Theta tech history NOTES:

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.

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 contaminates the sound. (And no, these parameters are NOT quantified in measurements on the "spec sheets" you typically see.)

Although many highly regarded amplifiers provide XLR input connectors to receive balanced signals, the mere presence of "balanced inputs" is no guarantee that any balanced circuitry is present in the amplifier itself.

Full balanced differential circuits are rarely used in amplification. This no-compromise technique 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.

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. There are two basic categories of this sort of negative feedback. The impossibility of instantaneous correction is one factor that makes this distinction important.

Local Feedback:

Local feedback is very common in almost all analog circuits. It stabilizes, sets operating points, limits unwanted oscillation, reduces distortion, and protects delicate devices from potential damage. Local feedback is applied almost immediately back to the input, with very little delay.

Global feedback:

Global feedback is also very common in circuit design. It is usually applied to reduce distortions and lower output impedance. It can be used to stabilize circuits that are unstable on their own. There is significant time delay between the input signal and the feedback signal, due to the number of stages the input signal must pass before being applied back to the input in the form of feedback. Additional circuits must also be used in the feedback path to make sure the negative feedback never becomes positive feedback at any frequency. Because of the significant time delay, global feedback can cause a smearing of imaging and an upper midrange with harsh or glare. The audible effects of global feedback vary, mostly depending on the amount of feedback but also on the circuit they are correcting. Nearly all power amplifiers use global feedback in large proportions.

Theta’s goal is to create very sonically accurate components. Measurements typically published as "specifications" do not reflect some of the most important aspects of sonic performance. It is quite possible to design circuits that measure well but sound bad. In fact, it’s done all the time.

Time delay created by global feedback creates audible problems. 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. Since Theta is able to offer circuitry that is inherently stable, there is little incentive to trade actual performance for measurements.

The decision was easy, if radical: Theta’s amplifiers use no global feedback!

Jitter Jail
Theta’s Jitter technology

What is Jitter?

From the very beginning of the research project that led to Theta being created, the engineering team was aware that Jitter is one of the major factors in the degradation of digital signals.

As data streams make their way through circuits, connections and components, they have a tendency to become temporally misaligned. This is an inherent vulnerability. The errors tend to be cumulative, and as the misalignment worsens, it can become impossible to correct.

These timing errors, which we call jitter, translate into harmonic distortions to the music. Once incorporated into a musical signal, these errors are inextricably part of what you hear, and since they are mathematical artifacts unrelated to music, the sound can be quite stressful to endure. Even when jitter is reasonably low, rather than providing you with a headache, these residual errors can cloud the music and dirty up the sound.

Maintaining signal integrity

Theta uses many methods to minimize jitter at points where the signal is particularly vulnerable. All digital signals are reclocked as they come in to any of Theta’s D to A converters. Theta deals in other ways with jitter problems originating in transports. Maintaining signal integrity all along the digital chain is the goal.

"Jitter Jail" technology, introduced in 1998, was a revolutionary new tool developed as an even more effective technology than reclocking the signal "on the fly". A custom-manufactured low-jitter crystal oscillator at its heart, Jitter Jail acts as a buffer, actually storing the digital signal long enough to reclock it much more perfectly than we can do as it is streaming by.

First incorporated in Theta’s Jade CD transport, the circuit’s master clock realigns data, correcting raw data as it comes from the disc.

This high-precision buffer becomes your source, rather than the CD, which has jitter problems of its own. CDs are not physically perfect and the recording and editing leading up to the CD has its own accumulated jitter. The pits are unevenly aligned, there are deformities in the material of the substrate, and the clear plastic is not perfectly clear. By reclocking the data from your CDs you can actually recreate a truer representation of the music, as it was created, than the raw data coming direct from the CD.

The Jade transport was the first Theta product to take advantage of this kind of digital buffering, but succeeding processors and transports all make use of this technology.