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Old 18th August 2014, 12:12 PM   #1
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Default Switch on/off protection for active speakers

Passive xovers just can't stand up to active ones, but obviously,active xovers create a number of design challenges.

One that I frequently encounter is that of switch off/on pops. After all, in an active system, an electronic crossover and power amps are integrated into one system. Ideally, the power amps should only switch on after the electronic xover has completely booted. On switch off, the power amp should switch off before the electronic xover switches off.

The need for an adequate switch on/off sequence is excesserbated when building systems without separate ground, that is, single supply systems. The reason is that the xover and the amplification stage are bound to have different internal virtual ground levels. Coupling caps can deal with this, but the charge/discharge pulses on switch off or on can be severe enough to kill drivers.

Therefore, I designed a simple circuit which:

-shall delay switching on the power amp untill the xover has booted.
-shall immediately switch off the power amp in case the power is abruptly removed.
- shall switch off the power amp in case the power supply gets below a certain level.
- shall charge the coupling caps to the respective internal virtual ground levels of the coupled devices before the power amp is switched on.

delay.jpg

The relay is of the double throw NO type, with one pole required for each channel, plus one for the amplifier mute circuit. Since I don't have LTSpice model for a relay, it is not in the schematic, but in words: the common terminal of the relay is connected to the coupling cap. In the open condition, it connects to the virtual earth of the xover, in the closed condition, it connects the output of the xover to the coupling cap.

It may not be necessary to disconnect the xover from the power amps by means of the relay. It is just a safety and a wise thing to do if the virtual ground levels of the two connected devices are very different.

The input of this circuit is fed from the power supply common to the xover and the power supply, from the left hand side. The electronic xover is supplied from the right hand side of the circuit. The 1000 uF cap should be large enough to enable the xover to function while the circuit has time to mute the power amp.

Please let me know if there are any questions, but I can ensure you it works flawlessly and is completely fail safe.

Last edited by vacuphile; 18th August 2014 at 12:51 PM.
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Old 18th August 2014, 01:04 PM   #2
AndrewT is offline AndrewT  Scotland
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What signal tells the circuit to power up?
What signal tells the circuit to start timing?
What is turned on by the relay contacts closing?
What signal turns off the power amps/protects the speakers?
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Old 18th August 2014, 02:02 PM   #3
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Hi Andrew,

Power up is by applying the supply voltage to the circuit, the power amp and the xover through D1.

As the circuit is powered up , the node between R3 and R4 goes without delay to 6/7*(V+ minus the diode drop). The node between R1 and R2 powers up to 6/7V+, but delayed, because C2 is charged through R1. Once the node between R1 and R2 goes above the node between R3 and R4, the opamp working as a comperator goes high, driving the relay.

The diode drop provides hysterisis to the comparator, but to keep sensitivity to slow loss of power, best use schottky.

The relay contacts closing will de-mute the power amp and will connect the outputs of the xover to the power amp, see #1.

If power is lost instantaneously, the relay looses its current and disconnects, muting the amp. If power is lost gradually, such as in the case of PS caps unloading, the node between R3 and R4 stays high longer than the node between R1 and R2, switching the comparator to low and the relay off, thus muting the amp.
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Old 19th August 2014, 06:40 AM   #4
AndrewT is offline AndrewT  Scotland
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Power ON and power OFF activates the circuit.
Relay acts on Power Amp mute/unmute.
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Old 20th August 2014, 12:12 PM   #5
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Andrew,

It is a bit more complicated than that.

1. The decive that is protected by this circuit has an external psu. In case this psu is plugged out, a sudden loss of power is experienced by the protected device. This is easy. However, when the psu is connected, but power is cut to the psu, there is a gradual drop of supply voltage to the device. This circuit mutes the power amp before the power supply drop threatens to power off the xover when the power amp is still powered through its capacitors (an idling classD amp can go a long way on reservoir caps).

2. it brings both ends of the coupling caps to the level of the respective virtual grounds before the power amp becomes active. It is not just pops. I fried the input circuit of a classD module before coming to this solution.
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Old 20th August 2014, 03:37 PM   #6
AndrewT is offline AndrewT  Scotland
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I asked for
Quote:
What signal tells the circuit to power up?
What signal tells the circuit to start timing?
What is turned on by the relay contacts closing?
What signal turns off the power amps/protects the speakers?
and you gave post3.
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Old 20th August 2014, 07:27 PM   #7
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Sorry, can't explain any better without explaining what a diode does or what a comperator is.
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