Extensive compressor reviews and FAQ

Viva Analog Envelope Lock: Viva Analog is best known for their pedal-format clone of the Acoustic 360 preamp. If you haven't checked that unit out yet, you should. Viva is also an old-school deity among the DIY pedal-building community. Recently a reader let me know that Viva has a lot of other interesting products that haven't gotten as much attention as the 360 preamp, including this compressor.
First thing I should tell you is this pedal is not for beginners. It is capable of great-sounding results, but it is also capable of destroying your speakers if you aren't careful. Also, you must be willing to very patiently tweak knobs until finding the sweet spot of ideal performance; the settings outside that sweet spot can sound crazily broken. This pedal is not for the plug-and-play user, but the effort of careful experimentation does pay off.
Controls include Sense, Ratio, Thrsh (threshold), and Boost. These knobs are all highly interactive; they do "what they say they do", however each of them has an impact on the behavior of the others. Ratio and Threshold are relatively straightforward, and each has a very wide range. Sense controls the strength of the signal going into the sidechain that feeds the optical gain reduction element. So it is like an input gain control, except that it doesn't add gain to the audio path. Similarly, Boost controls the uncompressed gain level of the gain reduction circuit, but there is no extra gain stage at the audio output. The designer made these choices to reduce the noise, clipping, phase distortion, and frequency rolloff that can occur when using multiple gain stages. For the same reasons, there are no capacitors in the entire circuit.
There is a purple LED that indicates whether your signal is over the threshold. Even though I said the Thrsh knob is "relatively straightforward", actually it can be maddening because there is only a tiny, tiny sweet spot where the LED indicates the threshold status the way you'd expect, by flashing only on your signal peaks. Above that narrow point, the LED stays off and your signal is not compressed. Below that point, the LED stays lit solidly, no flashing, because your signal is entirely over the threshold. The Sense knob helps to fine-tune this setting. I found it works best to start with Sense at 12:00 and Thrsh near minimum, and slowly turn Thrsh up (while playing your instrument) until the LED just starts to black out; then spend some time fine-tuning between the Thrsh and Sense knobs to get the correct operation. You may find that the LED stays on all the time even at settings where the compression sounds best to you. The LED does not indicate bypass status! There is no on/off LED.
There is also a toggle labeled "C/L" to select compression or limiting modes. In the limiting mode, the Boost knob sets the maximum output level, in the way you'd expect from a "normal" compressor's level or volume knob. The L mode is easiest to use and understand, and it does a very good job of controlling big signal peaks--though it will flatten a larger percentage of your signal than some people would want. It's not a squashy sounding effect, but the threshold is tricky to set, so it will likely end up being a low threshold once the peaks are under control. The action is clean and smooth.
In the compression mode, Boost still affects the levels during compression, however as the comp releases, the circuit will try to apply nearly infinite increasing gain in inverse proportion to your signal level. Yes, I said nearly infinite gain--meaning the volume output can easily blow the hell up out of control, clipping the holy crap out of everything in its path, and blowing your speaker cones right out of the cabinet. At many settings of the various knobs in C mode, this pedal can seem like it is on a mission to destroy everything rather than to nicely smooth your dynamics or regulate your peaks. And I don't mean "destroy" in a good way, I am not being cute about some cool distortion sound. But here's the thing--when you finally dial in the sweet spot, the C mode can provide wonderfully smooth compression with maximum sustain, without adding any noise or altering your tone at all.
It has zero loss of highs or lows, and the tone is completely transparent. At settings where the compression is heavy and the gain is blowing up, the lows and mids get boosted drastically stronger than the highs, so in those cases the highs will sound rolled off in comparison; however those are bad settings you will not want to use anyway. At "proper" settings, the highs are all there. As mentioned before, it adds no noise. The circuit design also has very high headroom, so even line-level gear or very high-output instruments will not clip it.
There are in and out jacks for the sidechain, so you can use external processing to affect the compression response. The most common use would be to put an EQ pedal in that loop, which allows you to adjust the intensity of the frequencies that the comp engine "hears", without applying any EQ to your audio signal. For example if you turn down the lows on the EQ, the compressor will react less strongly to the low-frequency peaks in your audio, minimizing the amount that those peaks cause overcompression that can "swallow" the highs and dull your tone. You can also experiment with other types of processor in the sidechain jacks. The jacks are 1/8", so you will need special adapter cables to connect them to most other gear.
It runs on 9V AC only, so you will need a special power supply for it. The construction quality looks more like a DIY project than a commercial-looking product, but it uses very good components. One quirk to note is that the pedal may glitch or act funny when you first power it on, but it will settle down and behave once you start running audio through it and finding the correct settings. The housing is the small MXR size. The footswitch is "true bypass".

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