Why Crown CTs Amplifiers Sound Better
Crowns new CTs series of power amplifiers are remarkable for their loud, punchy sound; sustained, tight bass; and superior protection against overheating and overload. This paper explains the design features behind those advantages. After reading this information, youll know why Crown CTs amps sound better than the competition.
CTs amplifiers can provide large amounts of power into various loads, such as 2, 4, 8 ohms and 70V. With most power amps, the maximum output power tends to drop as the load impedance increases. But with the CTs series, the power stays the same at almost any impedance.
For example, the Crown CTs 2000 is unusually adept at providing significant power levels into various loads. In dual mode, it delivers 1000 watts into 2/4/8 ohms and into a 70V line. In bridge-mono mono, it delivers 2000 watts into 4/8/16 ohms, 2000 watts into a 140V line, and 2000 watts into a 200V line.
The power charts below show power vs. load of all the CTs 2-channel amplifiers.
Before we can explain how CTs amplifiers can provide so much power into various loads, we need to give a little background on the "V-I plane," a simple graph that displays amplifier power capacity at a glance.
|Suppose you wire a load to the speaker terminals of a power amplifier. The amplifier will produce a certain amount of voltage across this load, which in turn produces a certain amount of current through that load. By Ohms law, the power provided to that load is voltage times current, which is power in watts.
Suppose an amplifier produces 100 volts peak across an 8 ohm load resistor. According to Ohms law, the current through that 8-ohm load is
Current = Voltage/Resistance = 100/8 = 12.5 amperes.
So 100 volts across 8 ohms produces 12.5 amps through the 8-ohm resistor.
At 50 volts, the current through an 8-ohm resistor is Current = V/R = 50/8 = 6.25 amps.
At 0 volts, the current throught the 8-ohm resistor is V/R = 0/8 = 0 amps.
Lets plot those three data points on a graph of current vs. voltage (Figure 1) and draw a line through them. This gives whats called the load line (the current vs. voltage curve) for an 8-ohm resistor.
Figure 1. Load line (I vs. V curve) for an 8-ohm resistor.
|Since voltage and current can be negative as well as positive, we need to show those values as well. The complete graph is shown in Figure 2.||
Figure 2. An 8-ohm load line including negative voltage and current.
|Similarly, you can calculate a load line for a 4-ohm resistor. At 0 volts, the current is 0 amps. At 50 volts, the current is V/R = 50/4 = 12.5 amps. At 100 volts, the current is V/R = 100/4 = 25 amps.
How about a 2-ohm resistor? At 0 volts, the current is 0 amps. At 50 volts, the current is V/R = 50/2 = 25 amps. At 100 volts, the current is 100/2 or 50 amps.
Lets put the 8-ohm, 4-ohm and 2-ohm load lines on the same graph. We get the family of curves shown in Figure 3.
Figure 3. Load lines for 2, 4, and 8 ohm resistors.
|Now lets take a closer look at the 2-ohm load line. Notice that at 100V, the amp is trying to put 50 amps through the load. But what if the amplifier can produce only 25 amps maximum? You get the V-I curve shown in Figure 4. It cuts off when the current produced by the voltage exceeds 25 amps (when the amplifier goes into current clipping).||
Figure 4. I vs. V at 2 ohms with a real-world amplifier (the current and voltage are limited to 25A and 100V respectively).
|Similarly, the voltage that the amplifier can produce is not infinite. It is limited by the voltage rails (the DC supply voltage) that powers the amplifier circuitry.
So the V-I plot of a real-world amp has boundaries or limits. The right- and left-side boundaries in Figure 4 are the amps rail voltage. The top- and bottom-boundaries are the maximum current the amplifier can deliver. Figure 5 is the same as Figure 4 with those boundaries added.
Figure 5. 2-ohm V-I plot with boundaries added.
|So far weve seen a V-I plot for a resistive load. What happens when we hook up an amp to a speaker, which is a reactive load? Remember that a speaker voice coil has some inductance, so it has some inductive reactance. Because of this reactance, the current will be a number of degrees out of phase with the voltage at certain frequencies (Figure 6).||
Figure 6. Current and voltage are out of phase at many frequencies in a reactive load, such as a loudspeaker.
|Notice in Figure 6 that sometimes the voltage is positive while the current is negative, and vice versa. If you make a V-I plot of Figure 5, you get the ellipse shown in Figure 7. Its like the Lissajous patterns youve seen on an oscilloscope.||
Figure 7. V-I plot of the current and voltage
|Different frequencies have different phase shifts. So each frequency results in an ellipse with a different shape, ranging from very narrow to almost circular. Some frequencies will have little or no phase shift, resulting in an upward-sloping load line. If you plot the ellipses and in-phase load lines of all the audio frequencies, you get a big fuzzy ball in the V-I plane (Figure 8). This fuzzy ball is skewed along a diagonal line because several frequencies have V-I plots that are diagonal load lines.||
Figure 8. V-I plot of current and voltage at all audio frequencies.
|As we said earlier, a real-world amplifier has limits on the maximum current and maximum voltage it can produce. So if we take the fuzzball of Figure 8 and add those limits, we get a typical V-I plot of an amplifier driving a loudspeaker (Figure 9). The V-I plot is measured at a single load impedance say, a loudspeaker that is 8 ohms nominal.||
Figure 9. V-I plot of current and voltage into a loudspeaker at all audio frequencies, limited by the power amps maximum output current and maximum output voltage.
The overall size of this V-I plot is set by the rail voltage on the left and right sides, and by the maximum current on the top and bottom. So, the higher the rail voltage, and the higher the maximum current, the bigger the V-I plot.
An amp with a large V-I plot sounds louder and punchier than an amp with a small V-I plot. In other words, an amp with a large V-I plot can produce higher peak voltage and current. High-voltage, high-current peaks sound louder, resulting in punchier transients. Thats part of the Crown sound.
The area of the V-I plot is directly related to the size of the amplifiers power stage. If you want to know at a glance how big an amplifiers power stage is, compare its V-I plot to those of other amplifiers.
Under certain conditions, Crown CTs amps can produce power levels far above the published specs. The specs shown in data sheets are very conservative. The data sheet power spec is the rated maximum power per channel with all channels driven with a sine wave.
But in the real world, not all channels might be playing the same program. You might be playing Disco music on channels 1 and 2, while channels 3 and 4 are inactive except for announcements. In that case, CTs amps can generate significantly higher power than the published spec. That Disco music will really kick.
Whats more, if you drive the amp with a signal having a short duty cycle (a short transient like a kick-drum hit), the peak output power is much higher than the specified sine-wave rating. You get all this extra power for no more money.
Lets show where this extra power is hidden in the V-I plane.
|Figure 10 shows a simplified V-I plot for a typical amp designed for constant-voltage applications. Its high-voltage and low-current. Note: The actual load impedance of a distributed speaker line, containing many speakers with step-down transformers in parallel, is about 4 to 8 ohms.||
Figure 10. V-I plot of constant-voltage amplifier.
|Figure 11 shows a V-I plot for a typical amp designed for 2/4/8 ohm loads. Its low voltage and high current.||
Figure 11. V-I plot of amp designed for low-Z loads.
|Figure 12 shows a V-I plot for a CTs amp, shown with the two preceeding V-I plot. The CTs V-I plot has enough voltage and current reserves to handle both high-Z and low-Z loads. Also, notice the shaded areas in the four corners of the V-I plot. Those areas represent power capabilities that the other two amplifier types dont have. CTs amps operate in those high-power areas when not all channels are driven, and when signals are short duration.||
Figure 12. V-I plot of CTs amp, shown with V-I plots of high-Z amp and low-Z amp.
High-Current Design In The CTs Series
High power is the result of high-voltage rails and high current capability. How do CTs amplifiers achieve such a high output current?
CTs 1200 amplifiers use six 250W output devices per channel, for a total of 1500W per channel. The competition uses eight 150W devices for a total of 1200W per channel. So the CTs amps can produce more watts of power. Note that those wattage ratings are taken at 25 deg. C.
Also, the output devices (power transistors) in CTs amps have metal cases. In competitive amps, the output devices have plastic cases. Metal cases permit device heat up to 200 deg. C, while plastic cases permit device heat up to 150 deg. C. Since Crown devices are not as limited by heat, they can pass higher currents without meltdown of the junctions. At 100 deg. C, a device with a plastic case can handle 60 watts; a device with a metal case can handle 142 watts!
If a device can handle a lot of power, it can handle a lot of current. Thats why the output transistors in CTs amps can produce the high current needed for low-impedance loads.
An amplifiers maximum output current is determined by the number of output transistors and their current capacity. So CTs amps, by design, can generate high current in almost any load. This helps the amp produce high-voltage, high-current transient peaks that sound amazingly punchy.
Supperior Power Supply
So far we explained the V-I plane, and we showed how the CTs circuit design (high rail voltage, high current capacity) results in bigger V-I plots than the competition. Bigger V-I plots equal louder sound.
Whats more, CTs amps can deliver rated power into almost any load, partly due to their power supply design. Lets explain.
1. CTs 2-channel amps have regulated (stiff) power supplies whose power does not sag with low-impedance loads. Thats because the power supplies themselves are low impedance, thanks to the large semiconductors, efficient switching and regulating control.
2. CTs amps use huge electrolytic filter capacitors to store energy received through the AC power cord. This stored energy helps the amp deliver sustained high power.
|Lets explain. AC power is not continuous, like DC is. Thats because power is voltage times current. Since the AC voltage is approximately a sine wave, twice per cycle the voltage is zero, and so is the power (Figure 13). The amplifier needs some energy storage to draw on during those brief periods of no power from the AC mains.||
Figure 13. Power vs. time of the AC mains.
|Amplifiers with small, cheap electrolytic capacitors have little energy storage. They also provide poor filtering, so the power supply will have a fairly high ripple voltage. If you drive such an amp with a tone burst, the level of the tone burst will drop over time, and will show some power-supply ripple (Figure 14).||
Figure 14. Tone-burst response of competitive amp. (Graph is simulated. Actual data will be coming in the future.)
|In contrast, Crown amps use large electrolytic capacitors with lots of energy storage. When you drive a Crown amp with a tone burst, the level of the burst stays constant over time, with very little ripple (Figure 15). Same for a sustained bass note. Thats why bass sounds so awesome through Crown amps.||
Figure 15. Tone-burst response of CTs amp. (Graph is simulated. Actual data will be coming in the future.)
Overheating and Clipping Prevention
CTs amplifiers, and some other Crown amps, have built-in signal-management circuitry that protects the amp from overheating and from clipping. This signal processor is called TLC or Thermal Level Control.
How does the TLC know how hot the output devices are? Its impractical to put a thermometer inside the output transistors to measure their junction temperature. In the CTs 600/1200/4200/8200, the junction temperature is simulated with a circuit called a JTS or Junction Temperature Simulator. It predicts how hot a transistor is, based on the power it is producing. In the CTs 2000/3000 which use BCA design, the heatsink temperature is measured.
In addition, another circuit senses the short-term average power in the power supply, and converts that to a DC voltage which controls the TLC circuit.
In summary, the TLC signal processor is controlled by
The Junction Temperature Simulation (JTS)
The Input/Output Comparator (IOC)
The power in the power supply.
|The TLC manages the output power of the amp. The TLC works by programming a window detector, which adjusts the allowable output signal. When the amplifier is cool, the window is wide open, allowing all signals within the V-I operating space to play at full level (Figure 16).||
Figure 16. When the amp is cool, the window of allowable output signal is wide.
|If the signals exceed the V-I space and overload (clip), a compressor/limiter will reduce the signal, causing audibly distortion-free amplification. The signal is kept inside the window.
When the amplifier gets hot, the window progressively closes to reduce the output signal to where the amplifier can run continuously without shutting down or distorting (Figure 17). The show goes on where lesser amplifiers would have shut down and waited to cool.
Figure 17. When the amp is hot, the window of allowable output signal is narrow.
Part of the TLC processing is compression. Some competitive amps use built-in compression that is controlled by the temperature of the amplifier. This scheme is unusable for live sound. Heres what happens:
1. The amp starts to overheat, so the compressor kicks in and makes the sound quieter.
2. To compensate, the mixer operator turns up the mixer level.
3. Due to the higher signal levels, the amp heats up again, adding more compression.
4. To compensate, the mixer operator turns up the mixer level, and so on.
During a pause in the show, the amplifier cools off, so it stops compressing, and the system goes into feedback!
In contrast, the compressor in Crown amps is driven by the signal not by the amplifier temperature. Temperature is used only to set the window of signal voltage and current limits.
The TLC compressor is normally driven by IOC overload outside the V-I space. If the signal is outside the temperature-set window, the compressor gently comes on to control the signal level and prevent overheating.
If the amp gets very hot, the window will start to come into the V-I space (Figure 17). The compressor keeps the signal in the window, which varies with temperature.
Amps without any compression shut off if they overheat. But with Crown amps, the show goes on.
|Some competitive amps use outmoded V-I limiters that dont do the job nearly as well as Crowns sophisticated JTS/TLC system.
Whats more, the competitions old-style V-I limiter results in a smaller V-I plot that is pinched in the middle (Figure 18).
Figure 18. V-I plot of competitive amplifier, showing pinching near 0 volts.
|Figure 19 shows an actual V-I plot of a Crown CTs 2000. A special signal was used to drive the amplifier (sawtooth waves of two different frequencies).
In the top-left and bottom-right corners, the plot is truncated diagonally because of the way the amplifier was measured.
Note that the CTs 2000 can produce over 40A at 160V peaks. By Ohms law, R = V/I or 160/40 = 4 ohms. This shows that 4 ohms is the best load for getting the most bang for your buck on these units. The power produced at 40A, 160V peaks is 40 x 160 or 6400 watts of unadvertised power! The average power would be 3200 watts.
Figure 19. V-I plot of a Crown CTs 2000 amplifier
|In contrast, Figure 20 shows the V-I plot of the nearest competitive amplifier. Note that the V-I plot is smaller because its peak voltage is not as high as in the CTs 2000. Smaller V-I plot equals smaller peak levels less punch.||
Figure 20. V-I plot of competitive amplifier.
Heres why Crown CTs amps sound better than the competition:
LOUDER SOUND due to a bigger V-I plot. That bigger V-I plot is a result of higher-voltage rails and high current capability. The high output current is due to well chosen output devices with high-temperature metal cases.
For the BCA versions, high-efficiency switch mode operation reduces the heat that contstrains high output levels.
SUSTAINED POWER on long-duration signals due to regulated, stiff power supplies with incredible energy storage (huge filter capacitors).
EXTRA HIDDEN POWER during high-level transients and during incomplete use of all the channels.
OVERHEATING AND OVERLOAD PROTECTION thanks to the sophisticated TLC circuitry which compresses the signal only when needed. Compression is controlled not by temperature, but by signal voltage. This keeps the loudness up during compression more than with competitive amps.