Possibly Dumb Question re:Battery as a power supply

millercarbon: Never misunderestimate the ability of the forum to overthink even the simplest things. Full disclosure: I converted AC to DC battery, did none of that, and it was audibly quite a bit better. Just not that hard. Unless you make it so.

Truer words were never written. If the Clearaudio table uses a DC motor, it is not driven directly from the wall wart power supply. At 33 RPM, it is most likely driven by 3-4VDC and from an internal controller which not only regulates the voltage but has active current feedback (negative output impedance) to compensate for the motor’s negative-slope torque/speed curve, without which, the motor will slow under load.

Regulating an AC-to-DC supply is done to remove any AC ripple from the output. The turntable should work just fine from an unregulated battery supply as there will be no AC ripple present. Regulating a battery supply in this case would do nothing other than complicate the application and waste heat and power. The rest of this discussion is just noise.

FYI, MC7812 regulators have a dropout voltage between 2.1 and 2.4V, depending on the device, so you would need 14.4VDC or greater for proper regulation. LDO regulators (Low Drop Out) exist with dropout voltages as low as 200-300mV, but they can become unstable and oscillate without careful layout and compensation.

Advice to the OP: As millercarbon alluded to, don’t over think it.

@terry9-

ELECTRICAL CHARACTERISTICS (Vin = 19 V, IO = 500 mA, TJ = Tlow to 125°C (Note 17), unless otherwise noted)

That is the standard disclaimer at the top of the chart of parameters. It is the operating point used in all of the measurements below it. One of those parameters is DROP OUT VOLTAGE which was listed as 2.0VDC (assuming 500mA load) on the datasheet I found on-line. The disclaimer does not mean the device has to be operated at those parameters, only that the published specifications were measured at that operating point.

Agree with your point about noise not being an issue. I don’t know that I’ve ever seen a measurement for this on a battery, but the internal resistance of a fully charged battery is extremely low so the equivalent noise resistance should be microscopic.

I would think a Sealed Lead Acid battery would be better (and cheaper) than a NiCad. SLAs tend to last longer and don’t suffer from memory effect if they are not fully cycled between charge and discharge as NiCads do. YMMV.

The LM317 has a drop out voltage of ~3V, so I doubt it regulates the battery output to the table. NiMH batteries need to be charged at 1C and negative delta V slope detection, thus the higher voltage input, the PIC processor and complex charging circuitry (another argument in favor of SLA batteries which are simpler to charge).

If the input of the table is designed for 12VDC, then 2 possible problems occur if you provide a higher voltage: Power dissipation and exceeding maximum voltage ratings on the input devices. Regulators (even low voltage output devices) typically have 18-26VDC input ratings. CMOS logic is usually rated to 15VDC (some CMOS devices are rated to 12V, but it would be bad practice to operate those at 12V so I doubt you find any devices with a max Vcc of 12VDC in a 12V table). Tantalum caps have standard voltage ratings of 6V, 10V, 16V, 25V or higher (6 & 10V devices couldn’t be used safely with 12V input so we can eliminate those). Power dissipation at 13.6 would only be ~13% higher than at 12V but 58% higher at 19V.

If the OP really wants to regulate the battery supply, then look at the LT3080 (or similar devices i.e. Micrel MIC29150 series which have fixed or adjustable outputs); the LT3080 is a LDO regulator (350mV) and 1.1A current rating. The output cap is critical for stable operation and should be a low ESR ceramic type located close to the output pin and ground.