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calx44 15th May 2012 03:52 PM

Basic Guitar Transistor Amplifier
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Hi all.

I am new to this forum and I have what I guess must be a pretty simple problem to start with.

I would like to make a guitar amplifier for my A2 level Product design project and so I thought I would start with a simple transistor amplifier to get things going. Basically, I have tried this a few times and watched videos, read books and seen schematics and I cannot get this circuit to work (see attachment for schematic).
I can hear the interference when I hook up the battery so it all must be connected up, right? But when I plug the guitar in, I get no sound out of the speaker whatsoever. I have tried to follow a schematic I saw on youtube for a transistor preamp where his input was a mic and his output a PA speaker, he ran it through an IC power amp stage though. Do I need to do this to get any sound?
If I need a power amp stage then I would prefer to make it myself out of transistors instead of an IC as I would like understand how it all works, so does anyone have a schematic for a basic power amp stage?

Any help would be appreciated as I don't really know where I am going wrong.
Thanks in advance

PS, the transistor is a 2N3904 if this matters

DigitalJunkie 15th May 2012 04:06 PM

First of all,you need more gain. The output signal of a typical guitar is very low,you simply aren't amplifying it enough to be heard.
Second,the circuit you posted is more of a 'low level' amplification stage,it doesn't have the 'grunt' to drive a speaker.You need a power amplifier of some sort,it could be a simple circuit with a couple transistors,and a handful of passives (caps,resistors) or a chip of some sort (LM386 comes to mind).

kevinkr 15th May 2012 04:08 PM

:cop: This belongs in Instruments & Amplifiers so I will move it there.

The circuit you have provided is just a simple voltage amplifier and cannot drive a speaker directly. A discrete transistor amplifier is a complex undertaking and without some experience you are not too likely to get it working.. A chip based power stage would probably be a better idea.

calx44 16th May 2012 06:13 PM

Hi, thanks for the replies, and thanks for moving it to right place :)

The only reason I wanted to build it using discrete components was to help gain some understanding on how it would work. Is there a basic schematic of an output stage that you know of that I could have a go at? I have struggled to find one.

Thanks again,

Loudthud 16th May 2012 08:51 PM

Go to this website: Solid State Guitar Amp Forum | DIY Guitar Amplifiers - Index

Register and then look at the Newcomer's Forum. Read the stickys. They will get you pointed in the right direction.

Simon B 17th May 2012 02:08 AM

The circuit you've posted has an input impedance of roughly 4k. For a guitar it needs to be more like 400k.

Guitar output varies a lot, but 50-100mV is a reasonable starting point, but only if it connects to a high impedance input - give it 4k and it'll pretty much disappear.

The speaker is very low impedance - 8R (ohms) is common, so it needs a circuit that is better at supplying current. I've never come across a one-transistor practice amp, and I'm old now!

Have a look around and ask again about whatever you find.

shanx 17th May 2012 03:14 AM

Simon is not old , we like to call it experienced. ;-)
Without using SPICE, would you like to go through the ''old school'' way of doing the design analysis? I have not used SPICE yet, I just started texting this year on my cellphone (to my daughter) and I still recall telex machines at my first job. Oh crap!!!
Seriously though maybe a step by step walk through is in order. BTW the 2N3904 really does matter, as it and consequently everything around it will define the operation. It is the active amplifying component .

shanx 17th May 2012 03:40 AM

You should consider the input signal level, and it's source impedance. The output load and what power you need to drive it with. Based on the maximum power dissipation and Hfe of the 2N3904 in class A operation I believe it will not be possible to make the voltage gain AND power gain required to get you to 500 mW at 8 Ohm load .
I would say at least 2 stages required.

calx44 17th May 2012 09:31 AM

Thanks again for the help I will give that link a look. I spoke to someone who said it needed an output stage and said it could be a push pull stage. Is this a good way forward?
Also, if i increase the resistors on the input side to keep the same bias voltage but increase impedance this should "boost" the signal?
Cheers again,

calx44 17th May 2012 02:17 PM

Actually, one more question, How do you work out the input impedance of this circuit? basic I know :L

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