Optimising the PT Too power supply to reduce AC motor vibration

Standard intro and disclaimer

High resolution digital recordings of hifi system upgrades can be used for subsequent comparison to provide a more reliable method of assessment than trying to apply and assess the update at the same time. Other benefits are the possibility of external validation of conclusions, and letting others hear the kind of changes that are possible for a given level of effort or expenditure.
It should be understood that no assessments are being made with regard to the relative performance of analogue vs high-resolution digital.

To avoid any copyright issues I’m restricting my recordings to 1 minute for vocal work, and 2 minutes for classical. My test recordings are as follows.

Female vocal, easy listening
Don’t know why – Norah Jones, Album: “come away with me”
Cold cold heart – Norah Jones, Album: “come away with me”
I’m alright – Madeleine Peyroux, Album “Half the perfect world”

Male vocal, pop
Paper walls – Marc Cohn, Album “The Rainy Season”

Classical
Schubert Symphony no.5 in B flat major, D 485, Album “Frans Schubert, Symphonien Nos. 5&6, The chamber Orchestra of Europe with Claudio Abbado”.


Test purpose
The DC motor tests I did last time showed the DC motor turntables having a cleaner overall sound, with more treble detail, cleaner bass and midrange. The accepted explanation is reduction in the vibration coming from the motor. Is it possible to optimise the drive signal to the AC motor to reduce vibration?
This might give the best of both worlds, as it won’t suffer from speed drift and will be cheap to implement (provided that you don’t blow up your power supply!)

The system under test was comprised of a Pink Triangle PT TOO turntable, SME IV tonearm, Dynavector DV10x5 high output MC cartridge and Graham Slee gram amp 2 SE phono stage.

Test preparation
Before each test session, the turntable was warmed up by playing one side of an album.
The phono stage was left running for 30 minutes before the start of the test.
Recordings hve been captured as 24bit, 96kHz WAV files using a SoundBlaster X-fi PCMCIA soundcard and a Dell Inspiron laptop.

Test method

  1. Adjust the power supply to minimise vibration at 33rpm
  2. Make the test recordings
  3. Return the power supply to its factory settings
  4. Repeat the test recordings

Results
Caveat: these are high resolution recordings taken from a high performance audio system, and are intended for download. The differences are unlikely to be audible through the embedded player and typical computer speakers.

Optimised power supply

dont know why – optimised AC
cold cold heart – optimised AC
Im allright – optimised AC
paper walls – optimised AC
Schubert-D485 – optimised AC

Standard power supply

dont know why – standard AC The standard power supply has slightly more midrange emphasis; voice and piano just a little more prominent. Both sound equally good on this track. The slightly leaner balance of the optimised power supply may make basslines a little easier to follow.
cold cold heart – standard AC Both sound good but the optimised power supply sounds more accurate – voice clearer, bass cleaner sounding.
Im allright – standard AC The optimised power supply is clearly better on this track. Gentle cymbal work is easier to hear, and voice a little clearer. This track has a powerful bassline which was leaner and easier to follow on the optimised power supply. Overall the optimised power supply has a punchier, clearer sound.
Paper walls – standard AC The optimised power supply is clearly better on this track for the reasons stated above. On this track the standard power supply had a slightly bloated bassline that merged into the voice. This was much tighter and more clearly separated on the optimised power supply.
Schubert-D485 – standard AC The standard power supply sounds quite nice; the little bit of extra body to strings and flute is nice, but the cello parts can be a little overpowering. On balance the optimised power supply sounds more accurate.

Conclusion
To my ears, a convincing win for the optimised power supply; this would be a very cost-effective modification for any PT too. I’d imagine Phil King’s vibration damping modifications would provide a worthwhile improvement too.

Details of the modification
The standard PT too (metal case) power supply synthesizes the AC drive signals, with the voltages of the phases being independently adjustable via the two small pots beside the output transformers. The standard configuration drives both phases at the same voltage (250v for 33rpm and 230v for 45rpm).

Warning

  1. The motor uses 250v drive signals, so be careful not to touch the live connections. You undertake this modification at your own risk.
  2. Never connect the power supply unit to the mains without the motor connected. This is reputed to damage it.
  3. Please note that while you can optimise the power supply by feel, you’ll need a voltmeter or oscilloscope if you want to return it to the factory settings.

I took the motor out of the turntable and adjusted the voltages of the phases to minimise the vibration I could feel while holding the running motor in my hand.

Playing around with these I found that it is possible to make 45rpm totally smooth, and to reduce the vibration on 33rpm significantly but not totally eliminate it. It is also not possible to optimise both speeds at once.

In my case the lowest 33rpm vibration was achieved with phase 1=200v, phase 2=210v. It was smoothest with the two phases unequal

factory 33rpm voltages
factory 33rpm voltages
factory 45rpm voltages
factory 45rpm voltages

This resulted in the following values for 45rpm : phase 1 varying between 170 and190v, phase 2 180-200v. This was very rough. Since I only listen to 33rpm I decided to optimise that just to see what kind of difference it would make.

adjusted 33rpm voltages
adjusted 33rpm voltages
adjusted 45rpm voltages
adjusted 45rpm voltages

This is likely to have to be done on a per-motor basis.

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