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Author Topic: Want to Make AMP...need HELP  (Read 4363 times)

jedisteve

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Want to Make AMP...need HELP
« on: January 17, 2007, 03:57:03 PM »
Hey everyone,
I am looking to build a small practice amp around (8-10 Watts) and I need some help.  I need help in how to actually go about doing this.  I would greatly appreciate it if someone could help me with a schematic and actually explain the details of the components so that I can understand why certain capacitors, op-amps, transistors and load resistances were used.  I am excited to try and build one and I hope you can help.  Thanks again.

teemuk

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Re: Want to Make AMP...need HELP
« Reply #1 on: January 17, 2007, 07:33:30 PM »
You are asking pretty much: I'm writing a book about this stuff and filled 133 pages (A4) so far. Yet I have not covered even half of all the stuff involved. If you want to know the basics I suggest you buy or loan a book that covers a nice deal of analog circuit design.

Basically guitar amplifiers,
1. Try to pick the signal from the guitar's pickup and therefore require a large input impedance not to cause signal losses (the load resistance thing)
2. Except the output stage, try to transfer as much as signal (voltage) from one stage to another as possible. Signal gain stages do not follow "power transfer" rule that tries to even the input and output impedances. (Again the load resistance thing)
3. Try to amplify this signal's amplitude, sometimes even higher than fourthousandfoldly (gain stages)
4. Need to alter frequency response to compensate the middle-frequency orientated signal of magnetic guitar pickups (tone stacks and other tone control circuits). Some times they alter frequency response just to create a certain "trade mark" tone.
5. Need to alter frequency response to get rid of excessive bass and high frequency content (low and high pass filtering)
6. Need to feedback inverted high frequency content to stabilize the gain stages (preventing oscillation)
7. Need to feedback part of the signal (inverted) to control gain, linearize the gain stages etc. (negative feedback)
8. Sometimes need to select between channels that have a different frequency response, gain etc. (solid state or conventional switching)
9. Need to buffer a voltage source with a current amplifier stage so that enough current can be fed to the speaker load (power amplifier)
10. In most cases, convert AC mains voltage in to proper amount of DC voltage to power the circuitry - and don't forget that a proper amount of current is needed as well...
11. Sometimes need to produce "special FX" like distortion, reverb etc.
11.1 Need to clip the signal with diodes, overdrive a gain stage etc. to produce distortion
11.2 Need to alter frequency response before this to make clipping sound nicer
11.3 Need to alter frequency response after this to make clipping sound nicer
11.4 Need to alter frequency response within the stage that distorts the signal to make clipping sound nicer

If you know basic circuit theory you can pretty much explain what different components in the circuit do based on the above list. Then there's stuff involving component layout, component parameters, speaker selection etc. No way anyone can give you a comprehensive reply. Maybe you should study a little bit yourself and then come back with more detailed and specific questions, i.e "In this circuit (schematic included), what do resistors a, b and capacitors c and d do in conjugation with compent e?". Or "I'm looking for an pre/power amplifier schematic that would have the following features (list included). Any ideas?"

The internet is full of schematics you just need to search harder. I have no particular interest in storing links for low power practice amplifiers; In my opinion most of them are are pretty "boring". Basically I have no interest in storing links for any schematics, they just tend to get lost and forgotten. This might help you though:

http://www.freeinfosociety.com/electronics/schempage.php?cat=1

By the way, best way to build a simple, low power practice amplifier is use a power opamp IC ("chip amplifier"). Look for TDAxxx or LMxxxx series, study the datasheets! Most of the datasheets contain a "demo" circuit that should be all you need to build the "power amp" part of a practice amplifier. TDA series datasheets even contain a PCB layout suggestion. This is just half way, though: you still need to figure out what kind of preamplifier you will use (if any) and at least find out how to build a proper power supply for the thing.

Yeah, I was planning to keep this as a short post.

Edit: With that TDAxxxx and LMxxxx I mean ICs like: TDA2020, TDA2030, TDA2040, LM4700, LM4701, LM4730, LM4752, Etc. Anyway, if you decide to go with IC power amplifier you might as well build something with higher power. Granted, it does add a little bit of extra cost but you get an amplifier that is a lot more versatile than a puny practice amp. You probably will be spending a considerable amount of time to this project regardles of whether you build a high or low power amplifier. Basically, I advice you to think whether that time is worth some extra money spent on the project. If you are going to build a discrete design then it's a different thing. Building a high power discrete power amplifier that performs well and will not oscillate to death is a tough job unless you build a kit offered by various sources.
« Last Edit: January 17, 2007, 07:55:30 PM by teemuk »

joecool85

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Re: Want to Make AMP...need HELP
« Reply #2 on: January 18, 2007, 07:00:36 AM »
Wow thats a lot of good info there. 

The most important thing (to me) is the power amp.  The power amp is what actually drives the speaker and makes sound.  Most of the amps we deal with here are "chip amps" meaning that the main part of the circuit is an IC.  For instance a LM3886, LM1875 or TDA2030.  The schematic shown on the datasheet is normally what we go by for our power amps.  If you want to look at some helpful information, check out the wiki at the top of the page here, click "contents" then click on a few of the chips (IE - LM3886 etc) and read.  After you get a decent basic idea and have more questions (or if you are so confused you don't get any of it) go ahead and ask us some more questions and we'll help from there.  Good luck, it's a fun and rewarding hobby.
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