DC-Servo Vs. Speaker protection with relay

sebastian_s

Member
2006-09-29 10:05 am
Hi Tyimo,

I don't believe a servo can replace a proper protection circuit. If for example a faulty output device shortens the output to either main rail, the servo will not do much about that.

If you have a DC servo, it might be easy to do the following:
- Add a relay that, if closed, grounds the output terminal (so it is not in the signal path)
- Add a comparator that closes the relay in case the servo's integrator approaches the rail voltages, i.e. when the servo can no longer keep the offset at 0V.
That solution is easy to implement and will protect your speakers. It depends on the design of your amp (is the output current limited, does it have fuses in the main rails, etc.) whether it will work without causing too much damage to the amp.

Coming back to your question about sound quality: A servo which is not designed properly can degenerate the sound, and so can a relay in series with the output. But fuses in the main rails can well, so there is no panacea I guess.

Hope these ideas are helpful for you.

Sebastian
 
If you have a DC servo, then that implies you have a DC coupled amplifier.
If you have a DC coupled amplifier then I very strongly suggest you incorporate DC output detection and Speaker isolation on detection.

You can also incorporate input muting on detection.

I don't think sound quality is a criteria once the DC coupled amplifier decision has been made.
 
I don't think this is a valid "either-or" question. The DC servo is intended to minimize the DC offset at the output of the amp, during normal operation. Under fault conditions, its not going to be of much help.

The speaker protection circuit is intended to disconnect the speakers from the amp under fault conditions. It should do absolutely nothing during normal operation.

The two can work together when turning the amp on and off. That is, the relay ckt can be designed to not connect the speakers to the amp until the servo has minimized the output offset.

In a general sense, there may be situations when you need one or the other, or both, but they have fundamentally different purposes, so one can't take the place of the other.

regards,
Mike
 
The two can work together when turning the amp on and off. That is, the relay ckt can be designed to not connect the speakers to the amp until the servo has minimized the output offset.

In a general sense, there may be situations when you need one or the other, or both, but they have fundamentally different purposes, so one can't take the place of the other.

I agree this makes sense. Even a relay that grounds the speaker terminal can arc and weld under fault conditions. The relay is helpful at startup for a direct coupled amp with servo in preventing turn on transients to the speaker. You can set up the logic so that there is a delay before the relays engage and this can be trumped by a DC detection circuit so that the relays don't connect the speaker if there is a DC output. In this circuit I use Trench type fets to switch the power rails on/off to the output stages. They have a particularly low Rds on and are more suitable than Hex type or Planer stripe type fets for this purpose. Fuses with fault detection is a good idea too. If a fuse blows due to a short you would also want the amp to cut off. I have found that AC input detection for the power supply is useful in switching off the speakers (output stage) before any turn off transients might affect it.:cool: