Hi Frogman,
Thanks for the feedback. You have highlighted the nub of the issue in your last few sentences - when the arm moves laterally to accommodate eccentric records it becomes very complex. In addition to the bearing tube moving back and forth we have -
1. The arm mass pushing the cantilever back and forth through its pivot point
2. The counterweight, if partially decoupled with a spring, starts to oscillate as the bearing tube moves back and forth.
My approach to the problem has 3 elements all of which work together -
1. Lighten the arm to minimize resistance to lateral movement. Remove any soft spongy material - shrink wrap/foam if using the old aluminum arm tube.
2. TOTALLY decouple the counterweight ( no spring, no fixed coupling )
3. Minimal magnetic damping
Counterweight Set Up
In my view fixed, coupled counterweight is wrong because it adds inertia, a resistance to any correction for eccentric records.
I also think a spring is suboptimal, because although by tuning it we get a lower resistance, and maybe better bottom end on round records, on eccentric records the in and out motion means the counterweight will oscillate and feed back into the bearing tube, affecting its horizontal stability.
I would surmise that when you tune the spring, you are probably synchronizing the motion of the counterweight with the arm motion to minimise these conflicting forces. This will vary with the cartridge compliance and how eccentric the record is.
With both coupling and spring ( partial coupling ) the arm movement is inhibited – causing cantilever flex that is disconsonant with the music.
The key question is - What do we want the counterweight to do on eccentric records ?
The answer in my view is nothing.
We want it to continue to load the tracking force.
We want it to remain absolutely still horizontally whilst the arm moves in and out so it does not inhibit the arm movement. ( we don’t want the tail wagging the dog ).
Think of a hinged counterweight, rigid vertically, but free to move so that when the arm moves the counterweight stays still.
My solution was to remove the spring action by loosening off the end cap such that the counterweight beam just flopped at a touch; then inserted teflon wedges loosely either side of the spring to provide the tiniest dampening of that I could apply.
The combination of the "floppy" counterweight in the horizontal direction and minimal magnetic dampening gave me the best result – very quick bottom end.
A bass drum has harmonics and overtones up in to the high frequencies – the highs tell you how a drum is hit – so for me I trade off a little bottom end weight for accurate, clean and extended high frequencies.
Funnily enough this approach gives me the quickest and most tuneful bass.
How much magnetic damping to apply ?
My experience is as little as possible. What I have found is that I set the amount effectively by tuning the bass for optimum speed. Remember that the eccentric arm movement is affecting all frequencies. The dampening will affect all frequencies.
My view is that if you use too much magnetic dampening you will stiffen up the bottom end, but at a cost over over dampening the high frequencies. That’s why I was interested in Chris testing the eddy clamp – which I suspect is just too much.
I wanted to clarify that for me the use of minimal magnetic damping goes hand in hand with how I set the counterweight up and the low mass/minimal resistance.
Thanks for the feedback. You have highlighted the nub of the issue in your last few sentences - when the arm moves laterally to accommodate eccentric records it becomes very complex. In addition to the bearing tube moving back and forth we have -
1. The arm mass pushing the cantilever back and forth through its pivot point
2. The counterweight, if partially decoupled with a spring, starts to oscillate as the bearing tube moves back and forth.
My approach to the problem has 3 elements all of which work together -
1. Lighten the arm to minimize resistance to lateral movement. Remove any soft spongy material - shrink wrap/foam if using the old aluminum arm tube.
2. TOTALLY decouple the counterweight ( no spring, no fixed coupling )
3. Minimal magnetic damping
Counterweight Set Up
In my view fixed, coupled counterweight is wrong because it adds inertia, a resistance to any correction for eccentric records.
I also think a spring is suboptimal, because although by tuning it we get a lower resistance, and maybe better bottom end on round records, on eccentric records the in and out motion means the counterweight will oscillate and feed back into the bearing tube, affecting its horizontal stability.
I would surmise that when you tune the spring, you are probably synchronizing the motion of the counterweight with the arm motion to minimise these conflicting forces. This will vary with the cartridge compliance and how eccentric the record is.
With both coupling and spring ( partial coupling ) the arm movement is inhibited – causing cantilever flex that is disconsonant with the music.
The key question is - What do we want the counterweight to do on eccentric records ?
The answer in my view is nothing.
We want it to continue to load the tracking force.
We want it to remain absolutely still horizontally whilst the arm moves in and out so it does not inhibit the arm movement. ( we don’t want the tail wagging the dog ).
Think of a hinged counterweight, rigid vertically, but free to move so that when the arm moves the counterweight stays still.
My solution was to remove the spring action by loosening off the end cap such that the counterweight beam just flopped at a touch; then inserted teflon wedges loosely either side of the spring to provide the tiniest dampening of that I could apply.
The combination of the "floppy" counterweight in the horizontal direction and minimal magnetic dampening gave me the best result – very quick bottom end.
A bass drum has harmonics and overtones up in to the high frequencies – the highs tell you how a drum is hit – so for me I trade off a little bottom end weight for accurate, clean and extended high frequencies.
Funnily enough this approach gives me the quickest and most tuneful bass.
How much magnetic damping to apply ?
My experience is as little as possible. What I have found is that I set the amount effectively by tuning the bass for optimum speed. Remember that the eccentric arm movement is affecting all frequencies. The dampening will affect all frequencies.
My view is that if you use too much magnetic dampening you will stiffen up the bottom end, but at a cost over over dampening the high frequencies. That’s why I was interested in Chris testing the eddy clamp – which I suspect is just too much.
I wanted to clarify that for me the use of minimal magnetic damping goes hand in hand with how I set the counterweight up and the low mass/minimal resistance.