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Old 6th August 2012, 02:00 AM   #1
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Default LM4780 Guitar Amp Question

I'm in the process of building a mono LM4780 bridged amplifier identical to the application circuit in the datasheet, and I have a few questions:

I want the amp to be able to take a signal from either a CD/MP3 player, or an electric guitar. I assume that since the guitar signal is much weaker than the CD/MP3 signal, they will need to have separate inputs.

Can anyone point me in the direction of a pre-amp that I could use for the guitar input, and would it be best to have 2 separate volume adjustment pots at the pre-amp and LM4780 stages?

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Old 6th August 2012, 02:09 AM   #2
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I've moved to instruments and amps forum (with a one week redirect) hopefully that should get the attention of the right people.
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Old 6th August 2012, 02:41 AM   #3
Einric is offline Einric  United States
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If you want to keep it cheap I would pick up a pair of cheap op-amp preamps from ebay.
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Old 6th August 2012, 10:05 AM   #4
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an MP3/CD shouldnt need too much gain, a single op-amp stage with a fairly flat frequency response should be fine, a pot can be used in the feedback. A guitar preamp will need more gain, especially if your looking for some distortion, which will also require a frequency curve unsuited for music.

with a 120 watt output, are you looking for something to gig with? and are you looking for a fairly complex pre-amp that will get you a range of tone?
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Old 6th August 2012, 10:37 AM   #5
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I would choose a schematic from a well known brand or one of your favorites.

My two favorites are the Marshall 8100 Valvestate and the Peavy Renown 400.

Either can be duplicated very easily !!!


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Old 8th August 2012, 01:47 AM   #6
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Any hints on where I can find schematics for the Peavy?

Looking at the controls for it, it seems like they have a 'pre-amp' built in if you use the high gain input, unless I'm mistaken...
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Old 8th August 2012, 03:01 AM   #7
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Yes it can be found here,

Peavey Renown 400 Manuals, Support and Troubleshooting - Amps

And the Marshal 8100 as well as many others can be found here,

Marshall Amps Info & Schematics
Enjoy !!!


I have owned both of these amps and their sound second to none except to the fully tubed amps, I still have the Peavey Renown400 but my Marshall 8100 was stolen.

Last edited by geraldfryjr; 8th August 2012 at 03:06 AM.
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