Quick Power Supply question

Regardless of the supply the regs still need input and output caps to ensure stability. The datasheet will tell you what's needed. A typical 7815 regualtor will need say 0.1uF input and output caps minimum. Your problem will be drop-out voltage, which is around 2V for 7815 regulator. This means you need 17V on the input to maintain a regulated output, and that will happen very quickly with two 9V batteries, wasting most of the battery capacity. A 9V battery's end life is around 4.8-6V depending upon how you take it, so at end of life two 9V batteries will only be +/-12V at best.

You can get "low dropout" regulators, but that won't solve your battery discharge problem. BTW, low dropout regulators are much more unstable than the normal 7815 regs, they need sepecific input and output cap requirements - beware. Get it wrong and they can oscillate badly.

Dave.

Don't bother, the ripple rejection of those regulators is very high. Paying attention to correct star point earthing (multiple if required) is the key.

I don't know what the Jaycar kits use but op-amps reject PSU noise pretty well too.

Graham

batteries

Volt

supply?

First ask yourself if you need 4 * 9V batteries to record vinyl to a computer? You do realise most soundcard inputs overload at ~2V RMS or so, therefore a single 9V battery providing a quasi +/- 4.5V supply is probably adequate, but 2 * 9V batteries is definitely adequate, and four is just a waste of money. Simply adjust the phono pre-amp gain to suit your cartridge output voltage to avoid overload. Make the batteries rechargeable NiMH, and your hum & earthing problems are solved.

What is the perceived benefit of adding regulators to the batteries BTW?

MrT.

"Mr.T"

** The LM833 is not speced to work with rails less than +/- 5 volts making single 9 volt battery power operation not practical at all. But using two should be fine.

As would using the regulated PSU - long as the AC tranny is kept far enough away from the PCB and input wiring to avoid hum injection.

..... Phil

** I'm just following the written instructions for this preamp, quote SC mag 1994,

"The required power supply is a regulated source of + and - 15VDC"

-end quote.

I'm using the regs for the reason stated above, in any case, I would have thought the benefits of using regs were obvious with this experiment.

Experiment is now complete.

After replacing the transformer and bridge rect. with the four 9 volt batts. (with regs and caps in place) to simulate original conditions as close as possible, I ran some tests and now know that the original PS is fine. I dont have hum issues, I just wanted to compare the two methods of supply.

Thanks to all for the comments.

Mark Kelepouris

mag

Which is only a benefit if you plug it into a pre-amp with a suitable overload level.

No, there is a benefit in *not* using regs with batteries.

MrT.