Continuing on my quest to understand all there is to know about how the Quilter Micropro does what it does, I turn now to the Limiter. Depending on what comments you might have read in forums, the Limiter is either the most useful feature ever created, the most misunderstood, or the biggest waste of space on an amplifier front panel. I will readily admit that I am firmly in the first camp - I love the Limiter feature and use it all the time.
My goal here is to explain and demonstrate what the Limiter does, how it does it and hopefully dispel some of the misunderstandings surrounding this feature. Of course, tone is in your ears and YMMV so while I get that I may not change your mind, I at least hope you'll get a clearer idea of how this feature works.
What is a Limiter and isn't it just a compressor?
Before I get into specifics regarding the Quilter Limiter, I think it's important to talk briefly about limiters and compressors in general. Both devices affect the dynamic range (the difference between the loudest signal and the softest signal) but they do it in different ways, and for different reasons.
A limiter's job is to keep the peaks of an audio signal from exceeding a certain maximum level, most usually to prevent clipping. In sound reinforcement applications, a limiter is used to protect high-frequency transducers (tweeters and horns) by preventing excess heat from clipped signals burning out the voice coils (a clipped signal has significant high-frequency content). This maximum level is called the "threshold level" and any part of the signal that exceeds that threshold is reduced in level. The limiter does nothing to a signal that is below the threshold.
A compressor, on the other hand, limits high-level signals but adds a second feature - it boosts low-level signals. The result is that the overall signal level is "squished" - the peaks are lowered, and the softer signals are boosted to even out the volume. This can add sustain to a note or chord by increasing the level as the signal begins to fade, keeping it at a higher level for a longer period of time. Since a compressor is a time-based device - in other words, its response changes over time - it helps to look at its performance with a dynamic signal, such as a since wave burst.
The upper trace is the input, the lower is the output. We can easily see the characteristic performance of the compressor: The initial attack gets through but is quickly clamped after about 25mS and the level remains constant for the remainder of the burst but at a slightly lower level. When the burst ends at 100ms, it is still being clamped, so we see the release time of about 80mS as the level increases. The effect of the added gain is clear - while the input signal is fading, the output actually increases in level for nearly 500mS before finally beginning to fade. We can hear this as "blooming" of the note and increased sustain. Toward the end of the signal, the output fades to zero.
Of course, compressors have various controls for gain, sensitivity (some have a knob called sustain) so the output can be adjusted for more or less "blooming". The settings I used were chosen to demonstrate the various qualities of a typical compressor. It is entirely possible to adjust the output so that it is completely "squished" with no difference in level between the burst and the sustain/fade, or to set it with practically no boost at all.
So, what's in the Quilter? The circuit in the Quilter is most definitely a limiter as it does not boost low-level signals the way a compressor does, but it does have some unique qualities. After these experiments, I now understand why some people perceive it to be behaving like a compressor.
Let's see it in action
Here, the Limiter is off, and the output level (bottom trace) set to clip into an 8ohm load. Every peak in the bottom trace is clipped the same amount.
Below, the limiter is set to about noon, and we can see that the initial couple of peaks are still clipped, but close examination will show that the rest are gradually rounded, and "softened up". The level hasn't changed much though.
Finally, the limiter is set at max, as shown below. The first peak gets through at a high level, but it isn't clipped. The rest of the burst is reduced, but also not clipped. That initial peak is important in preserving the attack of the note.
OK, so that's a sine wave, what does it look like with an actual guitar signal? In order to show the difference - I hope more clearly - I used a feature on my scope that only displays the peaks (or the envelope) of the signal, and then I overlayed the non-limited and limited outputs. In all cases, the Micropro was driving an 8-ohm load. This was a single note (an A-440 this time), capture by a looper pedal and played back into the Micropro at a fixed level.
There are two envelopes in the scope trace below. The outer envelope is with the Limiter off, and the signal level was set for heavy clipping. The inner envelope is with the Limiter set at about 3 o'clock (where I normally have it set).
Both envelopes start off at the same high level in the region marked by the red arrow. This preserves the attack of the note. We can see that at the yellow arrow, the signal with the Limiter ON is at a much lower level - the limiter has clamped down and is preventing any further clipping. By the time the signal reaches the green arrow point, the Limiter has begun to release a bit, which I believe we hear as a blooming of the note - similar to what the compressor was doing. It is important to note that while the signal level is increasing there is no additional gain, the inner envelope is still below the max envelope.
So, in summary, the Quilter has a dynamic Limiter, with a defined attack and release time which we perceive as blooming or gain when the Limiter is hit hard.
For comparison, below are two envelopes set for lower overall output level, and less clipping:
We can see that the Limited signal (the inner envelope) follows the non-limited signal pretty closely, except with a slightly lower level. What we do not see is that heavy clamping and subsequent release that was evident in the previous scope capture. So at lower signal levels, the "blooming" effect of the Limiter's release time does not come into play. In this example, the Limiter knob was set at about 12 o'clock.
The performance of the limiter is dependent on the Gain setting of the Micropro, so at a lower Gain setting, the Limiter control may seem like it isn't doing much if anything at all. This all follows what I've heard while using the MP2. When I first played the amp, I kept the gain a bit lower and just set the Limiter to either allow clipping (when it was Off) or to prevent clipping and clean up the signal (when it was On) in order to get a basically 3 channel amp - clean (limiter on), crunch (limiter off), and creamy (with the Boost engaged). Lately, I've been preferring this blooming-sustainy kind of sound so I've got the Gain set to about 3 o'clock and the Limiter at about 3 as well, and just really driving the output stage simulation hard. I still turn the Limiter off on occasion if I want full-on output stage saturation sounds.
I hope this helps clarify what the Limiter in a Quilter amp is doing, and encourages some of you to experiment with the Limiter. I've found this feature to be quite useful, and I really can't imagine using any amp that doesn't have a dynamic limiter.
this is just fantastic, and fascinating. thank you.
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