Help with a DD circuit

[analog_sa]

My first TT project is built around an ESD1000 DD motor from a Dual 701. The purpose is mostly to allow experimentation with different plinth materials and because of the DD simplicity i chose this motor unit rather than one of the 301/401/124 that i have patiently waiting for a new plinth.

Apart from the plinth it is my first glimpse into direct drive circuits and this one is surprisingly simple. Still, there is a lot i don't understand and hopefully some more knowledgable members will be able to throw some light upon its workings.

As far as i can see this circuit's simplicity doesn't come for free. Each hall element's sensitivity will have to be adjusted and each driving transistor current gain also be accounted for. That's why there are so many pots.

My question is: how would they perform these adjustments at the factory or if a driver transistor needs to be replaced? Measuring wow&flutter may be one way but is too complicated and for me obviously impossible. Connecting a scope to each collector and adjusting the pots for equal amplitude is the only method that makes some sense to me. Unfortunately these signals are of very low frequency and this is difficult with a normal scope.

Btw T10, T11 and the mysterious relay are absent from my unit.

It already sounds better than my LP12 and i still don't even have lead shot in the plinth

Any suggestions how to improve upon the circuit other than upgrade the PS? I suspect improving the symmetry of the drive between the four coils and possibly re-adjusting the integrator time constant may be beneficial but i honestly know nothing about motors.

Any suggestions will be welcome.

[ingvar ahlberg]

Hi

As far as i can see this circuit's simplicity doesn't come for free. Each hall element's sensitivity will have to be adjusted and each driving transistor current gain also be accounted for. That's why there are so many pots.

Yeah, and using AC188 germanium transistors which have as diverse characteristics as stray dogs on tin pan alley, so if it works let that part be. Making the supply stiffer is propably a good idea. I think that DD can work and sound as good as belt or idler wheel drive, its just using the chosen method right that counts. Those who claim that belt drive is the only way to make a record listenable has never been able to tell me which record masters were cut on a belt drive lathe, not even Ivor Tiefenbrun.

[analog_sa]

Making the supply stiffer is propably a good idea.

Replacing a tiny wall-wart with a 60VA transformer made an astonishingly small difference; not sure if altogether positive. Raising the filter capacitance to a 6800uF Nichicon Gold Tune made an even smaller difference. This must be the first TT drive system not sensitive to mains cabling i have ever encountered.

Still, no suggestions how to balance the drive to the coils?

[directdriver]

Any new progress on this front? Inquisitive mind wants to know.