The Electro-Harmonix NY-2A belongs to a class of signal processors known as “optical compressors.” Like any other compressor, the basic function is to provide a relatively constant output for a varying input. This description can be extended to include a certain timing to the onset and conclusion of the effect. This would be called the attack/decay time constant. Together, a range of dynamic changes can be added to a signal in order to limit or accentuate the relative amplitude. A Compressor reduces “dynamic range” so that a more consistent or more “forward” quality is added to a recording or performance. It can also add a percussive edge or soften the glare of plucked, struck or resonant instruments or voices.
As the signal level increases, the overall gain of a compressor decreases. In most modern circuits, a variable gain amplifier (VGA), an “active” design, is used to achieve this compression effect. This is most often accomplished by the use of a variable element in the feedback loop of a high gain amplifier. One difficulty for such circuits is that they have maximum gain at zero or at very small signal levels. Noise is also a small signal, and it becomes amplified by the full gain of the VGA when no other signal is present. This makes for hiss (amplified white or “Johnson” noise), or occasionally even whoosh or rumble (amplified 1/f noise).
Another problem is that the variable feedback element, the “active” part of the circuit, which controls the gain in the VGA, takes some finite amount of time to adjust itself. This means that short, quick transients are not handled equally as long, continuous phenomena. Basically, VGA based compressor designs often do not compress the high frequency signal content as much as the low frequency content. The larger the amount of compression, the more sibilance is added to the output. This in turn requires complicated “de-essing” circuitry to counterbalance the effect. In summary, many popular VGA based compressors sound very different between low and high amounts of compression.
The “optical compressor” takes another route to achieve the compression effect. The heart and soul of this approach employs two simple components that together create a passive attenuator, much the same as a volume control. These components are the photocell and the lamp. Photocells are components that exhibit a resistance to the flow of electric current dependent upon how much light they are exposed to: more light means lower electrical resistance, less light means more resistance. By closely arranging a light source to a photocell and then isolating the pair from ambient light, we create a third new component known as an “opto-coupler.” A passive attenuator made with an opto-coupler is like a volume control with light twisting the knob instead of your fingers.
In an optical compressor, we combine an opto-coupler/attenuator together with a lamp driver and a fixed gain amplifier. As the signal level increases, the lamp driver illuminates the photocell. The resistance of the photocell decreases, shunting more of the signal away to ground. This, in turn, lowers the output. A varying signal varies the amount of attenuation in inverse proportion to the input. The combined effect is that the signal level is kept more or less constant. Because all attenuators possess some insertion loss, we need some gain to make up for the loss. This is achieved by a fixed gain amplifier, which, unlike a VGA, behaves the same for any signal level.
The characteristics of the photocell/light source combination are what determine the attack/decay time of the attenuation and any frequency dependent changes to the signal. The Electro-Harmonix NY-2A has three selectable opto-couplers, which present the user with a range of options. The LED opto-coupler has the fastest attack time (about 10mS) and a flat frequency response. It can be saturated so that some bite can be added if desired. The Incandescent lamp opto-coupler has the slowest attack time and also has a little kink in the response. By switching in the attack selector, this can be slowed down a bit more. It responds slower to higher frequency signals than the LED, thus adding some brightness to the result. It is difficult to saturate and very clean, unless intentionally overdriven.
Finally, the Electro-luminescent panel of the Electro-Harmonix NY-2A optocoupler has an interesting frequency dependent characteristic. EL lamps change colour with frequency. Photocells have a range of sensitivity but most chemistries peak in either the red/yellow range or in the blue/ultraviolet. Most EL lamps go from yellow green at low frequencies (30 – 60 Hz) to blue (300 – 600 Hz) to purple (2 to 20 KHz). Additionally, EL lamps produce less light at low frequencies, more in the middle, and most at the top until the capacitance of the lamp lowers the impedance to a short circuit somewhere around 80KHZ. The right choice of lamp, together with a spectrum sensitive photocell, will create a natural “de-essing” circuit that compresses high frequencies more than low. This is excellent for vocals and for loud blaring electric guitar. The EL opto-coupler in the Electro-Harmonix NY-2A has 2 settings: normal, which is generally flat except for a small bump in the bass, and squash, which compresses the top more.
The point at which the compression effect begins is known as the threshold. In the Electro-Harmonix NY-2A, this is controlled by two means: the pre-gain control and the compression control. When the signal level is below the threshold, the unit behaves as an amplifier with a fixed gain. Above the threshold, the gain reduction begins, peaking just before the saturation of the photocell at maximum opto-coupler illumination. The Electro-Harmonix NY-2A has a FIXED THRESHOLD and a VARIABLE INPUT. This is crucial to understanding how to best adjust your compressor to get the right amount of effect where you need it.
The fixed gain amplifier in the Electro-Harmonix NY-2A is, in some ways, a classical tube preamp architecture. However, the components used are very special. The noise level and linearity are optimised without the use of any error correction or negative feedback at all. This makes for high-performance, beautiful sound while maintaining an inherently stable gain block that gives new meaning to the idea of “the sound of tubes.” We could not have achieved this with standard tubes or transformers. The overall level is adjusted with the post-gain control on the front panel. This gain block is completely independent of the opto-coupler attenuator and will not change its characteristics unless intentionally overdriven. We have left that possibility for creative use of the distortion in special circumstances.
Electro-Harmonix NY-2A Features
- True bypass for each channel using mechanical relays
- Swedish Luhndal transformers at both the input and output of both channels
- Switchable electroluminescent, LED or incandescent lamps for distinct and unique compressions
- Dual mono or stereo with the ability to assign different compressions on each channel
- Link mode to bridge channels 1 and 2
- Each channel fully independent when Link mode set to off
- Input and output gain controls
- High quality detented potentiometers to ease recall of settings
- Switchable squash mode for additional high frequency compression in EL mode
- Switchable attack mode for slower attack in incandescent mode
- Switchable 20 dB pad at the input of each channel
- 8 professional, low-noise Electro-Harmonix vacuum tubes
- Fast response magic eye tube meters
