What I'm trying to understand is, how would I be able to tell if my turntable was suffering from this problem. What would I listen for?
If you think about it, cogging (from the perspective of the symptom) is a variation in pitch commensurate with the variation in torque created by the motor whose action is not linear. If you have a 3 - pole motor running at say 60 rpm, you'll get 3 "cogs" per second.
Sonically, you're likely to get intrusion at the "cogging frequency", but perhapse just as importantly, the entire bandwith is affected with every "cog". Although with micro-cogging you'd perhapse not immediately identify the issue at hand, it would contribute towards a generally less coherent signal.
That's my best guess.
Sonically, you're likely to get intrusion at the "cogging frequency", but perhapse just as importantly, the entire bandwith is affected with every "cog". Although with micro-cogging you'd perhapse not immediately identify the issue at hand, it would contribute towards a generally less coherent signal.
That's my best guess.
Rythmic integrety, transient punch and bass slam are primarily affected or are the components that seem to show motor/'platter drive' issues the best. At least in my personal experience of examining the issue. Also, a overall blurring of the signal, yes.
On a LP 12, you can mess with the pot on the Vahalla board and you can put your ear on the chassis, listening for the changes in the motor's drive....and hear the differences in the sound of the table. You're not supposed to play with the pot on the valhalla board....but hey..I have a screwdriver..it was there..so....🙂
On a LP 12, you can mess with the pot on the Vahalla board and you can put your ear on the chassis, listening for the changes in the motor's drive....and hear the differences in the sound of the table. You're not supposed to play with the pot on the valhalla board....but hey..I have a screwdriver..it was there..so....🙂
Direct drives
Direct drives are not bad, really. But there are so many less good sounding DD turntables even from respectable companies, though. Take the Micro DD1500, which sounds really awful like it should be thrown out of the window immediately after the needle hits the groove, but it could be enhanced with a standalone motor unit into a much better string drive turntable. Then take the Technics SP10II, which sounds OKish, when and only when used in its proprietary turntable base. I have had 40+ turntables at home so far, but the EMT938 based Thorens TD524 was the first and only one which I heard in my listening room, which could compete with a simple Garrard idler wheel turntable. And yes, it uses feedback. Remember that on European Triode festivals there is regularily an EMT 938 with FR-64S tonearm playing. Unfortunately, the EMT938 is handicapped by its own tonearm, so it takes another 10 inch tonearm to get the best out of it.
best regards,
Hartmut
Direct drives are not bad, really. But there are so many less good sounding DD turntables even from respectable companies, though. Take the Micro DD1500, which sounds really awful like it should be thrown out of the window immediately after the needle hits the groove, but it could be enhanced with a standalone motor unit into a much better string drive turntable. Then take the Technics SP10II, which sounds OKish, when and only when used in its proprietary turntable base. I have had 40+ turntables at home so far, but the EMT938 based Thorens TD524 was the first and only one which I heard in my listening room, which could compete with a simple Garrard idler wheel turntable. And yes, it uses feedback. Remember that on European Triode festivals there is regularily an EMT 938 with FR-64S tonearm playing. Unfortunately, the EMT938 is handicapped by its own tonearm, so it takes another 10 inch tonearm to get the best out of it.
best regards,
Hartmut
Did some experiments with a VCR drum motor and BLDC driver chip this morning. I built a PLL, and fed it a reference from my audio oscillator. I connected the PLL filter output to the BLDC chip control voltage.
This experiment definitely shows that these BLDC driver chips are not appropriate. Because they use trapezoidal drive, they generate harmonics that inject acoustic energy into the platter. It's very obvious cogging that can be heard just by listening a couple of inches from the platter.
Another problem is that the BLDC chips generate a tach signal (usually called FG, for some reason) from an inductive or hall sensor mounted close to the rotor magnet. However, this signal fails at very low speeds, and is quite erratic at speeds you would need for a DD motor. Once it gets above about 100 RPM it stabilizes, so it may be suitable for a belt drive. Also, the drum (at least the one I have) has a series of fine grooves machined around its circumference, which may be useable for a thread drive.
The cogging problem would be avoided by driving the stator with 3 phase sine waves, but I need to build a 3 channel linear amplifier to test that. Also, an optical tach signal would enable superior speed control, as you would get many more transitions per revolution.
However, in addition the abovementioned problems, the VCR drum motor contains ball bearings, and these of course inject acoustic energy into the platter.
This experiment definitely shows that these BLDC driver chips are not appropriate. Because they use trapezoidal drive, they generate harmonics that inject acoustic energy into the platter. It's very obvious cogging that can be heard just by listening a couple of inches from the platter.
Another problem is that the BLDC chips generate a tach signal (usually called FG, for some reason) from an inductive or hall sensor mounted close to the rotor magnet. However, this signal fails at very low speeds, and is quite erratic at speeds you would need for a DD motor. Once it gets above about 100 RPM it stabilizes, so it may be suitable for a belt drive. Also, the drum (at least the one I have) has a series of fine grooves machined around its circumference, which may be useable for a thread drive.
The cogging problem would be avoided by driving the stator with 3 phase sine waves, but I need to build a 3 channel linear amplifier to test that. Also, an optical tach signal would enable superior speed control, as you would get many more transitions per revolution.
However, in addition the abovementioned problems, the VCR drum motor contains ball bearings, and these of course inject acoustic energy into the platter.
FG = Frequency Governer.
Shame your experiment didn't work, but it's useful information nonetheless.
Shame your experiment didn't work, but it's useful information nonetheless.
Why not try the VCR-motor
with a small pulley and belt?
This way you can keep the revs up...
Arne K
with a small pulley and belt?
This way you can keep the revs up...
Arne K
yes, I'm sure that would work fine, It's just that I'm not building a belt drive at the moment. I'm building a DD.
Just today got a 30hp motor and controller that are stable down to 0.1hz. Ouch.
Kinda pricey, though. It's for a big mixer...
Oh yeah, there are some brutally high precision motors and controllers from that world, on ebay.
For example, we have a $15k (original price) lab mixer and PS, which is self adjusting under load, to maintain exact speed. You can run it at lower speeds..look at the RPM and current meters, grab the shaft and try and brake it. Then watch the current consumption rise and the RPM stay the same. It goes out to 20k rpm, and is definitely precision.
I'm sure there are used and simialr units on ebay.
Heck, I remember about 15 or more years ago, I complied info on the kinds of stuff that are used in the big Rockport and the like. (I had never heard of anyone using such beasts in turntables, at that time) I was looking at over $2k for the motor alone. Sure was a cool one, though!
Kinda pricey, though. It's for a big mixer...
Oh yeah, there are some brutally high precision motors and controllers from that world, on ebay.
For example, we have a $15k (original price) lab mixer and PS, which is self adjusting under load, to maintain exact speed. You can run it at lower speeds..look at the RPM and current meters, grab the shaft and try and brake it. Then watch the current consumption rise and the RPM stay the same. It goes out to 20k rpm, and is definitely precision.
I'm sure there are used and simialr units on ebay.
Heck, I remember about 15 or more years ago, I complied info on the kinds of stuff that are used in the big Rockport and the like. (I had never heard of anyone using such beasts in turntables, at that time) I was looking at over $2k for the motor alone. Sure was a cool one, though!
Re: Re: Teflon does make a stable trust plate ...
Hello EC8010,
Yes, indeed. But as i had my shaft (as well as the bushing) precision-ground to dimension, it had to have conical centering bores at the shaft's end faces. As the whole cylindircity and concentricity of the shaft is **referenced** to those centering bores, the centering bores are indeed very accurately centred.
And i use one of the centering bores as the reference face to center the ball.
No wonder you don't remember the TT. It's home-brewn, looking intentionally unspectacular, IOW it is living-room-compatible 🙂 but it has inner values indeed.
Hello EC8010,
EC8010 said:
Yes, indeed. But as i had my shaft (as well as the bushing) precision-ground to dimension, it had to have conical centering bores at the shaft's end faces. As the whole cylindircity and concentricity of the shaft is **referenced** to those centering bores, the centering bores are indeed very accurately centred.
And i use one of the centering bores as the reference face to center the ball.
No wonder you don't remember the TT. It's home-brewn, looking intentionally unspectacular, IOW it is living-room-compatible 🙂 but it has inner values indeed.
Re: Re: Re: Teflon does make a stable trust plate ...
Hello Bernhard,
I fear you have failed in the ethos of DIY Hi-Fi. It has to be big, ugly, and inconvenient to use. If it can consume vast amounts of electricity and get dangerously hot as well, then that's all to the good. Otherwise, we might as well all use B&O.
dice45 said:
Hello Bernhard,
I fear you have failed in the ethos of DIY Hi-Fi. It has to be big, ugly, and inconvenient to use. If it can consume vast amounts of electricity and get dangerously hot as well, then that's all to the good. Otherwise, we might as well all use B&O.
Basicaly it must not be posible for your significant other to operate the unit without your supervision....
Re: Re: Re: Re: Teflon does make a stable trust plate ...
Hi,
Still my favourite: http://reviews.iwon.com/cat/audioreview/analog-sources/turntables/lirpa-labs/PRD_121333_1597crx.aspx
😀
EC8010 said:
Hi,
Still my favourite: http://reviews.iwon.com/cat/audioreview/analog-sources/turntables/lirpa-labs/PRD_121333_1597crx.aspx
😀
Re: Re: Re: Re: Teflon does make a stable trust plate ...
EC8010,
my coming TT-design is big, no question. It is a floor-standing unit (as no shelf would be able to carry it 🙂 ). But consuming vast amounts of N'R'G, no, it doesn't . It needs two tiny wall outlets, one for AC and one for pressurized air 😀 . And it has to be beautiful because i have to be able to stand it over the years.
My current TT is unobtrusively beautiful ... i had put a lot of effort into its design, appearance- as well as engineering-wise. And i was able (and still am) to like its looks.
And if i come to think of it, i didn't spent my design time for appearance at all. I simply strived to make things look "right", "plausible", "balanced". i do so when i started designing a component. So do i today.
Once there was a Swiss construction engineer; his specialty was designing and building bridges from steel-reinforced concrete. His name was Robert Maillart. When a journalist askend him one time to which percentage he spent his design time for architectural appearance, he answered: "Not a single minute, becaúse what is well designed is also inherently beautiful" .
Methinks, he's got a point with that.
EC8010,
EC8010 said:
my coming TT-design is big, no question. It is a floor-standing unit (as no shelf would be able to carry it 🙂 ). But consuming vast amounts of N'R'G, no, it doesn't . It needs two tiny wall outlets, one for AC and one for pressurized air 😀 . And it has to be beautiful because i have to be able to stand it over the years.
My current TT is unobtrusively beautiful ... i had put a lot of effort into its design, appearance- as well as engineering-wise. And i was able (and still am) to like its looks.
And if i come to think of it, i didn't spent my design time for appearance at all. I simply strived to make things look "right", "plausible", "balanced". i do so when i started designing a component. So do i today.
Once there was a Swiss construction engineer; his specialty was designing and building bridges from steel-reinforced concrete. His name was Robert Maillart. When a journalist askend him one time to which percentage he spent his design time for architectural appearance, he answered: "Not a single minute, becaúse what is well designed is also inherently beautiful" .
Methinks, he's got a point with that.
Nordic said:
quite the contrary. My technical specification for my tonearm requires "basic function must be layman-performable and evident without the layman having to consult the manual" ... and one of the consequences of that is that the linear tracker's active positioning must be fast enough (it is!) to follow the SO's hand trying to put the headshell from the lead-out groove to the parking position, as she would do with a pivoted arm.
After all, my (hypothetical) SO would have to stand my 190x120cm open baffle speakers and the amps driving it. Making the TT inoperable for her would be too much ... all the more as i never met a female preferring the CDP's sonics to that of vinyl 🙂
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