Stewart Audio World 21 Dual Mono Power Amplifier

I was looking forward to testing the latest power amplifier from Stewart Audio, the World 2.1 dual mono power amplifier ($1,395), which - like those previous models - features the Stewart high-speed switch-mode power supply.
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Most contractors are familiar with the Stewart PA50 and PA100 power amps. With their innovative power supplies, the amps offered a power-to-weight ratio that was unprecedented in the late '80s and early '90s. Musicians and contractors alike found many opportunities to take advantage of the small, lightweight, powerful units.

In my time spent as an FOH engineer I saw countless custom bass guitar rigs using the PA100 in bridged mono. It was also popular for backline acoustic guitar amplification.
Product PointsApplications: Live sound; install

Key Features: 650 W (RMS into 4 ohms); bridged; 2100 W (into 4 ohms); 17 lb.

Price:$1,395

Contact: Stewart Audio at 209-588-8111

Plus

+ Great sound

+ Plenty of headroom

+ Lightweight

Minus

- None

The Score: Stewart has produced a quality, great sounding amp in a lightweight, small package.
At the time, designer and company president Wade Stewart was approachable and more than willing to offer tech support. I remember building an installation around the PA100 using a 70-volt constant voltage design and Stewart spending time with me on the phone to make the application work. That is the kind of commitment I give to my own company, and it was refreshing to get that interaction from Wade Stewart.

Consequently, I was looking forward to testing the latest power amplifier from Stewart Audio, the World 2.1 dual mono power amplifier ($1,395), which - like those previous models - features the Stewart high-speed switch-mode power supply.

Features

More than anything else, it's their power supplies that distinguish Stewart amplifiers from components with otherwise similar performance characteristics. A conventional power supply recharges 120 times per second and consists of a large transformer and large storage capacitors. The Stewart power supply recharges 1,000 times faster - 120,000 times per second. As a result, it requires less capacitance for filtering and storage, drastically reducing size and weight.

To give you an idea of what this means in practical terms, this 650-W amplifier (RMS, into 4 ohms) weighs only 17 lb. A comparable amplifier with a conventional power supply would be in the 50-lb. range. At less than half the weight of a comparable amp, the Stewart makes a powerful statement.

The 2.1 is a true dual mono amplifier, meaning that it has two independent amplifiers, each with its own power supply, sharing only the power cord and cooling fan. The advantage of having two power supplies is maximum separation and greater dynamic capabilities. The World 2.1 features a S/N ratio of >100 dB (A-weighted, 650 W at 4 ohms), a damping factor of >500 and an operating bandwidth of 15 Hz to 45 kHz. Its bridged mono power output is an impressive 2,100 W into 4 ohms.

The World 2.1 is a two-rack-space unit, 15" deep, with a black steel chassis and anodized aluminum faceplate. For each channel, there is a master power switch, three-step LED signal level indicator (ö20, -3 and 0 dB) and 21-position level control on the front panel. These are arranged in mirror image layouts on either side of the cooling channel.

The back panel offers flexible input connectivity, with XLR, 1/4" TRS and barrier strips for each channel. All three can be balanced or unbalanced. In addition, they are all wired in parallel so that each can be used for through-connections to other amplifiers or components. The five-way output binding posts accept bare wire up to 6 AWG, spade lugs, hook lugs and single or dual banana plugs.

The binding posts are grouped so that when the amplifier is used in bridged mono, a dual banana plug spans the terminals. The stereo/mono switch and pushbutton circuit breakers are also on the rear panel.

In use

I chose to demo the World 2.1 while providing contract sound reinforcement for a live act since it was easy enough to carry the lightweight unit along with my main rack. For the first set, I used it to run the mid to high packs; in the second set, I swapped it out and ran the subs. In both applications the 2.1 ran cool, providing ample power and comfortable headroom for the 250-seat venue.

For critical listening I used my favorite all-around reference speakers, Audio Composite Engineering (ACE) Model 550s. Input was a CD player and I listened to a range of program material. In a side-by-side comparison with another similarly priced amplifier with a conventional power supply, the Stewart was tight and crisp. It yielded superior spatial cues, which I attribute to the dual mono design. Listening is subjective, but of the two I clearly preferred the Stewart.

Summary

Anyone in the market for an amp needs to give a serious look to this amp. Based on my experience, the Stewart 2.1 should vie for serious consideration in future pro audio blueprints.