Ok I'll take the bait.
Way back in this thread, I posted a resonance transmissibility graph.
interpreted correctly it tells us a great deal about how the ET2 responds to various stimuli.
For example it tells us how brilliant BTs design is in controlling unwanted resonant peaks at the natural resonant frequency of oscillation of the system.
The system being the arm and the cartridge. Just a guess here, but it is most likely that the ET2 was designed when med to high compliance cartridges were very popular. A look at the owners manual adds to this perception, when it uses a cartridge with a compliance of 30cu to calculate the horizontal resonance of the system. (Horizontal being where most of the low frequency information is encoded in the grooves.)The FR in this case comes out to around 5-6 Hz, from memory.
Now put this number, say 6 Hz, into the transmissibility graph I posted and we can see that when we approach the audio band ( 20 Hz and above) the transmissibility, depending upon the level of damping, will have fallen to around 10%. This means that 90%of horizontal groove modulation will be converted into useful cantilever movement. 10% of the modulation will cause the arm to SHIFT BACK AND FORTH.
In other words we lose some output amplitude from the cartridge at 20 Hz. and the arm moves in sympathy to the groove modulation . As we move up in frequency, the loss of cartridge output and arm shake reduce, until it becomes insignificant at around 6 times the resonant frequency, Fo
Since it is the arms job to keep the cartridge still relative to the groove, it would seem that we have failed in this goal.
Now look at what happens when we use a stiff cartridge, in my case around 9 Cu. If we keep the ET2 in its standard configuration with the decoupled counterweight the Fo now increases to around 8 Hz.
Put this number into the graph and we can see that the arm lateral oscillation has now increased to around 30% at 20 Hz. Only 70% of the groove modulation is converted into cantilever movement. A large 30% is converted into making the arm shake horizontally at 20 Hz. This situation is worse than before. Not only do we loose much more useful output from the cartridge but the arm is now booging down to the music, shaking about quite significantly.
This is not a good thing.
So so how to overcome this problem that exists most significantly when using stiff cartridges?
(if I was using a higher compliance cartridge I would not disable the counterweight decoupling)
Just as the makers of Morch, Dynavector, Walker, Kuzma, Rockport clearly understand. Make the arm heavy in the horizontal plane,
With the ET, this is simply achieved by disabling the counterweight spring. Now the cartridge sees the full weight of the arm all of the time. This lowers Fo back down to the nice range of 5-6hz. We minimise cartridge output loss and minimise arm shake, provided we take one additional precaution....
So what are the other consequences of locking the counterweight when using a low Cu cartridge.
1) much has been written in this thread that I risk impailing my neighbour with shards of diamond tipped boron as the cantilever explodes due to the effect of tracing an eccentric record. It has even been stated that such a setup as mine will result in an arm that will be 300% heavier than a standard ET, as seen by the cartridge. Back to the transmissibility graph. The ET manual tells us that with a single counterweight spring the counterweight will swing horizontally at around 1.5hz. With an input frequency of 0.55hz for 33,1/3 rpm or 0.75hz for 45 RPM, we can see that under these conditions the transmissibility approaches 1. In other words the counterweight spring does NOT flex. The cartridge sees the full arm/ cartridge weight. Wand, spindle, cartridge, AND COUNTERWEIGHT. Under these circumstances my arm and a standard ET appear to the cartridge to be more or less the same weight. Under this stimulI, my arm and a standard ET behave in the same way.
Adding more springs to the counterweight assembly shifts the spring frequency upwards and has no impact on the weight the cartridge has to push around. It still has to move all of the arm. One more spring increases the 1.5 Hz frequency by a factor of root2 Two more by root3
2) many more column inches have been dedicated to the myth that my arm has an enormous low frequency resonant peak at Fo ( around 6 Hz ) and this is disastrous to the sound quality. There are two words to dispel this "Critical Damping" . It is no accident that ALL of the arms I mentioned above have independent horizontal damping of some form. Just like my arm, where I use an oil trough, The level of damping is then simply adjusted to be around critical. This eliminates all the low frequency bump. There is none.
3) so we are now achieving nearly full amplitude low frequency output of the stiff cartridge by simply coupling the counterweight and we have no disastrous low frequency resonant peak. All good things for full range systems, just as Morch discusses in their info on the DP8 arm.
4) but what if your system doesn't plunge the depts to 20 Hz. All this would seem to be a waste of time. Well actually not so. I did not realise this until I got the oil trough properly set up.
The clue is the standard ET arm actually shakes horizontally due to low frequency groove modulation when using a stiff cartridge. This in turn screws up all frequencies, not just the low stuff, because the arm is not providing a stable platform. So if we have low frequency information present, any other higher frequency music which is playing at the same time will be distorted.
This is the real benefit of locking the counterweight. But only do it if you are using a stiff cartridge say around 10Cu and you are using an oil trough.
The image becomes solid, stable. A very appealing attribute.
It is not all peaches and cream however. This approach is very sensitive to the resonant characteristic of the structure that is used to carry the counterweight. For me this is still a work in progress.
Others have simply decoupled the counterweight by using blu tac. I have not tried this but it seems to be a good idea as it would help to damp the I beam as well.
cheers .
