind of motor type?

regards,
PeterW
 
You appear to be observing the minor variations of the AC power
supply frequency, which does vary slowly, usually to allow a
overall frequency correction.

Why ? A mains powered wall clock has a sychronous motor. For
it to remain accurate the overall average frequency of the mains
must be its nominal value, 50 or 60 Hz. The electricity companies
strive to acheive this, hence the minor variations.

IMO a standard wein bridge will be worse then the mains - IMO
you'll need a quartz crystal referenced phase locked loop to
achieve a consistently higher frequency stability than the mains.

But what is confusing me is the methods of fine speed adjustment.
I know the 401 has a magnetic eddy current brake speed
adjuster but this wouldn't change the speed of synchronous
motor, only increase its slip angle, perhaps it works by
increasing relative slip of the drive wheels.
I can't remember the exact 401 variable speed arrangement.
The TD124 I'm not familiar with.

Also just realised the above is completely wrong as presumably
you are using mains powered lighting to observe the strobe,
so any variations in mains frequency would be undetectable.

I have just painted myself into a corner ...............................


:) sreten.
 

fdegrove

diyAudio Senior Member
2002-08-21 1:20 am
Belgium
Hi,

Care to elaborate on this? My experience differs. Are you saying you'd prefer a Linn with a Basic rather than Valhalla or Lingo? Don't tell me to stuff that Linn

Assuming you know how synchronous AC motors work and you know what the powergrid is like, you'll also know that if anything can be called stable on the grid, it's the 50/60 Hz cycle.

When Linn brought these so called regenerated supplies to the market, we sat down and listened....
We, that is, a mixed group of engineers and audiophools, found that the sound was degraded.

I heard the same echo from other owners who were equally disappointed and I agree with them. Assuming the motor is well chosen for the task it should sound better run straight from the mains and IME it always does.

The Garrard 301 may be an entirely different kettle of fish, I'm not familiar with the inner workings of asynchronous AC motors.

Cheers,;)
 

analog_sa

Member
Paid Member
2002-08-14 1:47 pm
Cascais
Interesting. The 50/60Hz maybe very stable but at the time i was interested in all this i measured 6-7% distortion and plenty wide-band noise. Never bothered measuring the Valhalla but my creations managed under 0.5%. IMO this can really affect the vibration spectrum of a motor.

I also compared a Linn running from a resistor/cap network to a Valhalla, to a home-grown Lingo and to an Armageddon style transformer/cap. All this setups sound amazingly different and the synthesised supplies bring up so much more in detail/ambience retrieval and PRAT that it's not even funny.
 

fdegrove

diyAudio Senior Member
2002-08-21 1:20 am
Belgium
Hi,

All this setups sound amazingly different and the synthesised supplies bring up so much more in detail/ambience retrieval and PRAT that it's not even funny.

We all know that vinyl replay is highly dependent on correct platter speed...

I can only guess that something wasn't quite right, if the synthesised supply cured it than I'm happy for you.

What can I say?
A well set up LINN L12 should have great PRAT without the special supply but it does have it's colourations,the mat and platter being part of it, and dynamic range isn't what it can be.

As it is a suspended design it's bound to lose some micro-informations.

Mind you, CD isn't as dynamic as vinyl can be either albeit for different reasons....

Cheers, ;)
 

EC8010

diyAudio Moderator Emeritus
2003-01-18 7:57 am
Near London. UK
The Garrard 301 and 401 both take at least half an hour to settle to their speed (this is actually stated in their instruction manuals). Their motors are loosely locked to the frequency of the mains in that they can only apply torque if their rotor lags the rotating AC magnetic field. This is known as the slip speed, and means that whereas a 4-pole synchronous motor would run at exactly 1500 rpm, these motors run at about 1475 rpm.

Applying the eddy current brake makes the motor labour slightly, and slows it. Rather crude, but workable, although it increases vibration. The magnet in the brake tends to weaken over the years, so most Garrards run fast with the fine speed control knob set to the centre position. One cure is to fit a slightly under-size pulley to the motor.

A better cure is to remove the eddy current brake completely and power the turntable from an external supply. Because the motor no longer has to work against the eddy current brake, less power is required from it, so it needs less electrical power. If the voltage applied to the motor is reduced, the vibration is reduced.

Observing the strobe markings lit by a mains powered light is misleading because mains frequency is allowed to vary over the short-term.

My 301 is powered from an oscillator, and is stable after 30 minutes (it wasn't when powered from the mains). Is your 401 clean inside? Are the motor pulley and inside of the platter clean? Is the idler wheel clean, but not shiny?

The Garrard turntable doctor is Martin Bastin. He makes all sorts of mechanical bits like pulleys, control knobs (301), and idlers. He also makes a power supply. He can be contacted via Tom Fletcher of Nottingham Analogue.
 
EC8010,

Thanks for the reply.

Observing the strobe markings lit by a mains powered light is misleading because mains frequency is allowed to vary over the short-term.

I doubt if frequency variations in the mains are observable at all. Motor and the neon light bulb are powered by the same mains.

Is your 401 clean inside? Are the motor pulley and inside of the platter clean? Is the idler wheel clean, but not shiny?

The Garrard 401 is in immaculate condition. The unit has been serviced, relubricated and a new intermediate wheel has been fitted.

regards,
PeterW
 
PeterW said:
sreten,

Thanks for your reply.

Indeed, the Garrard 401 and the Thorens TD124 MkII are equiped with an eddy-current speed adjuster to change the slip angle. This works only in conjunction with an asynchronous induction motor.

Maybe this speed control-system is the cause of the fluctuations.

regards,
PeterW

I've done a litttle reading into asynchronous AC motors.

The fundamental difference between the two types is
synch operates with a variable slip angle - locking it to
the incoming frequency whilst asynch operates with a
slip speed - typically a few % below incoming frequency.
(Typical asynch operating point is at ~ 1% to 1.5% slip speed,
both types of slip vary in value with the applied load to the motor)

A synch motor is fundamentally immune to AC voltage variations
whilst asynch is not, both types are linked to the incoming mains
frequency, but the asynch is not tied to the incoming frequency.

The variation you observe implies the slip speed is reducing and
this implies the load on the motor is reducing.
The only way to stabilise the speed of the turntable is to make
the loading as consistent as possible.

I don't think that its related to mains frequency variation at all.

I'd suggest any form of additional power supply will not really help.
Of course if you make the power supply have variable frequency
you can remove the brake mechanism and speed adjuster.

:) sreten.

(To be strictly accurate the asynch motor operates with a slip
frequency, the difference between driving frequency and
operating frequency varying with the load and I assume the
amount of current driving the motor as this determines the
motor magnetic field strength.
I also assume that drawn power is constant for an asynch motor,
unlike a synch motor which draws more power with slip angle and
load - this is the basis of its excellent stabililty.
 

Attachments

  • gen_article_figure_1.gif
    gen_article_figure_1.gif
    11.7 KB · Views: 574
sreten

Thanks for the research you did on the working principle of the asynchronous induction motor.

From your reply and that of EC8010 I would conclude that an asynchronous induction motor is extremely sensitive to variations in mechanical load (eddy-current speed adjuster) and that an external power supply will only work if this load is minimal and consistent in its behaviour.

What a pitty that despite its musical capabilities Garrard applied this motor-type in such a wonderful machine like the 401.

regards,
PeterW
 

EC8010

diyAudio Moderator Emeritus
2003-01-18 7:57 am
Near London. UK
I'm not sure it's that sensitive!

Screten: Your post is extremely interesting - could you give a reference for the diagram?

PeterW: I'm wondering about your strobe observations. Bear in mind that a strobe beats two frequencies together, and is therefore a very sensitive measure of relative frequencies. As an example, using crystal-referenced illumination, it is possible to see the degradation in wow caused by fitting an unmatched platter to a centre spindle! (Platters and spindles were machined in pairs, so platters should not be exchanged.)

Driven from an oscillator, my 301's speed is a function of temperature. So much so, that I considered marking the oscillator's speed control in degrees Fahrenheit. Other than that, the only variation visible is a tiny amount of wow, and even that could be down to inaccuracy of the strobe casting.

Driven from the mains, my 301 meanders gently, but isn't bad.

Your 401 should be comparable, and produce less induced hum into the cartridge, but possibly more vibration due to all that horrible wiring between the vibrating motor and the chassis. Either you are being unduly sensitive in your observations, or your 401 has a problem. It ought to be a cracker!