by Greg J. Szekeres
Editor: The circuit described in this article is a MOSFET follower for driving headphones. FET followers can supply high current, but have a voltage gain of slightly less than unity. They are most suitable in applications where the input signal does not require voltage amplification – such as the output of a preamplifier or a portable stereo. If the input signal is too low, you can add voltage gain stage (see Designing an OpAmp Headphone Amplifier).
The class A headphone driver in figure 1 is ultra simple, except it does need a regulated supply and input and output capacitors. Most of the parts are not critical. I used a pot instead of the 100K bias resistors, so I could vary the output side of the MOSFET at 1/2 the supply. This maximizes the ouput potential of this driver. R1 limits the input current. The diodes are reversed-biased protection only. They short out spikes greater than 9 volts and negative 1 volt. In the original I used a 9 volt zener diode instead, but I figured it’s harder to get zeners. The circuit will work without the diodes.
The volume control, Rp, is optional, because the driver is meant to be fed from a source with a volume control. If the volume control is not used, then it would be a good idea to put a discharging resistor in front of the input coupling capacitor C1 to ground – around 100K ohms. Sometimes the preamp is capacitor outputted, but may not have a discharging resistor. So, in the worst case, there would be capacitors back to back, and the voltage bias in between the capacitors would not be necessarily predictable (e.g., at or near zero volts to ground), or could cause some pops when connecting with the power on. A volume control accomplishes the same thing with its resistance to ground.
All capacitors should be the highest quality, especially the output capacitor. The output capacitor is an easier-to-get size capacitor of 470 microFarads, but one could at least double that for better low response. Bypass it with a quality 1 microFarad polypropylene cap for the best sound. I included the discharge resistor R5 to help eliminate pops when inserting the headphone plug. The input resistor would do the same if the input was inserted live. It might also help eliminate turn-on thumps.
The MOSFET (Q1) should be mounted on a heat sink. Almost any heatsink could be used. Q1 dissipates up to 2.5 watts. I think about 2-3 square inches surface area is minimum, and probably should be more. I just mounted them to the chassis with insulators in my model. A LITTLE silicone grease is nice. The MOSFET and R4 will get warm during operation, which is normal. With R4 = 20 ohms, the source resistors and MOSFETS will dissipate up to 7 watts in the stereo pair using a 15 volt supply. R4 could be changed to even lower values – its simply a matter of delivering enough current out of the supply and providing enough heat dissipation.
The IRF513 is an N-channel power MOSFET, rated at V(ds) = 80V, I(d) = 4.9A and r(ds) = 0.74 ohms. I chose a device with a reasonably small resistance.
A higher resistance, lower-rated device could also be used. There is a large selection from which to choose. Q1 draws less than 200 ma. at 9 volts. One thing I didn’t try was to use two circuits per side, so one could use direct coupling on the output side by feeding the signal to one side, and using the other side to provide a positive but equal resting voltage creating an effective zero-DC operating point. I wanted something simple though.
A 15V supply could also be used, but then the power rating of R4 should be increased a little. [Editor: Radio Shack sells a pre-assembled 13.8V regulated supply (RS 22-504). If this supply is used, make sure the MOSFET heatsinks can dissipate 5W.] C3 (usually a ceramic type) decouples the power supply from the circuit.
I put the amp in a Radio Shack box with vents (about 5″ x4″ x2″). Since the boxes sold change so often, I don’t know if Radio Shack still carries them. The binding posts on the front were used as auxillary drive amp for experiments. There is a 1/4 stereo phone jack on the front and RCA input jacks hidden on rear. The power supply is built-in.
This headphone driver design is simple, but it sounds pretty good to me. Using a pair of Grado SR60s, I compared the MOSFET amp to the built-in integrated phone amp in my modified Hitachi HCA 8300 preamp. The MOSFET amp was fed by the Hitachi preamp outputs. There was an immediate noticeable difference in sound. The MOSFET had a cleaner overall sound, not as muddy. Connecting the MOSFET amp directly to my Sony CD player, it sounded good and had more than adequate volume.
c. 2002 Greg J. Szekeres.