Calrad meter for monitoring bias aleph2

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Mattyo5

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Joined 2002
Ok, yesterday i bought a couple of the Calrad meters that monitor amperage, 0-10A.

http://www.calrad.com/calrad/index2.htm

there is a pdf of a catalogue... p91 of the catalogue and 100 of the pdf.

part # 60-171

there are only 2 connections on the back.. + and -

where to I connect these on the my aleph 2's to monitor the bias? the meters will be on the back panels...not on the front :)

thanks guys!

-Matthew K. Olson
 
Holger Barske

Holger Barske

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Joined 2001
holgerbarske.com
Basically, I'd just put them into the positive (ore negative, doesn't matter) supply rail, betwwen PSU and amplifier board.
What you measure there is the current through the output stage plus a negegible amount of supply current for the input stages.
But:
I am not sure if it was a good idea to buy ampere instruments.
The problem is: They add a certain amount of restiance, which may not be a good idea there. The catalogue says, the meters come with a high power parallel shunt. What resistance does this shunt have?
 
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Mattyo5

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Joined 2002
It says in the catalogue that its 10mA/3ohm +par shunt. I dunno what that means. Voltmeter huh? Measures the voltage drop across a .1 or .01 resistor? Will that be accurate especially if the resistors are a little off. ?? Anyway, if i can't use the meter, let me know, and i'll return it and get a voltmeter. what value meter? doh!

-Matthew K. Olson
 
Holger Barske

Holger Barske

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Joined 2001
holgerbarske.com
Let's see:

The meter itself shows 10 mA full scale and has a resitance of 3 ohms. So, there will be 30 millivolts drop. To retain those 30 millivolts at 10 amps, you will need a resistance of 3 milliohms, about 1000 times less than the meter's resistance. This is about the value the shunt sould have.
3 milliohms isn't much resistance and won't hurt anywhere, I would go for it
 
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Mattyo5

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Joined 2002
Ok, so i just put the + on the meter in series with the + capacitor and the - to the pcb? sounds intuative to me, but i'm making sure :) it basically is going ot measure how much current the amp is drawing from the caps ...no?

do i want a 3 ohm resistor on the opposite rail just to even things out?

how about putting it right after the inductor in my CLC filter...might make the wiring easier and would also allow me to use a 3 ohm resistor on the oppposite rail. Just a thought :)

-Matthew K. Olson
 
G

GRollins

Member
Joined 2001
Okay, back up...what is it you're trying to do? If you just want something to watch, then either the rail or the Source resistor will do. If you want this to be a serious I'm-really-monitoring-something-here meter, then I'd go for the rail. Why? Because that will show the total bias current, which is the sum of all the individual output pairs. If you measure the voltage drop across the Source resistors, you'll find that they vary, one from the other, even though you matched them. If you're watching one, it'll always be in the back of your mind that the others are doing something different. By monitoring the total current, you'll have a more accurate idea of what is happening inside the amp.

Grey
 
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BDP

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Joined 2002
Ok, so i just put the + on the meter in series with the + capacitor and the - to the pcb? sounds intuative to me, but i'm making sure it basically is going ot measure how much current the amp is drawing from the caps ...no?
Click to expand...

Put a resistor in series with what you want to measure and then put your meter across this resistor.

The meter itself shows 10 mA full scale and has a resitance of 3 ohms. So, there will be 30 millivolts drop. To retain those 30 millivolts at 10 amps, you will need a resistance of 3 milliohms, about 1000 times less than the meter's resistance. This is about the value the shunt sould have.
Click to expand...
:nod: :nod:

This will give you a full scale reading at 10 amps of current draw.

BDP
 
M

Mattyo5

Member
Joined 2002
Ok.... good, i like the rail idea. So, if i put just the meter alone in series with the rail...i'll measure the current. Since aleph 2 has 6A bias, 0-10A would be appropriate range correct? I'm assuming I do not need any resistors across this meter since it should already have a shunt resistor inside it ...no?

-Matthew K. Olson
 
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