The Line 3 is the top model in a series of three new tube preamplifiers from Sonic Frontiers. Like the companyıs Power 3 tube power amps, the Line 3 takes a serious shot at the state of the art. The three Line series preamps share the same functions and front-panel layout, differing only in the extent and sophistication of their circuitry and power supplies. All three units come with an unusual, circular remote control, a mere 3 inches in diameter, that looks cute and feels good in your hand.

All three models have fully balanced, differential active circuitry, which I consider a big plus for sonic performance. It ensures that both phases of a balanced input signal are represented in each output phase and thus are represented in the unbalanced output, too. Further, this arrangement enables balanced output from single-ended input signals without additional circuitry. The Line 3 has several interesting, useful features. A memory system enables you to store startup volume and balance settings for the various inputs. Although the Line 3 does not have built in surround decoding, it has a surround sound processor mode, which bypasses all the preampıs functions and routes the signals from your surround processorıs mainchannel outputs directly to the preampıs output jacks. Instead of providing a simple headphone amplifier of its own, Sonic Frontiers incorporated one from HeadRoom with spatial processing that makes stereo recordings sound more natural through headphones. The rear panel also has an input jack for commands from infrared repeater pickups in other rooms and an output jack for a relay trigger that can be used to operate retractable screens or other home theater accessories.

Dimensions: Two chassis, each 19 in. W x 41/2 in. H x 141/4 in. D (48.3 cm x 11.4cm x 36.2 cm). Weight: 60 lbs. (27.2 kg). Price: $4,995. Company Address: 2790 Brighton Rd., Oakville, Ont., Canada L6H 5T4; 905/829-3838;

The display window in the front panel shows each channelıs volume setting and the status of the switches for the input selector, stereo/mono mode, output polarity, and muting. The display is flanked by the eight input-selector buttons on one side and the volume control and ³Standby/Operate² switch on the other. Below the display are the headphone jack and pushbuttons for output polarity, balance, mono/stereo mode, and muting. The main power switch is on the front panel of the power supply, which has the same size and general appearance as the preamp.

On the Line 3ıs rear panel are two sets of balanced and four sets of unbalanced normal inputs, a tape monitor, and the unbalanced bypass inputs for your surround processor. Each channel has two balanced and two unbalanced preamp outputs and an unbalanced tape output. Also on the rear panel is a multipin connector for the cable from the separate power supply and the infrared-repeater and relay-trigger jacks.

All input and output connectors are mounted to a circuit board at the rear Line 3ıs interior, together with the signal-switching relays. Most of the interior is taken up by the main board, which holds the tubes and associated signal circuitry, the power regulator circuitry and heat sinks, and the Headroom headphone amplifier. Sorbothane mounts protect this board from external vibrations. All control circuitry is in a shielded subenclosure just behind the front panel.

The interior of the Line 3's power supply is almost completely occupied by a large circuit board, whose mirror-image layout shows that this is a dual-mono supply. Each channel's supply has a shielded and potted toroidal power transformer, two high-voltage filter chokes, four Solen metalized-polypropylene filter capacitors, and heat sinks for its low-voltage regulators. Between the two channels' heat sinks is a third power supply, for the control logic, with its own power transformer. Parts and construction quality are first-rate in this beautifully made component.

Measurements

Frequency response with the volume control fully clockwise and instrument loading is plotted in Fig. 1. With balanced input and output (Fig. 1A), the channels act very much alike, but with unbalanced input and output (Fig. 1B), there is a noticeable difference between the channels at frequencies above about 40 kHz. Changing from instrument to IHF loading dropped the output by 0.05 dB for unbalanced connections and 0.07 dB for the balanced ones, but left the shapes of the curves pretty much unchanged. For unbalanced input and output, the frequency response did not change with volume-control setting down to -40 dB, but there was some high-frequency rolloff at lower settings, hitting 10 dB down at 200 kHz when the attenuation reached 70 dB; with balanced input and -70 output, the rolloff was not as great.

Rise and fall times with unbalanced input and output were 0.8 microsecond in the left channel and 0.4 microsecond in the right at an output level of ħ1.25 volts; IHF loading did not change these times appreciably. At ħ5 volts, some slewing became apparent and rise and fall times increased to 1.1 microseconds and 0.8 microsecond, respectively. Slew rate was about ħ10 volts per microsecond. A 20-Hz square wave exhibited a just noticeable tilt of about 5%. The common-mode rejection ratio (CMRR) is shown in Fig. 2 for the left channel; the right channel performed similarly which is to say, quite well. For the unbalanced outputs, CMRR is an indirect measure of the circuit's ability to represent both phases of the balanced input signal equally in the unbalanced output; the better the CMRR, the more balanced the two phases of the output are. As mentioned earlier, only a circuit with fully balanced, symmetrical topology can represent both balanced input phases equally in an unbalanced output signal. Output impedance was a low 50 ohms for the unbalanced main outputs, about 78 ohms at the balanced outputs, and less than 1 ohm at the tape outputs. Input impedance was 10.4 kilohms for the unbalanced jacks and 20.8 kilohms for the balanced ones.

Figure 3 shows total harmonic distortion plus noise (THD + N) as a function of output level with IHF loading. Compared to instrument loading, the IHF load did not increase distortion very much and had very little effect on maximum output. Distortion was generally lower with balanced input and output (Fig. 3A) than with unbalanced (Fig. 3B). As you can see, the Line 3 has the desirable property of maintaining virtually the same low distortion at all audio frequencies. The sudden increase in distortion at high levels, which normally indicates clipping at a circuit's output, is actually a function of input level in the Line 3, caused by overloading of its digital volume control, not its tube circuitry. With balanced input and output, the first measurable effect occurs at an input level of about 7.85 volts, and the onset of clipping becomes visible in the output waveform at an input voltage of 8.2 volts. For unbalanced input and output, the corresponding figures were 3.9 and 4.2 volts. I'm not stating this to find fault - the Line 3's input acceptance is certainly adequate - but merely to clarify which portion of the circuit is doing what. Interchannel crosstalk, with the volume control at its maximum, was lower in the balanced input and output mode, less than -110 dB up to 2 kHz, rising to about -90 dB at 20 kHz. For unbalanced input and output, crosstalk was less than -110 dB up to 200 Hz, rising at 6 dB per octave to about -73 dB at 20 kHz. Results were essentially the same at lower volume settings. In the right channel, DC offset at the preamp outputs was -5.0 millivolts for the positive phase and -0.7 millivolt for the negative; right-channel measurements were +2.3 and +5.1 millivolts. The Line 3 drew 0.7 ampere of AC from the line in the standby mode and 1.56 amperes when switched on. Input sensitivity measurements for various input/output combinations are enumerated in Table I. Noise measurements, which were very similar for the two channels, are given in Table II.

Use and Listening Tests

The Line 3's ergonomics and sound impressed me when I first put it in my system and kept on impressing me as I used it. I found myself enjoying music with the Line 3 and having no desire to go back to another preamp to improve the sound. Reproduction was very clear, spacious, and musically believable. Bass was tight, articulate, and solid; mids and highs were open and airy, with very little irritation. This is a very fine preamp, indeed.

The design of the Line 3 and its remote control make it a delight to operate. The clear, easy-to-read display of volume settings also makes it easy to repeat previously set levels and to adjust balance for particular recordings. It's also a great reviewing tool, as it's equally at home with balanced or unbalanced inputs and outputs. There were no surprises or glitches in the Line 3's operation, just perfect behavior.

If you get the idea that I like the Sonic Frontiers Line 3, you're right. It is one of the best preamps I've had the pleasure of using in my system, and it's become my current reference preamplifier. I highly recommend an audition - though it might also be rewarding to look into Sonic Frontiers' less expensive Line 1 and Line 2 models.

In the Line 3, all input and output switching is handled by relays. All inputs except the selected one are grounded. The input jacks are therefore connected to the relay contacts through series resistors, so that sources connected to the unselected inputs won't see a brutal short. The value of each resistor is 475 ohms, high enough to give the desired effect yet low enough to keep from slowing down the circuit and making it roll off high frequencies. The balanced inputs, of course, have such a resistor in each signal leg.

If a balanced input signal is selected, both of its phases are taken to the tape monitor selector relay; when an unbalanced input is selected, an additional relay switches the monitor input's ground connection from the negative phase of the balanced input circuit to ground. The tape output, which is also unbalanced, receives the selected input's positive phase through a buffer. From the tape monitor relay, both phases of the signal go to the volume control. Between that relay and the control is the relay for the stereo/mono mode selection; in mono mode, it connects the corresponding phases of the two channels together through resistors. Because of these resistors, the "mono" mode actually leaves about 8 dB of stereo separation, for natural stereo reproduction through headphones and, Sonic Frontiers says, to avoid altering the timbre of stereo recordings through unpredictable cancellations and reinforcements of different frequencies.

The volume control consists of two digital controlled attenuator ICs made by Crystal Semi conductor, one per channel, that adjust the Line 3's volume and balance in 0.5- dB steps over 95.5- dB range. Each of these ICs is a stereo unit, to handle the two signal phases. This attenuator has tight tracking between its sections, which insures good common-mode rejection over the volume control's range. A technical paper from Sonic Frontiers states that this volume control, together with its new tube circuitry, yields even better sound than its past designs that used four-gang, switched-attenuator volume controls. Next in the signal chain is the heart of the preamp, the vacuum tube line amplifier circuit. Its input stage is a differential amplifier consisting of one 6922 twin triode per channel, with an AC balance control between its cathodes. This control's wiper is connected to the plate of a 6GH8 pentode/triode tube's pentode section (more about its triode section later). This pentode functions as a constant - current source for the input differential pair and is connected to a regulated -125-volt supply. The plate outputs of the differential pair are connected through 12-kilohm plate-load resistors to a regulated 176-volt supply; each input triode conducts about 4.7 milliamperes.

From the input stage's plates, signals are capacitor-coupled to the output stage, which is configured as a cathode follower with a constant-current source. In this case, the cathode follower consists of paralleled halves of two 6922 dual-triode tubes while the constant current source uses the other halves of these tubes, also in parallel. Each channel therefore uses four 6922 dual-triode tubes, two per signal phase. This stage receives power at +125 and -125 volts from dedicated voltage regulators. The signal is directly coupled from the cathodes of these 6922s to the output.

A separate servo circuit for each output phase compares the DC level at the output to ground. The amplified error signal is applied to the middle of a voltage divider that is connected across the output stage's positive and negative 125-volt supplies. Another point, close to the -125 volt end of the divider, is connected to the grids of the cathode follower's paralleled current source. A third point on this divider, which is slightly negative with respect to ground, is connected to the cathode followers' grid-leak resistors. Thus, the servo keeps the output cathode at ground potential. It also keeps the output of the error amplifier at or near zero, maximizing the range over which the error amp can control the output's DC offset.

The two phases of the output are summed through matched resistors and applied to the grid of the 6GH8's triode section. Its cathode feeds a two-resistor voltage divider that ends at the -125 volt supply. The output of the voltage divider drives the control grid of the 6GH8's pentode section; because a capacitor bypasses this divider's upper resistor, the AC signal is coupled to this grid. This error-correction system helps keep the amplitudes of the two output phases equal, whether the input signals are balanced or unbalanced: Any difference between the two output phases modulates the pentode current source, modifying the effective drive in the input differential amplifier so as to restore the output balance. This error-correcting topology has been used in tube and solid-state instrumentation amplifiers, but I don't recall seeing it in tube audio circuits before. This is definitely not your everyday tube preamp!

Between the tube section and the output jacks, the balanced signal passes through three more relays. The first of these selects the surround-processor input, bypassing the Line 3's main input selector, tape monitor, volume control, and gain stages. Since the surround-processor input is unbalanced, the negative-signal leg of this relay is grounded when that input is selected. The second relay is used for polarity reversal. The final relay mutes the preamp by disconnecting its output jacks from the signal circuitry and grounding them. The Line 3's power supply regulators are as unusual as its signal circuitry-shunt regulators instead of the usual series type. In series regulators, voltage passes through the regulating element. In a shunt regulator, a fixed resistor is connected between the unregulated input and the regulated output and its output end is shunted to ground; the shunt is a variable element that passes just enough current to keep the output voltage constant. In both designs, the regulator is controlled by an error amplifier, which senses the regulator's output and feeds a corresponding error signal to the regulator's control input. For the concept to work, the amount of current flowing through the shunt element when the circuit is idling is typically about equal to the output load current. When the load current increases, the current in the shunt element decreases by the same amount. Since the total current flowing in the series resistor is therefore the same, the output voltage doesn't change. The disadvantage is that shunt regulators dissipate more power than series regulators do. Sonic Frontiers chose this topology because shunt regulators are said to maintain a very low output impedance out to high frequencies; the regulators in the Line 3 are said to hold their output impedance to less than 0.05 ohms up to 200 kHz, no mean feat. This low regulator output impedance helps the lube circuitry perform optimally over a frequency range wider than the audio band. Each channel's active circuitry has four of these regulators, two for its input stage and two for its output.