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Old 24th December 2013, 02:45 PM   #1
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Default ACA as 5W guitar power amp

Hi,

I know that this topic should be propably in "Instrument amps" forum, but this forum is more relevant because of the design and designer.

I would like to tweak the ACA amplifier for mixed-mode feedback to better mimic tube amplifiers.

I donīt know if I have connected correctly the feedback paths and how to calculate the resistors with question marks to obtain same voltage gain to the 8R load as original - 14dB of gain.

I have found some document how to calculate resistors values for mixed-mode feebdack, but donīt know how to appy it to ACA topology

Thanks.
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File Type: png ACA 5W SE MOSFET.png (11.6 KB, 349 views)
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File Type: pdf mixed mode gain.pdf (55.7 KB, 12 views)
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Old 24th December 2013, 09:50 PM   #2
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I think I have it done. I have simulated schematics in Multism and both types of feedback are set to reduce total gain evenly, they are mixed in 50 to 50 ratio. Closed loop gain of the amplifier is unchanged - 5x. Donīt know how much output impedance is raised.
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Old 25th May 2014, 06:41 AM   #3
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After a little bit of tweaking, this schematics sounds the best to me as guitar power amplifier. Gain and the input impedance of the circuit is unchanged - 15db and 10k.
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Old 16th January 2015, 10:03 AM   #4
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Default Latest schematics

Hi,

here is the latest schematics of Amp Camp Amp modified as guitar power amp.
Apart from little tweaks I have added presence filter control as many guitar preamps do not include it. For more bass increase 1uF after opamp. It sounds great!

Enjoy.
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File Type: png ACA 5W SE MOSFET Guitar.png (16.2 KB, 106 views)

Last edited by kacernator; 16th January 2015 at 10:05 AM.
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Old 16th January 2015, 10:46 PM   #5
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So you gave up on the mixed mode feedback and just added a resistor in series with the speaker? Have you tried disconnecting the feedback? Just remove the 68K. Have you tried changing the 0.22 and 0.33 resistor ratio?
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Old 17th January 2015, 07:15 AM   #6
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It is easier in this design to add output resistance then to mess with mixed-mode feedback.
I have tried disconnecting the feedback and I didnīt like the sound - it was harsher.
I dndīt mess with resistor ratio. I think it sounds pretty damn good as it is right now.
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