I have been using an external motor to drive my TT platter. It is a DC motor by reputed swiss brand (Maxon). I am using it direct, connected to a 12V battery. Initially the sound was not very clean, the soundstage was hazy, not much seperation of instruments. Then I added a 10 microfarad capacitor between the battery and the motor to smooth out the power noise. This improved things significantly. I was a bit surprised because I thought using DC directly off the battery should be a very clean solution already, but adding a cap in between showed that there is a lot of scope to improve the power further.
As I hear it now, there is still a lot of scope to improve the noisy power. I am looking for some suggestions on what else can be done to the battery power so as to refine it further before it reaches the motor ?
As I hear it now, there is still a lot of scope to improve the noisy power. I am looking for some suggestions on what else can be done to the battery power so as to refine it further before it reaches the motor ?
How do you set the rpm then?
There are a number of threads dealing with dc motor control and even one group buy for Mark Kelly's controller board. There is plenty to read.
the cap is in parallel?
ought to have next to zero effect.
a bit depends on the type and nature of your battery.
If it is drawing a lot of current, and the battery is barely up to drawing that current, you have a problem.
Also, if the current draw from the motor is not very very even, in part based on the mechanical load, you have another problem in addition.
You ought to put a scope into this and look at both the voltage across the motor, and the current (very very small resistor in series with the motor) drawn, the waveforms.
Sounds to me that there may be a problem in the DC motor.
Is it brush type or brushless?
_-_-
ought to have next to zero effect.
a bit depends on the type and nature of your battery.
If it is drawing a lot of current, and the battery is barely up to drawing that current, you have a problem.
Also, if the current draw from the motor is not very very even, in part based on the mechanical load, you have another problem in addition.
You ought to put a scope into this and look at both the voltage across the motor, and the current (very very small resistor in series with the motor) drawn, the waveforms.
Sounds to me that there may be a problem in the DC motor.
Is it brush type or brushless?
_-_-
The exact motor I use is Maxon RE-Max 226774 DC motor. It is brush type. The datasheet is available here:
http://catalog.gaw.ru/index.php?page=document&id=38415
It may be simple but is still unclear. There are two possible places for the cap: in parallel to the battery and in parallel to the motor. The logical place is, of course, in parallel to the motor. Which doesn't mean, that it won't do whatever you say it's doing in parallel to the battery... but i don't think there may be any engineering reason to place it there.
Most people drive these motors from a low impedance source, i.e. the output of a regulator. Have you tried this as well?
Most people drive these motors from a low impedance source, i.e. the output of a regulator. Have you tried this as well?
Maxon motors generally have a small cap already in parallel with the motor windings, they call this "CLL concept". Increasing the capacitance will decrease commutation noise. It's worth doing even with a low Z source - the DC drives I designed many years ago had negative Zout but still benefitted from capacitance on the output.
Last edited:
I have one cap parallel to the battery. So, is increasing the cap the only possibly improvement path ?
If you mean have I developed the concept further, then yes.
The I x R compensation scheme (aka negative impedance) has a practical accuracy limit, the circuit I published all those years ago is at that limit and I haven't seen anything better out there. There are three areas which contribute to the limit: change of compensation value required with change of motor winding temperature, feedback limitation because of commutation noise and long term drift.
I believe I have practical answers for all three areas but at the moment I lack the means and the motivation to pursue it further.
The final problem with DC motors is wear: the best available motors are expensive but still suffer bearing wear issues. I do not have solution for this.
Thanks for this. I can imagine a micro with an ADC can help in at least two of these cases, possibly all three.
And the bearings, at least on Maxons, are really not meant for lateral loads, no matter what the datasheets say.
And the bearings, at least on Maxons, are really not meant for lateral loads, no matter what the datasheets say.
You know, I'd never thought of that, you might be on to something. The three fixes mentioned above were done using analogue techniques but pushing some or all of it into the digital domain may well be the way to go. I think I know just the person to do the development on the digital side too.
IIRC Maxon specifies a maximal axial load but the spec is at full speed (several thousand RPM) and needs to be reduced at lower speeds such as those seen by a TT motor. Not sure if the reduction required is linear.
We dont know what type of "noise" you are experiencing. In lieu of thbis info put the capaqcitor (what is the mfd value) in parallel with the motor after the capacitor. You may want to experiment with an inductor before the capacitor and the potentiometer. A second order filter. Info on noise waveform would help greatly
- Status
- Not open for further replies.