Most Rega arms, RB300 included, have a curious way of setting downforce. There is a dial, graduated in grams, that operates a spring. However, the spring does not add downforce. On the contrary, it pushes the front of the arm up, assisting the counterweight. Thus at 0 grams the spring is at maximum tension, and at the other end of the scale, slightly above 3 grams, the spring is not exerting any force.
There is a widely held belief that disengaging the spring and setting the downforce only with a counterweight improves the sound. Supposedly, the spring "rings" when coiled, and setting the downforce dial to maximum gets rid of this ringing. Is there anything to these claims? Let's find out.
First, I've attached the accelerometer to the headshell and did 2 measurements: with a dial set to zero and to maximum. Actual downforce in both cases was set to 2 grams.
Oops. Nothing, or nearly nothing. There's a tiny bit of improvement on the second harmonic of the main tube resonance, and on the counterweight stub resonance. But both improvements are minuscule and, really, lie within the possible measurement error.
We need another approach. To get closer to the downforce spring, I've attached the accelerometer to the yoke. There is a fixed axis that is right by the downforce mechanism, and I thought I would be able to get a good reading.
Please note that the vibrometry charts below are not calibrated. I had to do some postprocessing to get rid of 1/f noise, and the absolute values in the end result are approximate.
Basically there's no difference. The most noticeable peak is counterweight resonance. It's a bit lower with the spring "upforce" at minimum. In this case the counterweight is about 10 mm farther from the center of rotation.
Ok, let's try a different approach. Another way to capture the bearing chatter and other things going on close to the center of rotation is to place an accelerometer on top of the central part of the arm. RB300 is more or less a single casting, so I've put an accelerometer right on it.
That's a bit better. With the decrease in spring force pushing the front of the arm up, our familiar counterweight resonances improve a little bit. I don't think it's audible.
Another curious thing is the way the nasty headshell resonances behave. Remember, they were the same when measured at the headshell. Here, there's a slight (and quite certainly inaudible) improvement with spring force set either to 0 or to max. Again, I think it has nothing to do with the spring itself, but rather with a particular position of a counterweight and its rubber O-rings. When it's closer to the ends of the stub, it is probably damping this racket a tiny bit better. Here's the photo of counterweight with downforce of 2 grams and the spring dial set to, left to right, maximum, 2 and zero grams.
So, whatever you set on a spring dial does not result in significant changes to the vibration profile of the arm. Unless there are other more esotheric channels of influence, I would say that there should be no audible differences between a spring-set and a weight-set downforce on an RB300.
I am not actively using an RB300 now, but my past experience with stock and modded RB300 arms confirm this. I could never hear the supposed improvement of setting the downforce dial to maximum and thus "disengaging" the spring.
For me, the case is closed.
Next week, we are returning to our prototypes with the next arm tube!
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