This time, we are going to help you figure out how to set the limiters in your DSPs in order to protect your investment in Danley loudspeakers. Of course, you can use this approach to protect other speakers as well, but this is
Part of the problem is that there are many different brands of DSPs out there and there is no DSP police making them all agree. We will try to give you a solution that will apply to most every situation. Of course, the simple solution is to use our DNA series amps/processors, then you could ignore the rest of this Tech Corner!
There are 3 bits of information that you will need. 1) the power rating of your loudspeaker and the rated impedance, 2) the gain
of your amplifier expressed in dB, and 3) what convention is used by your limiter for expressing the limiting threshold, i.e. the level at which the limiter kicks in.
Let's start with the first bit of information, the loudspeaker rating. With Danley speakers, we give you 2 ratings; a continuous rating and a peak rating. It is beyond this Tech corner to fully explain the difference, but to be conservative, let's use the continuous rating. For now, let's assume that the speaker has a 1000 watt continuous rating and the rated impedance is 8 ohms. The limiter will be controlling the voltage that goes into the amplifier, so we really need to move from thinking about POWER
to thinking about VOLTAGE.
We need to know how many volts that 1000 watts represents. In other words, when the loudspeaker is dissipating 1000 watts of amplifier power, how many volts will the amplifier be producing? The formula for Power is P=V^2/R,
therefore V=√(P×R). So, on your calculator multiply P by R: 1000 X 8 = 8000. Then hit the Square root key followed by =. The answer should be 89.4 volts (approximately). Ok, this is a very important number to jot down someplace. Remember, what we are trying to do is end up with a system where the limiter kicks in and won't let the continuous voltage that the amplifier produces exceed 89.4 volts.
OK, are you all still with me? The next thing we want to do is to determine the gain
of your amplifier. Once again, we are going to be talking about voltage
gain, not power. Sometimes the gain is called out in the spec sheet. Other times it is not and only the sensitivity or drive level is called out. If the gain is called out, you are 2/3 of the way there, and you can skip the rest of this Tech Corner and wait for the conclusion next month! If the gain of your amplifier is not called out in the spec sheet, we need to slog on! OK. Let's assume that you chose an amplifier that is slightly larger than what you need… which is always a very good strategy. In our example, we have been talking about wanting 1000 watts into 8 ohms, or 89.4 volts. So let's say you chose an amplifier rated at 1500 watts into 8 ohms. According to our formula V=√(P×R) the amplifier is capable of delivering 109.5 volts into the 8 ohm load. Remember we don't need all of that 109.5 volts. We only need 89.4. Now look for the sensitivity rating. This is a rating of how many volts into the amplifier will result in full output. So let's say that your amplifier spec says it has a sensitivity rating of 3.2 volts for full output. Just to be clear, that means that 3.2 volts in will result in 109.5 volts out, into 8 ohms, with the volume control of the amp wide open.
The gain of an amplifier is normally expressed in dB, so the gain of your amp can be found using the following formula; gain in dB=20 ×log〖(output volts)/(input volts)〗, or dB=20 log 109.5/3.2. On your calculator, type 109.5 / 3.2 =, then log = then X 20 =. Your answer should be around 30.7 dB. This is the voltage gain
of your amplifier in dB. Whatever you put into the amplifier will come out around 30.7 dB higher. We want to set the limiter so the DSP will put out a voltage 30.7 dB below 89.4 volts. If you do the math, that turns out to be around 2.59 volts. Maybe this graphic will help:
We are out of space for this time, but you are 2/3 of the way there! Good Work! Next time we will look at the settings in the DSP. They can be a bit confusing so it deserves its own Tech Corner. Until next time!