This page is my shrine to all things vinyl!!
Belt drive turntables are preferred by the audio elite but there is small problem - how to vary the speed of one?
First - why are belt drive turntables considered better? Primarily because the belt isolates the platter from motor noises, secondly because the AC motors used to drive the belt turn continuously at a speed determined by the power line frequency (nominally 60Hz or cycles per second). Since the AC motor does not have a commutator it's rotation is very smooth, and since the power line frequency is very stable the motors speed will hardly change at all.
So why mess with "perfection"? Well - if your a guitar player and you just spent an hour tuning your guitar so you could practise along with your favorite old record only to discover your guitar is not perfectly in tune pitch wise with the turntable...arghhh!! Or you want to correct the pitch of an old record known to be wrong. Or you want to play a 78 RPM record when you don't have a 78 RPM speed. Or...you get the idea...
This small gadget generates a source of 120VAC at around 25W (enough to power most turntables...) and lets you change the frequency using small push switches. The power line frequency
is normally 60 Hz...as shown in the picture the switches are set to "60.0". By pressing the small push buttons above and below each switch you can raise of lower the output frequency
in 0.1 Hz steps from a low of around 30 Hz to a high of around 95 Hz. Cool!! A PLL (phase locked loop) using an internal crystal for reference controls the frequency - its short term
stability is as good as the power line - but over days or months not so much. Practically speaking you'll not hear the difference.
Contact me at geoff@bandonelectronics.com if your're interested in building or buying this item.
You've probably noticed most turntables with adjustable speed feature a set of strobe markings cast into the platter that are illuminated by a small bulb. This bulb is actually flickering 60 times a second (too fast for your eye to perceive). The markings are spaced such that they appear to stand still when the platter speed is set precisely.
If your turntable does not have these strobe markings and you want to check its speed you can print out a paper version (see link below), lay this on top of the platter and use
this gadget as the light source. A bright white LED flickers at precisely 60 flashes per second substituting for the bulb. Makes a great companion to the variable AC power
source project.
Contact me at geoff@bandonelectronics.com if your're interested in building or buying this item.
Print sheets for 50Hz strobe discs can be found at Sound Exchange