# Differentiator help...

Discussion in 'General Electronics Chat' started by chunkmartinez, Jun 1, 2013.

1. ### chunkmartinez Thread Starter Senior Member

Jan 6, 2007
180
1
I am designing a device that detects harmonic distortion. While playing music within musical limits(say 20hz-20khz) it will not trigger unless the frequency is outside the hearing range(over 20khz).

My highest input voltage can go upto about 200ACV(before attenuation) So I want to attenuate the input voltage with a voltage divider, then use an opamp as a differentiator to trigger an LED when harmonics develop(they develop close to clipping threshold). Can someone help me with the capacitor value, and what resistance values should be used to attenuate the input? The circuit is generally simple but I have never designed for something that has a range of say 200-300ACV RMS max. I want as little power to be wasted through the circuit as possible as I am going to use it during music play(a speaker connected and playing).

My Max ACV ratings can change to go lower some...

I realize an HPF has a slope which is why I don't want to try one because then it would be tough to design within limits due to another voltage in a different music setup having a higher voltage output(I want to be able to use it for more then one setup). Is a differentiator more dependand on frequency? I don't want to have to worry too much about designing for a max voltage value but then moving it to a higher voltage setup and now the slope isn't sharp enough, but I don't want to use inductors for a multi order filter, just something like a capacitor or few.

The speaker impedance will varry due to using a circuit for each speaker channel, 1, 2, and 4 ohms.

Last edited: Jun 1, 2013
2. ### crutschow Expert

Mar 14, 2008
17,022
4,670
A differentiator is basically a high-pass filter with an increasing gain slope of 6dB/octave out to the limits of the amplifier. I don't think that will do what you want.

Better might be to use an active HP filter and perhaps compare the output of that to the average rectified output of the amp.

3. ### chunkmartinez Thread Starter Senior Member

Jan 6, 2007
180
1
I just did a test with a voltage divider and a simple polyester capacitor but I forgot to add a resistor to that. Is it mandatory to add a resistor with the cap for a simple first order filter?

4. ### crutschow Expert

Mar 14, 2008
17,022
4,670
Yes you need a resistance. In your case it's likely the equivalent resistance of the voltage divider. The corner frequency of an RC first-order filter is 1/(2$\pi$RC).

5. ### chunkmartinez Thread Starter Senior Member

Jan 6, 2007
180
1
Okay after a few days of expirimenting and trying to dial in components I simulated a differentiator last night, a simple one, the schematic is shown in the attatchment below.

Let me go over my idea as clearly as possible again but be specific and rule thigns out:

First, I will not consider comparing the output to the input at all, why? Because that is simple and has been done over and over, my concept is different. I will be designing a product to sell on a market. Another guy created a product that only requires connecting aligator clips to the output terminals of a car audio amplifier and running test tones. I will need to only connect the amp output to this device so it needs to detect harmonics.

How am I going to do it? As said in original post, I will be using a differentiator to give me an output depending on frequency. Only harmonics will be high enough in frequency to trigger it. 20khz and below will not give output from the differentiator in the design. I will be creating about 4 LED channels to be able to detect harmonics from the headunit, and 3 other amplifiers since the average car audio setup can use upto 3 amps on average for the different frequency ranges such as subwoofer amp, mid amp, and trebble range amp.

My circuit will use VCC and GND for the op-amp power supply so that all my output from the opamp will be above GND to be friendly with an LED...Well, I will probly send the op-amp output to a comparator to compare the out voltage to a reference voltage since the circuit won't be perfect and will still produce some voltage even at 20khz, just a little. That comparator output will also saturate so It may work good. I can drive the LED from that comparator. I may use a peak detector(diode and capacitor)to take the ac output from the first opamp and turn it into a DC voltage.

Rightnow I am just testing and testing to try and get my real world results close to my simulation results. If I can get the differentiator to saturate or get close due to harmonics created measured by my oscilloscope then I will be a haaapy!

Do I sound like i am on the right track here?

couple more details:

I am first testing on my 80w x2 caraudio amplifier. I am simple paralelling my circuit to one of the two channels. I am using a voltage divider to bring the max amp output before clipping to aroung 800mV to 1 Volt. My amp output before clipping is about 17V.

My opamp power supply is 12V from my headunit's turn on REM lead which dosn't worry me because this circuit will draw practically no power. Giving that the opamp will swing about 9V my virtual ground created by my monopole opamp application should be about 4.5V with "no" differentiator output.

I am testing with a 100hz tone, a 1khz tone, and a 20khz tone. The first two tones should yield about 4.5V where the frequency is too low to give any differentiator ouput. 20khz will be the same or close, maybe a tad higher. Since I will have multiple LED's and sections to detect different frequency ranges my design limitations open up a little I think. Also, that 200-300V maximum requirement I stated int eh original post changed since not all frequency ranges will be near that at most. The higher "trebble" range LED section will have a max possible power of about 1000Wrms or so, so the voltage swing isn't near the oginal requirment(I know that sounds rediculous but I want it to be compatible and there are people who actually use that much power just for their tweeters but it's because they use a whoole lot of them in show vehicles).

One last thing, I will want the device to be able to work during music play unlike the product the other guy has that I was refering to earlier.

For now I will be focusing on only the trebble LED channel and I could use some advice for the circuit. My circuit I am testing now is very simple it is shown below.

• ###### proto.png
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Last edited: Jun 7, 2013

Jan 6, 2007
180
1