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Theta Digital Dreadnaught II Multi-Channel Power Amplifier

Part I

January, 2004

Steve Smallcombe

 

Specifications:


Output: 225 Watts into 8 Ohms per mono module, or 2x100 Watts into 8 Ohms per stereo module; Configurable with up to 10 channels

Zero Global Negative Feedback

Fully Differential Balanced Design

Input Impedance: 50 kOhms

MFR: 0.3 Hz - 250 kHz ± 3 dB

THD: < 1%

Size: 8 7/16" H x 17 5/8" W x 23 1/2" D

Weight: 98 Pounds

MSRP: $6,000 Typical Configuration

 

Theta Digital

www.thetadigital.com

Introduction

Secrets reviewed the original Dreadnaught Multi-channel Power Amplifier in December of 2001, and so for some readers, this review may serve as an update. For me, it was a revelation.

I didn’t really want to review this amplifier – I was happy with my current system, and I did not think that there would be that much difference. Frankly, an amplifier that cost $6,000 or more seemed, well, excessive.

However, it didn’t take much listening time at all to discover that the Dreadnaught II sounded considerably better than my then reference amplifier setup – a combination of an Acurus 200x3 for the front channels, and two Hafler stereo amplifiers for the four speakers in the rear.

So, I found that amplifiers do sound different from one another, an idea that is controversial with many consumers.

To make a long story short, I bought the Dreadnaught II in a seven-channel configuration while reviewing it, and it is now serving as my reference amplifier for reviewing other components, as well as for daily music and Home Theater (HT) usage.

To understand why, read on.

A Modular Design

One of the nice things about the Dreadnaught II is the ability to configure it for your exact needs today, while having the ability to upgrade as your needs change. Basically the Dreadnaught has five slots that can be used with any combination of 225 watt “mono” modules (top photo at right), or 2x100 watt “stereo” modules (bottom photo at right). Both types of modules feature XLR balanced and RCA single ended inputs. The amplifier modules are fully balanced.

While one can imagine the Dreadnaught in configurations as simple as a stereo amplifier, or as complex as a ten-channel amplifier for multi-room usage, it is the five and seven-channel configurations that are of the most interest for HT.

I first experienced the Dreadnaught configured with five 225 watt channels. At that point, I used two channels of my Acurus 200x3 for the rear surround channels of my 7.1 setup. I used it this way for several months, and it sounded great. It seemed clear to me, however, that my ultimate setup would be the seven-channel configuration with three 225 watt modules for the front speakers, and two stereo 100 watt modules for my four surround speakers.

In most setups, including mine, the surround speakers are considerably closer to the listening area that the front speakers, and therefore need less power to stay balanced in level. Surround effects are not usually very intense anyway, and so do not require 900 watts (4x225) to make their presence felt. Therefore, 4x100 is plenty. So, I had the amplifier configuration changed to the seven-channel package partway through the review period. (The term “review period” is a bit ambiguous here as it soon became apparent that I would not be able to let this amplifier go, it was just a question of when I would pay for it and when I would write the review.)


Ergonomics

The Dreadnaught is a very nice looking amplifier in burnished aluminum with beautifully curved surfaces. It is of impressive size and weight, especially if it is sitting on the floor and needs to be put into an equipment cabinet or rack. While it has handles on the back and nice lifting areas on the front corners, at 111 pounds, or so, moving the Dreadnaught II is a two-person job. I ended up moving it by myself – not recommended.

Inputs and Outputs

The XLR connectors lock, and this feels very reassuring. The speaker connectors for the mono 225 module are very solid and very easy to use, but it is obvious that they are designed only for use with spade connectors.

Getting two sets of inputs and speakers wires connected to the stereo module is a bit of a challenge. I should mention that on “my” amplifier, the top speaker terminals on the stereo module are positioned so that if the speaker wires and spade connectors come in from the top, as one might naturally do, there is possibility of shorting the spade connectors, and hence the amplifier's outputs to the chassis – not good. So make sure that either your spade connectors are well insulated, or do as I did, come in from the bottom with the connectors.

Controls

Controls for audio power amplifiers are typically fairly simple. With the Dreadnaught II there are controls to change the amplifier from “Operate” to “Standby”, and from “Stereo” to “Surround”. This later feature is a consequence of the user’s ability, via a switch on the back of each module, to assign the various modules to either the “Stereo” or “Surround” buss. Thus if you are using a five-channel Dreadnaught for two-channel listing, the unused modules are essentially put into standby mode, thus lessening the load on the power supply.

Three mechanisms for provided for switching modes: front panel buttons, 1/8-inch mini jacks for each function, and an optional RS-232 control. Unfortunately, mode control via the 1/8 mini jacks only respond to a 5-12 volt pulse, not the typical level supplied by most preamps for amplifier control. This was a source of some frustration for me as you will read in the comments section. I have since upgraded to the RS-232 control for these functions and found a marvelous device from Celadon to convert IR commands into RS-232 codes in the process. That combination works well and provides reliable control of the various amplifier controls.

Design Considerations

It is hard to decide whether to call the Dreadnaught II a high-end audio amplifier that can be configured for multi-channel usage, or a multi-channel amplifier that has been designed using techniques typically reserved for high-end mono or stereo amplifiers. Both are good descriptions for the Dreadnaught II.

So what is the big deal? Why is designing a good amplifier a challenge, and why would anyone spend this kind of money on an audio amplifier? After all, it’s not even “digital”, despite coming from a company with “Digital” in its name, and that is known for its high quality digital products – Theta Digital!

It turns out despite all the advances in “digital” audio, it still takes analog components to provide the voltage and current gain (amplification) necessary to drive our necessarily analog loudspeakers. Analog designs need to deal with two issues, noise and distortion, as well as the vagaries of driving “real” loudspeaker loads with a wide range of frequencies, at low and very high sound levels, and while trying to reproduce complex, dynamic musical passages.

So, despite the fact that virtually all amplifiers measure flat as a ruler across a wide frequency bandwidth, and have distortion specification far better than the loudspeakers they drive, amplifier reviews end up sounding like wine tasting notes with subjective comments about warmth and openness or transparency. The simple fact is that all not all Zinfandels taste the same, and amplifiers don’t really sound the same when driving real loudspeakers loads with real music. Let’s examine some of the reasons that the Theta sounds so good.

Amplifier Design and Distortion

One major issue with amplifier design involves audible distortion that arises in the amplification process. One common type of distortion is Harmonic Distortion, a type of distortion that produces frequencies that are integer multiples (e.g. 3x) of the original frequency.

Harmonic distortion occurs when the signal experiences any non-linear element in the amplification process, typically at either very high or very low signal levels. Amplifiers are often specified in terms of the Total Harmonic Distortion (THD), typically measured using sine waves. However, THD is not the whole story when it comes to audible distortion.

Other types of distortion include transient distortions that can occur at all signal levels with rapidly changing or dynamic signals such as experienced with music. The design of the Dreadnaught II places more weight on minimizing these transient distortions than on a low THD specification. While this is perhaps a matter of some controversy in the audio industry, the excellent sound quality of the Dreadnaught speaks for itself – it sounds very clean with music and movie soundtracks.

In traditional amplifier circuits, a device such as a transistor works with a constant supply voltage, often called the “rail” voltage, and the current through the transistor is ideally proportional to the voltage on the transistor’s input. In practice, power amplifier circuits typically involve multiple stages that have been configured to provide the necessary voltage and current gain with the “load” on the final stage being the loudspeakers.

Harmonic distortion can happen if the output voltage necessary to accurately reproduce the loudest passages approaches or exceeds the rail voltage. In such cases, the amplifier is said to “clip” the output waveform, and the loudest notes in that passage will go “splat” or seem fuzzed. The naďve listener may say, “Wow, that sure is loud, what a great sound system” as that is the way “loud” is supposed to sound, right? Wrong! With an amplifier capable of reproducing the passage properly, the loudest notes don’t sound “loud” at all – no different from the others notes - nothing exceptional.

Good power handling starts in the power supply – large and heavy are the key words in this respect. It is worth remembering that the Dreadnaught weights more than 100 pounds! Another clue to the Theta’s excellent power handling is that all of the Dreadnaught’s modules are rated for twice the power into a 4 Ohm load as they can put into an 8 Ohm load.

I have also been impressed that at the Home Theater shows I have attended, e.g. CEDIA, that the Dreadnaught seems to be the most common amplifier used in the various HT demo rooms by a variety of exhibitors showing their own products. This is not an “official” count, but one does notice when the amplifier being used to demo high end HT equipment is the same one you have at home. It feels good, very good. And let's just say the demos typically involve very high sound pressure levels in venues that are too big to be called “rooms”. A Dreadnaught can put out an impressive amount of sound. Power handling is not likely to be an issue with the Dreadnaught II in any venue I can image.

Distortion at Low Signal Levels – Class A Design

Ironically, low signal levels can also be problematic for amplifiers, especially “solid-state” amplifiers. This is because transistors, by their nature, don’t pass or amplify signals below a few tenths of a volt on their input. Since sine waves, and musical signals in general, are always passing through zero, the non-linear behavior at the zero crossing can be a significant cause of harmonic distortion. As a consequence, amplifier circuits must be “biased” with a bias voltage so that small signal levels can be amplified without significant distortion.

Traditional amplifier designs are often characterized by the nature of the strategy used for this bias voltage. Class A designs use a bias voltage above this cutoff so that the transistor is always conducting or always “on”. Class A designs have the lowest possible zero crossing distortion, but are inefficient, as they are always drawing energy from the power supply, even with no input signal. Class A audio amplifiers therefore run warm even when played at very low volume levels.

The alternative is to lower the bias voltage to the point that the transistor is not conducting in the absence of an input signal, the so called Class B or the Class AB designs typically used in audio amplifiers. The problem here is that there will necessarily be some zero crossing distortion with these designs, and therefore harmonic distortion at low signal levels.

The Dreadnaught II is biased heavily towards Class A, no compromise here. In the 5x225 configuration, the Dreadnaught II draws 275 watts of power at idle, i.e., with no input signal. It runs warm – it is designed to do so.

With traditional amplifier designs, there is necessarily a tradeoff between sound quality and efficiency. It should be noted however that there are several newer amplifier designs, such as Class H, that vary the rail voltage in response to, or hopefully, anticipation of, the demand, thus significantly improving efficiency – Class H designs draw very little power with no input signal. There are also “digital” amplifier designs today (Class D) that use pulse modulation to construct the output waveform, that also achieve remarkable efficiency. The object of these designs is efficiency however, and not necessarily sound quality.

The Dreadnaught is a traditional amplifier design that clearly goes for sound quality at the expense of efficiency and makes no apologies for it. As a consequence, you will need to provide adequate ventilation for the Dreadnaught II, with 1/2 inch of clearance on the sides and 3 inches on top. I have my Dreadnaught installed in a cabinet (closed door in front, open in back) with no problem, and while it is likely not necessary, I have a whisper fan blowing air through the 5 inch airspace above the amplifier.

I was surprised however, to find that even when in the “Standby” mode, the amplifier (5x225) still used 150 watts of power and remained quite warm. Again this is by design, as the Dreadnaught II is meant to provide excellent performance as soon as it is taken out of standby. If you choose to leave the amplifier on, and in standby when not in use, this will have a small, but noticeable effect on your electric bill. To save energy, not to mention the earth, I turn the amplifier on and off as needed. Theta says that turning the amplifier on and off is not a problem for the Dreadnaught, but that it might take a half an hour or so for the amplifier to warm up to the point that it achieves its ultimate performance.

If you want to try switching the Dreadnaught on and off, don’t expect the relays in your average power conditioner/AC switching unit to handle the surge of current when the amplifier switches on – it won’t, trust me. I now use a 30-amp contactor to give a fairly “hard” connection to the wall outlet.

Click HERE to go to Part II

 

 

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