05-14-13: Richardkrebs
The cartridge still has to push this mass sideways. Either a relatively low mass with the ET2 or heavier with a Rockport , Kuzma, Walker or Krebs arms. In an undamped system with a fixed counterweight we see a large rise in amplitude at resonance. This is easy to control with damping.
The adding of lead mass will increase distortion due to the higher inertia of the arm. Basically you have added a peak resonance in the bass, then tried to tame it by adding damping. Both the added mass and fluid damping will increase inertia, and compromise the ability of the cartridge to track the grooves accurately. On an eccentric record this will be even worse. The Krebs arm has some 65g of horizontal effective mass added - an increase of inertia close to 300%. This increases cantilver flex and distortion as per Bruce Thigpens commentary.
05-14-13: Richardkrebs
With a stiff cartridge; the only way to get Fr low enough to avoid interaction with audio frequencies was to make the arm heavier. The Fr of my arm/cart is 5.2 Hz.
This is not true. Most tonearm designers target a FR of around 8-12hz. This is based on an assumption that most systems dont produce much response below 20hz and footfalls are in the zone of around 2-3hz, dangerously close to your 5hz.
Secondly, the increased distortion from the added inertia of the arm resulting from the increased horizontal effective mass certainly impacts the audio frequencies.
The phase anomalies from the increased distortion on the cantilever can be heard quite clearly in a system that is reasonably transparent and quick. Furthermore the peak resonance at 2-5hz that you quote can invoke instability in tracking, its the same as walking past a turntable on loose floorboards.
05-14-13: Richardkrebs
In past I have damped the resonant peak by adjusting the air pressure and deliberately dressing the lead out wires to damp horizontal movement. This method is fussy and not particularly robust.
Basically what you are saying here is that you like the sound of the ET2 with additional drag inhibiting the free movement of the arm to track the grooves. In my experience this slugs the sound. Other users have found improvements by removing this drag - they have removed their fluid damping and replaced it with electromagnetic damping which has the benefit of not inhibiting movement until the movement happens.
05-14-13: Richardkrebs
My counterweight is 32 grams.
I have added 30 grams of lead inside the spindle in the center of working travel, such that it does not leave the bearing sleeve when tracking modulated grooves.
The ET2 can be optioned with the heavier magnesium wand. 8 gms heavier.
It comes with up to 40 gms of counterweight. 8 gms heavier.
In terms of bearing load carrying capability I have added 30 gm to a lightly configured ET2. If we option it with the magnesium and use all the supplied weights my weight adder is now reduced to 14 grams.
Your mathematics is incorrect :
The bearing spindle without arm/counterweight is 14g
The aluminium arm is 11g
The CF arm is 17g
The Magnesium arm is 19g
So the total horizontal effective mass of your 9gm cartridge on a standard ET2 arm with a 32gm decoupled counterweight is:
9g(cart)+11g(arm)+14g(bearing spindle)=34gm
The total horizontal effective mass of your 9gm cartridge on your altered ET2 "Krebs arm" with 30g of lead added and the 32gm counterweight coupled is:
9g(cart)+11g(arm)+14g(bearing spindle)+30g(lead that you added to the spindle)+32g(counterweight that the cartridge now sees because you removed the decoupling)=96g
Therefore you have increased the horizontal effective mass from 34gm to 96gm, an increase of 62g.
I cannot see how you get to 6gm in your maths.
Coupling the 32g counterweight adds 32g to the horizontal effective mass.
Adding 30g of lead to the spindle increases the horizontal mass by 30g, even when placed inside the bearing.
05-14-13: Richardkrebs
If we option it with the magnesium and use all the supplied weights my weight adder is now reduced to 14 grams.
This is another area where the underlying assumptions are misunderstood. Bruce's recommendations documented in the ET2 manual on low compliance cartridges is carry as little weight as possible as far out on the I Beam as possible. This keeps the horizontal effective mass low but increases the vertical effective mass. It is not recommended to load the I Beam with all the counterweights if one can shift the counterweights further out.
