JBL MPC - Manual (Page 9)

8
Loading an amp channel—Be careful not to overload the channel when combining low-
impedance and distributed loads on the same amplifier channel, because either load affects
the amount of power available for the other. The next two paragraphs explain how they load
the amplifier output.
Internal output signal flow—Each T-version amplifer output connects directly to the LOW
IMPEDANCE output terminals and to the primary of the audio output transformer. The
transformer secondary and its tap connect to the channel’s AUDIO TRANSFORMER
terminals labeled 0V, 70V, and 100V. As with all transformers, there is some insertion loss, so
slightly less power comes out of the transformer than goes into it from the amplifier circuitry. If
nothing is connected to the AUDIO TRANSFORMER outputs, the transformer will look like a
very high impedance to the amp output, and therefore virtually the entire power capacity of the
amp will be available to the low impedance load. If a lightly-loaded distributed line is connected
to the transformer outputs, the transformer will look like a somewhat lower impedance, and less
power will be available for the low-impedance load. The more heavily loaded the distributed
line is, the lower the impedance the transformer presents to the amp’s output circuitry.
Minimum impedance considerations—The minimum load allowable for an MPC amp
is 2 Ω per channel, whether low impedance, distributed line, or a combination of the two.
Therefore, if a 2 Ω load is connected to the LOW IMPEDANCE terminals, then no
additional power is available for the AUDIO TRANSFORMER outputs. Likewise, as the
distributed line’s power demand grows, the channel’s low-impedance capabilities diminish.
1) Computing maximum allowable distributed line loading with a known low-impedance load
The formula is:
MaxATP = [RatedLZP – ((2 × RatedLZP)/Impedance)]/2
MaxATP is the sum of the power taps of the speakers connected to the AUDIO TRANSFORMER output(s).
Rated LZP is the maximum rated power of the amplifier into a 2 Ω load.
Impedance is the load impedance connected to the LOW IMPEDANCE output.
Example: One channel of an MPC300T has a 4 Ω load connected to the LOW IMPEDANCE output.
What is the maximum power left available to drive a distributed line? The answer is:
MaxATP = [450 – ((2 × 450)/4)]/2 = 112 watts
Due to frequency-dependent impedance variations, “constant voltage” loudspeakers can sometimes draw
more power from the distributed line than their transformer tap labels indicate. Although driving too low an
impedance will not harm your MPC amplifier, it might activate its protection circuitry or cause less-than-
optimal performance. When in doubt about actual loudspeaker characteristics, we recommend you use a
slightly higher nominal load impedance on the LOW IMPEDANCE output and/or a slightly lower total power
demand (i.e., sum of all the power taps) on the AUDIO TRANSFORMER outputs than what the following
formulas suggest.
Computing channel loading
These two formulas can help you determine how to combine a low impedance and a distributed line on an amp channel
without overloading it. The first starts with a known low-impedance load and calculates the maximum power available for the
distributed line; the sum of the power taps on the speaker transformers should not exceed that amount. The second formula
starts with your knowing the total power draw on a distributed line and calculates the minimum load impedance, if any, that you
can connect to the LOW IMPEDANCE output terminals.