Product Review - Niles SI-250 Stereo Power Amplifier - January, 1997

By J.D. Moretti


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Niles SI-250 Amplifier; Two channel amplifier; Rated output 125 watts/ch rms into 8 Ohms, 185 watts/ch rms into 4 Ohms, 250 watts/ch rms into 2 Ohms, both channels driven; Bridgeable to 395 watts rms into 8 Ohms; Frequency response 20 Hz - 20 kHz 0.1 dB; THD<0.1%; Turn on modes: audio sense, constant, trigger; Two grounded AC outlets, one switched, one un-switched, 750 watts total; Two RCA jack unbalanced inputs; Two RCA jack cascade outputs (for daisy chaining to other amplifiers); 12 volt trigger output; 30 volt trigger input; Ground lift switch; Two pairs of 3-way binding posts; Size 5"H x 17"W x 15"D; Weight 25 pounds; black aluminum sheet metal chassis; $729; Niles Audio Corporation; P.O. Box 160818, Miami, Florida 33116-0818; Phone 305-238-4373; Fax 305-238-0185.

When my wife and I purchased a new condo last year, she asked me if I could arrange the stereo system in a way that we could listen to it in different rooms. And, she didn't want to go around turning all the individual preamplifiers and amplifiers on and off. She wanted something more convenient and less conspicuous now that we are home owners (actuallly, the bank owns 80% of it, and it is REALLY a cool condo). Although, I am pretty handy with carpentry, I thought I might contact a professional installer to help us out here.

Niles Audio makes what are called Systems Integration Amplifiers, which are used mostly by professional installers for home audio/home theater applications. Niles makes the SI-1200 which has 12 channels at 25 watts rms/ch into 8 Ohms, and which can be used for multi-room audio systems. We decided to take a look at the SI-250 because it is a relatively inexpensive two channel power amplifier, delivering a respectable 125 watts into 8 Ohms (sufficient for digital surround sound), and it is also rated into 4 Ohms as well as 2 Ohms in case you want to have more than one set of speakers playing at the same time (VERY few amps in this price range are rated into 2 Ohms).

Part of the reason this amp is not very expensive is the fact that it is mounted in a very simple sheet metal chassis. The amp is designed to be hidden rather than showcased, so there is no reason to put dollars into chrome plate, heavy front panels, or any other flashy looks. However, the amp does have a very large number of features. The front panel has an on/off rocker switch, but there are extensive sockets on the back for remote control on/off by several means, including 30 volt trigger and auto sensing of audio signals. An "Active/Protect" LED indicates power on or that the protection circuitry has come on (short circuit). Left/right Clipping LEDs and L/R Rail Fuse LEDs (indicates blown fuse - each channel separate) are situated to the right of the on/off toggle. There are also two small holes into which a screwdriver can be inserted for adjusting the maximum output of each channel (if you have teenagers, this would be a nice feature for preventing them from playing the system too loud when you are away from home!)

The back of the unit has two pairs of 3-way binding posts, for use with banana plugs, spade lugs, or bare wire wrap. Besides the two RCA input jacks, there are also two RCA output jacks for use in daisy chaining of multiple amplifiers. Input and output jacks for triggers by remote control are also on the back. A ground lift slider switch is there in case of ground loop hum problems (likely with multi-room setups). The amp can be bridged, and there are two grounded AC outlets (one switched and one un-switched) for powering associated equipment; 750 watts are allowed here. The AC power cord is permanently attached.

The electronics include a heavy duty toroidal transformer with two secondary windings each rated at 0.3 kVA, two 22,000 F capacitors (63 volt), and eight bipolar output devices (four per channel). [
Click here for photo of inside of amplifier.] The power supply will store 73.9 Joules, based upon 58 Volts DC on the capacitors. The heat sink fins are inside the chassis. When we tested the unit for maximum output at clipping, the unit became warm but not hot. It is truly designed for heavy use, and is well protected against abuse.

We listened to the SI-250 using our McCormack Audio CD system, McCormack preamp, Nordost Flatline cables, and the Anthony Moore Ribbon hybrid speakers. Ribbons can be a tough load for an amplifier, but they also can sound terrific when properly driven. One of our favorite albums is The Classical Film Music of Bernard Hermann (Silva Screen Records America; SSD1051). It puts extreme demands on the entire system. The opening track contains instruments, such as trumpet and kettle drum, that produce sounds throughout the entire audio spectrum . Brass was crisp but not harsh, and the deep rolls of the drum were tight. We were really quite surprised at the performance for this price level. Sibilants were natural (the "s" in the human voice). When we cranked the volume until the clipping lights just started to blink, the bass was still tight, mid-range full but not mushy. Overall, really a nice performer.

Instead of the usual output testing with sine wave pulses into a computer system, we decided to test this amp into the speaker load itself, and inserting the oscilloscope and rms volt-ammeter into the circuit. At the edge of clipping, using pink noise into both channels (all frequencies in the audio spectrum simultaneously, each at equal energy), the amp produced 19.3 volts rms and 3.97 amps of current into the Anthony Moore speaker load, measuring the left channel, and both channels driven. This represents about 80 watts rms per channel. Although the amp is rated at 125 watts rms per channel, that is a rating using individual sine waves, one at a time, and into a testing device. Our methods use an actual speaker load - one that is not easy to drive - and many frequencies simultaneously. This is an unconventional means of measurement, because there are other variables at large, such as the back EMF, but we feel that a "real world" approach to performance is more informative. At the edge of clipping, with both channels driven, we measured 98.7 dB of SPL eight feet away from the speakers (the Anthony Moores have an 86dB/w/m sensitivity). This represents plenty of volume.

If you are planning to have someone install a multi-room audio or home theater system, we suggest you speak to them about using the Niles. In my case, I am going to let my wife take care of this, because she knows exactly where she wants everything to go. Although, in our opinion, 25 watts/ch (the output of the SI-1200) is not enough for anything more than background music playing, the SI-250 is another animal entirely, and should do the job for most applications. It is well constructed, sounds excellent, has good flexibility, and the price is right.

J.D. Moretti

Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
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