Phono3S it's humming again.

[bikerhifinut]

Originally Posted by John Caswell

Hi Andy,
Placing a die cast case around the Gyro PSU will not help as it is an electromagnetic field the is causing the problem. Only mumetal or distance will help and that is a seriously expensive exercise.
Why not do as I did (cos I am lazy and cannot be bothered to change over the belt) and build yourself a new PSU with a toroidal transformer and switched voltages to change speeds, assuming that you have the DC motor, as opposed to the DC Papst motor. It is not very difficult using a standard 3 pin regulator. Of course if ll you play is 33 rpm is is even easier.

John

Intriguing John, I imagine all it is is a voltage adjustment? I would need a strobe mat and light to adjust/check it I guess. The original is a small EI transformer and a 317 or similar regulator, doesn't look anything special.
So do you put the belt on the big motor pulley or the small (33rpm) one?
And I guess for 2 speed operation a switch between two trimpots on the adjustment leg of the device.?

A

[John Caswell]

Hi Andy,
Yes exactly that, I keep the belt on the small 33 rpm pulley, otherwise the voltage required for 33rpm on the large pulley is quite low.
Use something like a 9-12V 15VA toroid and an LM1084/LM317 regulator on a heatsink, using a relatively high voltage makes the regulator work a bit harder.
Measure the output from the PSU then do the calcs for the adjustment R to give you about 2V more than the 45rpm voltage. Then place two adjustable 20 turn R in parallel with the adjustment R via a toggle switch and adjust each to give you 33rpm and 45rpm.
The reason the adjustment R is used is that if the switch/pot goes O/C the volts will be limited to whatever you set with that R. Use high stab Rs for all the voltage adjustments and 20 turn cermet pots for the adjusters.
Mine has been like this for years and I have never had to adjust it since setting up.
You can download a strobe from Vinyl Engine and 50Hz readily available from your house lights.
If you get stuck, pm/email me.
John

[bikerhifinut]

I am minded to build up just a basic new PSU with a quiet toroid.
In truth changing speeds is no problem on the Gyro SE as the belt and pulley are exposed so knocking it down or up a pulley notch is a quick and easy task.
but the idea of a more mechanically quiet transformer with a lower hum field does appeal. I reckon I just have to measure the output volts of the existing PSU and use that as a starting point. It's just a standard 317 type setup. I don't see how the HR supply can really improve the performance apart from emptying my wallet effectively.
Thanks for the excellent tip John.

A