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Old 14th November 2016, 12:34 PM   #11
ubituks is offline ubituks
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Quote:
Originally Posted by AndrewT View Post
When the gain switch is closed, the NFB loop is operating and you get a gain = 4k7/1k5 + 1 = 4.13times (+12.3dB)
When the switch is open the NFB loop is not operating and the opamp acts as a Follower/Buffer. Now the gain is +0dB

If you also add in a plugable 2k2 parallel to the 4k7, you get a gain of +6dB switchable back to +12.3dB, when the 2k2 is opened.
So you mean like this? Well thats better than 0dB / 12.3dB gain switch.
By the way, what's the purpose of Q (R6 in schematic) ?
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Old 14th November 2016, 01:46 PM   #12
AndrewT is offline AndrewT  Scotland
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R6 & R7 vary the roll off frequency of the high pass filter. This protects the bass driver from over excursion when you apply that +6dB or +12.3dB gain stage that you probably don't need when you already have +6dB from the bass summimg channel to input to your bass only speaker.
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Old 14th November 2016, 03:47 PM   #13
ubituks is offline ubituks
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Thanks. Okay, so current schematic and pcb are in attachment.
What about input over-voltage protection?
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Old 14th November 2016, 05:49 PM   #14
AndrewT is offline AndrewT  Scotland
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That would need a protection "system".
Many professional PA rigs include a system to ensure the paying audience hear the audio even though an idiot operator mis-sets the controls.
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