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Turntable not spinning fast enough on 45
powerstone05
post Jan 5 2013, 01:48
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I have (now) a GemSound BJL-1150. The problem I'm having is that the 45 rotation is quite slow and is noticeable in playing. The 33 seems to be fine though. I checked the belt and it's still in working order. Any ideas?
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powerstone05
post Jan 6 2013, 04:42
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Probably a stupid question but I know that playing a 33 LP at 45 and then slowing it down compromises the quality, but wouldn't the same thing happen if I play a 45 at 33 and then speed it up? (Yes I do plan to record some records off of it)
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Glenn Gundlach
post Jan 6 2013, 06:11
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QUOTE (powerstone05 @ Jan 5 2013, 19:42) *
Probably a stupid question but I know that playing a 33 LP at 45 and then slowing it down compromises the quality, but wouldn't the same thing happen if I play a 45 at 33 and then speed it up? (Yes I do plan to record some records off of it)

If the speed is only off a few percent 'tweaking' it in digital is OK. I don't know how big a problem it is but the RIAA curves crossover points should really track the speed difference but it's probably more of an OCD thing.
Question: You say the speed control is still slow when at +10%, is the speed _range_ about right? Does it seem to go to -20% less ? A 20% span would be a little more than 3 semitones.

G
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powerstone05
post Jan 6 2013, 06:52
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QUOTE (Glenn Gundlach @ Jan 6 2013, 00:11) *
QUOTE (powerstone05 @ Jan 5 2013, 19:42) *
Probably a stupid question but I know that playing a 33 LP at 45 and then slowing it down compromises the quality, but wouldn't the same thing happen if I play a 45 at 33 and then speed it up? (Yes I do plan to record some records off of it)

If the speed is only off a few percent 'tweaking' it in digital is OK. I don't know how big a problem it is but the RIAA curves crossover points should really track the speed difference but it's probably more of an OCD thing.
Question: You say the speed control is still slow when at +10%, is the speed _range_ about right? Does it seem to go to -20% less ? A 20% span would be a little more than 3 semitones.

G


Well I just posted a similar question over at the Audacity forums, and used the formula to determine the actual RPM. I did a test with a 12" single I have and put it on 45 with pitch adjustment at max (10%). The result was approximately 44.826 RPM, which bugged me, so close but not there. But its enough that I can notice.
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mzil
post Jan 6 2013, 07:57
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You can increase the speed slightly by applying a twist to the belt before it loops around the pulley. [One high-end TT maker a decade or two ago, Well-Tempered, used to do this as a normal course of action, claiming it reduced flutter.] Depending on if you flip it around one way or the other [top edge flipped away from the main platter vs toward the platter] the belt may have a tendency to ride around the pulley rather high or rather low. Try both ways and see what gets you to the speed you want.
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DonP
post Jan 6 2013, 16:22
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QUOTE (mzil @ Jan 6 2013, 01:57) *
You can increase the speed slightly by applying a twist to the belt before it loops around the pulley


Unless the problem is slipping, I don't see how this would increase the speed. The ratio between capstan and platter pulleys will be the same.

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mzil
post Jan 6 2013, 17:53
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The turntable designer Bill Firebaugh described the belt linkage of turntables as such, in this Stereophile interview:

"...if you try and calculate the platter speed from the pulley speed and the two circumferences, it doesn't work out right. The platter doesn't run at the predicted speed; it will always be slow. I'll tell you, this drove me nuts until I found out what was going on. I wasn't the first person to discover this by any means. I was explaining my speed problems to my belt manufacturer, and he knew all about it. He confirmed that this is a known phenomenon in belt-drive systems, especially ones with relatively compliant belts. The pulley goes faster than the belt." [emphasis mine]

The belt twist(s) slightly reduces the effective length of the belt and therefore changes it's "stretch" and compliance.

Powerstone5 will hopefully try this quick, simple and free test and then share with us his measurements. [Also, more than one twist might be in order, to tighten the belt even further.]

This post has been edited by mzil: Jan 6 2013, 18:03
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