Author Topic: Amplifier Schematic  (Read 1655 times)

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Offline Sabe

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Amplifier Schematic
« on: January 08, 2012, 07:06:31 am »
Need some help he guys, I need a schematic (wiring diagram whatever you want to call it) for an amplifier for two 8 ohm 2 W Speakers.
Thanks!


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Offline Foxpup

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Re: Amplifier Schematic
« Reply #1 on: January 08, 2012, 08:13:41 am »
Since decent simple amplifier circuits are strangely hard to find on the Internet (as you've probably already discovered), here's one I came up with off the top of my head even though I don't really know what I'm doing which ought to do the trick:
Code: [Select]

                  +-----------+-------+--------------------+
                  |           |       |                    |
                  |           |       C ][       _  /|     |
                  |         __|__     C ][ C----| |/ |     |
                  |         _____     C ][ C    | |  |     |
                  | 0.047uF   |       C ][ C----|_|\ |     |
                  |           |       C ][          \|     |
                  |           |       |   200-8 ohm        |
                  |           +-------+                    |
                  |           |       |                    |
               1k \      100k \       |                    |
                  /           /       |                    |
                  \           \      /                     |
                  |    | |    |    |/                      |
          +-------+----| |----+----|                       |
          |       |    | |         |\                      |
     470k \       |   +              >                  ___|___
          /       |    10uF           |                   ___
          \      /                    |                    |   6-12V
    | |   |    |/                     |                 ___|___
----| |---+----|                      |                   ___
I   | |        |\                     |                    |
N      +         >                    |                    |
P  10uF           |                   |                    |
U                 |                   |                    |
T                 |                   |                    |
------------------+-------------------+--------------------+

Obviously, you'll need two for stereo sound. Also, it probably won't sound very good, which is why I don't usually design amplifiers of the top of my head. :P (Hopefully there's someone here who actually knows what they're doing who can help you)

EDIT: Reduced resistances, my initial guesstimates were (probably) too high. Well, I did say I didn't know what I was doing.
« Last Edit: January 09, 2012, 12:30:16 am by Foxpup »
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Offline Kobuk

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Re: Amplifier Schematic
« Reply #2 on: January 08, 2012, 10:23:52 am »
Moved to Furry Tech Talk forum from the GNFD forum since the topic seems better suited here. ;)
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Offline McMajik

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Re: Amplifier Schematic
« Reply #3 on: January 08, 2012, 05:05:44 pm »
Unfortunately, I only know anything about valve amps really, and they tend to get expensive :p

Offline redyoshi49q

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Re: Amplifier Schematic
« Reply #4 on: January 08, 2012, 09:55:42 pm »
If you're able to use op amps in your circuit, I might be able to help.  I got force fed some electrical engineering stuff at school (and looked at the circuits on this page for a refresher).  (*edit: I cite that page as my source, *not* as a good page to learn op amps from.  The material, to me, is more confusing than it needs to be.  The diagrams were what I primarily used from that page; I used the text much less.*)

Below is a non-inverting amplifier circuit built using op amps that I pulled from that page, with voltage equations written in:



The part in red, if you can't read it (I don't have a tablet), is this:

Vout = Vin * R2/(R1 + R2) * (R3 + R4)/R3

To multiply the input voltage by a desired scalar, just choose resistor values for R1 through R4 such that the ending constant in the equation above is the scalar you want.  For example, to double the inputted signal, you could make R1 through R3 equal in value and R4 three times the value of the other resistors (this is most certainly not a unique solution).

Also, I should note that given the rating of the speakers, you will want to keep the maximum (absolute) value of Vout below 4V.  Anything more than that will give the speakers more power than they're able to handle.  I'm drawing a blank right now on how to build a circuit that would output +/- 4V if the input's magnitude was greater than 4V and output the input otherwise.  If you really need this, maybe somebody else will be able to help you?

The circuit I drew above might work.  I only did somewhat well in my EE classes (I'm a CE major, and more biased in favor of CS material), and since electrical circuits is not my area of expertise, there are almost certainly important considerations that I haven't accounted for.  Whatever way you decide to tackle your conundrum, I wish you luck!
« Last Edit: January 08, 2012, 10:05:46 pm by redyoshi49q »
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Offline Foxpup

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Re: Amplifier Schematic
« Reply #5 on: January 09, 2012, 12:38:08 am »
Also, I should note that given the rating of the speakers, you will want to keep the maximum (absolute) value of Vout below 4V.  Anything more than that will give the speakers more power than they're able to handle.  I'm drawing a blank right now on how to build a circuit that would output +/- 4V if the input's magnitude was greater than 4V and output the input otherwise.  If you really need this, maybe somebody else will be able to help you?

I don't know much about op-amps, but I'm fairly certain the output voltage will never exceed the power supply voltage under any circumstances. :P In my above design, with the transistors at saturation (input voltage in excess of 900mV), the full 12V will be pulled through the 200 ohm transformer primary, drawing 0.72 watts (which is less than the full 2 watts the speakers are rated for, but my design is limited by the fact that the transistors tend to explode when you put more than 100mA through them). Line level is 1V @ 0dbV, so that works out just fine. Of course, the transistors will blow up if the input voltage exceeds 5 volts, so don't do that.
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Offline Avan

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Re: Amplifier Schematic
« Reply #6 on: January 10, 2012, 12:28:33 pm »
I don't work with speakers, so I'd have no idea what the specific criteria would be without googling, and I'm too lazy to do that. I do recall you will need a current-limiting resistor, as under certain circumstances in their normal operation, speakers can act almost like shorts (well, more like saturated inductors, which have some parasitic capacitance and resistance, but its low enough that it will allow too much current to pass through in most circuits)

HOWEVER, I do know about op-amps.

Foxpup is right; the output voltage will never exceed the powersupply rail voltages. In fact, because op-amps are semiconductor/ICs, they experience a fixed voltage drop (Actually dependent on multiple factors, ranging from semiconductor choice to temperature, but its 'fixed' in that its ideally independent of the actual properties of the circuit) (where the power supply voltage is greater than the drop, or a drop equal to the power supply voltage when it is less than the maximum drop) on the output from the powersupply inputs. So if the negative PSU rail was -5V, and the positive PSU rail was +10V, the maximum outputs might be something like -4V to +9V. if you had +.5V and -.5V on the power supply, you'd end up with an output range of 0V to 0V.

Also, its important to note that the op-amp has a maximum voltage rating on the power supply rails, for example, +-18V. You'll want to check the datasheet for the specific model of op-amp you have, or else you might fry it.
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Offline McMajik

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Re: Amplifier Schematic
« Reply #7 on: January 10, 2012, 05:09:34 pm »
I don't work with speakers, so I'd have no idea what the specific criteria would be without googling, and I'm too lazy to do that. I do recall you will need a current-limiting resistor, as under certain circumstances in their normal operation, speakers can act almost like shorts (well, more like saturated inductors, which have some parasitic capacitance and resistance, but its low enough that it will allow too much current to pass through in most circuits)

Speakers typically have between 4 and 16 ohms of resistance