ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Some of the tangential turntables by B&O had a laser sensor arm immediately to the left of the tone arm. The others had a fake sensor arm to the right of the tone arm, and this fake sensor arm just showed the record type, e.g. 33rpm. Presumably the turntables with the fake sensor just moved the tone arm on some sort of ratchet or worm drive, and the tone arm fixing had sufficient float in it to compensate for any small position errors.
Have I got this simple analysis right? Does the sensor do anything else other than detect the start and finish of a record?
Are the turntables with a sensor actually better than those without, or is this just a complicating feature that is likely to fail?
Graham
The sensor arm (real or not) doesn't actually have anything to do with tracking. In both cases, the tracking control input comes from a photoelectric sensor that is located at the pivot end of the tone arm. So there's no tracking accuracy advantage to either of the methods.
The sensor arm (when present) just detects the presense of a record under it to know when to lower the arm. Or more correctly, it detects the reflected light pulses from the platter stripes to know when there's no record under the arm.
The last tangential Beograms without a real sensor arm weigh the record on the platter to tell whether it's a single or an LP. Both methods have their shortcomings.
--mika
tournedos: The sensor arm (real or not) doesn't actually have anything to do with tracking. In both cases, the tracking control input comes from a photoelectric sensor that is located at the pivot end of the tone arm. So there's no tracking accuracy advantage to either of the methods. The sensor arm (when present) just detects the presense of a record under it to know when to lower the arm. Or more correctly, it detects the reflected light pulses from the platter stripes to know when there's no record under the arm. The last tangential Beograms without a real sensor arm weigh the record on the platter to tell whether it's a single or an LP. Both methods have their shortcomings.
Thanks Mika.
So they are simpler than I thought in one sense, yet more complicated in another!
At least on the modern tables, the difference between a 45 rpm 7 inch and a 12 inch LP was determined by the ring at the center of the platter. It wouldn't be depressed with a 45, but was with a 12 inch record, as far as I can tell. I've put a 7 inch record with a small center hole on it and had to do it manually as the arm on my BG3000 tangential would try and go to the outer edge and then refuse to drop because of the light sensing no record there. Had to manually run the arm over and lower it.
Jeff
I'm afraid I'm recovering from the BeoVirus.
The older decks had a lovely machined worm drive to move the arm with a high spec motor. The last of the tangentials used the same motor as was used to open the cassette draws of the 5500 and a cord drive. Made little difference but nothing like as engineeringly pleasing!
Peter
Jeff:I've put a 7 inch record with a small center hole on it and had to do it manually as the arm on my BG3000 tangential would try and go to the outer edge and then refuse to drop because of the light sensing no record there. Had to manually run the arm over and lower it.
That should not be necessary – as Mika said, these beograms weighs the record. If it's heavy, it probably is a 12" but if it's light it probably is a 7". The size of the center hole does not matter. Be careful when playing 10" records :)