Compressor Tests Part 3

In the first two parts of the compression tests, it has been shown that many of the guitar pedal compressors are actually acting as limiters, and will clamp down hard on the signal level once it has exceeded a set threshold. Tests of a buffer and the Dynacomp were compiled onto the same graph to better illustrate what is happening to the signal.

The signal level of an op amp buffer is drawn in blue. It is very linear so the line is straight from the lowest input level to the highest. The input and output signals are equal at every point. The red line on the graph above is the Dynacomp measurement. It can be seen that the signal is linear below the threshold, which is set at about -30db. Once the input signal has exceeded the threshold, the control signal causes the vca to clamp down on the output and limit the signal level so that it can rise only slightly, even though the input is still increasing. The limiter action is easily seen in the red line that is close to horizontal, while the blue buffer line is steadily rising.

In this part of the compressors comparisons, the next pedal to be tested is another classic model that is probably second only to the Dynacomp in popularity. Of course I mean the Orange Squeezer, and a clone of it was made for testing.

Up to this point, all of the circuits have had voltage-controlled amplifiers that are getting a DC feedback signal derived from the output to control the gain and the resulting compression. The Squeezer is also a feedback design but it uses a jfet transistor to produce the compression effect. It has only a single knob to regulate output volume so the threshold and ratio are fixed in this design and cannot be easily adjusted. The gain curve is shown below.

The test results from the Orange Squeezer reveal it to be a peak limiter also. The output/input curve shows the same type of limiting effect that was produced in the previously tested pedals. The threshold is close to -20db and is determined in part by the setting of the trimpot inside the pedal. Above the threshold, the signal is hard limited at a 10:1 ratio.

As far as compression is concerned, the Orange Squeezer will respond much the same as the Dynacomp derivatives. There are other factors that have an impact on the sound, so this pedal is not going to sound exactly like the Dynaclones.

A different compression response is available with the Keeley Compressor Pro shown in the first picture on this page. It is a more sophisticated design that offers actual peak compression instead of the limiting that was obtained with the pedals shown previously. In the graph below, I have included responses for the buffer (blue), Dynacomp (red), Keeley Pro set for 4:1 (green) and the Keeley Pro at 2:1 (pink).

Below the threshold, the output voltage equals the input voltage, so they were adjusted for unity gain (gain=1) in all cases, though I varied the thresholds slightly so the lines on the graph would be easier to detect. The gain above the thresholds are not the almost flat response of a limiter, but instead the compression lines are angled up due to the lower ratios. The lower ratio compression settings will give a sound that is less squashed on the peaks as compared to the Dynacomp or Boss clones. Lower ratios are less percussive and may have a more natural sound, while still taming the extreme signal spikes.

Clearly the Keeley Pro Compressor is a more advanced design with superior control over the peaks of the signal.

Another advanced design is the Empress Compressor, shown on the right side of the first photo on this page. Like the Keeley Pro, it has more controls to manipulated the response of the compressor. The Empress has a jfet control device as was used for the Orange Squeezer, but the compression curve is vastly different.

The first striking thing about the Empress response is the very soft knee transition. The knee is the speed at which the compression engages the signal and starts to reduce the gain. The Dynacomp has a sharp knee since the transition from linear response to the highly limited signal takes place over a very short input signal range. The Empress, on the other hand, has a very soft knee because the transition from linear to compressed is brought in gradually over a several db range. To further investigate this response, a scan was run with a lower ratio selected (shown below).

The response is very soft as the -30db threshold is exceeded, but interestingly, the compression ratio increases at stronger signal levels above -5db. Low compression ratios with a soft knee are hard to hear while playing, even though the signal peaks are being compressed. I would consider this type of response to be very transparent,

The AMZ Q&D 2 compressor, based on the SSM2166 chip, was brought back to demonstrate an entirely different type of compression. All of the previously tested pedals have been peak limiters or peak compressors, which means they perform gain reduction on signals that exceed a certain input signal level (or threshold). The Q&D 2 works differently as shown in the graph below.

The gain control of the Q&D is radically different because it is not a peak compressor, but instead is a full range compressor. The threshold is set for -16db and above that level the signal is compressed at about 2:1 until limiting kicks in at -6db. However, a significant difference is that the signal that is below the threshold is amplified at a 2:1 ratio, unlike the peak compressors that do not affect the dynamic range of low level signals. With the Q&D 2, all of the dynamics are being squeezed into a smaller voltage range.

Since full range compression amplifies low level signals, it will tend to pull tiny sounds out of the background and make them more distinct. This can be good or bad, depending on what is trying to be achieved. Because it reduces the dynamic range of the entire signal instead of just the strong peaks, the compression may be more obvious but with the advantage that it can smooth out the volume of the playing by making soft notes louder and strong notes quieter. It is definitely a different style of audio compression when compared to the many Dynacomp and Orange Squeezer clones that are on the market.

Lastly, a compressor was made with the NE570 compander chip that is widely used in BBD chorus and delay pedals. The values for this test board were taken from the Boss DC-2 Dimension C chorus pedal, and are typical of a common implementation of the NE570 as a compressor. The graph below shows the response drawn on the same scale with the buffer results.

The NE570 has a 2:1 compression ratio across the full audio range similar to that shown with the Q&D 2 immediately above. However, the NE570 has a higher threshold, which is around -2db. Low level signals will be boosted but with a reduced dynamic range, therefore soft sounds will be significantly boosted but stronger signals will have less boost.

It can be seen that the slope of the two compressors is very similar, but the Q&D has an adjustable ratio while the NE570 is fixed at the 2:1 value. Even so, it could make a very good fixed compressor pedal that sounds different from the many Dynacomp and Squeezer clones.

As an aside, the extensive testing that I have done with compressors, and particularly my own Q&D 2, has given me a better understanding of the workings of these circuits and presented me with ideas on how to improve the next version of the Q&D-SSM2166 compressor.

If you have not seen the prior articles on compression testing, you should read Part One and Part Two for additional information.

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