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I want to build my own guitar amp and I know a little about electronics (handling op-amps and little more) and I would appreciate if anybody could explain me these circuits,

http://mad-science.wonderhowto.com/how-to/build-your-own-mini-altoids-guitar-amp-for-about-5-0135412/

enter image description here

and this one

https://sites.google.com/site/amplificatoare/12w-amplifier-using-741-op-amp enter image description here

I would like every detail that I should know in order to understand those two circuits

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    \$\begingroup\$ This question is probably a bit broad in it's current form. It might be better to start building and ask specific questions about areas you get stuck with. To explain every detail about building even a basic guitar amp would take a lot of time to do thoroughly, and since we don't know exactly what you already know some of the info presented may be redundant. \$\endgroup\$ – Oli Glaser Apr 23 '13 at 22:42
  • \$\begingroup\$ Ah, sorry, didn't look at the links. I assumed a far more complex design, these shouldn't be too difficult to cover, at least in a basic fashion. Still it would be good if you let us know which specific areas you need help with, or ask some specific questions about the designs. \$\endgroup\$ – Oli Glaser Apr 23 '13 at 23:15
  • \$\begingroup\$ Both these circuits are low power and won't give you a great sound - is a low power practice type amp what you are looking for? For a "real" amp, there are many Marshall/Fender/Vox/etc schematics available on the 'net. Some nice simple valve based designs, or modern all transistor versions. I used to use a very basic little switchable 7W/20W 4 or 5 triode based valve amp for studio work as it naturally started to overdrive at low volumes (and an Orange AD120 for gigs) \$\endgroup\$ – Oli Glaser Apr 23 '13 at 23:31
  • \$\begingroup\$ Might want to take a peek at coursera.org/course/audiomusicengpart1, a coursera course some of my colleagues will be teaching soon \$\endgroup\$ – Scott Seidman Apr 23 '13 at 23:32
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    \$\begingroup\$ The first thing you need is to take "LM386" and double up the 8 digit. \$\endgroup\$ – Kaz Apr 24 '13 at 0:25
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The first circuit is just the LM386, which is a low-power IC amplifier. The datasheet has a decent explanation of how the circuit works.

The second one is more interesting. It uses an op-amp to drive some complimentary Darlington pairs. The interesting part is its output connected to ground (through a small resistance R6). Instead of using its output to modulate the power transistors, it uses the power rails. So when the opamp tries to pull its output high, it can't really (because the output is essentially grounded) but it pulls high current through its positive supply pin. Likewise for pulling the output low. This is a kind of messy way to do it, but the overall feedback (R3 / R1) should reduce distortion somewhat.

One advantage of the output configuration is that the output can pull nearer the rails than a traditional emitter-follower output stage. You are limited by T2 (and T4)'s saturation voltage, maybe 0.4 V. In the traditional configuration you can't get closer than 1.5 V to the rails without special tricks. That means this amp can get closer to the supply rails and play slightly louder.

A interesting construction feature of this amp is that the collectors of all the power transistors are connected together. If the heatsink can be electrically isolated from the case and the rest of the circuit, the transistors do not need to be electrically insulated from the heatsink. In fact, the heatsink can form one of the speaker output terminals...

This will not be a wonderful-sounding hifi amp, but it might serve well enough for guitar. It will definitely lend some "character" to the sound.

If you want to build a clean-sounding amplifier, consider an LM1875-based circuit. The datasheet has the basic circuit, and you can find DIY forums where circuits using this chip are discussed pretty thoroughly.

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  • \$\begingroup\$ +1 - nice answer. The second circuit is indeed quite interesting, I'd be interested to see how well it works in practice. \$\endgroup\$ – Oli Glaser Apr 23 '13 at 23:24
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    \$\begingroup\$ The circuit is not really different from an emitter follower. It has just reversed what is ground and what is supply. But that only matters from the POV of DC and DC biasing. The emitter followers are driving an AC ground which just happens to be called +12 and -12. This reminds me of an amplifier designs where the output where the speaker is normally found mysteriously goes to ground, and the speaker is connected to the power supply midpoint. ssguitar.com/index.php?topic=2812.msg20622#msg20622 \$\endgroup\$ – Kaz Apr 24 '13 at 0:42
  • \$\begingroup\$ It is different from an emitter follower, in that the output stage has voltage gain. In the standard circuit, there is a carefully designed low distortion voltage gain stage, then the darlington pairs just amplify the current. The difference should be immediately apparent when you calculate loop stability. \$\endgroup\$ – markrages Apr 24 '13 at 2:21
  • \$\begingroup\$ I've seen this setup more often, I guess it works quite well if many people are using/copying it. It's worth the try I guess. R6 is for bias current (R4/R6 and R5/R6). I expect you can adjust the rest current (and quiescent power) in the output stage by changing its value slightly. \$\endgroup\$ – jippie Apr 24 '13 at 6:45

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