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Model 650 Amplifier Review

Auto Sound and Security
by Chris Lewis

The Model 650 is a five-channel amplifier including crossovers. Four of the channels are identical and are rated to deliver 50 W per channel. The 5th channel is rated at 250 W. The four low-power channels can be combined into two high-power channels, resulting in a high-power three-channel amplifier.

One end of the amplifier contains the RCA inputs and crossover controls. The other end is for speaker and power connections. Since the amplifier can draw over 100 amperes under full load at full power, the power connections are large enough to accept half-inch spade lugs. The speaker terminals accept quarter-inch spade lugs. And how very nice, DEI supplies them. The adjustable crossovers are of the 12 dB-per-octave variety built with the 4559 op-amp. Each of the front, rear, and subwoofer crossovers are independent. The low power channels can be selected for high pass or low pass, ideal for putting together a system using front and rear speakers with a subwoofer.

The double transformer power supply is based on the TL-494, and construction is excellent; no point-to-point wiring, and all of the heat- generating semiconductors are clamped to the heat sink. The 250-watt subwoofer channel has its own power supply, and there are separate rectifiers and filters for the op-amp power supply. Input and output filters include inductors. Three IRF-Z44 MOSFETs per phase drive each of the two transformers, which is plenty. The amplifier section is all discrete, using TIP-35/36c 25-ampere plastic TO-3 outputs. The low-power channels use one pair per channel, while the 250 W channel uses four pair. Looks like good parts and the right number of them.

The power supply is unregulated and the model 650 specs are at 14.4 volts. I measured the small channels at 57 watts at 0.026% distortion. The large channel delivered 248 W at 0.07 percent distortion. With all the channels driven, the current consumption is 60 amperes. This matches very nicely with the 2 ATC-30 amperes fuses mounted on the chassis. Full power with two-ohm loads draws just over 100 amps. That would be 80 watts per small amplifier and 0.0358% distortion, and 380 W at 0.09% out of the large amplifier. In the three-channel mode, I got 162 W and 0.05% distortion out of each of the bridged channels. The owner's manual specifies 150 watts, but you won't mind having additional power. The box says 800 W dynamic power at maximum load. Such testing of the subwoofer amplifier revealed a 200 Hz EIAJ dynamic power of 400 W. The smaller channels at 1000 Hz EIA came in at 80 W each.

Small signal results were good. The noise floor was a constant 74 dB below one watt, no matter where the gain was set. Distortion at one kilohertz, one watt comes in at 0.042%. This consisted primarily of a slight crossover distortion. Naturally the 5 kHz distortion was higher, 0.103%. The subwoofer amplifier crossover cannot be bypassed, therefore the subwoofer amplifier was tested at 200 Hz. At one watt the distortion was 0.120%, and again there was a slight crossover notch.

With the crossover set for full range, the front channels have a very good frequency response. The response is flat with less than one dB of attenuation at 10 Hz and 40 Hz. Testing the damping factor at 1 kHz, I measured 192, which is very good for a discrete amplifier design. The input stage was also well designed, at minimum gain full power was reached with a 7.7V input. Crank the gain wide open and you can get full power with as little as 157 milli-volts.

The sample I received came with a one-page preliminary instruction manual. For those of you who know what you're doing, this will be sufficient. However, I followed the instructions and went to the web site, www.directed.com. The 12-page owner's manual downloaded very quickly and the Web site was easy to navigate. Perhaps I should thank John Durbin; I know he spends a lot of time monitoring the car audio news group. The manual is well written with examples and very good advice on a number of subjects. The warranty is two years if an authorized DEI dealer performs the installation. Like many companies, the warranty is shorter (one year) if you do the installation

Congratulations to DEI for a fine amplifier.

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