Loudspeaker protection circuit De Luxe

Loudspeaker protection circuit De Luxe

Posted by: Alexis

On: 23 Nop, 2020

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Category: Circuit

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PCB 152x91mm 70µ Cu 2 DK Fr4
Thermal dynamic fan control 12 - 24V via NTC
Opto coupler
Nf in max level monitoring
Nf in mute
LS monitoring
Voltage monitoring, amplifier
Switches off when the current is too ...

PCB 152x91mm 70µ Cu 2 DK Fr4
Thermal dynamic fan control 12 - 24V via NTC
Opto coupler
Nf in max level monitoring
Nf in mute
LS monitoring
Voltage monitoring, amplifier
Switches off when the current is too high, adjustable
Shuts down if the heat sink temperature is too high.
3 Power relay unit 3x16A (223 mA current consumption 12V)
Functions can be selected and combined using dip switches
Switch-on delay

loudspeaker protection circuit de luxe

Professional protective measures are necessary so that your audio power amplifier does not dissolve in smoke due to incorrect operation. Power amplifier concepts Stand and fall with the power amplifiers and loudspeaker protection circuit.

The most common mistakes that happen again and again are: Short circuit in the cabling. Output stage is called or is overloaded. And so on.

The power amplifier can then quickly say goodbye and maybe the speakers too.

The printed circuit board is correspondingly impressively large, but also necessary for all the solutions.

To ensure that all measures work properly and properly, other areas can be switched off using a dip switch.

In some cases, connections must be made at certain points in the output stage so that the unit can also evaluate important components of the output stage.

The easiest way to integrate is by showing all functions, including examples and background knowledge of the individual components, one after the other and explaining some of them with sketches.

The switch-on delay against switch-on noises

We start with the usual switch-on delay.

The NE555 serves as the heart of the loudspeaker protection circuit. via two comparators in which the threshold value is set via a resistor network. The delay time is set to approx. 3 seconds via C11 and R30. Pin 2 tri. Is positively charged. As soon as the positive charge has fallen below a certain level, the status is set to 0 so that the relays drop out. All faults run together on pin 2. Q2 Controls a total of 4 relays. Led2 Indicates the state D7 protects the transistor from the reverse voltage.

the following picture shows the relay unit

loudspeaker protection circuit 2

loudspeaker protection circuit 3

temperature overlord

The loudspeaker load relays are connected in parallel so that even heavy loads can be switched.

The line from the output stage is connected to x11 1-2

x11 3-4 Connects to the speaker jack.

K is responsible for muting the input. The audio signal is connected to X8-1 and from x8-3 it goes to the output stage x8-2 is GND for the audio line and has no connection to ground GND to prevent hum loops Common!

Thus, in the event of an incorrect evaluation, the amplifier is decoupled from the input and from the loudspeaker.

Power supply monitoring

The symmetrical transformer voltage is controlled via X5. The transformer voltage is queried via X5-1, X5-2 or D1 and D2. At X5-3 you make sure the center tap at S1. The voltage is added up via diodes D1 and D2 and the adjustment can be carried out with the aid of R10.

The circuit itself is quite simple. A voltage of approx. 1.09V is present at OK1. Thus the photo transistor conducts in OK1 and conducts the positive voltage to 0V. The following transistor is therefore non-conductive. If one of the two transformer voltages is missing, the voltage is no longer sufficient to make the OK1 conductive. As a result, the base of the transistor is made conductive via R13. Now the transistor becomes conductive and sets the control line to 0V. The threshold voltage on the comparator of the NE555 is then not reached. And the relays fall off.

For this unit to work, proceed as follows: S3 Switch on and everything else off! Put the transformer voltage of the output stage in AC! At. The LED of the NE555 should now light up. Unplug X5-1 and set the potentiometer so that the LED of the NE555 goes out. After it has gone out, reconnect the line. The LED must then light up again after 3 seconds. Then do the whole thing with X5-2, X5-3. The adjustment for checking the transformer voltage is then adjusted.


Temperature overlord

A remedy can also be found here so that your final stage does not die from heat.

Mount an NTC resistor on the heat sink. To do this, drill a small hole in the heat sink so that the NTC fits well into it. They prepare the NTC accordingly. Insulate the wiring and then glue the NTC in the hole. To do this, you can fill the hole with something conductive and seal the upper Tein with a silicone proppen. Alternatively, the NTC can also be glued directly into silicone. S4 switch on! Now heat the heat sink to 60 degrees and set R31 so that the LED of the NE555 goes out. Another maximum temperature can be selected as required.

If you do not have a temperature measuring device at your disposal, you can heat the heat sink until it is so hot that you can no longer place your palm on it permanently. Then calibrate R31.


Current monitoring of the output stage


Current monitoring is one of the most important tasks of protective equipment. Here it is defined how high the maximum current of the output stage may be before the output stage then switches off.

In order to be able to determine a meaningful value here, one should deal with the output stage transistors at this point. To do this, use the information on the data sheet.

In the drawing you can see a section of the output stage and the current detector. Connect each directly from the emitter of the NPN and PNP transistor. Turn on S1. Now you should consider how much current you want to use the power amplifier. To do this, follow the manufacturer's instructions.

Make sure that the power ratings of the power transistors are not exceeded! When planning, you should also think about whether the amp. Can still function sensibly with low resistance without breaking it. As soon as you have determined how much power the amp is allowed to deliver, the easiest way is to constantly generate the maximum power using a sine wave generator. Instead of a loudspeaker, hang a high load resistor at the output of the power amplifier. Select the lowest impedance that the amplifier should still convert. Now set the power to maximum. An oscilloscope is ideal. Also check the heat sink temperature. If the maximum power is in the limit range, set R27 so that the protection circuit responds.

Thus, the maximum work performance is limited.

In the event that one of the power transistors breaks, the voltage on the OK3 also rises and switches off the amplifier as well.


Line in and line out control


Another protective measure is the control of the entrances and exits

The maximum permitted level can be set via R 19 via Nf. Furthermore over LS again the maximum level.

Both signals are passed on via the OPs to OK2, which is activated via S2. Set the potentiometers so that the circuit responds as before when the maximum is reached.

The fan control

The fan control switches on depending on the temperature and is supposed to cool down the heat sink with a fan. Both 12V and 24V fans can be selected here. As already discussed, prepare the NTC in the same way and install it on the heat sink in the same way.

Use R7 to set the desired temperature from when the fan should start. The fan can start running at 30-35 degrees. If the temperature continues to rise. So the fan runs faster.

The speed can then be influenced via R8. Up to the maximum. As soon as the temperature drops again, the fan also runs slower and stops running again when the heat sink is cold enough again. LED1 Signals the status of the control.


Coupling of both channels

Since the protection circuit is set up for one channel, it is required twice for stereo operation. A coupling of both channels is not absolutely necessary for this. If both output stages are mounted on the same heat sink, the fan control is only required once. Theoretically, the overtemperature circuit is also required. So that the temperature monitoring does not have to be installed again for both channels, there is also the possibility of coupling both units, if that is also desired.

If you want to operate the amplifier as a bridge amplifier (monoblock), the coupling is definitely an advantage, so that all parts are equally protected.

The coupling takes place via X12, so both units are connected to one another.

So now everything is done.

Below is the complete circuit of the monitoring control as a complete circuit.

Since this is a safety device, it should be considered at this point whether all OK or ICs should be plugged into a socket or perhaps better soldered directly. The second option makes more sense for me. The large number of print terminals that are required for securing are impressive.

All of this may look like a lot at the beginning. But if you work your way through it should be possible to get everything working.

As you can see, all conceivable important signals are queried. It is complex, but this is the only way to secure an amp correctly against interference.

loudspeaker protection circuit 4

loudspeaker protection circuit 5

loudspeaker protection circuit 6

loudspeaker protection circuit 7

loudspeaker protection circuit 8

loudspeaker protection circuit 9

loudspeaker protection circuit 10

 

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