Phoenix Gold Ti2500.1 Monoblock Amp Review

Part 2: Class D Monster Amplifier

In the first installment of this test, I related how Phoenix Gold was one of the golden brands in the heyday of car audio, then fell on harder times. Venerable car audio accessory maker AAMP of America bought the brand as a move beyond accessories, and came out with the Ti series in a bid to rejuvenate the Phoenix Gold image.

They're certainly on their way. Our testing found the Ti800.4 to exceed its power spec while running quietly with low distortion. Now, we turn our attention to its big brother, the Ti2500.1 monoblock. For $1099 retail, this Class D monster purports to pump out more power than a lot of pro sound amps do! Let’s see if it can hold up the laurels earned by the impressive 4-channel amp.

Beauty?: This amp matches the dark color scheme discussed in the previous part.



The Hook-Up: As with the Ti800.4, power, turn-on, and speaker leads all run into direct-in receptacles; three sizes of Allen keys come in the box to tighten hex bolts down onto the bare wire. Two solid input RCAs accept the signal from the head unit. A second set of RCAs allows cascading to another amplifier, useful when using multiple amplifiers. A single RCA with a master/slave switch allows two Ti2500.1's to strap together into an even more humongous monoblock. Compared to the same amps unstrapped, total power goes up just a decibel or two, but maximized into a two ohm load instead of one ohm. Two sets of speaker terminals are provided for convenience, since most users will connect at least two subs to this amp. One thing to watch out for: unlike its sister amp, the terminals on the Ti2500.1 are arranged - - next to + + instead of the more typical + - + -. Logical from an internal layout point of view, but I miswired the outputs initially. No harm would occur, but if you get no sound this could be one reason.



Crossin’ Over: The crossover setup follows the Ti800.4 as well, with minor differences. For instance, there is no FLAT setting, since this is a sub (or midbass) amp. The crossover frequencies adjust over a smaller range as befits the mission of this amp. Highpass or “subsonic filter” adjusts from 10 to 55 Hz; the lowpass from 30 Hz to 300 Hz. Should you need to bang your head some more, bass boost of up to 18 dB may be had (you can see the curve in Part One.

Construction: Like other Ti models, one the front panel three blue LEDs indicate power on, and a recess holds a protection light and a peak level indicators. The optional remote lowpass level control and optional remote monitoring display mentioned in Part One also connect to this amp. The construction features have been explained in Part One as well. Of course, being a mono amp and Class D, the guts differ in detail versus the Ti800.4. But the same theme of nicely laid out high quality parts holds on this model as well.



Power?: 2500 watts from a typical Class AB amplifier could suck 300 amps out of a 14.4V battery, so Phoenix Gold designed this amp with a Class D topology for greater efficiency. We’ll see just how efficient it is, but they recommend a 200 amp fuse, so weak alternators need not apply! But first, an important question:

If A Mosquito Buzzes Distantly In The Forest, Can You Hear It?: OK, probably not. What does that have to do with this amplifier review, you ask? Well, that necessitates a brief explanation regarding Class D operation versus Class AB. You can read a lot more about Classes A, B, AB, C, D, etc. on the internet, but for readers who don’t know about them let me give some basic explanations. Let’s confine ourselves to transistor amps for simplicity.

Amp Classes
Class A: The transistors amplify the input and apply the output to the speaker. They are on all the time, which means the amp constantly draws full power. Efficiency? Yuck, don’t ask.

Class B: In a Class B transistor amp, there is a positive set of transistors and a negative set of transistors. When the input signal is positive, the positive transistors amplify it and apply the output to the speaker. When the input signal is negative, the negative transistors amplify it and apply the output to the speaker. Each set of transistors is on only half the time, improving efficiency from yuck to medium. Unfortunately, nasty distortion results around the crossing between positive and negative. (This is because transistors have a kind of “dead zone” of about 0.7 volts before they turn on, leaving a hole in the music).

Class AB: To remove the zero-crossing distortion, the Class AB amp runs mostly like a Class B, except that both sets of transistors stay on a little bit all the time. This eliminates the “dead zone.” Most AB amps run just a little bit of Class A so that efficiency remains moderate. Some designs do run more Class A in an attempt to yield better sound. The downside is somewhat less efficiency and therefore more heat. I suspect the Ti800.4 runs more Class A than usual since it was warm even with no signal.

Class D: In Class A/B/AB, when the transistors are on they are rarely 100% on. Usually they are kind of partly on, amplifying the input, with some power flowing to the speaker and the rest of the power turning to heat inside the transistor. Class D runs a different way. The amplifier controller switches between the positive and negative transistors at a very high frequency. The audio signal modulates the switching, becoming superimposed on the switching frequency. It’s vaguely like a radio station-the audio rides on a much higher frequency carrier wave. Notice I did not use the word “digital”…

The "D" in Class D does NOT stand for “Digital”! Just because the transistors are switching on and off does not at all mean they are somehow ones and zeros like digital data. The switching actually is controlled by analog modulation, not digital at all. Even amps with direct digital inputs end up converting to analog inside, hidden away from your prying eyes. Well OK, there was an amp from Sharp and also the TacT Millenium which were truly digital throughout. Essentially giant D/A converters, they still end up converting from one digital form to another. So far, there is no pure PCM digital amp.

Class D Advantages: The basic Class D advantage is efficiency, because the transistors switch between full on and fully off, without sitting in-between. This manifests itself in different ways. You can either get more power from a given size amp, or shrink the amp size for a given power. You can suck less current from the alternator for a certain wattage amp, or you can get more watts to the speaker before breaking the alternator. Of course real designs still have losses and heat, but much less than Class AB.

Class D Disadvantages: Complexity aside, the carrier wave is the basic disadvantage for Class D. In some imaginary universe from an Einstein thought problem, positive and negative transistor sets are perfectly matched, switch perfectly 100% on and off in an absolutely instantaneous way, and thus the output with no music playing is zero. But in the real physical universe, some amount of switching noise exists. This switching noise exists right at the output power stage which connects to the speaker. Some feel this noise modulates the music, causing sound quality of all Class D amps to be poor. This is more likely due to the fact that early incarnations of any technology are simply not perfected yet. My longest experience with a Class D car amp was with an Alpine MRD-F752 which sounded quite nice. Nevertheless, many manufacturers put an output filter-basically a passive lowpass crossover-inside the amp, to cut down on how much switching noise reaches the speaker.

We Don’t Need No Stinkin’ Filter: It appears Phoenix Gold may have skipped this. That’s a valid choice as well, since the output filters reduce damping and can increase some distortions. If the additional 0.001 or so watts applied to your subwoofers are a problem…you need better subwoofers! Phoenix Gold told me the Ti2500.1 switches at 100 kHz, and indeed the Audio Precision I was using showed 105 millivolts at right around 100 kHz. That’s enough to mess up distortion and noise readings, as audio analyzers are not designed to have very high frequencies applied to their inputs, so I ordered an AUX-0025 filter from AP. Which reduced the noise down to…36 millivolts?!? A phone call to their ever helpful tech support established that the filter was not designed for this application. They assume a higher switching frequency, meaning Class D car sub amps are out of scope. Grrrr. What to do? In the end, I punted. I stuck a 12 dB lowpass crossover in line. It wasn’t textbook perfect, but reduced the switching noise to 2 millivolts. Since it was for a component set, and the filter resistance was under 0.1 ohms, it had no negative effect on the low frequencies.

Noise: Because of the switching frequency, a proper noise measurement was not possible. We ran a check of A-weighted signal-to-noise: -58 dBW. Wow, that’s terrible. Or is it? That’s 58 decibels below one watt. It is almost certainly all the 100 kHz switching frequency, into a driver crossed over to below 300 Hz tops. Concern level? Zero.

Frequency Response: The Ti2500.1 features the same impressive lack of low frequency rolloff as its little sister Ti800.4, with a response free of any ripple. Three sets of measurements appear in the graph. The first curve I ran with the crossovers set at the 10 and 300 Hz indications. For the third curve I set the lowpass to the halfway setting, which should be about 100 HZ (the minimum setting of 30 Hz would mix with the 10 Hz highpass and not show the rolloff shape clearly). The second curve was with the highpass at the maximum 55 Hz. You can see the passband becomes very narrow, so you could run some kind of specialized midbass but not a woofer. Phoenix Gold says the crossover slopes are 24 dB, which is consistent with the measurements. Now, sometimes a very low crossover frequency like 30 Hz lets the tuner essentially dial in a slowly rising low frequency response, when combined with the subwoofer rolloff and the vehicle cabin gain. But a 12 dB slope serves much better for that purpose, so in this case I wonder why the lowpass range isn’t, say, 60-600 Hz. Then it would be more useful for a woofer crossing over to a midrange.



At any rate, the amp appears to do what Phoenix says and does so smoothly. P.S. We measured this without the 12 dB filter, since that won’t affect these curves.

Distortion: We ran distortion tests at 3.162 volts RMS, which is 10 watts into 1 ohm. The results can be seen in this chart:



The results look a little odd. With the 12 dB filter, the 2 millivolts of 100 kHz leaking through should be equal to 0.0002% THD+N. But, it’s still possible the test setup affected the results somehow. Or, this amp, being Class D, is really less clean than a Class AB. It doesn’t matter either way-levels of 0.1% simply won’t be audible with bass cranking into your subs.

Feel The POWER: And crank you shall! Ever wonder why the tablets Moses brought down from the mountain don’t exist any more? It’s because some crazy guy let this amp loose on his subwoofers and the bass pulverized the tablets. OK, car stereo didn’t exist back then, but if it had, let’s imagine it could have happened like that. This little chart shows the power output several ways, as explained below.



Watts: is the output power into the load.

dBW: this is also watts, but converted into decibels.
· A 1 dB volume change is barely noticeable, and requires 25% more power.
· A 3 dB volume change seems like a mild increase, and requires double the power.
· A 10 dB change seems “twice as loud” but requires 10 times as much power.

Power droop: how much the power drops in decibels as the load gets tougher. A perfect amp would keep doubling the output power as the load dropped from 8 to 4 to 2 to 1 ohm. This would be a 3 dB increase for each time the load drops to half the impedance. By subtracting the power output in dBW, we can see how much the amp output power “droops” compared to the reference. In this case, we used 4 ohms as a reference, representing an easy load for a 1 ohm rated amp. This graph puts the power on a logarithmic axis, like decibels. A perfect amp would show a straight line, and the Ti2500.1 looks really good.



Efficiency: How much of the power taken from the vehicle actually get passed to the speaker. Here we see why Class D is slowly taking over all of car audio. Instead of the 50% or so of Class AB amps, here we run around 80% near full power. The Ti2500.1 also ran completely cool during all these tests, quite a contrast to the Class AB Ti800.4 which was always uncomfortably warm to the touch. That’s what Class D gets you.

Power?!: As for the 2500 watts implied by the model number, where are they? As noted in Part One [hyperlink], the big number refers to dynamic power for brief musical peaks, which we are not set up to measure at this time. 2500 watts into 1 ohm dynamic would require 50 volts, and we saw 47.9 volts into 4 ohms continuously. It’s reasonable to assume 2500 watts should be possible for brief peaks. And for the eagle-eyed amongst you perusing the manual and noting we measure 1610 watts at 1 ohm instead of the 1650 watts Phoenix says, that’s measured at 100 Hz. We made a check at 100 Hz, and cranked out 1717 watts, exceeding PG’s spec. By the way, at that point 146 amperes of current flowed out of the supply, so 2500 watt peak capability does indicate a 200 amp fusing as the manual suggests. You’ll want minimized runs of big cable for both the amp and speakers.

Loading Down Your Amp: In the Ti800.4 review I basically said loading down your amps to real low impedances is often a waste. Well, the Ti2500.1 is a bit of an exception. The power does droop into 1 ohm, and the efficiency becomes somewhat less as well. But the drops are not so much, implying a very robust design, and indicating the amp was designed for 1 ohm operation from the get-go.

Conclusion
So, is Phoenix rising?: We'd have to say so. This Beast sure has enough power to start a fire! The amp looks nice and clean, the measurements are good even with less-than ideal testing filters, and power abounds. Thanks, AAMP of America, for breathing some life back into this brand.

Coda: “But hey! How do the amps SOUND?!?” you may be asking. Patience, grasshopper, patience. Due to some craziness going on, we haven’t had a chance to find the right vehicle to put these in. That would be someone with roughly comparable amps who could render a good opinion. Stay tuned—if we find a proper comparison we’ll let ya know - Eric Guarin