Designing An Opamp Headphone Amplifier

TUBE-BASED OPAMPS Unlike transistor amplifier design, tube amplifier design is more dependent on the electrical characteristics of the tubes themselves. Tube opamps attempt to bring the simplicity and higher preformance of amp-block design to tube audio. In audio applications, they can aspire to the same high performance as their solid state cousins and have the additional benefit of even-order distortion harmonics. There has been a revival of interest in these devices with the publication in recent years of several amp-block circuits, ranging from basic AC feedback amplifiers to tube-MOSFET hybrids all configurable with the familiar opamp feedback scheme.

Not widely available back in the heyday of glass audio, tube opamps are very hard to find today. The following circuits develop the tube amp-block concept with increasing complexity. They all have limited current output and may need an output buffer stage to comfortably drive headphones (see the section on output stages below). To adjust the closed-loop gain of any of these amp-blocks, simply add a feeback resistor from the output to the inverting input and an input resistor just as with solid state opamps. Some of these amp-blocks may prefer higher magnitudes of resistance than are typical of solid state opamps, so sample gain resistances are included. It´s a good idea to construct several of these at a time to have a handy supply for experimentation.

Figure 2

Eric Barbour´s "1+1 Cascade" amp-block is an AC feedback amplifier (figure 2), consisting of a common cathode gain stage and a cathode follower output stage. This amp-block has a single inverting input and limited open loop gain, but is entirely suitable as a front-end of a headphone amplifier. The open loop figures are less than spectacular: G ~ -50, Fh ~ 30 kHz, THD > 2%, Rout ~ 2K ohms. When configured for a closed loop gain of -10 (Rf = 100K ohms, Rin = 10K ohms), the situation changes dramatically: Fh > 100 kHz, Rout ~ 500 ohms and THD drops below 0.4%. Since the 12AX7 is a dual triode, this design uses only 1 tube per channel. 12AU7s can also be subsituted, but the open loop gain will be lower. Purists may want to add an inverting output stage for correct output phase.

Figure 3

Keywords : Opamp, Operational Amplifier, Headphone, P-amp, Configuring, Opamps, For, Voltage, Gain