Chris, thanks for the tip. I will try it and report back. |
As we have been discussing the pros and cons of the decoupled counterweight on the ET, I have this question on my mind (admittedly a non-technical one) for a long time!
The ET uses leaf springs to decouple the counterweight so the arm doesn’t “see” the additional weight! To my thinking, this can only be possible if the counterweight actually doesn’t move during the initial movement (milliseconds?) of the arm, due to the compliance of the leaf spring. Then, after the arm has moved for a certain range, the counterweight would need to start “rebounding” to follow the arm.
If that is true, then would it create some delayed effect that could be detrimental to the tracking?
Or is my reasoning totally false? |
Thekong - yes you are correct. That "rebound" as you call it has a resonance. The idea behind the variable spring rate and shifting the position of the lead counterweight is to place that resonance below the horizontal resonance of the arm so that the 2 resonances ( arm and beam ) do not couple together to produce a large peak at the arms resonant frequency.
All tonearms will have a lift or peak in response at their natural frequency. The tuning with the I beam ( decoupled ) is designed to reduce this natural bass peak at the resonant frequency of the arm to produce a flat bass response. This is illustrated in the graphs on Bruce Thigpen's website where he has published some of his test results.
The idea behind loosening the I beam that I outlined in my earlier posts is to lower this "rebound" resonance, by allowing the I beam to swing freely and also keep the horizontal arm mass low. Some wag above says you get no response below 30hz but that is not correct. You get a low fall off in bass below 30hz.
Rigidly coupling the I beam will increase the bass, adding more bass lift at the resonant frequency and beyond. If you run 30 year old Acoustat 2+2's in a room barely 10ft x12ft, listening position 5 feet from the panels and you like humpa humpa one note bass, then that is your ideal solution. |
Hi Dover,
Thanks for the explanation, will certainly try all different set ups and see the difference myself! |
Hi theKong – answer to your question from Bruce. Thanks for that info as well Dover. Acoustats ? won't go there.
03-27-13: Thekong
As we have been discussing the pros and cons of the decoupled counterweight on the ET,
I have this question on my mind (admittedly a non-technical one) for a long time!
The ET uses leaf springs to decouple the counterweight so the arm doesn’t “see” the additional weight!
To my thinking, this can only be possible if the counterweight actually doesn’t move during the initial movement (milliseconds?) of the arm,
due to the compliance of the leaf spring.
Then, after the arm has moved for a certain range, the counterweight would need to start “rebounding” to follow the arm.
If that is true, then would it create some delayed effect that could be detrimental to the tracking?
Or is my reasoning totally false?
Thekong (Threads | Answers | This Thread)
********************************************************** Answer from Bruce. Chris,
The counterweight is decoupled in the horizontal plane so it only affects lateral motion of the tonearm. The counterweight always moves if the spindle is moving, it does not move at the same rate, the time constant is about 300 milliseconds. If the weight is coupled the system resonant frequency would be extremely low, a resonant frequency at 3Hz with a significant rise in response (6-12dB) results, which would affect tracking slightly because of the asymmetric position of the cantilever, we opt for splitting the horizontal resonance frequency into two points and lowering the "Q" which improves tracking. The motion of the weight is damped so that it has a low "Q" resonance with a rise in response of about 3-4dB at 3hz and 2dB at 10-11Hz, these frequencies do not coincide with the vertical resonance.
More important than tracking, the intent was to reduce the modulation effects of low frequency energy (FM and AM) that increase distortion in the cartridge, amplification chain, and especially in the speakers woofer at a few Hz. I hope this helps.
brucet
Boy I wish I had someone like Bruce for when I go into these board meetings trying to sell services to CEO’s and CFO’s. You know the ones where they bring their own in house techies to the meetings to ask questions. Cheers |
