output transistor gets hot, you’ve done something wrong.
Assuming no smoke, measure the output voltage to
ground. If it is not within 20 or 30 millivolts, there is a
problem. My breadboard circuit measures close to 0 mV.
If your voltmeter doesn’t go down that far, set it to the 20
volt or more range, then connect it between output and
ground. If the needle doesn’t move, switch to successively
lower ranges. On a one volt range, you might see the
needle move a little.
Assuming low voltage, connect an audio source to the
input. A CD or MP3 player is convenient. If you have a
scope, connect it to the output and adjust the volume
control for a few volts peak-to-peak output and note that
the peaks are not clipped.
Headsets come in a variety of impedances and
sensitivities. A search on the Internet found headsets with
impedances from 600 ohms all the way down to 24 ohms.
This amplifier is current-limited at the output. It should
not damage any headset due to high signal levels and the
output is protected against a short circuit. High-end
headset users worry a bit about the damping factor. (In an
audio system, the damping factor gives the ratio of the
rated impedance of the loudspeaker to the source
The amplifier has very low output impedance and
provides a high damping factor for any headset. If you
have a low impedance set, be aware that the volume
control must be turned to a low level. I’ve left the gain
QTY POSSIBLE SOURCE
1 Transformer 18 volts CT at one amp Marlin P. Jones
4 1N4001 rectifier diodes
2 1,000 µF 25 volts or more
Resistors all 1/4 watt 5% carbon film
5 2N5401 Tayda
4 2N5551 Tayda
2 BD139 BG Micro
2 22 at 25 or one 22 at 25 non-polar
1 220 at 25
Capacitor, ceramic NPO or silver mica
1 100 pF at 25 or more
1 Perfboard 0.1 in hole centers Jameco
2 TO-220 heatsinks
1 Dual ganged potentiometer,
50K linear (for two channels) Tayda
Stereo headphone jack, your choice to match your phones
Project box Jameco
2 RCA jacks for input
All Electronics www.allelectronics.com
Allied Electronics www.alliedelec.com
B&D Enterprises www.bdent.com
BG Micro www.bgmicro.com
Marlin P Jones www.mpja.com
Tayda Electronics www.taydaelectronics.com
32 September 2013
SUPPLIER DETAILS SUPPLIER DETAILS
All Electronics — Van Nuys, CA. Has resistor and
capacitor kits with a number of values.
Allied Electronics has a good selection of
B&D Enterprises has hard-to-find semiconductors.
BG Micro is in Garland, TX. They have surplus
capacitors, diodes, etc. The BD139s are available at
BG Micro for about 40 cents each.
Digi-Key is in Thief River Falls, MN. They seem to
Jameco, in the San Jose, CA area, tends to cater to
hobbyists. Vector board is available at Jameco.
Marlin P. Jones in Florida has a good selection of
transformers. They have one rated at one amp for
$4.95. Your local RadioShack may have one for about
the same price if you include shipping charges.
Mouser is more of an industrial supplier but has a
lot of good stuff.
Tayda has the 2N5551 and 2N5401 for very low
prices. Note this supplier is in Bangkok, Thailand but
the prices are very low and shipping is not costly if you
are not in a hurry. I receive orders from them in less
than 10 days. They have a limited selection of items.