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Turning PC speakers into a small guitar amp

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moody07747

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I have an old set of Altec Lansing computer speakers, they really have nice bass. the system runs on 120V 120W.

Anyways I want to convert this system to work with guitar.

If I plug my guitar into it now I have to turn it up all the way just to get good volume out of it but even then is sounds good.

So what should I do to get this to work?

I was thinking of adding a basic buffer/booster but don't have a link to any
 
Assuming your powered speakers have a decently high input impedance, any sort of amplifier should work. If you don't have any stomp boxes around (a decent distortion box would have the positive side effect of coloring your sound some), you can simply make a basic opamp circuit. An inverting (or noninverting) opamp circuit with a gain of 3-4 might do a decent job as a preamp.

If the problem is low input impedance on the speakers, I think a 1:1 noninverting opamp might make a decent buffer, and significantly raise the gain of the system, although I'm not 100% sure about that.

With a few diodes, you can probably convince the circuit to clip a little bit. Sounds like a good project to me!
 
Dave,
If you have have an old audio system that you don't use, you could disconnect the output stage and connect the output of the pre-amp to your speaker box.
 
I take it these speakers already have a built-in amplifier, if so all you need is a preamp a standard non-inverting op-amp amplifier with a gain of 20 with probably do, and don't use the crappy uA741, use a TL071.
 
Hero999 said:
I take it these speakers already have a built-in amplifier, if so all you need is a preamp a standard non-inverting op-amp amplifier with a gain of 20 with probably do, and don't use the crappy uA741, use a TL071.

I was worried about hitting the rails. What is the output voltage level of a guitar? I was thinking ~1V, but I can't for the life of me remember what gave me that idea.
 
**broken link removed**

That circuit should do well as a buffer or a booster (and its very simple). The gain is Rf/Rin + 1. A small gain may be sufficient, but if you find you need more it would also be pretty easy to increase the gain by increasing Rf. I would recomment decoupling capacitors (10uF range would be decent) on the input and the output to block your biasing. Try something like Rf = Rin = 1k, with the resistor on the positive input = ~10k (its not very important... what ever you have handy). Should have a gain of 2ish, and the opamp has a huge input impedance and a very small output impedance.
 
You could buy and build an amp kit. I don't know what is sold in the US, but in Australia, the electronics shops JayCar, Altronics and Dick Smith sell kits. They all sell the "Champ" pre-amp for the same price (very cheap) and have mail order sales via the internet.

See www.jaycar.com.au search for catalogue number KC-5152, www.altronics.com.au, catK 2112 and www.dse.com.au cat K 5608.

The gain of this amp can be set by selection of a resistor. The maximum gain is 40 dB.
 
dboyer said:
**broken link removed**

That circuit should do well as a buffer or a booster (and its very simple). The gain is Rf/Rin + 1. A small gain may be sufficient, but if you find you need more it would also be pretty easy to increase the gain by increasing Rf. I would recomment decoupling capacitors (10uF range would be decent) on the input and the output to block your biasing. Try something like Rf = Rin = 1k, with the resistor on the positive input = ~10k (its not very important... what ever you have handy). Should have a gain of 2ish, and the opamp has a huge input impedance and a very small output impedance.
3 comments.
1. this needs a dual supply. This could be avoided by connecting another resistor of the same value (10k was suggested) from the + input to the Vcc.

2. the input resistance will be = 1k if Rin is 1k.

3. the gain will be Rf/Rin if the input is connected as shown. If it is connected to the + input (with the left hand end of Rin connected to gnd), it will be Rf/Rin + 1. If you connect the input to the + input, Rin will have to be 2 resistors, one connected to Vcc if you want a single supply.
 
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I have used the front part of this guitar pre amplifier to boost up to line level before. It worked very well, and if you fancied more of a project, you could keep all the tone controls in place too:

**broken link removed**
 
Notice that the guitar preamp uses a non-inverting opamp at the input for a 1M ohm input impedance and has a max voltage gain of 210.
When played loudly sure the 1st opamp will hit the rails, and the clipping diodes will clip good and hard. That's what guitar "fuzz" is all about.
 
Dr.EM said:
I have used the front part of this guitar pre amplifier to boost up to line level before. It worked very well, and if you fancied more of a project, you could keep all the tone controls in place too:

**broken link removed**

as most here probably know, im not that advanced in electronics. I can follow basic layouts but other than that i have no idea what im really doing.

I should really read up on it all one of these days

something like that circuit is too advanced for me.

now that dse link one seems like a good buy, only $7.

I will look around here some more for a preamp circuit...maby the shack has one.

maby i can even run my mixer with this PC set to get some gain.
 
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It's probably not as complicated as it may seem to use the front part of the design. I removed the surrounding parts and allowed it to run off of a single polarity supply (I think that was a requirement). I hope this is all correct :)
 

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Don't order stuff all the way from Australia's "Dick Smith WW2 Surplus Electronics and Used Car Parts" store. It even sells a record player made 40 years ago for RadioShack.
 
audioguru said:
Don't order stuff all the way from Australia's "Dick Smith WW2 Surplus Electronics and Used Car Parts" store. It even sells a record player made 40 years ago for RadioShack.
But if he can't buy it in yank land, then he will have to buy from a real country.
 
In Yank Land, people buy big guitar amps and speakers for guitars, not little amplified computer speakers.
 
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