Elliott Sound Products

Project 53

Audio Amp Output Power Limiter
Rod Elliott
(ESP)
Updated 13 Feb 2000

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Introduction

If you hire out audio equipment, or just don't want the kids to blow up your
speakers when you are not home, this is the project for you. It is a very simple
little project, but will protect the speakers from being overdriven. Any attempt
at overdrive will simply cut the amp gain back - the more overdrive, the more
the input signal is reduced.

This is a simple peak limiter - performance is quite respectable, and it can
be used with conventional amps using bipolar transistors, MOSFETs, valves, etc,
as well as BTL (Bridge Tied Load) amplifiers in car audio systems or for hi-fi.
It will work with any amplifier from about 10W up to the highest power you are
likely to encounter.

Description

The gain control element is a Light Dependent Resistor (LDR, aka
photoconductive cell). These are blessed with a few very useful features for our
purposes, one of which is low distortion even at quite high signal levels. Being
light activated, all we need is a LED to provide illumination when the preset
power level is reached.

Once this point is reached, a very small increase in amplifier output voltage
(and power) will cause the LED to provide much more light, reducing the
resistance of the LDR, and thus reducing the input voltage. The effect is to
keep the level more or less constant. This will prevent the amp from clipping
(although a small amount on transients is unavoidable), and increase the
apparent loudness because the signal is compressed.

The entire circuit can be built inside the amplifier chassis, or can be in a
small external box - For obvious reasons, I suggest the former, since it cannot
be defeated as readily. The circuit is simplicity itself, but some precautions
must be taken to ensure that the amplifier's output signal is not coupled back
to the input, as this will cause (potentially highly destructive)
oscillation.

Any oscillation may cause the instant destruction of
tweeters, and may damage the amplifier by overheating. Do not try to
simplify the mechanical construction unless you are sure of what you are
doing, and can test the results with an oscilloscope.

The mechanical (light pipe) assembly is simply a piece of clear plastic rod
(e.g. Perspex or similar). The LDR is glued to one end with clear glue, and the
LED fits into a hole drilled in the other end. Aluminium foil around the tube
provides an earthed protective barrier, preventing any of the output signal from
reaching the amplifier's input. Two are needed for stereo.

Figure 1 - The Light Pipe Assembly

It will be seen that this is identical to the light pipe used for the bass
guitar compressor, except a LED is used instead of a light bulb. We could use a
small filament lamp here, but the response is too slow for a protective circuit.

When the light pipe is completed, wrap the LDR end with aluminium foil, and
tightly twist a bare wire around the foil to make good contact. Tape the
assembly firmly so that nothing comes undone. This acts as a shield, and is
connected to the earth (ground) connection on the input connector. Make sure
that the foil does not short circuit the LDR leads, or you will get no signal at
all. Note that one of the LDR leads will be connected to ground anyway - it does
not matter which one.

The complete assembly must be totally shielded from light. I will leave the
exact method to the individual constructor, but you might consider heatshrink
tubing, a black 35mm film can, or anything else that comes to hand that is light
proof. If metal, it must be earthed along with the shield around the light pipe
- make sure that the LED leads are well insulated - a short to earth may damage
the amp, and will almost certainly do something unpleasant and/ or undesirable.

Figure 2 shows the circuit of the unit. A 10k resistor was selected for the
input, and although this is lower than I would like, many power amps have a
relatively low input impedance and too much signal would be lost. The LDR simply
shunts the signal to earth when it is illuminated. A single unit should control
both channels of the power amp as shown. If only one channel is needed, then
delete the components for "Right", including the associated light pipe. Use the
input circuit shown in Figure 3 to improve limiting by using a higher input
resistance.

IMPORTANT- If your amp is operating in bridge mode, the
terminal marked "Com" must go to earth - not one or the other of the
speaker leads. For automotive use (or any other single supply bridged
amplifier), you must use two completely separate circuits, since
the speakers operate with a quiescent voltage of about 7V above the
chassis (earth) voltage, and there is no usable common terminal
available.

Figure 2 - The Complete Limiter Circuit

The value of R3 must be selected based on the amplifier power. For a 100W
amp, a value of 1.8k is about right, but it is likely that a little
experimentation will be needed.  As a rough guide, the table below will be
helpful, and it is probable that the value from the table will be OK. The idea
is to limit the current through the LED to a sensible maximum.

Amp Power (8 ohms)

Peak Voltage

R3

20 W

17 V

820 R / 0.5 W

50 W

28 V

1.5 k / 1 W

100 W

40 V

1.8 k / 1 W

200 W

56 V

2.7 k / 2 W

500 W

90V

4.7 k / 2 W

Table 1

Amplifier power in Table 1 for an 8 Ohm load. All diodes are 1N4004 or
similar. The voltage rating for both capacitors should be 63V, and R2-L and R2-R
should be rated at at least 1W for any amp over 20W. VR1 should be a multi-turn
trimpot. High brightness LEDs will improve sensitivity and are recommended (but
not essential).

Amplifiers with more than 200W will need a
MJE340 transistor to get the voltage rating, which will be too high for a
BD139. The caps will also need to be higher voltage - 100V is
recommended.

LED current is set at a maximum of about 20mA by R3, however this should
never be reached for more than a few milliseconds - if that. In most cases, the
LED current will be zero, and there will only ever be just enough LED current to
reduce the LDR resistance sufficiently to lower the amp's input signal to retain
the maximum preset power level.

The transistor may need a heatsink, but if so, this will only need to be a
simple flag type affair for even the most powerful of amps. Remember to check
that the transistor and heatsink are not overheating while you are testing the
circuit operation. If you can't keep your finger on the transistor, it is too
hot!

The polarity of the connection to the power amp output does not matter (but
see Note 1, above),
since a bridge rectifier is used. Very little current is drawn from the power
amp output, and the whole circuit is self limiting, so it is not critical. When
complete, advance the volume until you figure that this is a loud as you want
the amp to be, and adjust the trimpot until the external LED just flashes. Use a
multi-turn trimpot, as the setting is quite touchy.  It could be made much
less so, but at the expense of circuit flexibility.

Now, if you try to drive the amp harder, the external LED shows that the
circuit is working, by flashing brighter, but the volume should remain quite
stable. This can be checked with an oscilloscope (ideally), but otherwise just
set it by ear. As more signal is driven into the amp, it may sound
louder, but this is only because the input signal is being compressed.

One of the great benefits of this circuit is that it will be fairly
unobtrusive even for hi-fi applications. Since the amplifier should never be
driven to the extreme, the circuit will never operate, so nothing is lost. On
those occasions when "someone" winds up the volume too far, then the limiter
will do its job, protecting the rest of your precious equipment. You might like
to label the external limit indicator LED as "Overload" or something similar.
This will frighten the timid user (others will naturally ignore it completely).

Update 13 Feb 2000
The circuit has been changed to incorporate C2,
as I found that with fast LDRs the distortion at low frequencies is excessive.
The connection to the base of the transistor was also changed to increase the
circuit gain and provide a better limiting action. This includes the reduction
of R4 from the original value to 10k. With this arrangement, I was able to set
the limiter on a 100W amp to about 80W, and could drive up to 10V RMS into the
amp without clipping.

I also found that the resistor power ratings are probably over the top. I
used 0.25W resistors, no heatsink and nothing got even slightly warm ....
however, I suggest that the power rating on R3 be 1W, since there is a
likelihood that with less sensitive LDRs (or standard LEDs) the average power
will increase. Drive the amp hard, and make sure the transistor does not get
hot. If it does, a heatsink is needed.

The LDR I used is an encapsulated unit with integral high brightness LED
(VTL5C4), and is extremely sensitive. These are usually hard to get (they are
made in the US by Perkin Elmer - formerly EG&G Vactec) and are not stocked
by most suppliers. Unless you need 100 or more, you will have a tough battle! As
a result, you will probably be limited to using my light pipe, and this will be
less efficient. Note that if the light-pipe is kept fairly short (say 15mm) and
a high brightness red or green LED is used, you will probably come fairly close
to the VTL5C4 for sensitivity.

The sensitivity of LDRs varies with wavelength, and it's not always easy to
get the information so you can decide which colour LED to use. Given the high
light output from even rather lowly white LEDs, these are not a bad choice. Some
LDRs have their most sensitive region centred around 550nm, which is a
green-yellow colour. If you can get enough light without excessive current, most
colour LEDs will work just fine. Blue is not a good choice though, as it's at
the high frequency end of the spectrum and most LDRs are not very sensitive to
blue light. The same applies to infra-red (IR) and ultra-violet (UV) LEDs - they
are too far from the optimum colour and won't work well.

For those who don't mind a bit of extra work, you can use an opamp buffer
after the circuit to drive the amp's input. This will allow R1 to be a higher
value, which will improve the unit's limiting action, but will not reduce the
amp's sensitivity. If you decide on this, make R1 (Left and Right) 47k, as shown
in Figure 3.

Figure 3 - Using A Buffer To Improve Sensitivity

The opamps can be whatever you want. Since they are acting as buffers only,
even the 4558 dual opamp will be quite adequate for PA or mobile disco work. For
hi-fi you might want to use something a little better, such as TL072 or NE5532
devices.

This circuit cannot protect equipment from distortion
caused by an overloaded preamp. I suggest that the gain of the preamp be
set so that no sensible input level can cause overload. This might require
that you modify the preamp's gain structure to ensure that clipping is not
possible.

It is vitally important that the (limited) amplifier
power is within the maximum continuous power rating of your
speakers. As the input level is increased, the average power also
increases. A 500W amplifier will cheerfully destroy 500W (peak) speakers,
even if the amplifier never clips. Please refer to the article Why
Do Tweeters Blow When Amplifiers Distort? for a full
explanation.

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Copyright Notice. This
article, including but not limited to all text and diagrams, is the
intellectual property of Rod Elliott, and is Copyright (c) 1999.
Reproduction or re-publication by any means whatsoever, whether
electronic, mechanical or electro- mechanical, is strictly prohibited
under International Copyright laws. The author (Rod Elliott) grants the
reader the right to use this information for personal use only, and
further allows that one (1) copy may be made for reference while
constructing the project. Commercial use is prohibited without express
written authorisation from Rod Elliott.

Updates:  Page Created and Copyright (c) 12 Feb 2000./
13 Feb 2000 - added Fig 3 and update text, modified Fig 2./ 13 Feb 2006 - page
reformat, added final note.