Since the 1.3 amps bias through the .47 ohm Source resistors provides at 0.6 volts already,
we have to divide that voltage down so that limiting occurs at a higher current. We do this
for Q5 with divider resistors R17 and R19, and for Q6 with R18 and R20. R21 and R22
allow us to also adjust the limit point based on some information from the output voltage.
Where do we want to limit? The amplifier with 24 supply rails should be good for a 50 watt
peak into 8 ohms, or about 2.5 amps. For a 4 ohm load we would want 5 amps, and for a 2
ohm load, we want 10 amps. Since we only have two output devices, we probably are best
off stopping there.
We will calculate the values as follows, starting with arbitrary values of 1 Kohm for R17 and
R19. At 10 amps peak, the voltage across R11 or R12 will be about 4.7 volts. R19 and
R21 will be chosen to divide that down to .6 volts, and we solve the formula
.6V/4.7V = R19/(R17+R19) Since R17 is already 1 Kohm, we simplify to
.127 = R19 / (1000 + R19) And we get R19 = 150 ohms
10 amps into a dead short is about 200 watts dissipation on an output transistor, which will
probably exceed its rating. R21 adds a “foldback” characteristic to this so that the current
value will be lower into a shorted output. To ensure against damage into dead shorts, we
want to limit the zero crossing current to about 5 amps, keeping the dissipation just this side
of destruction.
Current limiting gets a bad rap in general, but I think it’s a matter of where and how the
limits are set. If you build this amplifier, you are of course welcome to delete this portion of
the circuit, and of course you will not complain if a shorted output lets the smoke out of your
amplifier. As it is, you will still be taking some chances with a shorted output.
Part Selection
The input JFETs used are 2SK170 or 2SK370 for the N channel parts (Q1), and 2SJ74 or
2SJ108 for the P channel parts (Q2). In these cases the Idss selection code is BL, although
V and GR types will also generally work fine. The primary thing about these particular parts
is the transconductance figure of 20 mS – many of the potential substitutes are much lower
at 4 to 10 mS.
The output MOSFETs are the IRF240 N channel type (Q3) and the IRD9240 P channel type
(Q4). They will want to have a voltage rating in excess of 50 volts, a current rating of about
15 to 20 amps, and a dissipation of about 150 watts. Comparable parts are widely
available, and here I used Fairchild FQA12P20 and FQA19N20C.
The remaining parts can be generic or special as you see fit. I used Dale RN55D type ¼
watt resistors and the 3 watt Panasonic power resistors from Digikey, where you can also
get the 4.7 Kohm thermistors and trim pots.
Audiophiles are often concerned about the effects of capacitors in audio circuits, but a quick
examination of the schematic will put your concerns to rest.
Power Supply
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