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audio power indicator

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Jaydu1904

New Member
Hi friends, first of all i thanx for answers given to my previous questions.

Now i am making a power level indicator with circuit diagram and description below.

I am not getting how R2 is working as it is connected directly(not as a pot) and what is a purpose of diode D2. please explain...



Circuit description:

Parts:

R1__________100K 1/4W Resistor
R2___________50K 1/2W Trimmer Cermet
R3__________330K 1/4W Resistor
R4____________1M2 1/4W Resistor
R5__________470K 1/4W Resistor
R6,R7_______500K 1/2W Trimmers Cermet
R8____________1K5 1/4W Resistor
R9-R11______470R 1/4W Resistors

C1___________47pF 63V Ceramic Capacitor
C2__________100nF 63V Polyester Capacitor
C3___________47µF 25V Electrolytic Capacitor
C4____________1µF 25V Electrolytic Capacitor

D1______BZX79C5V1 5.1V 500mW Zener Diode
D2_________1N4148 75V 150mA Diode
D3-D5________3mm. Yellow LEDs

IC1_________LM339 Quad Voltage Comparator IC

SW1__________SPST Slider Switch

B1_____________9V PP3

Clip for 9V PP3 Battery

Circuit operation:

This circuit is intended to indicate the power output level of any audio amplifier. It is simple, portable, and displays three power levels that can be set to any desired value. For a standard HiFi stereo power amplifier like the 25W MosFet Audio Amplifier described in these pages, the power output values suggested are as follows:

* D5 illuminates at 2W
* D4 illuminates at 12.5W
* D3 illuminates at 24.5W

The above values were chosen for easy setup, but other settings are possible.
IC1A is the input buffer, feeding 3 voltage comparators and LEDs drivers by means of a variable dc voltage obtained by R5 and C4 smoothing action. In order to achieve setting stability, the supply of IC1 and trimmers R6 & R7 is reduced and clamped to 5.1V by Zener diode D1.
Notes:

* The simplest way to connect this circuit to the amplifier output is to use a twisted pair cable terminated with two insulated crocodile clips.
* Setup is best accomplished with an oscilloscope or an audio millivoltmeter like the one described in these pages. Precision Audio Millivoltmeter
* A 1KHz sine wave generator with variable output is also required (see a suitable circuit in this website also). 1KHz Sinewave Generator
* Connect the generator to the amplifier's input and the Audio Power Indicator to the output of the amplifier, in parallel with the oscilloscope probe or the audio millivoltmeter input.
* When using high power outputs disconnect the loudspeakers to avoid Tweeters damage and connect in their place an 8 Ohm 20-30 Watt wirewound resistor.
* Remember that VRMS output is equal to output Peak-to-Peak Voltage divided by 2.828.
* RMS power output in Watts is equal to VRMS2 divided by speaker impedance (usually 8 or 4 Ohm).
* Example: set the output of the 1KHz sinewave generator to read 14V on the audio millivoltmeter (24.5W @ 8 Ohm). Set R2 until D3 illuminates, and be sure that D3 turns-off when diminishing a little the generator's output.
* Do the same with R7 for D4 and R6 for D5. The readings of the audio millivoltmeter must be 10V (12.5W @ 8 Ohm) and 4V (2W @ 8 Ohm) respectively.
 

Attachments

ccurtis

Well-Known Member
I am not getting how R2 is working as it is connected directly(not as a pot) and what is a purpose of diode D2. please explain...

The implication is that R2 is connected as a variable resistance with the wiper connected to one end such that the resistance with vary from zero to maximum resistance.

D2 acts as a ≈0.6 v (-) reference for the second comparator by virtue of the input bias current that flows through D2, supplied by the second comparator. The output saturation voltage of the first comparator is non-zero. D2 ensures that the second comparator will turn on when the first comparator turns on (when its output is saturated).
 
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Jaydu1904

New Member
thanks, ccurtis..
got the working of D2 but i still have doubt in working of R2, its variable terminal is kept open, so how the resistor will vary??

is it a mistake?? or i should connect the variable terminal instead of fixed one??
 

ccurtis

Well-Known Member
Either it is a mistake or a funny hoax because the instructions indicate that R2 should be "set" until D3 illuminates.

My vote is that it is a mistake in the schematic and that R2 should be shown connected such that its resistance can be "set" to calibrate D3 for either an 8 ohm or 4 ohm speaker impedance.
 

audioguru

Well-Known Member
Most Helpful Member
R2 is shown as a variable resistor (rheostat).
Its middle pin connects to a pin at one end.
 

bountyhunter

Well-Known Member
Not sure IC1A is going to be happy with a 1uF cap sitting directly on it's output. It looks like IC1A is trying to be some kind of pseudo-averaging circuit? It's a comparator, but the schematic shows it wired as an op-amp with the output tied back to the inverting input (?)

Power meters are a lot easier to build using an LM3914 or 3915, ie they are voltage reading meters which translate into power if the speaker impedance is known. Since you actually care if an amplifier clips, peak voltage levels are more interesting than an averaged level.
 
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bountyhunter

Well-Known Member
I am not getting how R2 is working as it is connected directly(not as a pot)
It's simply a variable resistor to ground. It forms an attenuator of the input signal with R1. The higher the amount of resistance at R2, the more signal goes into the comparator input. Less R2 resistance and more voltage drop across R1 = less signal at the input of U1A1.
 
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bountyhunter

Well-Known Member
what is a purpose of diode D2. please explain...

.
It's a lazy man's way to bias that input up to approximately 0.6V. The designer is relying on the bias current flowing out of the input of the LM339 to cause a VBE across the diode to ground. Since that determines the comparator's threshold, it's certainly not a precise design.
 
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audioguru

Well-Known Member
Most Helpful Member
Not sure IC1A is going to be happy with a 1uF cap sitting directly on it's output. It looks like IC1A is trying to be some kind of pseudo-averaging circuit? It's a comparator, but the schematic shows it wired as an op-amp with the output tied back to the inverting input (?)
The datasheet of the LM339 from ST Micro shows the output of the comparator with a huge compensation capacitor to ground so that it performs like a very low frequency opamp.
 
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bountyhunter

Well-Known Member
The datasheet of the LM339 from ST Micro shows the output of the comparator with a huge compensation capacitor to ground so that it performs like a very low frequency opamp.
It just looks like a squirrely circuit to me. If I want an op-amp, I'll put one in and adjust the compensation/gain components to set the bandwidth.

The other thing is: exactly what is it trying to measure? Given the "input buffer", it sure can't be true RMS power, it's not even RMS voltage, and it sure isn't peak voltage. How can any of that be related to an RMS power meter? Looks flaky to me, it's basically an idiot light. The calibration is to send a 1kHz sine wave in and turn the pots until the lights go off? Junk.
 
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Jaydu1904

New Member
OK, I will connect the R2 as R6 and R7 as it is not working for given circuit diagram. I have another doubt... Is it necessary to connect IC1A as buffer??

If i connect input directly(removing buffer) to the comparators (IC1B,C,D)
will the circuit work properly?? what is exact requirement of Buffer(IC1A)?
 

ccurtis

Well-Known Member
OK, I will connect the R2 as R6 and R7 as it is not working for given circuit diagram. I have another doubt... Is it necessary to connect IC1A as buffer??

If i connect input directly(removing buffer) to the comparators (IC1B,C,D)
will the circuit work properly?? what is exact requirement of Buffer(IC1A)?
IC1A is more than just a buffer. It works as a clever peak detector owing to the fact it has an open collector output. The circuit will not work without it.
 

stainlessbrown

New Member
new guy...

I realize this is an old thread, but I'm wonder if it would have any effect to eliminate all but one of the final op-amps (specifically the higher power indicators)?

This circuit appeals because a). it's pretty simple, and B) the 9VDC battery- I want to trigger a LED based on an audio signal/click track, converting it to a visual (blinking) LED (red) the output from the software driven source is basically a sloppy square wave (I say sloppy, because the audio can be varied from sounding like an old Pong game to a cowbell, wood stick "clack"... among other sounds)-

the output is at a low level intended for a pre-amp input. It needs to be output driven so it syncs with the software (recording)

I've looked at several LED driver IC's, but they seem to drive 8 - 10 LED's in sequence depending on the strength of the signal... and all i need is a single LED... and ideally, something that could operate on a single 9 V battery

so am I off base by thinking this circuit might work? and if so, could I just use 2 single 741's?

thanks for anyones patience!

Stainless
Sacramento
 

bountyhunter

Well-Known Member
I wouldnt use a 741 op amp for a battery powered circuit since it wastes quiescent current. There are a lot of CMOS low power op amps available.
 

audioguru

Well-Known Member
Most Helpful Member
Some 741 opamps do not work from a supply as low as 9V. A little 9V battery drops to 6V during its life.
 

stainlessbrown

New Member
Thanks for the replies

741, understood, ...I'd considered using 2 - 9V battery's so I could have a +9 and -9, but still a bad idea?

any issues from eliminating 2 of the 3 final output "stages" from the circuit above
 

audioguru

Well-Known Member
Most Helpful Member
The circuit uses 3 output stages so it can indicate 3 different levels of noise. You can use only one output stage if you want.
 

stainlessbrown

New Member
so if D5 is the lowest level, I could eliminate R7, R9, R10 (and D3 and D4) and D2?... or should D2 be connected to the ungrounded side of R6
 
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