What’s the difference between opto, FET, VCA or tube compressors? Learn this and more in our primer.

"Making the needles stand on end” is how one recording engineer described the heavily compressed sound of ’80s rock bands. What is compression and how can we make the needles stand on end? For starters, we should clarify that we’re talking about dynamic range compression, using devices like compressor pedals, and not digital-data compression, such as MP3 encoding.

Essentially, compressors do what I do when I’m watching a movie after the kids are asleep. I turn up the volume to hear the dialog and quickly turn down the volume when the car chase and explosions kick in. It’s important to note that turning the volume up and down should not introduce distortion, as this volume change is done over many audio cycles. This is opposed to clipping the signal, which compresses the signal but introduces harmonic distortion.


Fig. 1. One of the great studio compressors, the Teletronix LA-2A Leveling Amplifier, first appeared in 1965 and was produced until 1969. This tube-driven electro-optical attenuator system is now available again from Universal Audio, which builds LA-2A repros in California using old-school, point-to-point wiring.

Let’s take a look at a classic studio compressor, the Teletronix LA-2A (Fig. 1). See those two big knobs on the front? One is marked gain and the other is marked peak reduction. If we start by turning up the peak reduction knob, the level of the loud parts of the signal will be reduced and the overall perception is that the program material is quieter. That’s when we can reach for the gain knob and increase the volume so that the overall perception is that we are back to where we started in terms of level, but with a reduction in dynamic range.

Essentially, compressors do what I do when I’m watching a movie after the kids are asleep. I turn up the volume to hear the dialog and quickly turn down the volume when the car chase and explosions kick in.

Dynamic range is the measurement of the difference between the quietest parts and the loudest parts. Metallica’s “Creeping Death” would have a fairly low dynamic range while Nirvana’s “Smells Like Teen Spirit” would have a larger dynamic range.

What about one-knob compressors? It’s possible to combine the peak reduction and gain knobs into one knob by using a pot that has two elements. These pots are usually used for stereo signals but we can coerce it into turning up the gain and peak reduction at the same time. This is what I’ve done with the Strymon OB.1. Since a compressor is typically near the beginning of the pedal chain, we can adjust the amount of increased gain to match the amount of peak reduction when using a medium-output pickup. This results in an experience where everything gets smoother, but not louder or quieter.

But what about low- or high-output pickups? The low-output pickup is not going to trigger the peak reduction as often and the signal will get louder as the comp knob is turned up. The opposite happens with the high-output pickups as they drive more peak reduction. This is where the level knob can be used to provide a fine adjustment on the amount of overall gain.

What’s the difference between opto, FET, VCA or tube compressors? The peak reduction circuit needs to be able to turn down the volume in response to a control signal—standard audio circuit components like resistors, capacitors, and op-amps can’t do this.

Opto compressors use a light-dependent resistor (LDR) and a fixed resistor to create a voltage divider that reduces the signal level. The resistance of an LDR goes down as it is exposed to light. Incandescent bulbs, LEDs, and electro-luminescent panels have been used to control LDR. One of the properties of an LDR is that it can be turned on (gain reduced) quickly, but turns off relatively slowly. This has a natural musical property and creates some the character of opto compressors.

FET compressors use JFETs or MOSFETs (two types of transistors) to create a voltage-controlled resistor that is used in a voltage divider like the LDR. But FETs are lightning fast and can clamp down or release as fast or as slow as the peak reduction circuit dictates. These compressors are usually associated with the pumping kind of sound you get from many compressor pedals.

VCA (voltage-controlled amplifier) compressors use an integrated circuit multiplier that can be very fast and precise. You are more likely to see a VCA in a rack unit than a pedal.

Many “tube compressors” are really opto compressors with tube gain stages, but there are circuits that use a tube as the variable gain element. The LA-2A we discussed earlier falls in this category.

What about blend knobs? Some compressors will allow you to mix your uncompressed signal with the compressed signal. If the compression circuit has a very aggressive squash, then mixing in some uncompressed signal will get some of the dynamics back while maintaining overall compression.

Do you need a compressor pedal? Before deciding, take some time to explore the various available technologies to discover which devices work best with your music and guitars.

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Some effects loops let you attenuate the signal level heading to the input of the effects device. Effects loops: What are they and why would I want one? Back


Some effects loops let you attenuate the signal level heading to the input of the effects device.

Effects loops: What are they and why would I want one? Back in the day, I went down to the local music store and bought an Echoplex tape delay, plugged it into my amp, and had a lot of fun. (Ironically, I also dreamed of the day when technology would bring us the “perfect delay”—one without warble and with perfect copies of the input signal. Live and learn.)

After a while, I noticed that something wasn’t quite what I was looking for, and it didn’t have to do with the recording limitations of the tape. I was using a decent amount of distortion in my tube amp and as the repeats trailed away, they got cleaner. In nature, echoes are copies of the original, and they get quieter and darker, but never less distorted. A trip down to my local amp-mod guy yielded an effects loop, version I. The idea was that you plug straight into the amp, and then break the signal chain after the preamp but before the power amp section, thus creating a loop in which you can insert effects. Using this loop, the input signal to the Echoplex now contained preamp distortion and the repeats didn’t clean up as they decayed.

Problem 1 solved.

On to problem 2. In general terms, the internal voltage levels in effects are 1V to 2V; in tube amps, they’re 100V to 200V. In order not to “smoke” the input stage of the Echoplex, the signal from the preamp stage of the tube amp needs to be attenuated by about a factor of 10 (20 dB). But now when the signal goes back in to the power amp, we have lost 20 dB of gain and can’t drive the amp to full volume.

Okay, back to the amp guy for more mods and effects loop, version II. The first mod is a send level control so we can adjust the amount of attenuation of the tube preamp stage to match the expected level of the effects input stage. The second mod is a return level control and an additional tube to add back the gain we lost sending the signal out. The biggest problem with effects loops is getting these two different animals to play nice with each other—and to do that we need someone in the middle.

So far we can see that the advantages of an effects loop are that we get to use the tube amp’s preamp distortion and plug the guitar straight in to the amp. But the level matching can be problematic without send and return level controls. Short of hacking up the insides of the amp, what can you do if you have an effects loop with no send and return controls? There are companies that sell level shifters that are basically transformers in a box, and these devices provide a fixed gain or attenuation of the signal. Unlike electronics, there is no defined input and output with these level-shifting boxes.

The loop we have been considering so far is a series loop. At the time I got my amp modded, parallel loops, if in existence, were not well known. In a series loop, the dry signal goes out and is mixed with the wet signal within the effect. In a parallel loop, the dry signal stays in the amp and the effect’s mix control is set to 100 percent wet. Now we can control the effect amount with the loop’s return level and thus preserve the dry path.

Some delay effects have an effects loop. A loop in a delay effect is not the same as what we have been describing here. The simplest of delays would record the input signal and play it back some time later. This would give you one repeat. I used to do this with a 3-head cassette deck. If we take the output of the delay and send it back to the input we will get more repeats—this is the feedback loop of the delay. (This control is labeled feedback on some delays and repeats on others.) Increasing the level of the feedback signal increases the number of repeats. If we break the loop, we can insert effects that will only act on the repeats. For example, if we insert an overdrive, the repeats will get distorted each time they pass through the loop. Or a low-pass filter will make the repeats successively darker.

What about an effect loop for a looper? That’s a topic for another day. Happy looping.

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