Also added gate resistors (Rg) to both MOSFETs in figures 2 and 3. With some brands of MOSFETs (such as those by International Rectifier), the gate resistors help to keep the amplifier from oscillating. They should be installed as close to the MOSFET gate leads as possible.

7/7/2000: The Harris RFP15N05 MOSFET has been discontinued. The Intersil HUF75309P3 N-channel MOSFET is reported to be a good substitute.

7/14/2000: Because different brands of MOSFETs can have different biasing requirements, the author has incorporated an adjustable bias scheme (Vbe multiplier) in the amplifier's output stage, so that DIYers can can set the MOSFET idle current via a trimmer pot. The amplifier schematic in figure 1 has the MOSFET gates biased at 8V. The Vbe multiplier replaces D3 to D6. With the 20K pot shown, the multiplier can provide a bias voltage range of about 0 to 12V. A 15K pot would have a range of about 0 to 10V. The NPN transistor can be any general purpose type: 2N2222, 2N3904, 2N4401, etc. The author writes:

A 15K pot (or 20K) will give a max of 16 diode drops. For 8 volts across the FETs (just conducting), you'll need 13 diode drops. To set the bias, I'd turn the pot so that it is as low a resistance as possible. Then with a voltmeter across one of the two 10 ohm resistors hooked to the sources of the FET's, I'd slowly turn the pot up until I read 0.6 - 0.8 volts across the resistor for an idling current of 60 - 80mA. I'm planning to build another amp for my office, and I am going to try out different FETs. I'll also add the Vbe multiplier to the PC layout.

5/22/2002: Sijosae (from the forums) built this mini version of the SDS amplifier, which he uses with his Philips HP-890 headphones. The wood frame of the chassis has been covered with a natural wood veneer and varnished. The power supply is a separate 24V commercial unit. The SDS amp is also powerful enough to drive very efficient DIY speakers, also shown below.

The speakers are 8" full-range drivers mounted on large 5-gallon plastic water bottles. Sijosae used a jig-saw to cut off the neck (to mount the speaker) and the bottom of each bottle. The interior of the speaker is stuffed with a roll of plastic foam wrapped in felt. The back of the speaker remains open.

The speakers are located on top of a bookshelf. A small brick placed inside each bottle adds mass to the enclosure and keeps the speakers from rolling away. The speakers cables are made from LAN cables and electrical clips. He says "The sound of the speakers is good. But with my system, headphone sound is much better. I think all DIYers love their own work. Therefore in spite of many flaws in my speaker, I love sound of mine."

5/22/2002: Algar_emi (from the forums) completed this SDS headphone amplifier. At first, the amp oscillated. He had omitted the 100-ohm gate resistors on the MOSFETs. When he put them in, the oscillation went away. Here are the actual performance measurements of his amp:

Output DC offset: +1mv and -2mv DC
Noise (Max volume, no input) (Vrms): 0.2mV
S/N (Max volume): 64dB
Crosstalk (Max Volume in 32ohms): 68dB
Gain Voltage: x2
Zin: 100K ohms
Max Vinput (for load 32 ohms): 2.88Vrms or 5.8dBm in 600 ohms
Pwr Out (load 32 ohms, max volume, Vin max 2.88V): 1W
THD: lower than 0.05%
Heatsink Temp at full power: 42 deg C
Supplies ripple noise (Full load): <2mV RMS

Return to Main Article

Questions or comments? Visit the HeadWize Discussion Forums.