  |
|||
| R
e v i e w s |
|||
|
Digital
A/V Controllers Power
Amplifiers Transport
& Players |
|||
|
Privacy
Policy | Terms of Use |
|||
|
 |
|
POWER
AMPLIFIER? THETA?
That was my initial reaction when, in 1999, I
first heard the news that Theta Digital was preparing a power amplifier
for release. In recent years Theta has become well known for its
digital products - CD, LD and DVD transports, digital-to-analogue
converters and surround processors (Casablanca and Casa Nova), so
news that Theta was striding out into the power amplifier market,
which at the moment is almost entirely an analogue domain, was a
little surprising. After reading some of the promotional material,
it sounded like this amp was going to be a little out of the ordinary
and any amplifier named - albeit incorrectly spelt - after the early
twentieth century British battleship,
To design their first power amplifier, Theta enlisted the services of Charles Hansen of Ayre Acoustics, whose designs were refined and turned from a concept into a construction design by Theta engineer David Reich (previously of Classé). The Dreadnaught has a somewhat unusual design for this sector of the power amplifier market - it is described by Theta as a "zero feedback, fully balanced differential" amplifier. Let me explain what they mean (most of the following is derived from Theta's technical description or "white paper" for the Dreadnaught). Feedback is a term used to describe the application of the output of an electrical circuit back to its input. Most of us are aware of feedback in the form that exists at rock concerts and live TV broadcasts, it's that loud, high-pitched, squealing noise caused when a microphone is too close to a loudspeaker - the speaker output "feeds back" into the amplification circuit - think of it as the audio equivalent of holding one mirror up in front on another. In this context feedback is bad - very bad, and can quickly damage components in the signal path, and it sounds pretty nasty too. In the context of an amplifier, however, feedback is used in two different contexts, both categories of "negative feedback". The first is local feedback: most power amplifiers consist of several amplification stages and feedback is "local" when it occurs only within one stage. The second form is "global feedback", where the feedback encompasses two or more amplifier stages. Local feedback is good and is used in many analogue circuits to do several things: limit unwanted oscillation, reduce distortion, and to protect components from damage. It is applied almost immediately back to the input with very little delay. The Dreadnaught uses local feedback in each of its three stages. Global feedback is also commonly used in circuit design - to reduce distortion and lower output impedance. With global feedback there can be a significant time delay between input and feedback due to the number of stages that the signal must pass through - this can cause audible artefacts such as smearing of the sonic image. The Dreadnaught does not use any global feedback.
Remember I said the box was heavy? It is. Weighing
in at 115lbs (52.2kg) for the 5 channel version, it posed rather
a problem for me - how to get it down into my basement home theater
space. Usually I get my wife, Kris, to help out with the heavy-lifting,
but she was away in Europe on a business trip. I certainly couldn't
carry it downstairs myself, so I improvised. I managed to manoeuvre I asked for a sample in Theta's "ebony" colour, which is basically a brushed black finish, the same as my Theta DaViD DVD transport, and it looks equally attractive. My review unit came with racks ears for fitting the amp into a standard 19" rack, but my rack was full so I detached the ears and opted for placement on the floor and then later on a Sound Anchors stand (I cannot speak highly enough of the quality of Sound Anchors stands - really solidly built and the perfect base for a heavy amplifier - http://www.soundanchors.com). As I mentioned above, the Dreadnaught is modular. There is space for 5 amplifier modules, tucked in behind the power supply. The "standard" amplifier module is a balanced, single channel rated at 200 watts into 8 ohms, 400 into 4 and "even more into 2 ohms". You can order a Dreadnaught with as few as 2 amplifier modules for a stereo configuration, the full five or any number in between. Theta has also recently released a two-channel module (which I have not had the opportunity to take a look at) so in theory you could order 5×2 channel modules for a 10 channel configuration. A quick word about the power supply - the Dreadnaught has a single 2.2KVA toroidal transformer, Theta supporting the theory that with this configuration any single channel confronted with a full level input will have the full current capacity of the transformer available to it. The transformer has three sets of secondary windings, one each for the output stage, the input and driver stages, and the control circuitry.
Each channel has its own current bridge and capacitor bank, and fully discrete regulation circuits for the input and driver stages.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||
H.M.S.
Dreadnought starts out with an advantage in my book! So I made a
request for a review sample. Theta and Bryan Stanton of Theta's
PR agency duly obliged and one day way back in June 2000 a large
(and very heavy - more on that later) box was waiting for me on
my return home from work.
the box onto a mat and drag it over to the basement stairs. From
there, I was able to slide the box down the stairs, standing below
it toprevent it speeding out of control - sounds ugly but it worked,
so well in fact that I have used this method subsequently for other
big boxes. The only problem is that it doesn't work too well for
getting equipment out of the basement!
