Theta tech history continued:

Power Amplifiers:

Dreadnaught, Intrepid, Citadel, Dreadnaught II

Theta had always concentrated on doing digital signal processing, pressing the boundaries to derive the finest analog music signals possible, and left it to others to amplify them. Having great respect for the engineers who created some truly wonderful two-channel amplifiers, Theta’s engineers concentrated on areas that needed their expertise. It seemed reasonable to assume that those same great designers at those audiophile-oriented companies would take up the challenges of multi-channel sound, and provide what was needed.

Not so great expectations
What actually happened instead seems to have been a lot of compromises and lowered standards. Two kinds of multi-channel amplifiers came into being, both presuming that standards for home theater amplification are not very demanding.

Many multi-channel amplifiers are available from companies that do not make music-oriented amplifiers. For music, these amplifiers are highly uninspiring, but they are considered acceptable for home theater.

Some "high-end" companies offer multi-channel amplifiers better than those from companies whose main orientation is home theater. Yet in comparison with their own high-end stereo amplifiers these sound lackluster and pallid. This could be a result of cynicism and a desire to capitalize on a market geared to people presumed to be sonically naïve, or a disdain for home theater as a less exalted form of entertainment than music listening alone. The reasons are mysterious.

Granted, multi-channel amplifiers are designed to take price into account. It is tempting to reason that there is a natural enmity between quality and quantity. Aware of economies made possible by combining channels on one chassis, Theta’s engineers felt there should be BETTER quality per channel in a given price range, in a well-designed multi-channel amplifier.

Where was a multi-channel amplifier that took musical performance seriously? Baffled that no one else was meeting this need, Theta set to work in new terrain.

A number of radical ideas went into development, notably a foray into the elimination of global negative feedback. This work evolved into the Dreadnaught power amplifier, which began shipping in early 1999.

The Dreadnaught is not a compromise. It is not a downgrade of any pre-existing two-channel design. Dreadnaught was conceived and created from the ground up to deliver flexibility and performance; to meet needs that simply have not been met before.

Modular
Casablanca’s modular, custom configurations were inspirational; Dreadnaught was designed for total flexibility. Dreadnaught owners aren’t forced to get everything all at once. They don’t pay extra to start with a stereo amplifier and add channels later.

The quality of a Dreadnaught amplifier doesn’t go down when you add more channels.

The Necessity of Power
Home Theater amplifiers are generally right at the borderline of offering sufficient power. Speakers starved for power can’t perform up to their capabilities, and an amplifier that is strained to its limit produces fatiguing distortion.

Each Dreadnaught channel is rated at 200 watts into 8 Ohms, 400 into 4, and even more into 2 Ohms.

Dreadnaught can be a two-channel amplifier, or loaded with up to five channels, each with enough power to properly do home theater.

The Two-Channel Module makes it possible for the Dreadnaught to generate up to 10 channels of fully balanced power amplification.

In September of 2000, Theta introduced a new module that could fit into any of Dreadnaught’s 5 slots, producing two channels, each rated at 100 watts into 8 Ohms. These modules can be combined with single-channel modules, extending the flexibility of the Dreadnaught still further.

With multichannel formats now exceeding the conventional 5.1 channels, a typical configuration in a Home Theater environment might consist of three single-channel and two two-channel modules. The single channels drive the front left, right, and center channels and the two-channel modules power the left/right surrounds and left/right side channels, resulting in a seven-channel setup, not counting subwoofers.

The new module extends the application opportunities of the Dreadnaught for multiroom use, providing audiophile-grade audio for whole house sound systems. Where it might have been cost or installation prohibitive to have a Dreadnaught power a separate whole house system, the new two-channel module can effectively drive a whole house system and a 5.1 Home Theater without draining or compromising the Dreadnaught capabilities.

Each Dreadnaught module has single-ended and balanced inputs for each channel, along with buss selection for stereo or surround modes. Dreadnaught modules produce a fully balanced output with either a balanced (XLR type) input or a single-ended (RCA type) input.

Power Supply
Dreadnaught’s power supply acts as a power reservoir. Its massive 2.2 KVA power transformer is common to all channels. Any single channel confronted with a full-level signal will have the entire current capability of the transformer at its disposal.

In fact, in single-channel power tests, the power transformer does not limit the output power. Unless all modules are driven to full power, which never occurs in real-world situations, the channels not being driven hard effectively lend their portion of the transformer's power to the active channels. This is true for all Dreadnaught modules.

One of Dreadnaught’s features is a button that selects channels that have been assigned to stereo or surround busses via a switch on each module.

Balanced Differential all the way
Signals coming in are balanced at the input to the very first gain stage.

What does that mean? In essence, each amplifier module is really two mirror imaged signal paths for each channel. The signal is cloned, and the phase-inverted duplicate sent through a path identical and proximate to its twin. Then, at the amp’s output, the two signals are reconciled. Anything not a perfectly "mirror imaged" between the two signals is discarded (called "common mode rejection"). Those discontinuities are noise picked up in the course of amplification. Eliminating noise* this way preserves the integrity of the amplified signal nearly perfectly.

This no-compromise technique is rarely used, however, because it requires almost twice as many devices as conventional, single-track amplification. Most companies just don’t do this rigorous a job. We wouldn’t invest this much in high quality components either, if we could figure out any easier way of getting such pristine results.

*Other means of dealing with noise are crude in comparison, "dirty" in that they leave artifacts in the signal; phase anomalies (which impact spatial cues) and intermodulation distortion (which make timbres sound wrong, and otherwise clutters the sound). (And no, these parameters are NOT quantified in measurements on the "spec sheets" you typically see.)

Feedback
Dreadnaught uses no global negative feedback.

What is feedback? Like a snake biting its tail, a negative feedback loop sends some of its output signal back to its input. To cancel out the errors that have crept in during the amplification process, a compensation signal is applied at the input. Obviously, this correction can not actually take place instantaneously.

Small amounts of feedback, within a single gain stage (called "local"), help ensure circuit stability and keep the circuit’s processes on track. Delay is miniscule in local feedback. Nearly all analog circuits rely on this kind of benign feedback.

However, when applied to an entire amplification chain, often comprising several gain stages, time delay becomes a significant problem. The "envelope" is too big, resulting in serious phase shift and intermodulation of the signal with its own error products. This fantastic complex of distortions goes unmeasured (in all the usual specifications), and is not correctable.

Dreadnaught uses none of this kind of "global" feedback!

Intrepid Power Amplifier, a five-channel, 100 Watt per Channel Amp, was introduced in October 2000.

Delivering a powerful 100-watts/channel into an 8-ohm load, each of the Intrepid’s channels has single-ended and balanced inputs. Building on the foundations of the knowledge gained in producing the Dreadnaught, Theta engineers produced a less powerful amplifier of similarly high quality.

The Intrepid Power Amplifier is a solid state, fully balanced power amplifier with five built-in channels. The nominal power rating into 8 ohms with one or all five channels driven is 100 Watts per channel, and 200 Watts into 4-ohms.

Each channel has single-ended and balanced inputs, and a high-current, dual binding post output connector. Like the Dreadnaught, Intrepid produces a fully balanced output with either a balanced (XLR type) input or a single-ended (RCA type) input.

Like the Dreadnaught, the Intrepid uses no global negative feedback.

Though it is less powerful than Dreadnaught, its actual performance is superior to what its power rating would suggest, because of its tremendous dynamic headroom. Similarly rated home theater amplifiers are grossly underpowered for serious music and home theater performance. Intrepid’s muscular power supply is fully capable of delivering 100 watts into 8 ohms, 200 watts into 4 ohms and even more into 2 ohms. The dynamic headroom of the Intrepid allows it to deliver even greater power than these steady state ratings.

Power Supply
The power transformer and output stage bridge rectifier are common to all channels in the Intrepid. This is the same "Power Reservoir" topology used by Dreadnaught.

There has been considerable debate on the subject of power transformers in multi-channel amplifiers. Some manufacturers provide a transformer for each channel; others divide the amplifier into one two-channel section with its own transformer and another three-channel section with its own transformer. Intrepid uses a single massive 1100 VA power transformer and a 35 Amp bridge rectifier for all five channels. The inherent benefit of this approach is that any single channel confronted with a full-level signal will have the entire current capability of the transformer at its disposal.

 

The Citadel is a monaural power amplifier. It is as great an amplifier as Theta’s engineers, building on the knowledge gained in the design and refinement of the Dreadnaught, know how to make. Citadel embodies a level of performance Theta can produce when the design team is not much constrained by concerns of cost, although it is also true that nothing spent on the Citadel is frivolous.

Citadel is built to stand up to the most demanding systems. More than an iron fist, Citadel is made for delivering exquisitely revealing two-channel music reproduction. And Citadel is the best choice for powering home theater systems, as it packs the massive power to recreate even the most spectacular, floor-shaking, chandelier-rattling soundtracks.

The heart of the Citadel is fully-balanced differential, zero-feedback circuitry. Component parts in every stage were selected for optimum performance, and for performance of the amplifier as a whole. Custom-made capacitors, Vishay precision resistors, copper buss bars, Litz/Teflon wires, and a custom-made output connector contribute to the performance.

Citadel’s power supply is an exercise in brute power and complete control. The output stage has a high-current Kilowatt power transformer, which feeds a unique dual-choke and capacitive filtering system. This technique is rarely applied because of the huge choke size necessary to avoid impeding high current demands. Citadel’s filtering system employs two massive chokes almost as large as the power transformer. These are fed from an ultra-fast/soft-recovery 35 Amp full bridge rectifier, which are then filtered by two capacitive banks totaling 80,000uF. The synergistic effect of this filtering system results in high-current power supply lines that are more than 8000% cleaner than conventional supplies. The sonic result of these super clean supply lines to the output transistors has not previously been achieved at high power levels.

The mechanical housing for Citadel is no less specialized than its internal components. More than an aesthetic statement, it’s seamless housing, free of traditional fasteners on top, front, and sides, provides a strong and stable environment for the operation of the amplifier. The power supply resides in its base, isolated from the amplifier module above.

Nominal power rating for the Citadel is 400 Watts into 8 ohms. What it really does is 425 Watts into 8 Ohms, 650 into 4 Ohms, and 800 into 2 Ohms.