This is a 100 watt basic power amp that was designed
to be (relatively) easy to build at a reasonable cost. It has better
performance (read: musical quality) than the standard STK module amps
that are used in practically every mass market stereo receiver
manufactured today. When I originally built this thing, it was because I
needed a 100 WPC amp and didn't want to spend any money. So I designed
around parts I had in the shop.
The design is pretty much a standard one, and I'm
sure there are commercial units out there that are similar. To my
knowlwdge, it is not an exact copy of any commercial unit, nor am I
aware of any patents on the topology. To experienced builders: I realize
that many improvements and refinements can be made, but the idea was to
keep it simple, and should be do-able by anyone who can make a circuit
board and has the patience not to do a sloppy job.
The input stage is an LF351 op amp which provides
most of the open loop gain as well as stabilizes the quiescent dc
voltage. This feeds a level shift stage which references the voltage
swing to the (-) rail. The transconductance stage is a darlington, to
improve high-frerqency linearity. The 2SC2344 by itself has a rather
large collector-base capacitance which is voltage dependent. The MPSA42
presents this with a low-z and has a C(ob) of only a few pf that is
effectively swamped by the 33pF pole-splitting cap. The stage is
supplied by the 2SA1011 active load (current source) which is about 20
ma. The current to the stage is limited by the 2N3094 to about 70 ma
under worst case.
The output is a full complementary darlington with
paralleled outputs. Although you could "get away with" only one if only
8 ohm easy-to-drive loads are used, this is not recommended. The use of
parallel devices increases the ability to drive reactive loads (which
can pull a significant current while the voltage waveform crosses zero
and puts a high voltage and a high curent across the transistor
simultaneously), gives the amp a higher damping factor, and reduces the
maximum current each transistor has to supply to peaks (remember, the
gain of a power transistor drops as the current increases).
Compensation is two-pole and one zero. The op-amp's
pole and the pole generated by the 33pf cap and the 470 ohm bias
resistor of the MPSA42 dominate. (the 33pF gets multiplied by the stage
gain.) The 22 pf feedback capacitor provides lead compensation, and is
taken from the output of the tranconductance stage rather than the
output itself. In this way, the phase lag introduced by the output
transistors is not seen by the high-frequency feedback. This intorduces
a closed-loop pole which limits the high-frequency response. The two
compensation capacitors must be type 1 creamic (NPO) or silver mica -
with ZERO voltage coefficient.
The amp was designed to run 2 channels off a +/- 55
volt unregulated supply, reducing to +/- 48 volts under full load. It
used a 40-0-40 volt, 5 amp toroid transformer, a bridge rectifier, and
10,000 uf of filter cap per side. If a standard EI transformer is used,
a 6-amp rated unit should be used. With this power supply, it produces
100 watts continuous, both channels driven into 8 ohms resistive with no
clipping. Dynamic headroom is about a db and a half. For more headroom,
unloaded voltages to +/- 62 volts can be used with no circuit
By the way, the schematic is in Postscript.
With no modifications the amp will drive 4-ohm
speaker systems with no current limiting. The short-circuit current
limit is set to about 4.5 amps peak, which will handle conventional
speaker loads.(It will, of course, produce higher peak currents as the
output voltage swing approaches the rail.) If you are going to be
running some of those high-end speakers with impedance minima of half an
ohm, or that stay reactive throughout most of the audio band ( ie, 0.5
+j3.2 ohms) you will probably already own a better amp than this. If the
higher-power Motorola power transistors are used, it will drive a 2-ohm
resistive load without problems (except heat).
I have never heard any slew-induced distortion on
this amp with a CD player's band-limited (22KHz) signal. I suppose that
real high-end freaks could pick it to pieces by hitting it with a TTL
square wave mixed with a 19KHz stereo pilot tone and crank it up. I
guarantee that there will be spurs all over the spectrum, but who
listens to that?
Possible Modifications: (What if I want mo' power???)
The Toshiba output transistors (2SD424/2SB554 pair)
shoud not be used with supply voltages above +/-60 volts. If you plan on
cranking it up, use more in parallel or use the 250 watt Motorola pairs
(MJ15024/MJ15025). If very low impedances are expected, raise the bias
in the transconductance stage to give more base drive to the output
darlingtons or add another current gain stage. Higher-Beta (and faster)
power transistors can't handle reactive loads worth a crap. Don't
substitute high-fT parts unless you are sure they have adequate
The NE5532 op-amp can be used in the input stage. If
more than one are used off the +/-15 volt shunt regulators (balanced
ins, anti-slew Bessel filters, etc.) the 2.7K dropping resistors may
need to be reduced to say, 1.8K ohm to maintain regulation. The 2.7K
resistors will allow up to 4 LF351 type op amps off the regulator (I
used a quad 347 for balanced inputs to avoid hum in a DJ setup).
The output transistors and thermal compensator
(2SC1567) will need to be mounted on a common heat sink - a finned unit
measuring 5 in. high by 8 in. wide with 1.25 in fins should do nicely
for one channel. (They look nice if you make the sides of the case out
of them). Most normal applications won't require more cooling than this.
The reason the 2SC1567 was chosen for the output bias regulator is
because it is fully insulated - the ECG version will require additional
mounting hardware. TO-3 hardware for the outputs is cheap and easy to
The driver transistors and voltage amps
(2SC3344/2SA1011 pairs) will all require heatsinking as well. Individual
TO-220 heat sinks on the circuit board will suffice - the voltage amps
dissipate about 1.4 watts each. A common piece of 1/8 in. thick 1 in.
wide X 4in. long angle aluminum will suffice for all 4 on each channel,
but bear in mind that it must be oriented to take advantage of natural
convection, and the transistors must be insualted.
Keep the imput grounds separate from everything else,
and return them at ONE point. Failure to do so WILL result in high
distortion (5% or so), or even oscillation.
The output stage bias should be set to about 25
milliamps in the output transistors. This value takes a while to
stabilize, and you may have to monitor it over an hour or so during
initial setup. To measure it, measure the voltage across the emitter
resistor and use Ohm's law. This way, you can check the current sharing
in the parallel output transistors at the same time and change them if
there is a serious discrepancy. With parts of the same date code, they
should not be off by more than 10% after it has warmed up. Higher output
stage biases can be used, but it takes more care in setting it. If you
want an idle current of more than 50 milliamps per side, increase the
value of the emitter resistors.
DO NOT just plug something like this in! A seemingly
insignificant error can set your house on fire! (As well as blow out $30
worth of transistors in a microsecond.) A variac will work in theory,
but the amp may latch to the rail if the supply drops too low. I suggest
the use of a ballast resistor - a 60 to 100 watt light bulb in series
with the AC mains. You get a bright flash when the caps charge, and then
it goes (almost) out as the idling supply current reaches its nominal
low value. The amplifier will then work normally at low volumes. If the
amp draws too much current for whatever reason, the lightbulb will glow
brightly, increase resistance, and limit the power to the circuit.
Usually, there will either be a mis-wire (use your DMM) or oscillation
(will show up on a scope or RF power measuring device). If the bulb goes
dim-bright-dim-bright... then the amp is marginally stable and the
grounding layout should be checked. Compensation capacitor values may
need to be adjusted if any significant changes were made. Mine is stable
the way it is.
The schematic is in postcript, so it should just be
able to be printed out. The emitters of the transistors are labelled by
an "e". I was too lazy to put arrows on the transistor symbols - and
I've been using it that way for over a year now.
Trouble finding parts? MCM (1-800-543-4330) has all
the transistors. Total cost for a stereo version should be between $150
and $250, depending on what kind of bargains you can find on the case,
transformer, and heatsinks. If you have to pay "list" for everything, it
will likely cost about $1000 to build.
The information included herin is provided as-is,
with no warranties express or implied. No resposibility on the part of
the author is assumed for the technical accuracy of the information
given herein or the use or mis-use of said information.
The equipment described in this article was designed,
fabricated, and tested on my own personal time using my own personal
HERE to get the postscript circuit diagram.
HERE to get the pdf circuit diagram.