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Old 23rd April 2004, 03:10 AM   #1
styoshi is offline styoshi  United States
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Question Automatic Gain Control - 50W amp needs a good feedback schematic!

I am trying to build a 50 W audio amp with automatic gain control. I was wondering if you might have any suggestions for a good AGC schematic. In short, the idea is that if I want to switch audio sources for my amplifier, my power output wouldn't fluctuate with loud or soft sources. My sounds source will be a CD player (~25 mV signal), so source switching would be between loud and soft songs on a CD.

I'm not sure if you have this book ('High Power Audio Amplifier Construction Manual' by G. Randy Slone), but I am using a Darlington array with two pole compensation for my Voltage Amplifier Stage (fig. 5.1h, p. 113) and a complementary feedback circuit for my Output Stage (fig. 6.3c, p. 164). This is closer to a Class B output.

I guess in particular, I'm not sure what kind of global feedback system to implement to maintain a constant power output. I'm thinking of putting in a feedback loop between the output of my voltage amplifier and the base of one of the transistors in my Input differential. Do you have any suggestions? (Also, if possible, I'm trying to stick to BJTs).

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Old 23rd April 2004, 05:26 PM   #2
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Default AGC Circuits

There are a number of good desins available at in the audio circuits archive.
Dan Fraser
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Old 23rd April 2004, 08:53 PM   #3
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You definitely don't want an AGC circuit in your amp's feedback loop or anywhere else for that matter.

You simply need individual level sets on the pre-amp for each inputted device that doesn't have it's own volume control. Since you can't have a level set for the phono input all other levels should be set to match the phono.

A dual 5K linear pot at each input would be a great start.
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Old 23rd April 2004, 09:11 PM   #4
johnnyx is offline johnnyx  United Kingdom
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Originally posted by Bill Fitzpatrick
You definitely don't want an AGC circuit in your amp's feedback loop or anywhere else for that matter.
Jukebox amplifiers have AGC circuits to ensure that all tracks have equal volume; this may be important in a pub, bar or restraunt, but it audibly mushes up the sound. Even a (expensive) broadcast - quality compressor/limiter is not something to have in your sound system permanently. JLL Hood designed a preamp where each input had its own level control. It had an inverting input configuration, so the series input resistor was made variable using a preset, thus adjusting the input sensitivity. This may be the way to go.
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