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Mad_K 27th October 2002 12:18 AM

battery powered RIAA preamp question

I am thinking about making my RIAA phono preamp battery powered. The preamp I itin question is ESP project #6 buildt with AD825's. (

PSU will be 2x8 LR14 batteries for a +/- 12V supply.

But, now I will never know when the voltage gets too low (under +/- 5V)

What I am thinking is this: There must be some simple way to implement a LED, which turns on when the voltage drops under a certain voltage(say, 6V)? Maybe a zener and a resistor or something?

Any hints/ideas/suggestions would be greatly appreciated...

Steve Eddy 27th October 2002 01:07 AM

Analog for analog
How 'bout a simple analog voltage meter? Radio Shack sells one that'll read 0-15 volts for $12.99.


dice45 27th October 2002 01:28 AM

Hello Mad_K,

how about a comparator wired as window comparator? LM339 has 4 comparators in one DIP, for a window comparator you need 2 ot them. Let one input look at a voltage divider with Zener as lower and resistor as upper elements and the other input look at a votlage divider made from two resistors. Tune resistors so that comparator turns Led on if supply voltage drops under 5V.
Should alos be possible with a NE555.

Mad_K 27th October 2002 01:42 AM

I was thinking the same, but I can't find any (not small enough size/price) at my parts supplier.

Also, the front-panel is only 40mm high....

But thanks for the input!

Mad_K 27th October 2002 01:55 AM

sorry Bernard, I was typing while you replied.

-That sounds like a great idea! They have the lm339. I'll have a go at it tomorrow, -now it's time to get some sleep before work again (every day is a weekday:().

PS- What do you think of the battery powered setup I described? I have done some estimates, and it looks like I can have quite a few hours of pure power from those batteries?

PPS- Part of the reason I'm doing this is because I'm building a new passive preamp, and then I came up with this brilliant(?) idea. (The RIAA-pre will go in the same box as the passive pre).

Steve Eddy 27th October 2002 03:15 AM

Here's something very simple but effective:

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Press the momentary switch and if you light up the LED you're ok. If not, time for a charge. And the rest of the time the circuit's out of the loop completely.


hagtech 27th October 2002 04:37 AM

Zener Circuit
Yes, the previous zener circuit is very good and effective. But swap polarity of zener, it is shown backwards. Can't get any more simple that this.


Steve Eddy 27th October 2002 05:29 AM

Re: Zener Circuit

Originally posted by hagtech
Yes, the previous zener circuit is very good and effective. But swap polarity of zener, it is shown backwards. Can't get any more simple that this.
Whoops. My apologies. I drew it up in AutoCAD copying components from an existing circuit. I just copied two bias diodes from a current source and added a couple of lines to make one a zener and one an LED and forgot to flip the zener around.

Thanks for catching that! The illustration's been updated.


Mad_K 27th October 2002 10:10 AM

:o Awww!

So it is as simple as i suspected...

-So I just stick a zener in series with a resistor and a LED tied to the rails, and then this arrangement stops conducting when the voltage is lower than the zener voltage?

(I know this is the opposite of what I asked, but it does the same. The LED stops lighting when the voltage is too low, rather than light up.)

-Time to have some fun! -Maybe we don't need a christmas tree this year!!:D

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