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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 2:38 am Post subject:
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fiilter Last edited by blackyellow on Sun Mar 03, 2013 8:46 pm; edited 2 times in total |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 2:55 am Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:49 pm; edited 1 time in total |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 4:25 am Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:49 pm; edited 1 time in total |
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wmonk
Joined: Sep 15, 2008 Posts: 528 Location: Enschede, the Netherlands
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Posted: Sat Feb 16, 2013 6:11 am Post subject:
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Did you use a virtual ground for the ground on the positive input of the opamp? _________________ Weblog! |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 8:25 am Post subject:
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R2 looks suspicious.
Even when at full resistance for 5K you will get only 1/3 of your signal as input voltage.
If the chosen pot is 1K, your input sees only 1/15 of the input voltage at best. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 10:53 am Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:46 pm; edited 1 time in total |
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wmonk
Joined: Sep 15, 2008 Posts: 528 Location: Enschede, the Netherlands
Audio files: 15
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Posted: Sat Feb 16, 2013 11:12 am Post subject:
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blackyellow wrote: | wmonk: I connected positive input to ground exactly as shown in the diagram.
my supply is +V= +5 volts and -V= 0 volts.
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For this filter to work correctly, the 'gnd' should be connected to 2.5V (that is, virtual ground between V+ and V-). _________________ Weblog! |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 11:19 am Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:50 pm; edited 1 time in total |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 1:22 pm Post subject:
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what about a simple transistor amplifier with an 2n3904 or something of the such? |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 2:08 pm Post subject:
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REMOVED _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
Last edited by JovianPyx on Sat Feb 16, 2013 6:08 pm; edited 1 time in total |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 2:19 pm Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:47 pm; edited 1 time in total |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 2:32 pm Post subject:
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you can increase the output one of 2 ways:
1) add an opamp as a DC amplifier stage
2) modify the right side LM324 to have gain more than 1 (which is what is shown in the drawing)
DC amplifiers are very common in synthesizer circuits. I would suggest reading about "Op Amp DC Amplifier", there are many sites which will explain the theory and give example schematics. This information will come in handy very often for you. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 3:13 pm Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:48 pm; edited 1 time in total |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 3:16 pm Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:48 pm; edited 1 time in total |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 3:42 pm Post subject:
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The reason I show 0V and a ground symbol is that they are different. 0v is the "minus" side of the 5 volt supply. It should NOT be the circuit's ground. If it is the circuit's ground, all AC signals will have their negative part clipped.
The virtual ground is created because you don't have a dual voltage or bipolar supply. It is a reference point halfway between 5v and 0v. This creates a situation similar to having a +2.5v and -2.5 supply. This allows you to pass a real AC signal into the opamp without distortion. Wmonk made this point as well, so I drew it into the schematic. The signal ground from the input and output circuits should connect to the virtual ground. Take the ground symbol for "system ground", that is, all grounds outside of this filter circuit go to the ground symbol point.
The ONLY place to connect 0v is the op-amp power terminal and the bottom resistor of the virtual ground circuit.
Note that the original schematic uses a 741 and a dual supply, that is +v went to some positive supply while -v went to a negative supply and where ground is not the negative supply, but rather a point of connection between the positive and negative supplies. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
Audio files: 26
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analog_backlash
Joined: Sep 04, 2012 Posts: 393 Location: Aldershot, UK
Audio files: 21
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Posted: Sat Feb 16, 2013 5:41 pm Post subject:
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I don't want to complicate the situation further, but when I was asking about lunetta mixing:
http://electro-music.com/forum/topic-55327.html
I was persuaded that the two resistors producing Vcc/2 should have a value of at least 100K as confirmed by this document:
http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf
That's what it says in section 1.2 (although it shows 100kW by a mis-conversion of the ohms symbol!). I had been using lower values (e.g. 10K or so), but ever since I've been using higher value resistors. I don't know how much difference it makes in the filter circuit above, but I've been using it as a rule of thumb ever since.
Any thoughts?
Gary |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 6:07 pm Post subject:
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Ah, I see, yes DO NOT USE MY SCHEMATIC ABOVE
One thing I see in the IGMF filter is that both cutoff and Q are affected by BOTH resistors. So the idea that the 330 K affects ONLY Q is not correct.
This is a simple, but non-ideal filter if you want to control ONLY Q or ONLY cutoff using a pot.
The pot as shown in the original schematic will affect both Q and cutoff. But it also affects overall gain.
Also, the IGMF filter does not include the pot (as shown above) for frequency control. This is a rather cheesey way to accomplish tuning IMO. As such, this filter deviates from the pure IGMF theory. When the pot wiper is at the top, there is no input signal at all... In fact, I'm not even sure it controls frequency. My first thought when I saw it was that it was a shunting mechanism to control input amplitude.
Also, the grounding looks wrong. You should not be connecting the input signal's ground to the zero volt side of the 5 volt supply.
blackyellow - did you put that pot in there as a modification or did you find the schematic drawn like that? _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 6:48 pm Post subject:
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as for the resistors in the virtual ground - The size of the resistors will determine how "stiff" the ground is. Larger resistors are less "stiff". It may well be that 100K will work in this circuit. Higher resistors will draw less current which is probably the point of what you read. With a regulated 5 volt and only this circuit - I think the difference is pretty minimal. 200K at 5 volts is 0.025 mA, 20 K is 0.25 mA.... really depends on the supply as to how far you push it. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 7:05 pm Post subject:
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filter
============== Last edited by blackyellow on Sun Mar 03, 2013 8:49 pm; edited 1 time in total |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Feb 16, 2013 7:21 pm Post subject:
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Last edited by JovianPyx on Sat Feb 16, 2013 11:33 pm; edited 3 times in total |
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blackyellow
Joined: Feb 12, 2013 Posts: 19 Location: Los Angeles
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Posted: Sat Feb 16, 2013 8:47 pm Post subject:
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filter Last edited by blackyellow on Sun Mar 03, 2013 8:51 pm; edited 1 time in total |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Sat Feb 16, 2013 8:54 pm Post subject:
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Self Oscillation, aka "screaming like a bitch" is sometimes desirable |
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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
Audio files: 26
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Posted: Sun Feb 17, 2013 2:13 am Post subject:
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blackyellow wrote: | SO I SHOULD KEEP THE POT GOING TO THE Virtual ground created for the positive input and ignore the red X he placed on the igmf bpfilter image? AND ignore the ground symbol under the pot?
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The circuit works fine if the pot is at ground.
analog_backlash wrote: | I was persuaded that the two resistors producing Vcc/2 should have a value of at least 100K as confirmed by this document:
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Hmmm. I had been using 100k until I read that the Vcc/2 source should be a low impedance. I've seen 10k used, which seems low enough, but as a compromise I've been using 47k, mainly because I have hundreds in that value.
JovianPyx wrote: | One thing I see in the IGMF filter is that both cutoff and Q are affected by BOTH resistors. So the idea that the 330 K affects ONLY Q is not correct.
This is a simple, but non-ideal filter if you want to control ONLY Q or ONLY cutoff using a pot.
The pot as shown in the original schematic will affect both Q and cutoff. But it also affects overall gain. |
Yes, I found there was interaction. When I've built this with variable Q, to limit the range I've used 220k in series with a 500k pot as Resonance.
Quote: | Also, the IGMF filter does not include the pot (as shown above) for frequency control. This is a rather cheesey way to accomplish tuning IMO.
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Yeah, but it works! _________________ Revenge is a dish best served with a fork... to the eye |
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analog_backlash
Joined: Sep 04, 2012 Posts: 393 Location: Aldershot, UK
Audio files: 21
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Posted: Sun Feb 17, 2013 2:57 am Post subject:
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JovianPyx wrote: | as for the resistors in the virtual ground - The size of the resistors will determine how "stiff" the ground is. Larger resistors are less "stiff". It may well be that 100K will work in this circuit. Higher resistors will draw less current which is probably the point of what you read. With a regulated 5 volt and only this circuit - I think the difference is pretty minimal. 200K at 5 volts is 0.025 mA, 20 K is 0.25 mA.... really depends on the supply as to how far you push it. |
richardc64 wrote: | Hmmm. I had been using 100k until I read that the Vcc/2 source should be a low impedance. I've seen 10k used, which seems low enough, but as a compromise I've been using 47k, mainly because I have hundreds in that value. |
Thanks for those points of clarification. I also noticed that the TI document wrongly describes Vcc/2 as a virtual ground, which I now know should apply to (e.g.) a shared op-amp inverting input in a mixer circuit.
Gary |
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