Wanted an easy project to build an also had a need for a small headphone amp to use at work.
The amplifier powered by a 18V rechargable battery pack.
An op-amp sets it's output voltage to the voltage difference between the inputs times a open-loop gain A. This gain is very large so even small difference between the inputs make the op-amp deliver the supply voltage to the output.
Vs+ | \ V+ ---| \ | \____ out V- ---| / | / / | Vs-
To reduce the gain a negative feedback loop. That is portion of the output voltage is then led back to the inverting input (V-).
This is how it's set up in the CMoy and the inout is put on the non inverting inout pin (V+) making this a non-inverting amplifier.
Vs+ | \ V+ ---| \ | \____ out----+-- V- +---| / | | | / | | / | | | Vs- | | Rf | +-----------|====|--/ | - | | | | Rg | | - | GND
The gain of this amplifier is then G = 1 + Rf/Rg.
To get the supply voltage rails there is a simple voltage divider with a pair of filtering capacitors. The virtual ground is tapped between the two resistors and capacitors.
There are more advanced designs using active components to make a virtual ground that is always exactly between the postive and negative rail. More info about this here.
This simple one works well for me. If you want to use a single 9V battery to drive the amp and/or use low impedans headphones you should look further.
To power the amp with 18V I built a small rechargeable battery pack. It contains 2 NiMH 9V batteries and is constant current charging using LM317. Needs a 24V supply to charge.
It has a small switch to turn the output off.
Looks a little messy inside but it works.
The LM317 is a variable voltage regulator. In this device it's setup as a constant current regulator. The regulator have three pins, in, out and adjust. The regulator holds 1.25 V between the out and adjust pin. This is used to get the constant current.
_______ out in --| LM317 |--------\ ---|--- | | adjust - | | | | | | R | | | | - | | +------------/ | v i | Constant current i = 1.25 / R
There is a 3V voltage drop over the regulator (we'll some heat here, P = 3V * i). That's why we need some extra voltage on the supply compared to the batteries to charge.
I set R to charge the battery in 10 hours (C/10) which is a fairly moderate rate that doesn't require anything more advanced. I usually let it charge over night after a day of use. Seems to work well.
Original article by Chu Moy Tutorial by Tangent
After the CMoy I wanted to build something with a few more parts and also have a look at vacuum tubes and discrete transistors. I found a simple design using a 12AU7 tube and mosfets at DIY Audio Projects.
I built it point-to-point in a plastic bento box for extra frugality points.
This is a twin triode vaccum tube. Triode means it has a cathode (filament), andode (plate) and a grid. Twin just mean that there are two in one, i.e. two channels, so only one tube is needed for stereo.
The tube is set up as a inverting amplifier. The voltage supplied to the grid of the tube controls the current flowing through it. Since a resistor is connected between the plate and voltage source the potential at the plate varies with the current flowing through the tube. The resistor in this case is a trim potentiometer so you can adjust the plate voltage to be between the supply voltage and ground. Then the output can make the biggest swing peak to peak and we get a maximum of voltage gain without clipping.
If the plate is biased to close to the voltage source we get clipping very fast like above.
Correctly biased we can get more than 5x voltage gain.
The plate of the tube is connected to the gate of a Mosfet configured as a source follower with LM317 as constant current source (see the battery pack). We lose a little voltage in this stage (maybe 5%) but it can supply a lot more current than the tube.
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