Alembic Preamps

Tone Dreams

What do you hear when you're dreaming of tone? Rolling thunder...punch and growl...melodic mids...clean and clear highs? Do you feel the earth shake or the warmth of glowing tubes? Do you see yourself dialing in every subtle nuance of your instrument's character? Do you want to own that dream tone now? Alembic bass preamps are the ultimate total tone control reality.
The Links of the Chain

Every component of your musical system has something to say about tone: the instrument, strings, preamp, power amp, and speaker cabinet holistically define your sound. But no link is designed to shape tone more than the preamp, and no company makes better sounding, higher quality preamps than Alembic.
Enduring Tones

All this talk about great tone is, well...great, but will these units really last? In all aspects, yes. Our preamps have withstood the test of time. Not only for ruggedness, but also for musicality. The first F-2B units shipped in 1969!

Alembic preamps sound as real and uncompromising today as they did all those years ago, and most units we built then are still in use today, many by the original owners. When ZZ Top's equipment was stolen, the first thing they replaced was their F-2B. Once you get a hold of tone like this, you don't let go easily.
Serious About Quality

Alembic products are built for extreme durability. We make our preamps, like all of our products , right the first time. We only use the highest quality components and meticulous assembly techniques to insure a durable and rugged unit you'll be proud to hear working in your rack any time and every time. The chassis are made from sixteenth inch thick powder coated steel. All cable assemblies are tie-wrapped and routed for minimum electronic and microphonic noise. All tubes are rigorously, sonicly checked so that no impurity or dull tone leaves the test bench. Every unit goes through a 35 point checkout before it is given a serial number and called an Alembic. To say we are serious about quality is an understatement.

The Models The F-1X and F-2B tube preamps give you everything you would expect from Alembic. The vintage tube warmth, intuitive bass-mid-treble controls, super-low noise floor, and precision engineering make these the finest preamps in the world.

The SF-2 can either be used as a preamp or as a stand-alone tone processor. The dual-channel Superfilter allows incredible, detailed control of the filtered signal as well the wet/dry signal blend. There is no other preamp, graphic EQ, parametric EQ, or filter unit anywhere from anyone that can shape tone like the Alembic SF-2.

    Jump to specific model: F-1X · F-2B · SF-2


The F-1X gives you a unity-gain impedance matching amplifier to feed guitar-level effects units. The effects return goes thru the tube gain stage and then to the tone controls. The signal from the tone controls is amplified and sent to the full-range output jack on the rear panel. This same signal is also fed to the crossover network. A transformer-isolated output feeds a 3-pin Cannon jack.
Circuit detail

Two input jacks provide the same functionality as found on the F-2B preamplifier. The input is buffered by a cathode follower tube circuit, which is a unity-gain impedance matching amplifier to feed the guitar level effects loop. This gives minimum loading on the output signal from the guitar while driving the effects loop gear with the same signal voltage level as the guitar but at a lower impedance, to isolate cable and other loading effects from the guitar pickups.

The other half of the 12AX7 tube provides the preamp's gain. This gain comes before the volume control and the 300 volt supply assures that even large transients are handled without overload or distortion. The tone controls are the interacting Fender circuit. The bright switch shunts high frequencies around the volume control. The deep switch changes a capacitor value in the bass control to change the frequency response (and a second section on the deep switch changes the gain of the following amplifier to compensate for the resulting change in signal level).

A solid state circuit restores the signal level after the volume control and provides impedance matching to drive the full range output jack on the rear panel. Additionally, it drives the crossover network. The active crossover network provides 12 dB/Oct slopes to divide the output for bi-amping. The crossover points occurr at 6 dB down so that the acoustic signals from the loudspeakers add seamlessly. A front panel control trims the high-frequency signal component to match the level of the low frequency output.

A transformer-isolated balanced output to an XLR type connector provides auxiliary Recording or PA feed. A switch selects the source to be the effects return (Pre) or the full range output (Post). The transformer is large enough to handle the low-frequencies without distortion to give good clear lows and a resistive pad isolates the transformer from loading effects of long cables to give clear highs.

The heater of the 12AX7 tube is supplied by a regulated direct current supply to assure low hum. The AC Line (Mains) connection is an IEC (computer-type) detachable cord. The supply voltage can be changed by moving jumpers inside the unit to operate from 100, 120, 220, and 240 volts for operation anywhere in the world. The F-1X is housed in a sturdy, road worthy, steel enclosure.


F-2B pic The F-2B contains two independant preamp/tone control sections. The amplification is entirely vacuum tube powered. It is normally used as a stereo preamp to interface stereo instruments to power amplifiers. Jacks are provided on the rear for outputs from each channel. In addition a Mono output jack is provided which combines both channels when the F-2B is used with a Mono power amplifier

Circuit detail

The F-2B was inspired by the Fender Dual Showman amplifier. In the late '60's, we routinely added preamp output jacks to Dual Showman amplifiers and substituted an external power amplifier for the built-in quad 6L6's, most usually a MacIntosh 75.

The F-2B came about to provide the same preamp functionality in a rack-mounting package, as the Dual Showman was considerably wider than the standard 19 inch rack.

We took the opportunity to make a small improvement in the input jack circuit, while providing identical functionality. Plugging into Jack 1 alone gives full sensitivity, while Jack 2 alone is attenuated 6 dB. If signals are plugged into both jacks simultaneously, mixing resistors combine the signals and give isolation to the two input sources. In the original circuit, the mixing resistors are always in series with the grid of the first stage, and contribute a small amount of Johnson (thermal) noise. Our jacks have additional contacts which completely switch out the mixing resistors when using Jack 1 and the ultimate quietness of the tube is preserved.

As in its predecessor, the instrument signal is amplified by the first stage before any volume control. Without the wide dynamic range provided by the 300 volt supply, the first stage could be easily overloaded by large transients which are characteristic of electric instruments.

The tone control section follows, with a circuit attributed to Tom Walker, wherein the Bass and Treble controls contribute mostly boost and the Mid control provides cut only. While there is no absolutely flat position, the response is reasonably flat with the controls set at 2 - 10 - 2, with the bright switch off. The circuit can be described as interacting, such that the frequencies affected by each control is changed somewhat by the settings of the other controls. Additionally, when all 3 tone controls are turned to 0 then no signal is passed at all. These effects, while not suitable for a music reproduction system, have nonetheless proved highly pleasing for electrical instruments and have been adopted by a wide group of manufacturers.

The Volume control immediately follows the tone control section. The Bright Switch connects a capacitor around the volume control, the value of the capacitor being selected to bypass only the high frequencies. The effect of the Bright Switch is also interactive with the Volume Control, and there is no effect if the Volume control is turned up to 10, and the magnitude of the effect changes, depending on the setting of the control.

The signal loss of the passive tone controls and the volume control is made up by the second stage, also a common-cathode vacuum tube stage. The second stage plate circuit feeds the output jack. Since the output impedance of the tube circuit is fairly high, the power amplifier(s) should be located adjacent to the F-2B so that short connecting cords can be used. While the output circuitry of the F-2B can satisfactorily interface with the vast majority of power amplifiers, the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity.

The mono output on the rear of the F-2B provides a facility to use a stereo instrument with a mono power amplifier. The mixing resistors associated with the mono jack are only connected together when a plug is inserted, thus preserving maximum stereo isolation when using the separate Channel A and Channel B outputs. Another application is to connect the two channels in cascade, by a cable from the Channel A output jack on the rear to the Channel B input jack on the front. The power amplifier is connected to Channel B's output. The signal from Channel A is high enough to drive the first stage of Channel B into distortion. The tone controls of Channel A are used to emphasize those frequencies which will be most distorted, while the controls of Channel B shape the color of the resulting distortion. Channel A's volume control sets the amount of distortion, while the Volume Control of Channel B sets the output level (Master Volume).

The power transformer has taps so that the F-2B can be operated from 100, 120 or 240 volt lines (Mains). It is necessary to open the cabinet and solder the correct tap to the power switch to make the voltage change, however. The Line (Mains) input connector is the IEC (computer type) so the power cord is detachable. The layout of the circuitry and the lead dress to the front-panel components are carefully planned to reject external hum fields. In addition, an internal hum-balance control rejects hum from the AC heater supply. The package is a single unit standard EIA rack mount steel enclosure designed to survive on the road.


The SF-2 Super Filter contains two tunable active filters of exceptional dynamic range. The filters can be configured individually as low-pass, band-pass, and high-pass. In the stereo mode, two channels are provided, each with a two channel mixer for combining the filtered and unfiltered signals. In an alternate mono mode, a three channel mixer combines the two filtered and one unfiltered signal paths, together with an input gain control. An instrument preamplifier with front panel input jack is also available for stand-alone use.

Circuit detail

The SF-2 filter circuit is a resistively tuned two-pole universal active filters, which yields 12 dB/Oct ultimate slopes in the high-pass and low-pass modes, and 6 dB/Oct ultimate slopes in the band-pass mode. The unconventional filter design was optimized for wide dynamic range, and achieves 20 dB better signal to noise performance compared with standard circuits. This increased dynamic range makes use practical with guitar level signals as well as line level signals. Additionally, the absolute polarity of the signals in the pass band of the filter in all three modes of operation is preserved, allowing the additive mixing of the unfiltered original signal with the filtered output without phase cancellation. A control calibrated in Reciprocal Damping Ratio units adjusts the response of the filter in the vicinity of cutoff, controlling the size of the resonant peak at the tuned frequency.

In the stereo mode, each channel is provided with a two channel mixer. The direct, or unfiltered, signal is adjustable from zero level to unity gain. The filtered signal is adjustable from zero to a gain of 3 times or 10 dB. The additional gain in the filtered section is provided for those settings when the filter is used to bring out softer spectral components of the signal. As mentioned above, in the passband of the filter, the polarity is preserved so that in combination with the direct signal, the resultant will always be additive, no matter whether in low- band- or high-pass mode. The amplitude of the filtered signal in the passband of the filter is also held constant when the damping ratio control is adjusted. The result is an easy to adjust unit, each control logically responding to its designated function.

In the optional mono mode, the output is derived from a three channel mixer, one channel for the direct or unfiltered signal, and a channel for each of the two filter circuits. As in the previous paragraph, the direct signal adjusts to a maximum of unity gain, and each filter can be mixed up to three times (10 dB) gain, needed when enhancing weak spectral components. The left-most control on the front panel serves as an input gain control (or master gain control) and provides unity gain when fully clockwise when the signal is connected to the mono line input jack on the rear panel.

An instrument preamplifier with 1 MegOhm input impedance is provided with an input jack on the front panel. The instrument preamplifier is automatically connected thru 'normal' springs when no jack is inserted in the line level input on the rear panel. A jumper block inside the unit provides signal amplification of 0 dB, 10 dB, and 20 dB for the preamplifier circuit by selection of feedback resistors. The unit is set for 10 dB amplification when shipped from the factory.

The Super Filter has many applications. As an effector for guitar and bass, an unlimited field of both dramatic and subtle spectral effects are available. And when the direct volume control is set fully counterclockwise, the filtered signal alone directs the ear's attention to the natural-sounding filter effect, unlike an equalizer where the input signal is always supplied to the output at unity gain.

Interesting vocal effects including band limiting can be dialed in. And the opposite of the last effect can be used to extend the frequency response of closed box speaker enclosures. The Super Filter is a very useful tool.