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Old 13th December 2010, 08:38 PM   #1
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Default How to add line-in to a guitar amp?

Hi!
I'm thinking about buying an small 12W amp, (orange crush). The thing is that, unlike it's bigger 20W brother, this one does not have a line-in jack (aux input).
Do you have an idea of how hard is it to add one?
I'm guessing it's just a jack and a couple of resistors in the right place... Click the image to open in full size.

Any help will be very appreciated Click the image to open in full size.
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Old 13th December 2010, 09:12 PM   #2
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Line in is a higher voltage so it is a jack and a couple of resistors as a potential divider to attenuate the signal.

Or just put the line in into the normal guitar jack but dont turn it up too loud.
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Old 13th December 2010, 09:19 PM   #3
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I'd just put it on the input of the power amp (provided there's enough gain).

Guitar pre-amps can have all sorts of eq in there. For example, mine cuts the bass and boosts the treble. Putting the aux in directly into the power amp means you can avoid this.
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Old 13th December 2010, 09:36 PM   #4
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Quote:
Originally Posted by chris661 View Post
I'd just put it on the input of the power amp.
I thought of doing it just like that but immediately before the volume pot (assuming that the combo has a pre-amp ---> volume pot ---> power amp).
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Old 14th December 2010, 03:53 PM   #5
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I was thinking about putting two resistors before the power-amp, one in series with the signal and the other to ground. Is this the right way to do it?
Any ideas of the values to use? About 10K?

Thanks
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Old 14th December 2010, 04:43 PM   #6
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It's hard to give specific advice, especially without a schematic.
This is a solid-state amp?
What do you want to play through your LineIn - CD player without a volume control, or something like an MP3 player with volume control?
For figuring out the values for a resistive voltage divider, one easy tactic is to temporarily wire in a pot, adjust to suit, measure the resistances from the pot, and then replace the pot with a pair of resistors.
I do what NigelWright has suggested - just plug it into the normal input and keep the volume control down.
A lot of the newer cheap amps have pretty flimsy PCB traces- I avoid hacking into them unless it is absolutely necessary.
John
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Old 14th December 2010, 04:52 PM   #7
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Think I'll try that first, I'm just woried it might get some distortion...
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Old 14th December 2010, 05:16 PM   #8
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Line level varies from ~0.3 - 1.5V rms depending on whether it's consumer (lower) or professional (higher) audio, and which country. They generally have a low output impedance (100R).

Guitar outputs range from 0.1 -1.0V rms. They are quite high impedance and guitar amps have a correspondingly higher input impedance of 500k or greater, so they will not load down the line out.

This means that a guitar amp will generally accept a line level directly into the instrument socket. Depending on the actual line level and amplifier the amp may be slightly underdriven or slightly overdriven, but it should tolerate the overdrive without damage. Just turn it down a bit if it sounds a bit rough.

If you want to run both simultaneously an opamp summer is the best solution.

w
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