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Old 12th August 2010, 03:52 PM   #1
graygem is offline graygem  United States
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Default Need help merging circuits

I need help merging two circuits together. I have a tone circuit and a automatic loudness (gain) control circuit that I would like to merge into one. I am using a LM358 for the tone control and would like to use the other half for the AGC. It is just mono input and output. I using a single 12v supply. I would also like to change the 20k tone control pot to a 25k, because I can't get a 20k to match. Any help would be greatly appreciated.
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Automatic Loudness Control - RED - Page67
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Old 12th August 2010, 04:48 PM   #2
macboy is offline macboy  Canada
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I don't know where the difficulty is really. Just connect Output of one to the input of the other. I would not expect any difficulty in running the second circuit from 12 V instead of 15 V.

I'd point out that the second circuit is definitely not an AGC as you stated. It is described by the author as an "automatic loudness" circuit, but you are perhaps confused by what the author means by that. He uses "loudness" not as gain or volume, but like that little "loudness" button on older hi-fi gear that boosts low and high frequencies. That is intended to be used when listening at low volume to overcome the human ear's lower sensitivity to those frequencies at low volume. This "automatic loudness" circuit is intended to be a kind of variable loudness control, controlled by pot P1. It only becomes "automatic" if you gang the pot P1 with your volume pot, so that the low/hi frequency boost is increased as you turn down the volume. If you truly need an AGC circuit, you had better keep looking. Hint: AGC circuits almost always include a FET or a photo-resistor to control the gain.
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Old 12th August 2010, 05:50 PM   #3
Mooly is offline Mooly  United Kingdom
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Your first circuit appears to show the audio "shorted" out via the 0.22uf cap.

The input needs to be AC coupled using single rail supplies.

Edit... and pin 3 should be biased to half supply voltage.
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Old 12th August 2010, 05:52 PM   #4
graygem is offline graygem  United States
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Thank you. I'll keep looking for a real AGC circuit. Can you point me towards any?
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Old 17th August 2010, 04:55 AM   #5
cbdb is offline cbdb  Canada
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Almost all ACGs suck! (although I havent really listened to any of the newer DSP based ACGs but thats probably not in the scope of this discussion.) Why do you want one? Theres always a better way (a real compressor, or a manual volume change between songs is a better way to go).
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Old 17th August 2010, 05:00 AM   #6
cbdb is offline cbdb  Canada
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Changing the 20k pot to 25k will affect the filter crossover freq.
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Old 18th August 2010, 04:46 AM   #7
graygem is offline graygem  United States
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It's for a project at work. We already have the best product on the market but we still get people not wanting it because "your competitor has it" kind of thing. Therefore I was asked to come up with a one knob tone control and AGC to add to our current product. My boss said he didn't care if it really worked or not. What do I need to do to the tone control so I can use the 25k pot?
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Old 18th August 2010, 04:48 AM   #8
graygem is offline graygem  United States
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Is This AGC better?

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Old 31st August 2010, 10:52 AM   #9
alexf is offline alexf  United States
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Better in terms it's a real AGC, yes. But expect a lot of distortion. To make a half-way decent volume control out of a JFET you need to at least feed half of the drain voltage to the gate.
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