Audible Illusions Modulus 3A preamplifier:
Page 4 So I did. I ordered a pile of dots from Marigo Audio Labs in Skokie, IL, and placed them as instructed on top of electrolytic caps, transistors, and ICs (power supply regulation), on the power transformer, on the chassis, on circuit boards at strategic locations, and on my forehead before listening to Ravi Shankar. And based on conversations with a number of Mod 3 aficionados, I substituted a Marigo power cord for the factory job.

I placed the 3 on a Bright Star Big Rock 3, mass loaded with a Bright Star Little Rock, which sat on three inverted cones to allow adequate chassis ventilation. Obviously I chose that option to leaving the top cover off.

The results of my tweaks? Significant improvement in bass focus, and transient snap, but still not enough to satisfy. I'm not an easy customer.

The Modulus 3A: a totally new preamplifier
More than another update or modification, the new Modulus 3A is a complete redesign: only the chassis and front panel remain unchanged, though the latter now features cool, green, backlit control knobs, making in-the-dark use much easier. The innards are all new.

When Audible Illusions offered to gut my unit and turn it into a 3A, I was more than happy to oblige—after removing all of those Marigo dots and saving them on a sheet of waxed paper.

What I got back a few weeks later looked the same on the outside, but not on the inside, where the new main board is densely packed with even more high-quality parts than the old one. While the basic circuit design remains unchanged—in tube amplifier design, Art Ferris claims, there's really nothing new except for better parts and power supply designs—there are major differences between the 3 and 3A, not the least of which is the price: the 3A without the M3 Gold MC board now costs $1895! This is still a bargain for what you get physically and sonically.

There are now seven separate regulated power supplies—each section of the preamplifier now has its own dedicated supply. Relay switching is now used to cut down signal-path length and to provide true monophonic blend when the stereo/mono switch is utilized. The 3's Stereo/Mono switch narrowed separation but didn't fold down completely—an important feature when you play mono recordings. The 3A features a much larger main power supply, the transformer of which is now manufactured by Audible Illusions instead of being sourced from the outside.

There are other differences between the 3 and 3A, but more significantly, there are bigger differences between either Audible Illusions preamplifier and most others. Take the one-tube/channel line-stage: each 6DJ8 is a dual triode, which in the 3A is operated with the two halves parallelled in high-current class-A mode with no feedback—"balls to the wall," as Ferris likes to call it.

Another commonly used tube circuit, the "cathode follower" that serves as an impedance transformer and output buffer, is absent in Modulus preamps. Followers do lower impedance and allow a preamplifier to drive long lengths of interconnect, but according to Ferris again, the extra circuitry adds noise, veils the signal, and can cause phase shift.

Without an output cathode-follower circuitry, a preamp can suffer from high-frequency rolloff with long or high-capacitance interconnects. To minimize the possibility of this happening, the Modulus uses a combination of unique circuit techniques and special parts to up the current drive and lower impedance. Because the tube is run in parallel, the 6DJ8's normal 3.2k impedance is cut in half to 1600 ohms—which, though closer to ideal (most tube preamplifiers have about a 600 ohm output impedance), is still high in absolute terms.

Other circuit innovations enhance the preamp's drive capabilities. Specially designed conductive-plastic potentiometers minimize the capacitive loading effects of conventional pots. The result is a low enough source impedance (1.2k ohms) and high enough current capability for the Modulus 3A to drive long lengths of cable into even low impedance (10k ohms) amplifier inputs without instability or high frequency roll-off.

Because the line-stage flips phase, the main output of the Modulus 3 and 3A is absolute-phase-inverted. To restore correct polarity, you simply reverse your speaker cables: hot to ground and ground to hot on both channels. The Curl-designed solid-state tape-output buffer circuit doesn't invert phase.