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AirPower Turntable Power Supply Kit
by Leigh Norton
Introduction
I
was planning to sell the kit commercially, but I am now unable to do this.
However, I would like to share the design with others for DIY purposes.
Circuit Diagram
Parts required
* Fuse rating will vary depending on the type and VA rating of the transformers.
Torroidals have high inrush current (a brief current surge at switch-on) so the
fuse need to be rated to account for this. The best recommendation I can make is
to experiment. The 1000VA transformer in production units required a 6.3A(T)
fuse. The 30VA (2 x 15VA) types in the Mk2/kit version require only 160mA(T). In
any case, an over-sized fuse is better than no fuse.
8-Pin DIN Output Socket
Parts availability
Most of the parts can be easily sourced from electronics suppliers such as
Maplin, RS and Farnell. It may be difficult to find transformers of the right
values. You could have some made to your specification or alternatively, buy
variacs. Capacitors should be high quality polypropylene. I tested lots of
different makes and found ICW Claritycap SA sounded best although this may be
less significant if you use variacs.
I still have a few parts available:
ICW Claritycap SA 0.22uF. £5
LCR 0.22uF Polypropylene Capacitor
8-pin DIN line plug. £2
All prices inclusive of postage. Please
email me to check
availability.
Optimising VARIACs (variable transformers) Every transformer, capacitor and motor has a tolerance - a
small acceptable variation from the published specification. To determine the optimum voltages, it is best to remove the
motor from the deck and hold it in your hand to feel for minimal vibration. It is
possible to perform the calibration procedure with the motor in the deck, but
you have to listen for best sound quality which is less precise and takes
longer. The idea is to test with TR2 set to a variety of output voltages
(60, 70, 80, 90, 100 and 110). In each case, adjust the output voltage from TR1
to minimise vibration (or maximise sound quality). Make a note of the optimum
TR1 value for each TR2 value. If you are particularly fussy, you can use 5v increments for TR2
(60, 65, 70, 75, 80, 85, 90, 95, 100, 105
and 110). Once you have your table of optimised values, re-install the
motor in the deck and start listening to which values sound best. Listen for a
deeper and fuller bass which has extra clarity and definition. In general, the
lower vibration values work best but do try all the combinations. The Linn
Lingo* outputs around 60v which achieves very low vibration but in my opinion is
the reason for it's overtly lean and lacklustre sound . For most LP12s, a TR2
voltage of 80 is the best compromise. One motor in a friends very early LP12
sounded best with around 110v. Note: The ratio between the voltages of TR2 and TR1 has a much
more profound effect on vibration and sound quality than the actual voltages i.e.
for most LP12s, where TR2=80v and TR1=60v, the ratio of 80:60 is
important, so using 112v and 84v (=80:60) would be much better than 80v and 70v
(=80:70). Contact * Linn, Sondek, LP12 and Lingo are acknowledged trademarks of Linn Products Ltd.
Philips, Airpax, Impex and Premotec are also acknowledged trademarks.
The AirPower kit is a turntable power supply for the Linn Sondek LP12 and other
turntables using a.c. synchronous motors. These motors were manufactured by
Philips Mechatronics under the Impex and Airpax brands. The company became
independent of Philips in 1997 and is now called Precision Motor Technology BV,
the motors being branded Premotec.
The kit is based on the same circuit as the production AirPower. For the sake of
size and economy, much smaller transformers are used but compromise in sound
quality is very minimal.
Most transformer based LP12 power supplies use a single transformer. During
extensive experiments when creating the original design, I found that due to
loading effects, reduced motor vibration and hence increased sound quality could
be achieved if you use two transformers whose output voltage can be determined
independently of each other. In fact, for absolute perfection it is possible to
optimise the voltages to each individual motor using variable transformers. I
describe the procedure to do this at the end of this article.
TR1 60v transformer
TR2 80v transformer
C1 0.22uF capacitor
F1 slow blow in-line fuse*
SW1 DPST switch
SW2 DPST switch (optional)
1 x 8-pin DIN chassis socket
1 x 9-pin DIN line plug
1 x enclosure
Cheaper alternative to the ICW. £2
There are many different model of Impex/Airpax/Premotec motor. The standard
AirPower values are good for most LP12 motors but some early decks have been
found to need different values. For these reasons, you can guarantee optimum
performance of your AirPower kit by using variacs.