Modern analog power amps, designed to cover any amplification need. Today we are seeing continuous growth in the pro audio amplification sector, with more and more manufacturers around the world and, of course, a big expansion of companies from East Asia, which compete with very low prices thanks to their reduced manufacturing costs.
We are going to analyze three amps from this brand, each aimed at a different market in terms of price and features. They are: the L1800, the most versatile; the CM20.5, close to the L series in features but larger and heavier, though more powerful; and the R500, aimed at smaller power needs and a tighter budget.
INTER-M L 1800
The L1800 is a power amp from Inter-M’s L family, whose specs are very strong, placing it at a good level within the professional range, despite the power it delivers (which is very respectable at low impedances). Its rugged finish and compact 2U size make it a good choice for mid-range systems where you want quality sound without a big outlay.
Technical specs
This amp delivers an output per channel of 360 W at 8 ohms, 600 W at 4 ohms, and 900 W at 2 ohms, per channel and measured at 1 kHz. In bridge mode (bridging the two halves of the amp to get a single, more powerful output) we get 1200 W at 8 ohms and 1800 W at 4 ohms, also measured at 1 kHz.
The input impedance is greater than 20 kohms for both balanced and unbalanced signals. Input sensitivity is 3.5 dBu for standard power at 1 kHz and 4 ohms. And it has a voltage gain of 33 dB.
As a theoretical refresher, and for anyone who hasn’t read some of my previous power amp tests, voltage gain is the result of dividing the output voltage the amplifier delivers by the input voltage it receives; in other words, how many times it amplifies the signal fed into it.
Voltage Gain = Output Voltage / Input Voltage. And to convert that to dB, we multiply the log of this gain by 20. The signal-to-noise ratio is › 100 dB, measured at 1 kHz. The inputs of this amp can be XLR or balanced jack via a combo socket, and the outputs are binding posts that let you use spade terminals, banana plugs, or bare wire directly; and, of course, Speakon. The damping factor is greater than 400 from 10 Hz to 400 Hz (well above the other amps we tested).
Remember that damping factor is the amplifier’s ability to control the reactive energy of the speaker. (For more information, see my article on digital power amps in issue 66 of this magazine.) What you want in a quality power amp is for the damping factor to be as high as possible, since the movement we’re going to drive in the speakers is going to be very large and we don’t want their reactive energy to affect the original signal too much (always advisable above 100).
“It has excellent sound quality thanks to a frequency response of 20 Hz to 20 kHz with variations of only 0.1 dB. It also offers channel separation greater than 80 dB and a T.H.D. below 0.03% (fantastic) at 8 ohms, at 4 ohms, and in bridge mode.”
Harmonic distortion is a technical parameter used to describe the audio signal coming out of a system. Harmonic distortion occurs when the output signal of a system does not match the signal that went into it. This lack of linearity affects the shape of the waveform, because the equipment has introduced harmonics that were not in the input signal. In every audio system there is always a small amount of distortion, since no piece of equipment processes the signal without introducing some modification. Harmonic distortion does not always mean a loss of quality. In fact, the harmonic distortion some gear introduces gives it an unmistakable color, as with old vinyl records. Without this harmonic distortion, the sound would be too pure or cold. When talking about harmonic distortion, people normally mean what is called total harmonic distortion, which is precisely the amount of harmonics that the equipment adds that were not in the original signal.
This total harmonic distortion is measured by feeding in a 1 kHz tone and measuring the output signal. In equipment specs, total harmonic distortion usually appears given as a percentage. It is commonly indicated by the abbreviation THD (Total Harmonic Distortion). For example, THD 0.3 @ 1 kHz. Total harmonic distortion should never be above 1%. If it is, instead of enriching the signal, the distortion starts to corrupt it, and the resulting sound starts to stop resembling the original. You have to be careful! Exceeding the level of distortion the system can tolerate means the sound can be altered to the point where it is different from the original or ends up “broken.”
It also has an HPF selector that lets us set a high-pass filter at 30 Hz or 50 Hz to protect any connected speakers that can’t handle such low frequencies, which could damage them.
It also uses two Class H power supplies (only on this model and the L2400 in the L range), which reduces AC current draw and cooling requirements.
Pros and Cons
The biggest difference between this amp and the others we’ll analyze is that, on the one hand, it lets us work at 2 ohms in stereo and 4 ohms in bridge, and I find the power response it gives us in this mode very interesting. Its greatest virtue: it’s an amplifier with very good value for money, since it offers a very clean and powerful sound (given the specs above) at a very reasonable price.
INTER-M CM20.5
Under the CM designation, Inter-M presents its Contractor series amps. At first glance, what struck me most were the enormous fans they have, which give it forced-air cooling far superior to other amps I know.
Technical specs
This amp delivers an output per channel of 700 W at 8 ohms and 1100 W at 4 ohms, per channel and measured at 1 kHz. In bridge mode we get 2100 W at 8 ohms. Input sensitivity is 0.775 V for standard power at 1 kHz and 8 ohms.
The signal-to-noise ratio is › 100 dB, measured at 1 kHz. The inputs of this amp can be XLR (with link) or jack, using the combo connector, and the outputs are binding posts and Speakon.
The damping factor is greater than 150. It has an extraordinary frequency response of 10 Hz to 40 kHz, though with variations of 0.5 dB. It also has a T.H.D. below 0.1%. It also has a limiter to protect any connected speakers from possible peaks that could damage them.
Pros and Cons
You really miss being able to run this amp at impedances below 4 ohms in stereo or 8 ohms in bridge, which is rather odd for an amp with this much power. Although the most curious thing is that they highlight the fact that it delivers a 10 Hz to 40 kHz response, which is interesting if you want to avoid “losing” upper harmonics. I think the sound quality is quite good, though I prefer the L series. What I like least about this amp is its size and weight, but that is made up for by its price, considering the power it offers. As for its external appearance, it’s very well done.
INTER-M R500 PLUS
Within its “Reference” range, Inter-M offers a series of low-power amps without forced-air cooling, though with more than acceptable sonic characteristics, while keeping a very competitive price in the market they are aimed at.
Technical specs
This amp delivers an output per channel of 170 W at 8 ohms and 250 W at 4 ohms, and in bridge mode we get 460 W at 8 ohms (500 W if we accept a THD of 0.5%, which is not very advisable; if we don’t “accept” it, we have a THD of 0.05% at all the power levels mentioned, which is quite good for an amplifier of this type and at its price).
The signal-to-noise ratio is › 100 dB, measured at 1 kHz. The inputs of this amp can be XLR or balanced jack, and the outputs are binding posts and Speakon. You can use the amp in either stereo or bridge, but it has no option for mono.
It offers channel separation greater than 70 dB, as well as a noise floor of only -80 dB.
It has no forced-air cooling and instead carries enormous heat sinks along the sides. Even so, the manufacturer advises leaving space between it and any other device you place above or below it.
Pros and Cons
I consider this amp to offer very high value for money, since for its price it gives us, with power equal to other brands, a very good signal-to-noise ratio and a very low THD. Tested at low power levels it behaves very well, giving us a very clear sound with no background noise at all. So if we are looking for an amp that we won’t push too hard and that we can put somewhere it won’t “choke” (since it has no forced-air cooling), and we want a “clean” sound, this is a good choice, as long as the power it has is enough for us.
