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Possibly Dumb Question re:Battery as a power supply

I have bought a new Clearaudio Performance DC Wood turntable.  I plan to drive to the dealer in Atlanta and pick it up next Wednesday.  I have seen the wall wart power supply which comes with the TT, and it is not a pretty sight.  Clearaudio sells an upgraded 12V battery and charger supply for "only" $1,200.00.  I don't wish to present myself as a parsimonious individual,  but $1,200.00 seems a bit steep to me.  I am wondering what problems would ensue if I bought a moderately sized (say 25 or 30 lbs.) , sealed, lead acid, 12V rechargeable battery; attached the proper wires to connect to the TT, and used it as a power supply.  When the battery runs down I could just take it to my workshop and use my electronic, automatic battery charger I use to recharge car, truck and tractor batteries to bring it back to full power.  As little current as the TT uses this shouldn't be too frequent a chore.  It is just a possibility, but if it is feasible I might try it.  I would really appreciate any and all comments, criticism or warnings.
kingharold

Showing 7 responses by terry9

terry9
2,354 posts
 
I salute you, Harold, and your attitude towards what awaits us all. Condolences for Mary.

About that battery supply, it’s not hard to engineer something like that, but it does have to be engineered. A DC motor’s speed depends on the supply voltage, and when a battery discharges, well, it discharges, and the voltage declines. That means the TT’s speed will be far from constant unless it is precisely regulated, and that means more than just an MC7812 regulator chip.

Unless you want to take it on as a DIY project, I’d buy the power supply, pricey as it is. To DIY, a good place to start is "Art of Electronics" by Horowitz and Hill. More I think about it, I’d buy that battery power supply if I were you.

Full disclosure: I built the power supply for my AC motor TT, full quadrature, silent. But it was a project, and it would have taken real time if I hadn’t been retired with the equipment to hand.
terry9
2,354 posts
 
@lewm , just so. An MC7812 requires 19VDC if I remember correctly. And it's hard to know the characteristics of any internal regulation or the motor, and it may not even be obvious about how motor speed relates to platter speed.

Doable, sure. How much of a project? Depends on your luck and how much test equipment you have lying around. If you don't have any test equipment, it'll take some pretty dumb luck not to void the warrantee.

Only way around it I can see is to measure (or otherwise determine) the voltage on the manufacturer's battery pack. Then replicate with a comparable rechargeable pack of your own. And don't forget to protect the pack with a diode and protect the motor with a fuse!
terry9
2,354 posts
 
I should have mentioned another issue: variability in parts. For example, the MC7812 is specced to regulate +/- 4% or so; which means that if you buy 5 of them, one should be within 1%. That’s 33 RPM to 33.6 RPM. Assuming you need it. Maybe you don't.

Like I said, it’s a project. You might have the time and inclination for it, or you just might want to listen to music.
terry9
2,354 posts
 
Thank you, @imhififan  for new information. This shows that a 12V battery should be fine. Still, I would never recommend connecting an expensive component to an unfused battery. And while doing that, why not protect the battery too? I mean, diodes don't cost much.

But then again, I wouldn't want to be accused of overthinking the OP's investment.

Phoenix, from the MC7812 data sheet:
ELECTRICAL CHARACTERISTICS (Vin = 19 V, IO = 500 mA, TJ = Tlow to 125°C (Note 17), unless otherwise noted)

One last thing: different battery technologies have different noise levels. It's probably not an issue; but if that Clearaudio pack uses NiCds, maybe it's for a reason, and $1200 for NiCds is not excessive.
terry9
2,354 posts
 
Phoenix, agree with what you say, but let me add that 19VDC is the voltage at which the part is known to meet spec. With obvious consequences - which, to an engineer such as yourself, pose no issues.

Battery noise was measured by someone in Germany some years back. I think they were looking at powering phono stages. He found that all technologies made a fair bit of noise, except NiCd, which measured at the noise floor of his HP instrumentation.

Anyway, I'm in the process of designing a new phono power supply based on NiCd's for listening and a linear PS for warming, and the unexpected issue is charging. Seems that no-one does it right - i.e. warning at 1.1VDC per cell to avoid memory effect and separate current source for each cell.

Expect to start building in a couple of weeks, will let you know if it makes a perceptible improvement over alkaline with LC filter.
terry9
2,354 posts
 
Previous experience with improving power has shown a worthwhile improvement in sound. This manifests as smoother with no shrill overtones or sibilance (dare I say 'more musical') as well as quieter. 

1. Isolation transformers
2. Brute force DC rails for amps: 200kg inductors and a Farad of capacitance
3. Alkaline battery with LC filter for phono/pre
4. Quadrature AC adjusted to AC motor

My experience. YMMV
terry9
2,354 posts
 
I think that you made a good decision. DIY is possible, but it has its costs in time, uncertainty, and risk.

Enjoy!
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