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Theta Digital Dreadnaught II Multi-Channel Power Amplifier

Part II

January, 2004

Steve Smallcombe


Transient Intermodulation Distortion (TIM) and Zero Global Negative Feedback

So far we have discussed harmonic distortion that arises when non-linear elements of the amplification path are experienced by the signal. The other form of distortion that needs discussion is called Transient Intermodulation Distortion or TIM.

One way of reducing harmonic distortion, and improving the THD specification, is through negative feedback. We have all the painful experience that happens when someone accidentally puts a microphone too close to a loudspeaker that is reproducing the amplified signal from the microphone. The resultant squeal is a result of feedback, positive feedback in this case. What’s happened is that some of the output signal get into the input, the microphone in this case, and the whole system goes into oscillation. Oscillation is a sign on an "unstable" circuit.

Negative feedback, where a portion of the inverted output signal is fed back into the input, can be used to avoid oscillation, provide a more stable circuit, and to reduce harmonic distortion. Most all amplifiers use “local” negative feedback to stabilize individual gain stages This is fine, up to a point.

One can also use negative feedback on the entire amplifier to further reduce harmonic distortion. In a multi-stage amplifier design, this is called global negative feedback. Unfortunately global negative feedback can produce other problems, such as TIM, and while audible, TIM does not show up on the specification sheet, as THD is typically measured with continuous sine waves, not dynamic signals such as music.

The problem with global negative feedback is that the feedback signal can be slightly delayed by the various amplification stages. With a rapidly changing dynamic multi-frequency signal, i.e., music, these delays and the global negative feedback can lead to the various frequencies and their distortion products, modulating each other to produce a series of transient sum and difference signals. These transient intermodulation products are called TIM and they sounds like “grunge” between the notes in a musical passage. Sometimes a reviewer will write about hearing the silence or space between the notes– a very good way to describe lack of TIM. Music sounds better and less tiring without "grunge".

The Theta Dreadnaught does not use any global negative feedback, and as a consequence, sounds very clean with music. You can hear the silence between the notes. (I am listening to solo acoustic guitar as I write this review, and it sounds great!)

True Balanced Differential

Noise is the other issue with amplifier designs. Amplifiers will amplify anything on their input, noise and signal, and pass through noise on their power supplies or rail voltages. The Dreadnaught II uses a balanced differential design to minimize noise. Just because an amplifier has XLR “balanced” inputs does not mean that it is a balanced design, in that some or all of the amplification stages may be single ended with half of the signal path connected to ground. The Dreadnaught II is a “true” balanced design with the use of symmetrical mirror-imaged signal paths all the way from the balanced XLR input to the speaker output connectors. Single ended signals from the RCA inputs are “balanced” at the first amplification stage by cloning and then phase inverting the signal.

The consequence of the balanced designed is that any noise or “discontinuities” that exist on both signal paths (common mode), are cancelled. The result is an uncommonly quiet, clean sounding amplifier. I can put my ear right up to my speakers, with the sound muted, and there is NO hiss or hum or any sound at all coming from the Dreadnaught II. The cost of a true balanced design is a more complex and expensive design that requires literally twice as many components in the amplification circuits. (You also need to be careful not to short either speaker output to the chassis or other "ground".)

Listening and Comments

I first hooked up the Dreadnaught II without putting it inside my equipment cabinet, as frankly, I was a bit skeptical as to how much I’d appreciate this amplifier. The improvement over my reference system however, was immediately obvious. The sound had more impact, better dynamics, and was surprisingly warmer – the mid-bass was definitely better defined and more obvious. The mid-bass seemed more up front, not just better defined or “controlled”.

My front speakers are Velodyne DF-661s, speakers that are specified to have exceptionally low distortion levels, but they also can sound a bit bright. In the years I have had these speakers, I have tried various equalizations and even modified the crossover, but to me they always had a bit of a weak sounding mid-bass. That is until I hooked up the Dreadnaught II. My wife and I noticed the difference immediately using the opening passages of "Lord of the Rings, Fellowship of the Ring". Later that evening, my wife asked how much this wonderful amplifier was, and with some trepidation, I told her, about $6000. She replied, “I guess that is what a good amplifier costs.” The next weekend, my older amplifiers moved over to make room for the Dreadnaught in the equipment cabinet.

Clearly when it came to driving my speakers, the Dreadnaught II did a much better job of keeping the various frequencies in balance while driving the complex impedance characteristics that the speakers present to the amplifier. The two amplifier setups in question, Acurus 200x3 and the 225 watts per channel of the Dreadnaught were of a similar power rating, and the distortion specifications of the Acurus are probably better than that of the Dreadnaught, but there was no doubt which sounded better with music and movie sound tracks.

For most of the listening, I used my B&K Reference 30, and, as it does not have balanced outputs, RCA-based connections to the Dreadnaught. That worked fine. I also had a chance to use the Dreadnaught II with the Sunfire Theater Grand Processor III, using XLR balanced connectors. That was a really magic combination with even better dynamics, a wider sound stage, and a more open and transparent high-end. Whether it was the nature of the Sunfire’s processing and electronics, or the use of the balanced inputs to the Dreadnaught II, or both, I can’t say, but there is definitely a new preamp with balanced outputs in my near-term equipment upgrade plans.

About half way through the review period, I upgraded from the 5x225 watt configuration with the rear surrounds receiving 2x200 from my Acurus amp, to the 3x225 and 4x100 7-channel configuration for the Dreadnaught II that I bought. I’ll have to say that even though the setup had less power available to the surround channels, the latter configuration sounded better, perhaps because all the amplifier channels had a similar “voice” and distortion levels, etc. Since the drive level from the preamp for my rear channels is down by 7 dB relative to the fronts, the fact that the rear channels have 4 dB less power (100 vs 225 watts) is really not an issue. And, by getting all seven channels into a single chassis, the Dreadnaught uses less cabinet space than my previous three-amplifier setup. (Of course, my old amplifiers still find use for multi-room purposes.)

Video material that we have particularly enjoyed recently includes "Lord of the Rings, The Two Towers". The opening scene where Gandalf and the Balrog are falling together played at anything near reference levels, will make a strong impression on any listener. The audio in chapter 5 of "Chicago" is also fantastic. Another AV treat, is “Pirates of the Caribbean”. This EX encoded soundtrack is awesome with a good sound system. Your woofers and well as your subwoofers will get a good workout with both the theme music and many of the special effects. The bass in the pirates theme music sounds very solid and well blended between my two 15 inch Velodyne subwoofers and the DF 661s woofers when they were driven by the Dreadnaught II.

Music is very satisfying with stereo reproduction using the Dreadnaught II in the Stereo mode, and that is how I currently listen to two-channel sources such as CDs and the music channels from DISH. The best music reproduction I have heard however, is with DVD Audio using all 7 amplifier channels and speakers. Emmylou Harris’ "Directors Cut" sounds particularly good with my Dreadnaught II-based setup.

So what’s not to like about the Dreadnaught II? No complaints about the sound, that’s for sure. It will chew up the power if you leave it on, and for someone who car pools daily in a gas/electric hybrid (Honda Insight), and is obsessive about recycling, this is an issue that gave me some pause. Turning it off when not in use seems a good compromise between great sound and saving the earth.

My only real complaint about the Dreadnaught is its control capability in the standard configuration – the use of a pulse instead of a level to select the Operate/Standby and Surround modes. The trouble is that a pulse is a toggle, and does not allow for a discrete selection of modes. Toggles are trouble for anyone wishing to write reliable macros.

I got the Operate/Standby selection “working” by assigning a 12V control output from my preamp to an unused input, and then selecting and deselecting that input in my “system on” macro. This was neither satisfying nor reliable. The issue became worse once I started turning the amplifier off when not in use. Now I needed an Operate/Standby pulse delayed by 6 seconds to assure that the amplifier was fully on and listening when the pulse arrived. Furthermore, macros with long delays are hard to do reliably using an IR-based remote control.

As a consequence, most of my time with the Dreadnaught II I used a “digital” device to poke the button on the front of the amplifier, but that was getting old quickly. After experimenting with an electronic circuit to produce a delayed pulse triggered by a level from my preamp, I decided that enough was enough, and sent the amp back to Theta for an upgrade to the RS-232 control module. I also switched to an RF-based remote for reliable long macros; more about that in another review on remotes.

Now the problem was that I needed some way of generating the appropriate RS-232 codes based on the IR (or RF) commands from my remote. There are several systems available that will do the job, but most are really designed for use by a professional installer and require the purchase of not only the device, but their (expensive) programming software as well. My salvation was a device from Celadon (www.celadon.com). They offer an IR to RS-232 device, the IRC-38, that comes pre-programmed for specific devices. If they have the device of interest in their database, then there is no charge for programming. (Let’s just say the Dreadnaught commands are in their database now.)

Using the Celadon IRC-38 I have now (photo on the right), not only the Standby/Operate modes work reliably after power on, but I also have all the surround modes selection buttons on my remote programmed to select the Surround mode appropriately. I never used to bother deselecting the surround buss for stereo listening when it required a manual operation. Now that the operation is automatic and reliable, deselecting the unused channels and placing them in standby for stereo listening make perfect sense, and I do so all the time. Of course, it would have been even nicer if the Dreadnaught responded to 12V levels, not pulses, and I would have not had to go to the trouble I did to get this working reliably, but given the great sound with the Dreadnaught II, it was worth the effort.

After I decided to go for the RS-232 upgrade, I pulled the Dreadnaught out of my equipment cabinet and shipped it to Theta, putting my old amplifiers back into their previous roles. That evening we put on "Lord of the Rings, Fellowship on the Ring", as I guessed there was nothing good on TV. As Galadriel was narrating the introduction, my wife looked at me in shock and said, “What happened to the sound, it's all trebly, there is no bass!" It was true. Kate Blanchart’s voice had lost most all its resonance. Even though the subwoofer level was properly adjusted, the bass, and mid-bass in particular seemed thin compared to the sound that we had become accustomed to with the Dreadnaught II. The difference was not subtle.

Over the next week or so while we were without the Dreadnaught II, we watched/listened to a variety of video material, including “The Italian Job”, a movie a friend dropped by to watch. She thought the sound was fine (I used to as well), but that evening I found myself constantly adjusting the volume control trying to lock in just the right sound. I never found it. Turning down the volume did not seem to make the sound any cleaner, and raising the volume didn’t really help that much with impact or mid-bass.

I also noticed that the dialog was not as clear as I had become accustomed to with the Dreadnaught II; it seemed more diffuse, like it was coming from more than one of the speakers. Several times I walked up to, and listened carefully to the various front speakers, but indeed the dialog was coming only from the center channel as it should. The dialog clarity was simply not as good with my previous setup as with the Dreadnaught II.

Needless to say, when the Dreadnaught II returned, it went back into its proper place in the cabinet in short order. The Dreadnaught was my amplifier now, no doubt about it – I had paid for it. While projector technology and preamp/surround technologies are constantly improving, I’m not too worried that the Dreadnaught will be obsolete in the foreseeable future. There may be more efficient amplifiers out there, but they likely won’t sound any better than the Dreadnaught II. I may upgrade most of my other components in the next few years, but I am planning of using the Dreadnaught for the foreseeable future.

Conclusions

The Theta Digital Dreadnaught II is a very versatile amplifier and can be configured with anywhere between one and ten channels. The five and seven-channel configurations are ideal for a high-end Home Theater. Details include a truly balanced differential design, with zero global negative feedback, and Class A characteristics, leading to a very quiet, clean sounding amplifier for both stereo and multi-channel listening. When driving my speakers, the Dreadnaught II produced a substantial improvement in sound quality over my previous amplifier setup. I bought a seven-channel version for my HT system.


- Steve Smallcombe -

 

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