Quick Power Supply question

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Without going into much detail....

I'm using the + and - 15 Volts, Power Supply kit (jaycar KC5038) to power
their KC5159 (universal preamp) kit, wired up for Phono use.

I've configured the PS to take a centre tapped 30 Volt transformer to a
bridge rectifier.
Of course all the Caps and regulators are in place and all is well.

But I am getting back into playing Records and the smoothest DC PS must
surely be an essential starting point. So i'm experimenting using batteries
as well.


My question is,

Do I need anything other than the (15V)regs if I were to use four nine Volt
batteries in series (centre tapping in the middle of course) for the supply?
Or would it be wise to include some Caps before or/and after the regs?
If so, which ones?

Thanks for any advice.

Mark Kelepouris






 



Re: Quick Power Supply question
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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.
http://data.energizer.com/PDFs/522.pdf

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.



Re: Quick Power Supply question



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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


Re: Quick Power Supply question

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batteries
Volt
supply?
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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.





Re: Quick Power Supply question

"Mr.T"

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**  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







Re: Quick Power Supply question

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** 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.


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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



Re: Quick Power Supply question

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mag

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

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

MrT.



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