You are here

Symetrix 606

Dual-channel Delay By Paul White
Published October 1997

With their new 606 delay, Symetrix have attempted to combine today's technology with the user interface of yesteryear. Paul White finds out how well they've succeeded.

No matter how sophisticated effects units become, it seems that everyone still has a soft spot for the old manually adjustable digital delay boxes with no program memories and lots of control knobs. A user interface makes a world of difference to how well you can operate a machine, so it made perfect sense when Symetrix announced their intention to build a programmable delay machine with a knob‑driven interface that would deliver pristine sound quality and include a few added features to bring it up to date. Indeed, the two‑channel 606 is far more than just a delay effect — it can also produce the full range of modulated delay effects, from delay and slapback to chorus, flanging and phasing, both in dual mono and stereo mode. This isn't so unusual, but the designers have also added variable Q, multi‑mode filtering, room simulation, delay diffusion and a few other twists.

Despite this extra sophistication, the user interface remains relatively straightforward, thanks to the 10 rotary knob encoders used to select and change the effect parameters (although, as we shall see later, the interface isn't perfect).

A conventional volume pot sets the input level, and each channel has its own 4‑stage input metering, though a few practical tests indicated that the meters aren't equipped to warn of internal overload caused by setting ludicrous feedback values. I know this because I am the man who set those ludicrous feedback values, and the result sounded like a tortoise in a blender with a little top boost added! Most of the parameters, the titles of which are printed above and below the knobs, have familiar names, but there are a couple of less obvious ones that I'll explain when I get to them.

The Package

The 606 is powered directly from the mains and is packaged in a tough, dark blue rack case. Both the inputs and outputs are on balanced jacks rather than XLRs, which makes connection to a patchbay easier, and there are also MIDI In and Out/Thru connectors as well as a footswitch jack for accessing a tap tempo facility. SysEx patch dumps are catered for, both bulk and single. Each channel also includes a LED that flashes at the rate of the currently set delay time.

Unusually, the 606 still relies on a basic 3‑digit, 7‑sector LED indicator window for displaying patch numbers and therefore resorts to displaying either cryptic abbreviations or straight numbers for the parameter names when editing. Turning any knob by one detent click will cause the display to show the current parameter without changing it — subsequent clicks then increase or decrease the displayed value. A similar system is used for the Preset knob — one click brings up the current preset, then you can dial in any preset of your choice. Once the preset has been changed, the display flashes until you load in a preset with (perhaps unsurprisingly) the Load button. Hitting Load twice bypasses the currently loaded preset.

Patches 1 to 99 are user memory locations, and patches 100 to 110 are filled by non‑volatile factory building block 'template' programs (which can't be overwritten); these may be used as the basis for user patches. These factory patches are also copied into the first nine user memories. The rest are filled with examples of what the machine can do, though you can of course replace or alter these. If you press the Store button, the current effect will be stored in the location shown in the display window, providing it's a user patch location. There's a memory protect function in the global menu to prevent accidental erasure or modification of the user patches.

The 606 may be configured as two separate mono units, or the two channels can be coupled to work together to create stereo effects. This is determined using the Mode switch, though the coupled mode can only be selected if the current effect includes links between the two channels. For example, there may be a feedback path that crosses from one channel to the other.

Though there are 10 control knobs in the programming section, there are rather more available parameters than this, so Symetrix have added a Select button that scrolls around three possible sets of editable parameters, printed above and below the knobs. The lower line accesses Mix, Time, Feedback and Modulation for the two delay lines, while the next row up addresses Filter Frequency, Filter Q, Diffusion Amount and MIDI Clock Sync, again for both delay lines. The top line of parameters gets you into Mod Select, Mode Source, Mod Destination, Mod Level, Oscillator Rate, Diffusion 1 Model, Diffusion 2 Model and Room Size (of which more in a moment). At the end of the row is the Parameter Select/Parameter Adjust/Output Level knob, and further sub‑parameters may be called up by number (or occasionally by abbreviation) and then adjusted. For example, the filters offer eight different high‑pass, band‑pass and low‑pass modes, and these are all denoted by a two‑ or three‑letter abbreviation. The Select button is used to flip between parameter select and parameter adjust modes, and the operation is very simple, but it helps enormously to keep the included A3‑size plastic sheet, which has the block diagram printed on it, close by, as this shows all the parameter numbers at a glance.

. . .you can keep things simple if you want to, or you can move firmly into anorak‑land, designing your own effects with complex modulation sources and MIDI control.

The basic rate/depth/shape modulation parameters should be familiar enough and are used to create chorus, flanging, vibrato, phasing and so on, but what are Diffusion and Room Size parameters doing in a delay box? The 606 has an early reflections mode that may be explored by loading the ERL preset. In this mode, the inputs are summed to mono, then fed into a virtual room, the dimensions of which may be changed. This is quite a complex piece of processing and the various reflections are generated by ray tracing, rather like the graphics programs used to render complex 3D images. Various other parameters may also be adjusted, such as position within the room, the reflective properties of the walls, the distance from the source, and even the space between the listener's ears. The manual recommends that, for most effective results using the Room Simulator, the dry/wet balance should be set up in the 606 rather than using the 606 in the effects loop, as you might normally expect.

Diffusion is a separate block that comes directly after the filter block, but no specific description of this function is included in the manual. As far as I can tell from listening while tweaking, the intent is to create a natural 'smearing' effect by adding early reflection‑type multiple delays to the main delay. The delay feedback may then be taken from the output of the Diffusion module, from the filter or from the delay's untreated output.

The Filter section is rather like that of a synth with various high‑pass, low‑pass, bandpass and notch options, as well as a Q control that can take the filter right into self‑resonance. Each channel comprises an identical chain of identical processing blocks, apart from the room simulator — you only get one of those.

Flexible Friend

If you think the 606 sounds a bit more complex than a delay line with knobs on, you're probably right, but the idea of the design is that you can keep things simple if you want to, or move firmly into anorak‑land by designing your own effects with complex modulation sources and MIDI control. To see just how comprehensive the system is, take a look at Figure 1, which shows a considerably simplified version of the block diagram on the printed plastic sheet that comes with the 606. The card also includes (on its other side) a list of the editable parameter numbers, along with a description of what each does.

The Effects

The subjective quality of the effects is very clean; there's a maximum mono delay time of 2740ms and a stereo delay time of up to 1350ms. Conversion is 20‑bit delta‑sigma, producing a dynamic range in excess of 90dB, and with a frequency response of 20Hz to 20kHz within 1dB. With a maximum input level of +25dBu, there's also plenty of headroom.

A brief skip through the factory patches gives a good idea of what the 606 is all about. Because there is a filter section after the delay block, it is possible to emulate vintage tape echo units where the delays get progressively duller, and resonance can be added to create more synth‑like sounds. A panning option also adds movement to stereo effects. The room simulator early reflections may be combined with the delay‑based effects, and crossfeeding delays from one channel to the other allows interesting stereo effects to be set up. I was particularly impressed by the depth and transparency of the flanging and phasing effects, and bringing the filters into play means that some radical sonic transformations can be invoked.

I was also pleasantly surprised by the room simulator, which is very convincing in the early reflections department. It doesn't have the density or smoothness of decay of a top‑end reverb unit, but for adding space and texture to sounds such as vocals, flutes and synths, it's excellent.


Sonically, this is an excellent unit capable of some very nice delay‑based effects indeed, and those wanting a super‑quality, modern delay line with old‑fashioned controls will find they can use it quite happily on that level. However, those prepared to delve a little deeper into its operating possibilities will be rewarded by a much broader spectrum of effects than you'd expect to get from a regular delay/modulator device. However, I do feel it's fair to say that the lack of a proper LCD display with full parameter names doesn't do the unit any favours when you get into serious editing. Indeed, if you don't have the manual or the plastic flowchart sheet, it's quite impossible to figure out what some of the parameters are, as they're only referred to by number. I suspect this decision was made to keep the price of the unit down, and I have to admit I was expecting the price to be rather higher than it turned out to be, but it still seems to fly in the face of the original ethos of returning to a simple control interface.

It's a little tricky to pigeon‑hole the 606, because it's not a multi‑effects unit in the traditional sense of the word. At the same time, it's much more than a simple delay box, and with such a comprehensive modulation system, room simulator and resonant filters, the more adventurous programmer should really have it jumping through hoops in no time. It's only when you play with a box like this that you realise how versatile delay‑based effects can be, and when you take the excellent sound quality into consideration, the 606 is — despite its flawed user interface — excellent value.


In true analogue synth fashion, the 606 has a choice of six modulation sources which may be used to vary parameters in real time. These are arranged as six blocks where each has a user‑selectable source, destination and level. In addition to the usual triangle and sawtooth LFOs, there are random mod sources, sources that track the input signal level and MIDI controllers, which allow the use of mod wheels, breath controllers and footswitches, as well as control over MIDI Volume and keyboard position. There are 18 possible modulation destinations with a total of eight MIDI controller options — virtually every relevant parameter within the 606 may be controlled via MIDI. Delay effects may be synchronised using the tap tempo function, and it is also possible to sync delays to MIDI Clock.


  • Excellent sound quality.
  • Huge range of high‑class delay, filter and early reflections effects.
  • Comprehensive modulation and editing system.
  • Separate knobs provide direct access to the most important effect parameters.


  • In‑depth editing hampered by parameter numbering system.
  • Display shows only patch numbers, not names.


This is a wonderfully flexible, nice sounding effects unit that offers something a little out of the ordinary. Used at a fairly basic level, the front‑panel knobs make effect editing very easy, but in‑depth editing is more complicated than it needs to be, due to the lack of a multi‑character display.