Frequency‑shifting is a unique and rare effect. If you search your favourite online retailer for analogue frequency‑shifting modules, you’ll likely come up empty. This is because it requires some complex, highly calibrated analogue circuitry. So the Doepfer A‑126‑2 is a very welcome addition to their fantastic catalogue.
Frequency‑shifting is a close relative of pitch‑shifting. Pitch‑shifting keeps the relative (musical) logarithmic relationship so that your ear hears, for example, a C major chord transposed to a D major chord. Frequency‑shifting moves frequencies by an equal amount, which turns a C major chord into something alien and weird. So if an audio signal contains two frequencies, one at 1000Hz and one at 2000Hz (an octave apart), and the frequency‑shifter is set to add 400Hz, the result will be two frequencies; one at 1400Hz and 2400Hz (no longer an octave apart). The results can sound similar to ring modulation and, in fact, ring modulation is a critical element in the frequency‑shifting circuit.
The A‑126‑2 (and the optional A‑126‑2Exp expander) is a redesign of their long‑discontinued A‑126. It’s a slender 8HP module in Doepfer’s usual utilitarian design, although you can also buy it in black. Because of the complexity of the circuit design and the 8HP restriction, three circuit boards are stacked behind the front panel, making the module 60mm deep and unsuitable for skiffs.
Audio is fed into the module via a single mono input into a preamp. A small pot sets the level, and an LED indicates overload. Frequency is set with Coarse and Fine controls. Instead of frequency being bipolar, as you would find on most synthesis pitch controls, the A‑126‑2 shifts up and down simultaneously. A Mix control blends between the up and down signals. The frequency and up/down mix can be CV controlled with attenuation.
The output signal is controlled by an internal VCA, which can be externally controlled via CV or via a built‑in envelope follower, delightfully named Squelch. A small control mixes between VCA fully open and the envelope follower, handy for setting how much you want the envelope follower to affect the output volume.
As is often the case with their modules, Doepfer have opened up some of the inner workings of the circuitry. The frequency‑shifting circuit comprises something called a dome filter, a quadrature LFO and two ring modulators. The dome filter is a collection of all‑pass filters and produces two copies of the input signal that are +45 and ‑45 degrees phase‑shifted. These signals are fed into two ring modulators and multiplied with two outputs from the quadrature LFO, producing sine and cosine outputs. The quadrature LFO frequency sets the frequency shift amount. If you’re wondering what on earth that all means, it doesn’t really matter. But Doepfer have added some extra inputs (to replace the quadrature LFO) and outputs (of the quadrature LFO), so you can experiment to your heart’s content. One obvious reason you might want to do this is to extend the frequency range, as the internal quadrature LFO is limited to a range of 20Hz to 5kHz. Doepfer’s own A‑143‑9 Quadrature LFO module would make an ideal companion.
Speaking of access to the inner workings, I suspect most potential buyers will want to purchase the A‑126‑2Exp expander. It’s a 1U module with seven outputs and one input. The seven additional outputs are envelope follower output, dome filter (x2), ring modulator (x2), up shift, and down shift. A second audio input also mixes with the main input and allows for feedback effects, although you could employ an external mixer to achieve the same tricks. The A‑126‑2Exp expander is only compatible with version 2 board revisions, so ensure you have the correct A‑126‑2 before purchasing. Sadly I didn’t have an A‑126‑2Exp to experiment with, but it opens up the possibilities immensely and is cheap enough to be a no‑brainer.
So what can you do with the A‑126‑2? Frequency‑shifting has all sorts of uses. It is an excellent drum synthesis tool when the frequency is controlled by an external envelope. It sounds great on both harmonic, inharmonic, monophonic and polyphonic material. Mixing the dry signal with a gently shifted version gives you beautiful chorus and phasing effects (sounds sublime on electric pianos). Inserting a frequency‑shifter into delay or reverb can produce stunning, otherworldly time‑based effects. It’s great fun on vocals too. In short, frequency‑shifting is one of those effects that can be incredibly useful, experimental, subtle or extreme.
There are discontinued frequency‑shifters that cost two or three times the price of the A‑126‑2. I cannot find another analogue Eurorack frequency‑shifter. There are a few digital options, but such comparisons would be unfair (and frankly, I think analogue frequency‑shifting is the gold standard). So the A‑126‑2 represents a rare opportunity at a more than reasonable price. Doepfer’s attention to detail is fantastic, and don’t even question whether to add the expander to your cart. Just do it.