Anthem Pre-1 Preamplifier

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Anthem is a new series of tube components built by Sonic Frontiers, priced more affordably than the company's flagship line. Besides the Pre-1 preamplifier, the Anthem line includes an integrated amplifier, a power amplifier, and a CD player with HDCD decoding.



ABOVE:. Anthem Pre-1 Power supply, inside and out.

Like preamps of yore, the Pre-1 has a phono stage; it also has three high-level inputs, a loop for a tape recorder or external processor, and two sets of main outputs (useful for bi wiring or to feed active subwoofers). All in puts and outputs are unbalanced and use high-quality phono connectors. Also on the rear panel are a ground terminal (mainly for use with the phono input) and a multipin connector for the external power supply.

The Anthem Pre-1's front panel has three knobs, one each for input selection, balance, and volume plus pushbuttons for switching to and from the tape/processor loop, muting, and switching between standby and full operation. The Pre-1's tube heaters are al ways on; pushing the power switch to "On" just turns on the high-voltage rectifier tube's heater and initiates a turn-on delay. During this delay, the preamp's outputs remain muted until the rectifier tube warms up enough for the high voltage to stabilize. A green pilot LED on the front panel dims during standby, glows during normal operation, and flashes as a reminder when muting is turned on.

The Pre-1's power supply is housed separately, to allow plenty of space for the pre amplifier circuitry and to eliminate the possibility of induced hum from the power transformer. The power supply's captive output cable plugs into the preamp chassis; the AC line cord plugs into an IEC socket on the power supply. A power-on LED is on the front of the supply.

The Pre-1 is an attractive piece of gear, its construction very neat and workmanlike. This applies not only to the outside but to the interior, which is nearly filled by the main circuit board. This thick, double- sided board is populated with many high- quality components, including capacitors by MIT, Solen, and Wima. A small p.c. board, attached to the rear panel, carries and connects the input/output jacks and the selector switch (operated by a long shaft from the front-panel selector knob). An other small board, attached to the bottom of the balance and volume controls, holds the gain switches for each channel's line-amplifier stage.

Measurements:

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Table 1--Input sensitivity at maximum and minimum gain.

Low-Pass: Setting Line Input to Main Out Line Input to Tape Out Phono Input to Main Out Phono Input to Tape Out; LOW GAIN: 324.7 mV 555.9 mV 1.19 mV 2.15 mV Sensitivity HIGH GAIN 19.45 mV 555.9 mV 730 mV 2.17 mV

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CIRCUIT HIGHLIGHTS:

A relatively new circuit topology, a mu follower forms the basis for most of the Pre-1's signal circuitry. This is similar to a cascode connection where the first of two devices drives the second device through the latter's emitter source or cathode In the mu follower the first tube s plate drives the second tube s grid via a coupling capacitor. Two resistors are in series between the second tube's cathode and the first tube s plate one resistor acts as a self biasing cathode resistor for the second tube and the other acts as a plate load for the first tube. A third resistor connects the midpoint of these two resistors to the second tube's grid serving as its grid leak resistor. The output of the circuit is taken from the second tube s cathode. This topology has two benefits: The second tube acts as a cathode follower and multiplies the effective value of the plate load resistor; And the first tube's gain unrestricted by output loading approaches its nominal amplification factor or mu -- hence the name, mu follower.

The phono preamp circuit s first stage is a cascode connected triode. (The tube used a 6922 can yield quite low noise and in the cascode connection is very linear). The plate output of this tube is capacitor-coupled to the second stage (a 12AT7 triode connected as a mu follower) through an interstage RC equalizer network that performs part of the RIAA equalization. The rest of the equalization is performed by an RC feedback network from the output of the 12AT7 back to its input and by the resulting interaction with the interstage RC network.

Signals coming into the Anthem Pre-1 pass through the selector switch to the switch for its tape monitor loop (or external processor loop) via a 1 kilohm resistor at the input to the monitor switch and another such resistor at the tape outputs. The wiper (output) of the monitor switch then passes to the balance control which is connected as a variable resistor in series with the volume control. Switching the Anthem Pre-1 to low gain mode inserts an additional resistor between the balance and volume controls. Switching in this resistor curtails the balance control s adjustment range. It also reduces the circuit s high frequency response and increases noise because it raises the impedance seen by the grid of the first stage.

Following the wiper of the volume control is the output amplifier which not surprisingly is a single mu follower stage. This stage inverts signal polarity (it's to Sonic Frontiers credit that this is mentioned in the Anthem's specs) so the Pre-1 inverts overall polarity from any input to the main output but not to the tape output.

Power-supply circuitry is quite extensive in this design In the high voltage supply the tube rectifier is followed by a three stage RC filter that feeds the main high voltage regulator. In this circuit which I call a zener follower a constant current source feeds a shunt regulator formed from a string of zener diodes. The output of the shunt regulator then feeds the gate of a MOS-FET series pass transistor whose source terminal is the circuit s final regulated output of 245 volts.

Following the main high-voltage regulator are six additional high voltage regulators, three per channel. A 24-volt zener diode and a resistor are connected in series between each regulator s high voltage input and ground the voltage drop through this zener goes through a resistive divider to supply +12 and -12 volts for the regulator s error amplifier op amp. The op amp s output drives the gate of a MOS FET series pass transistor whose source terminal feeds about 233 volts to the preamp circuitry the source terminal s output is also applied to the negative input of the error op amp In each channel separate regulators feed the phono section s first stage its second stage and the line output section. Each channel also has separately regulated tube-heater supplies for the phono section and the line output amp.

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IHF sensitivity measurements for the Anthem Pre-1's two gain settings are given in Table I. Frequency response varies some what with gain mode and volume setting, but mostly above 20 kHz (Fig. 1). When volume is up full (the "0 dB" curves), the rolloff above the audio band is more pronounced in the low-gain mode; with the volume set at -15 dB or lower, the high-frequency responses are about the same in both modes. However, if you use the low- gain mode with low-output sources, you'll need higher volume settings and therefore get more rolloff above 20 kHz.

Loading also affects the Pre-1's frequency response. With an IHF load, there's some noticeable rolloff at each end of the spectrum (Fig. 2, measured in high-gain mode with the volume control turned fully up). The low end rolls off because a high-pass filter is formed by the Pre-1's output coupling capacitor and the 10-kilohm resistance of the IHF load. The high end rolls off because of a low-pass filter formed by the Pre-1's output impedance and the IHF load's 1,000-picofarad capacitance. Still, this preamp does well with the IHF load and will therefore drive a reasonably long cable to your power amplifier and will drive most solid-state power amplifiers without a problem. Rise and fall times varied.

At worst, they were 5 to 6 microseconds (in low-gain mode with the volume fully up); they were only about 1 to 2 microseconds in either mode with the volume control down in the -20 dB range and reached a low of 0.6 microsecond (in the high-gain mode with volume fully up and with instrument loading). Volume-control tracking between channels was within ±0.5 dB down to -70 dB, which is very good.

The Anthem Pre-1's total harmonic distortion plus noise (THD + N) is plotted as a function of output voltage for the right channel with instrument loading (Fig. 3A) and with IHF loading (Fig. 3B). Note the prodigious output voltage this preamplifier can deliver with low distortion. With the instrument load, the distortion is admirably uniform with frequency; the 1,000-picofarad capacitance of the IHF load does cause somewhat more distortion at 20 kHz than the instrument load does, but that's to be expected.

Crosstalk generally increased with frequency, at about 6 dB per octave over the audio frequency range, and varied with direction and gain mode. The worst case was from the left to the right channel in low-gain mode, where it was -its dB at 20 Hz, -85 dB at 1 kHz, and -60 dB at 20 kHz. Switching to high-gain mode or measuring from the right to the left channel improved these figures by about 10 dB.

Output impedance did not vary with the gain setting, measuring 385 ohms at 1 kHz for the left channel and 411 ohms for the right. Input impedance, however, varied quite a bit with gain. In the high-gain mode, impedance with the volume control at maximum was just about 24.5 kilohms in either channel; with the volume lowered 20 dB, it rose to 48.3 kilohms. In the low-gain mode, the impedance was too high to reliably measure with my standard technique, but from the schematic, I estimated it as about 417 kilohms. In either gain mode, each channel's in put impedance increased by as much as 100 kilohms when the balance control was set to increase the output of the other channel.

The Pre-1's IHF signal-to-noise ratio in the high-gain mode was 86.1 dB for the left channel and 86.5 dB for the right. In the low-gain mode, the Pre-1 was noisier, with an IHF S/N ratio of 79.6 dB in either channel; this increase was caused by a series resistor used in that mode (see "Circuit High lights"). In high-gain mode, A-weighted output noise was 43.6 microvolts, worst case. It dropped to 24.1 microvolts with the volume control at maximum and to 17.1 microvolts at the minimum volume setting. In low-gain mode, worst-case noise (63 microvolts) occurred at the maximum volume setting; the noise fell to 16.8 microvolts at minimum volume. The channels matched quite closely, and neither was consistently the noisier.

The phono preamp section's RIAA equalization accuracy is presented in Fig. 4 for the left channel; right-channel response was almost identical. The admirably flat curve made with instrument loading is essentially what you'd get from the Pre-l's main outputs if there were no loads on its tape outputs. With IHF loading, the RIAA response rolls off by about 1 dB at either end. You're likely to get some of that rolloff with a tape deck or external processor connected to the Pre-1 but not as much as can be seen here. This is because the combined capacitance of the deck's or processor's in put and the cables feeding it will probably be lower than the IHF load's 1,000 picofarads. If the processor or deck has an input impedance of 10 kilohms or less, the bass will roll off as shown.


Fig. 1-Frequency response at various level settings.


Fig. 2-Effect of loading on frequency response.


Fig. 3-THD + N vs. output level for instrument load (A) and IHF load (B).


ABOVE: Anthem Pre-1 (top down inside)

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ASSOCIATED EQUIP. USED

Equipment used in the listening tests for this review consisted of:

CD Transports: Sonic Frontiers SFT-1 and Counterpoint DA-11A

CD Electronics: Genesis Technologies Digital Lens anti-jitter device; Sonic Frontiers SFD-2 MKII, Classé Audio DAC-1, and Manley Reference D/A converters

Phono Equipment: Oracle turntable, Well Tempered Arm, Accuphase AC-2 moving-coil cartridge, and Vendetta Research SCP-2C phono preamp

Additional Signal Sources: Nakamichi ST-7 FM tuner, Nakamichi 250 cassette recorder, and Technics 1500 open-reel recorder

Other:

Preamplifiers: Quicksilver Audio preamp, Forssell balanced tube line driver, and the reviewer's passive signal selector/volume controller

Amplifiers: Sonic Frontiers Power-3 mono tube amplifiers, Anthem Amp 1 tube amplifier, Quicksilver M135 mono tube amplifiers with Svetlana EL34 output tubes, and Arnoux 7B digital switching amplifier

Loudspeakers: Genesis Technologies Genesis Vs

Cables: Digital interconnects, AES/EBU balanced Illuminati DX-50; analog interconnects, Transparent Cable Music- Link Reference (balanced) and Music and Sound (unbalanced); speaker cables, Transparent Cable MusicWave Reference

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Fig. 4-RIAA equalization accuracy.


Fig. 5-Phono overload vs. frequency with instrument load.


Fig. 6-Square-wave response for 10kHz at 2V out, peak to peak (top); for 1 kHz at 2, 4, and 6V out, peak to peak (middle); and for 40 Hz at 2V out (bottom).


Fig. 7-Phono section THD + N vs. level


---Rear panel of the Anthem Pre-1.

Figure 5, phono overload versus frequency, indicates what output voltages can be attained at 2% THD -I- N with instrument loading and the input voltages needed to produce this output (left channel shown; the right was almost identical). In an ideal phono preamp, the output voltage would be flat with frequency and the corresponding input voltage would be the exact inverse of the RIAA equalization curve. In the Pre 1, the attainable output voltage is very large, with a corresponding input acceptance of 200 millivolts at 1 kHz. At high frequencies, the Pre-1 departs from the ideal, most probably a sign that the input stage overloads before the output stage does. With IHF loading, the attainable output volt age dropped about 80%, and 1-kHz input acceptance fell to 52 millivolts.

In the pre-equalized square-wave responses of the Pre-l's phono stage with instrument loading (Fig. 6), the 1-kHz traces (middle) reveal both a virtue and a vice of the Pre-1. I have overlaid three 1 -kHz traces, for output levels of 2, 4, and 6 volts, peak to peak. Some asymmetry begins to appear at 4 volts out and becomes quite noticeable at 6 volts, a sign that the Anthem's high-frequency acceptance is less than perfect. But the 4- volt square-wave output is still quite good and better than you'd get from many other phono pre amps. The 40-Hz trace's tilt (bottom) indicates that response falls off below 20 Hz.

The Pre-1 phono section's left- channel THD + N is plotted in Fig. 7 as a function of output level with instrument loading. (The 20-Hz curve is higher than the others at low outputs because its measurement bandwidth extends down below 10 Hz, whereas the measurements for the other curves are cut off below 400 Hz. This filtering is used when measuring phono stages so that THD readings won't be swamped by low-frequency noise, which is boosted by RIAA equalization.) With IHF loading, distortion was about twice as high, and maximum output was more on the order of 10 to 12 volts.

Interchannel crosstalk via the phono input was better than -80 dB up to 5 kHz and was 68 dB down at 20 kHz. With an IHF dummy moving-magnet cartridge load terminating the undriven channel's input, crosstalk from the left to the right channel was -53 dB at 10 kHz but only -70 dB at 10 kHz in the opposite direction.

The IHF signal-to-noise ratio from phono input to final output was 75 dB for either channel, based on an assumed input level of 5 millivolts from a moving-magnet cartridge and an output level of 500 milli volts. The Anthem's A-weighted noise, referred to its input, was 0.32 microvolt with the input short-circuited, 0.41 microvolt with a 1-kilohm source impedance, and 0.73 microvolt with the IHF simulated moving-magnet cartridge load, all for the worse (right) channel.

Use and Listening Tests

For my first listening to the Pre-1, I tried the Anthem in a familiar setup, where I use an Arnoux 7B switching power amplifier with either a Quick silver Audio preamp or my passive signal selector and volume control. Here, the An them sounded very good and similar to the other preamps.

I next set up the Pre-1 between a Manley Reference 20-bit D/A converter and a pair of Sonic Frontiers Power-3 mono tube amps, which fed a pair of Genesis V speakers. Because the Anthem Pre-1 has only un balanced inputs and outputs, I had to switch from my usual Transparent Audio Reference balanced interconnects to Music and Sound (MAS) unbalanced interconnects. I listened for a while with the Manley feeding the Power-3s directly via the MAS cables, and the sound was very good. I then inserted the Pre-1 between the Manley D/A and the Power-3s.

When evaluating components, I feel it's essential to compensate for any polarity reversals they intro duce. Because the Pre-1 inverts polarity and the Genesis V speaker has a multipin connector between its woofers and servo amp, I couldn't just reverse one end of each speaker cable; I also had to make up polarity-reversing cables to feed the servo amp's line inputs.

With the Anthem Pre-1 added to my system and polarity corrected, I could detect some loss of "thereness" in the reproduction, but the sound was still very good overall.

The noise level of the Pre-l's phono stage was low enough for me to try feeding a low- output moving-coil pickup directly to the phono input, without bothering to use a step-up transformer. At normal volume set tings, I could hear some noise right at the speakers, but the sound was so good that I felt no need for the transformer. I was very pleasantly surprised at how good my records sounded.

To see how the Pre-1 sounded with an amplifier in the same price category, I also paired it with an Anthem Amp-l, a tube power amplifier of 40 watts per channel. With CDs, this combination produced a smooth and spacious musical sound.

The Pre-1 operated flawlessly in the lab and in my various listening systems. All of its controls worked smoothly and had a nice feel. This preamp sent nary a click or pop to the amplifiers when I turned it on or off. Overall, the sound I heard from the Anthem Pre-1 was spacious and detailed, with natural tonal balance. Bass quality, extension, and impact were all excellent. Further, I was pleased that I never heard any undue edginess, in any of my listening systems; to me, that is a major positive attribute.

I liked the Anthem Pre-1 quite a bit. And my experience with the Anthem preamp and amp together convinced me that Sonic Frontiers' Anthem components do, as claimed, deliver high-end sound at relatively affordable prices.

[ Orig. publ. in Audio magazine/APRIL 1997]

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Updated: Friday, 2015-06-05 6:02 PST