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Alembic F2b meets KLON Centaur = Frog Fx1 Tube Preamp

I want to point out something that I very intentionally did, but really haven’t emphasized much at all and that is in regard to the 2 diode clipping options on the Frog Fx1 Tube preamp pictured below.  The clipping options were not in the Alembic F2b preamp or in the original Fender circuit.  I had some room available on the printed circuit board (PCB) when I designed it and decided I would add some diode clipping options to give a little bit of fuzz or overdrive tone to the preamp.  I was familiar with the Klon Centaur that was designed in the 1990’s and had built a clone of the circuit and liked the mild overdrive tones it gave me with guitar.  So, I researched the circuit diagram of the Klon Centaur (it is not tube based) and found where Bill Finnegan placed the pair of clipping diodes in the circuit.  They were right after the first opamp buffer stage and Bill only included one clipping setting.    So having found where they were placed, I tried placing the clipping diodes right after the first preamp stage of the Frog Fx1 (there are 2 preamp stages in the Frog Fx1) and found that they worked great in that position.  There were a couple different types of diodes I liked that gave distinctly different clipping characteristics so I added 2 different clipping options.  One is more of a mild overdrive and the other is a mild fuzz/distortion tone.  They are very touch sensitive, so their influence on the sound is related directly to the output/string attack of the instrument used.  I am sure there are many more that would sound great, but I only had so much room on the PCB.

Just so you know the street price of the Alembic F2b is about 1300.00 and the KLON Centaur has truly become mythological and runs between $2000.00 and $3500.00 when you find used ones online.  Makes the price of the Frog Fx1 downright reasonable!

Alembic, Fender, and KLON are the property of their respective owners.

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Why do I need a Preamp?

Why do I need a preamp?

What does a preamp do?

A preamp or preamplifier prepares the signal coming from a pickup or other instrument for additional amplification.

Here are 3 MAIN reasons to use an instrument preamp:

  1. It can boost a low signal
  2. It can clean up a signal so that it sounds better coming through the amp
  3. It can adjust the signal (e.g. volume control or equalizer)

For example, a pickup that’s plugged directly into an amplifier is a “passive” pickup. A pickup with a preamp, on the other hand, is an “active” pickup.  But then there is a cable (or some other device) between your instrument and your amplifier.

A note about cables:  A preamp can enhance tone that is lost by long cables, but make sure whatever instrument cable you use, it is fairly high quality.  I say fairly high, because, I believe you can pay too much for a cable.  There is the issue of diminishing returns on what you spend.  If you spend 10 times as much for one cable over another, but only gain 2% better/cleaner/ sound, was it worth it?  Can you even hear the difference?  That of course is up to you.  I do think you can buy a quality cable and not spend a fortune.

A good passive pickup should produce warm, full, tone.  Active pickups are usually louder and brighter and the preamp allows you to shape the sound of the pickup in a number of ways: volume, bass, mid, treble, gain, etc. You can assume, the more expensive the preamp, the more features and control it gives you over your overall sound.

In addition to controlling the output of your pickup, preamps can also improve their sound. If your pickup output is too low, the sound is thin, the signal is unbalanced, the signal is noisy, then a preamp might solve your problems. Most basic preamps will provide volume control and an equalizer (tone stack).  In other words, an instrument preamp is an electronic device that provides an end-to-end solution for taking the signal from a guitar and preparing a tone-shaped signal ready for further amplification.

It can be used to directly drive the signal into a power amp, meaning, a non-guitar/PA amplifier.  In fact, that is exactly what the Alembic F2b back in the 1970’s was used for.  They had Alembic F2b preamps directly connected into McIntosh audiophile power amplifiers.  Each instrument had its own stack of preamps, power amps and speaker columns.  Be careful though, some preamps are not able to drive a power amp based on the power amps input sensitivity.  The Frog Fx1 Tube preamp, based on the same Fender/Alembic F2b circuit is very much capable of driving a power amp directly.

One thing to watch out for when considering a preamp is noise. Whenever a signal is amplified, the goal is to keep the signal-to-noise ratio as low as possible. That makes sense because a little bit of noise from the pickup or the preamp can become a lot of noise when the signal is amplified and loud. In order to avoid introducing extra noise from a preamp, it’s good practice to place the preamp as close to the signal source as possible.

Additionally a preamp can be an effective way of overdriving an amp, where the instrument alone wouldn’t have enough power.  They are especially great for amplifiers that have a tube input stage as the first stage.  It can produce a nice creamy overdrive to sometimes an intense distortion sound depending on the output capabilities of the preamp.  The Frog Fx1 Tube Preamp has a lot of output capability for these features!

Many times, people confuse preamps with DI boxes.  A DI is not a preamp and a preamp isn’t necessarily a DI.  A DI, or “direct injection” box, converts an audio signal from unbalanced to balanced, and sets the output level and impedance of the signal specifically for connection to the input on a mixer.  Having said that, there are devices that have elements of both inside, but don’t assume that a preamp will function as a DI box or visa-versa.

A little terminology lesson:

Gain (or Drive) refers to the amount your signal level is increased. It is critical to understand how much a preamp can increase your gain, because each device that you send your signal to (power amp, mixer, instrument amp head, etc.)  has specific requirements for signal levels that are required for optimal results.  Not all preamps can drive a power amp. Some preamps have little or not gain. Others may be designed to boost the signal level enough to overdrive the input of a tube amp. Preamps have a gain control, while others have a fixed amount of gain. Typically, they will have a “volume” knob which just passively turns down the signal level at the end of the preamp circuit.

Tone can include anything from EQ controls, to “warmth” or other subtle qualities, to outright distortion. Some people want lots of tone changing and EQ controls, others want transparency, and others, everything in between.

Impedance can be described as the efficiency of the signal transferring from one piece of gear to another. An ideal impedance relationship is a very low output impedance number connecting to a very high input impedance.  If your instrument or device has an output impedance that is too close to the input impedance of the device you’re connecting it may result in your signal being too weak. That weakness may result in a lower signal level or a dull tone.

Another spec you can look for in a preamp is dB gain (amount the signal is boosted). A “clean boost” pedal may commonly offer 20 or 30 dB of gain, but it may take 50 or 60 dB gain to bring the output of a bass or guitar up to the level needed to drive a typical power amp.

Another factor to consider is if the output of the preamp is balanced or unbalanced. An unbalanced signal uses a regular instrument cable containing two wires, typically with a 1/4″ plug on the end. A balanced line uses three wires, and may have an XLR (microphone type) plug or a TRS (stereo) 1/4″ plug. You need to verify which of those types of connection is most ideal for the next item you’re going to plug the preamp into. There are some devices which can receive both balanced and unbalanced connections, but you cannot assume that about any one piece of gear. The instrument input of an amp head is unbalanced; most pedals are unbalanced; a DI output is balanced; many rack mount processors are balanced.

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Frog F1a Tube Preamplifier PCB for DIY


Class A vacuum Tube preamp based on the Fender Dual Showman preamplifier section which inspired the Alembic F2b preamp. It is high quality 1.6mm board with 2 ounce copper traces (double the normal printed circuit boards) with Gold plated solder pads.