DACs

Theta Generation VIII Series 2 Digital-to-Analog Converter/Preamp

ARTICLE INDEX

Installing the Upgrade

While I was able to install the Series 2 upgrade at home, guided by Theta's instructions and a little help on the phone, I had Theta's permission to do so. The upgrade must be performed either by the dealer or international rep; otherwise, the Gen. VIII must be returned to the factory to ensure proper installation and to maintain any warranty in effect.

Theta Generation VIII Series 2 Digital-to-Analog Converter/Preamp


Theta recommends 100 hours of break-in time. Since Gen. VIIIs do not sound their best until they are warmed up and played for 72 hours straight – I never leave the unit unplugged for more than 60 seconds when changing equipment around – break-in takes but one day longer than warm-up. Just turn off your amp(s), load your transport with a break-in CD, and run it on repeat for a little over four days. You can of course listen to music anytime during the break-in period, and monitor the improvements. Just remember that leaving the Gen. VIII Series 2 in standby mode is not the same thing as playing a break-in CD through it for four days straight with the volume turned way up.

Upgrade Specs and Improvement

The Gen. VIII Series 2 is a fully differential balanced DAC featuring custom-designed,
software-programmable digital filters feeding two digital-to-analog converters per channel, one for each phase. Theta claims that its linear power supplies ensure clean power resulting in ultra-low noise, and that its proprietary "Jitter Jail" technology eliminates most digital distortion. (I use the Jitter Jail during playback).

The Series 2's digital input section accepts 32KHz, 44.1KHz, 48KHz, 88.2KHz, and 96KHz signals. D/A conversion is 24-bit Ladder (8x oversampling). There are two DACs per channel for true differential operation. All DSP processing is 24bit with 56bit accumulator. Balanced output impedance is 25 Ohms, and single-ended output impedance is 12 ohms. The volume control is Theta's proprietary switched resistor network, and operates in the analog domain. The digital filter is an 8x oversampling Theta proprietary FIR filter running on Motorola 56362 DSP. Power requirements are 117 VAC, 50-60 Hz, 50 watts maximum, with all options installed. Fuse @ 100 & 110V = 630mA, Fuse @ 220V = 3/8A. Note that I use an after-market IsoClean audiophile grade fuse, which to these ears delivers a significant increase in musical detail for relatively low cost.

For help in understanding the many improvements to the Gen. VIII Series 2, I spoke with Dave Reich, Theta's General Manager and Design Engineer. Reich got his start in the high-end industry at Dayton-Wright Industries, a Toronto-based company that made electrostatic loudspeakers. He eventually founded Classé Audio in Toronto, where he focused on analog components. (The DR in old Classé models stands for Dave Reich).

After building up Classé for 10 years, Reich took some time off, then spent four years with McCormack Audio in San Diego. When McCormack was sold to Conrad-Johnson, Reich, in his own words, "very quickly got scooped up by Theta, because they wanted to add analog amplifiers to their line. It was a very good fit." As well as managing Theta, Reich has been the man responsible for Theta's Dreadnaught, Citadel, and Enterprise amplifiers.

Below are the improvements that Theta claims from the Series 2 upgrade. Dave's verbatim commentary is included in quotations marks:

1. New 24/192 DAC chip.

2. Up to 132 dB dynamic range (a 15 dB improvement over the Generation VIII).

3. Over 3X differential output current: "DACs are more than anything current devices rather than voltage devices. So current levels are very critical. There's now three times the amount of current available as it heads to the output of the DAC chip. I've always been big on current, whether it's at the amp or the chip level. I think it allows the device to convert the bits with greater accuracy and ease."

4. Stop-band attenuation of –130 dB (a 48 dB improvement over the
Generation VIII): "That's the built-in filter that shuts down the high-frequency bad stuff that we don't want getting in there. It's doing that much more efficiently, and probably generating less aliasing frequencies that might end up in the audio band generated from much higher frequencies. That translates into less noise, distortion, and hard digital edge."

5. Pass-band ripple +/- 0.00001 dB (a 200X improvement over the Generation
VIII): "That has to do with the rejection of noise on the supply, or even noise carried along on the digital signals. It's doing it a lot more cleanly."

6. Fully differential DAC follower op-amp with 150MHz bandwidth (a 70 MHz
improvement over the Generation VIII): "We're using a better op-amp following the DAC. It sounds better to us, and measures way better because it's so much faster. It works with more ease, and is able to handle more of what's coming along without generating any of its own spurious noise or distortion.

"An op-amp is an integrated circuit, made up of a lot of transistors, that allows you to do many, many things. It gives you good isolation at the input and reasonably low output impedance. Sometimes you can approach ideal specifications in them for input and output impedance and other factors. Op amps have come light years compared to where they were in the old days. There are more than a handful of them that sound so good that most designers want to be using them as opposed to discreet transistors.

"A DAC follower generally means an op amp that you'll put after the DAC to set the signal to go to the output without loading down the DAC. The main purpose is to not load down the DAC, so you want a fairly high impedance there that presents an easy load to the DAC and isolates what the DAC has to do, so that it keeps it constant no matter what you might be hooking up externally to the DAC."

7. Four new discrete analog digitally controlled volume cards with zero offset
nulling: "We now use the volume control card which we use in our other products – anywhere we need level control – be it in our Casablanca processor or our DACs. We came up with this card a long time ago. It's basically a set of discreet resistors that are controlled by digital switches. At any given time, the signal is only going through a pair of resistors. That's pretty much as good as you can get to control level.

"We did a new layout for the Gen. VIII, and added an adjustment to reduce any spurious DC that was floating around. It was always very low, but now we can make it even lower. That increases your symmetry and creates what we find greater realism in the soundstage."