Designing An Opamp Headphone Amplifier


Purists prize class A amplifiers as capable of reproducing audio signals with the ultimate fidelity, because the output voltage swing is under the control of a single transistor or tube. Class A amplifiers are inefficient, consuming up to 400% more power than they output, but are enjoying revived popularity for their simple topologies (especially single-ended class A amps). Whereas class A loudspeaker amps run hot enough to heat a room, headphone fans can indulge without guilt, since headphones require very little power.

The MOSFET source follower in figure 9 is a single-ended class A output stage. MOSFET followers (and their bipolar cousins, emitter followers) are current amplifiers, which have non-inverting unity (or slightly less than unity) gain. A voltage divider biases the MOSFET. The bias pot Rp adjusts the output voltage to 0V for DC coupling. An input coupling capacitor is necessary only if the previous stage has a DC offset.

The transistor can be any power MOSFET, so long as the voltage and current ratings are adequate. MOSFETs have the "soft" overload characteristics of vacuum tubes and are preferred in this type of application over bipolars. The gate resistor helps to stabilize the MOSFET, and the zener protects the gate from voltage transients. The VDS spec must be higher (at least twice) the idle voltage, which is usually about 1/2 the total supply voltage. Rs is a power resistor and determines the idle current. Starting with an idle current of about 100mA and -V = -15V, then Rs = 15/.1 = 150 ohms. The resistor´s power rating should be much greater than 15 * 0.1 = 1.5W (at least 3W to be safe). Also make sure that the MOSFET is heatsinked to dissipate a similar amount of power.

Instead of a resistor for Rs, a precision current source would improve linearity. Current sources are usually made with a transistor, but the version in figure 9 employs a LM117/317 floating regulator, which needs only one resistor to adjust current output from 10mA to 1.5A. The voltage differential between Vin and Vout (which is the 1.25V internal reference voltage) should be between 7 and 15V. At higher differentials, the current output starts dropping due to internal safe-area protection, in which case more than one current source can be paralleled for higher output. While not required, the output capacitor helps eliminate any instability. Again, heatsinking is recommended.

AC-Coupled Cathode Follower

Schematic for cathode follower output stage.

Figure 10

From a design point of view, tubes are less favored as output stages, because the output impedance is higher than can usually be achieved with transistors. Yet, there are many excellent headphone amps with tube outputs. The AC-coupled cathode follower in figure 10 (from Andrea Ciuffoli´s headphone amp project) achieves a relatively low output resistance of about 33 ohms by paralleling two sections of a dual triode. The cathode resistor is tapped to provide self-bias. Each section is biased at 26mA or 52mA total.

The output impedance of a single-tube cathode follower is calculated as: Zout = Rk / (1 + GmRk), where Gm is the tube´s transconductance and Rk is the total resistance of the cathode resistors. Therefore, when building a cathode follower, s e l e c t tubes with a high transconductance to get the lowest output impedance.
  Class B and AB Symmetric Emitter Followers
Schematics for Class B and AB output stages.
Figure 11

Keywords : Opamp, Operational Amplifier, Headphone, P-amp, Configuring, Opamps, For, Voltage, Gain
Writer : delon  |
27 Feb 2011 Mon   
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