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Old 31st January 2007, 07:04 PM   #1
adhoc is offline adhoc  Australia
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Default DC vs AC motors - a newbie needs help

G'day All,

I'm pretty new to the analog world (<6months) and have recently thought of heavily modifying my TT. A new motor would definitely be in the works, but

I did a search and the results went right over my head. I need help with understanding why DC motors are considered better than AC motors. I've got a few questions and have numbered them for ease-of-answering.

1. Doesn't AC have the advantage wrt ease of implementation of quality speed regulation? The Altmann DIY TT's motor setup is a model of simplicity to install.

2. Which motor emanates more spurious RMI/EMI? AC or DC?

3. What do I look out when I buy a DC motor? can anyone recommend any out there?

4. What do I look out when I buy an AC motor? can anyone recommend any out there?

5. Which motor type do most belt drive manufacturers use and why?

6. "All AC motors exhibit cogging, making them inferior to DC motors" - is this statement correct? What advantages do AC motors have then?

7. Weren't AC motors in fashion a few years ago? What precipitated the 'shift' to DC?

8. Which motors are more durable for a given price point? AC or DC?

Any help you could provide is much appreciated. Thanks all in advance!
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Old 31st January 2007, 11:47 PM   #2
kevinkr is offline kevinkr  United States
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I've had a number of TT some with DC motors and some with AC. Bluntly I prefer ac operated motors whether line operated or using a power weinbridge oscillator like in my Thorens TD-125.

Most of the dc motor tables (dd and belt) I've owned had less than stellar speed stability.

Incidentally at the physical level there is no such thing as a dc motor, the commutation is handled either electronically or with brushes as found in a lot of dc servo motors by Panasonic. (Technics and the notorious Pink Triangle) The Technics TT could at least maintain a more or less constant speed across one side, the PT absolutely could not -even with a new motor and controller..

Also I believe all motors except for the possible exception of some slotless coreless types cog to some degree. I think this more of an issue with DD tables whether ac or dc servo types.
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Old 1st February 2007, 12:22 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

some observations :

An AC motor needs an accurate pulley, a DC motor does not.
An AC motor needs the right size pulley for 50 / 60 Hz.
An AC motor pulley must match the diameter of what its driving.

Consequently a universal "upgrade" kit cannot use an AC motor.
The idea DC is chosen only because it is superior is marketing BS.

Anything will probably upgrade a tired / worn / unoiled motor.

Also some AC motors have recently gone out of production and
as DC is marketing flavour of the month, switches to DC motors
with inevitable claims of improved performance have been made.

/sreten.
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Old 1st February 2007, 02:03 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally posted by kevinkr

Also I believe all motors except for the possible exception of some slotless coreless types cog to some degree. I think this more of an issue with DD tables whether ac or dc servo types.
There is at least one type of motor totally devoid of cogging: the eddy current motor. And if I remember correctly, Thorens had one TT based on this type.
LV
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Old 1st February 2007, 02:32 PM   #5
EC8010 is offline EC8010  United Kingdom
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sreten: I fear all your comments are precisely on the nail.

adhoc: What sort of a turntable are you thinking of modifying, and why?
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Old 1st February 2007, 03:47 PM   #6
AMV8 is offline AMV8  United Kingdom
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Hi
I have used dc and ac driven turntables.

Only AC give good long term speed control. DC drifts over a period.

However if you use ac and want the best performance you do need to either clean the mains or electronically derive the frequency. ( I currently do the latter )

If you use ac from a "clean" source you will be surprised by the difference that makes over conventional ac power supply. It provides much cleaner notes particularly in the base.

Don
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Old 1st February 2007, 06:07 PM   #7
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While I'm no electrician I will add my 2 cents.

All electric motors offer errors, errors in speed accuracy and as such errors in wow and flutter etc. IMO, A.C synchronous or D.C. servo both can offer long term quality performance for audio playback use. The most important issue is making sure speed drift, speed accuracy and wow and flutter specs are all below the ability for most humans to detect. Once this is reached than anything below such is only guilding the lily. Poorer quality motors that were/are often used on cheap products may fail in reaching levels to where most humans could not detect their error. Better motors will obviously be engineered to surpass min. requirements for audio playback, regardless of A.C. or D.C. motor design.

I have heard turntables with A.C. belt drive, D.C. servo belt drives, and D.C. direct drives. Al offer their strengths and weaknesses. Neither is inherently better but only is better if the motor A.C. or D.C. was better designed and manufactured.

High end turntables use better motors because they pad the cost of development of these into their higher prices. Thankfully R& D costs of these generally filter down to lower priced motors that are used on more affordable turntables. But again the end result is the initial design and manufacture of any motor A.C. or D.C.

My current DIY turntable has a motor derived from an older Kenwood turntable (Sankyo is the motor maker) it is a D.C. servo motor and though far from high end it in my DIY design runs quite smoothly, with no real discernable inaccuracy to my ears or ears of others whom I have played the turntable for. I have other motors picked up from other thrift store tt buys that are D.C servo (Panasonic made) and A.C. synchronous motor (not sure what Japanese maker made it) and I have a Hitachi direct drive turntable from the mid-late 70s which I'm going to DIY into a better DIY direct drive. Each motor design will offer pluses or minuses and being used in future build will add to the sonic flavour of the design.

In the end IMO it's more important with how one uses a motor either A.C. or D.C. in a DIY design and how that DIY table is built. Plinth construction, tonearm, cartridge etc. will all add or detract more from the final sound than whether one uses an A.C. synchronous or a D.C. servo motors. The main point made earlier is that A.C. motors need an accurate pully if you want its speed to be more true. A D.C. servo can get away with an undersized or over sized pully as one just trims it out to get better speed accuracy.

Well that is my 2 cents
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Old 1st February 2007, 08:08 PM   #8
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You can control the speed of an ac motor if you build an oscillator so all that about the pulleys etc can be sidestepped.
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Old 2nd February 2007, 06:19 AM   #9
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Default Re: DC vs AC motors - a newbie needs help

Quote:
Originally posted by adhoc


1. Doesn't AC have the advantage wrt ease of implementation of quality speed regulation? The Altmann DIY TT's motor setup is a model of simplicity to install.

Yes, for a given accuracy AC is easier.

2. Which motor emanates more spurious RMI/EMI? AC or DC?

AC

3. What do I look out when I buy a DC motor? can anyone recommend any out there?

A low value of the ratio Ks / Kt where Ks is speed constant and Kt is torque constant. Maxon 110191 is my current favourite

4. What do I look out when I buy an AC motor? can anyone recommend any out there?

I'm not much help here unless you want to spend a lot of money on a three phase motor

5. Which motor type do most belt drive manufacturers use and why?

AC. see 1

6. "All AC motors exhibit cogging, making them inferior to DC motors" - is this statement correct? What advantages do AC motors have then?

All affordable AC motors cog. The rest is a matter for judgement. Again see 1

7. Weren't AC motors in fashion a few years ago? What precipitated the 'shift' to DC?

silence

8. Which motors are more durable for a given price point? AC or DC?

AC. DC motors must be considered as consumables due to brush wear. 10,000 hours is a good life.
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Old 3rd February 2007, 11:07 AM   #10
adhoc is offline adhoc  Australia
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Quote:
Originally posted by Elvee

There is at least one type of motor totally devoid of cogging: the eddy current motor. And if I remember correctly, Thorens had one TT based on this type.
LV
Thank you all for the replies - they are indeed much appreciated.

I had had some teeth pulled out under GA recently, and was in no condition to reply - but I'm back now!

I'll summarise my current situation:

- I recently came into possession of an early make LP12 in - for once - awesome condition. All the 'irreplaceable' parts (platter, shaft, bearing, etc) are in incredible condition considering the age of the TT - it has a serial number around 15,000, which places it sometime in the late 1970s.

- This is my first 'serious' TT, and while I understand it's basic design premise is arguably obsolete, I am in the process of modding the hell out of it. I have had wonderful experiences with modifying existing inexpensive DACs to get IMO wonderful performance out of them, and hope to replicate my experiences with this TT.

- I have some ideas so far for modification, and am in the process of putting them into place. Surprisingly, most of these 'modifications' were thought up independently by me, though you may recognise some of them as offerings from commercial entities. Like I said, I am very new to the analog world - a few months ago I would have not known a cartridge from a tonearm!

1. resonant top plate replaced with carbon fiber composite: stiff, light and from my experience with the material, not resonant. The current steel top plate rings like a bell when struck (surprisingly this is not so for the platters). I later found out that The Funk Firm (TFF) originally came up with this idea.

2. relocation of the motor to 7 o'clock position: to grossly oversimplify, a 'parallel' transfer of belt-borne motor-generated vibrations rather than a 'perpendicular' one - the former should be much more preferable to the latter, given the respective directions of travel and excitement in a stylus. I later found out that the now-defunct Pink Triangle originally came up with this idea.

3. stiffening of subchassis: the subchassis that came with my LP12 is a 'spot welded' version. I picked up a 'scrap' (seller's words) pre-Cirkus glued subchassis, and the former is definitely way more 'flexy' than the latter. I was thinking of bonding with epoxy some CF sheets to my subchassis. I later found out that Cetech, TFF and now Linn have adopted this idea, though implementation differs.

4. replacement of motor: I assumed (correctly) that an AC motor should be an EMI nightmare. I thought about going the DC route but then realised that the issue of 'hunting' and correct servo implementation was a serious setback, and difficult to get right. So I thought (admittedly rather simplistically) - hmm, why not keep a DC motor 'locked' onto a particular speed like how DACs are 'reclocked' with high-precision clocks? - but I lack the technical know-how (my speciality is medicine, not electronics!) and educational background necessary to translate my idea into reality.

So right now I'm torn between the effort needed to either:

A. take a higher-end AC motor, shield it and reduce vibration effectively.
B. take a higher-end DC motor and somehow figure how to implement a good servo.

Any suggestions?
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