I have recently built Merlin Blencowe's Engineer's Thumb compressor. There is several long threads to be found on the internet debating this unit. Opinions are manifold, we will not get into that here. The unit is a tool that serves its purpose, but it is best used in its original version (with hidden controls) and not compared to studio units, where some people want to see it.

Compressors are a bitch to work with, since their effect is transient and momentary. While the sermon "use your ears" is repeated ad nauseam by all self-taught experts, I personally like some visual feedback.

This is particularly helpful for dialing in a plugin version inside a DAW. Incidentally, it is comparably trivial to make a meaningful display in software. May I stress the word meaningful again.

Such a display should indicate the amount of gain reduction, so that the engineer can tell how fast the compressor is, how long it takes to recover, and how deep the amount of gain reduction is.

This is not easily accomplished on a hardware unit, such as a floor pedal, without having to resort to undue efforts.

Fortunately, such a fancy display is usually not needed for live work. In my opinion, even all the otherwise important parameters like attack and decay, are not necessarily needed to be externally accessible. Legendary stomp-box compressor units have been around for decades which are knitted just like this.

The Engineer's Thumb  is no exception to this. That said, I had it set to such low compression values that I often was not really sure if it was doing anything. In order to harness any such unit, you have to thoroughly know what it does, particularly what its time constants do. No two hardware compressors are reacting equal. So the demand for some visual indicator was growing.

Although Merlin (Blencowe) had created his compressor without visual indicator, people had obviously addressed him over such a tool. The choice he had was creating a very simple display or a very fancy one. The circuit's expected complexity was directly related to its usefulness.

Now his compressor is, besides being very well thought about and much improved, a very old technology, and much simplified compared to a studio unit (let alone digital units). In a way it is comparable to iconic vintage compressors, where timing settings (if at all specified) were not to be taken literal. Again, it was deliberately kept simple for live use.

So a fancy compression indicator that displays the exact number of decibels of gain reduction is neither possible, nor feasible. This is disregarding the fact, that very, very few working musicians out there will know what it tells them.

The solution Merlin came up with was brilliantly simple. It did not require huge modifications of his unit, and it was cheap:
a ultra-bright LED in the feedback loop of the OPA stage that buffers the mid-rail reference voltage, whose voltage drop was canceled out by the OPA. Its brightness was  a measure for the current drawn from the OPA's output (resp. from the reference voltage), which in turn was a more-or-less measure for "how hard the compressor was working".

Unfortunately, this type of visual feedback is not very effective, because

1. the LED is permanently lit (dimly)
2. LED brightness and supply current are not linearly interconnected
3. the overall brightness was either too little, or too much, dependent on the LED
4. the pulsing LED did not really manage to establish a feeling of the compressor's action and thus was not helping
5. it did not give a clue to the release action (which is better kept very long to avoid breathing effects)
6. wanting an  "effect on"  LED on the front panel would require a second LED
   

Other tinkerers have come up with different one-LED solutions, like the one shown in post#329 on the long diystomboxes thread. This idea is intuitive, however it produces substantial current shifts upon LED-turn-on that may cause audible side-effects by

1. corroding the Vref line (which should be avoided at all cost)
   
2. injection into other circuit parts
3. unknown side effects for the gain cell control current
   

(we will learn more about that in a second).

There exists a version of Merlin's compressor called Engineer's Thumb GTI, which is not easily to be found on the web (probably escaped eradication). It is not clear, if this was an experimental version authored by Merlin, or by somebody else.

It uses an LM3916 (LED) driver IC that can drive 10 LEDs in a row in dot or bar mode. Unfortunately, in the GTI, bar mode has been used, which will introduce enormous current shifts with each LED lit. So if one does not apply ultra-careful layout principles, this will be disastrous. This is not even touching the fact that the mentioned display is by no means gauged to anything, so it just shows that the compressor is doing something. It shows that the compressor is working, and increased values mean that it is working increasingly harder. Big deal, we could have told that without looking at the thing.

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In the lights of all that, I was close to give up on the matter, when I found an application note from THAT (DN113 and DN132, its successor), which presents a list of approaches to a simple compression display, comprised of a dual comparator and a bi-color LED. It is actually an above threshold indicator, that abruptly turns on when the compressor gets active. The one single LED (-change) shows clearly when, how fast and how long the compressor works. That is sharply outlined at least.

"As one might expect, the complexity of these different approaches is roughly proportional to their functionality."    - THAT Corporation Design Note 32, An Alternate Method of Indicating Compression

Assume that we have a single LED that turns on when a "below threshold" condition is reached.
Their concern is (and rightfully so...) that a suddenly activated LED instantly draws several milliamps from the power rails, potentially producing a current spike that is liable to get injected into the audio path. While changing from a given color (such as green) to a different one (such as red) does not yield a plus of information per se, they have cleverly used that for steering the current between the LEDs. So upon switching, no current spike is produced and the overall current draw is small and predictable.

All of THAT's above suggestions will yield the same indicator functionally, but the last one seemed like the one to go for.
This circuit, as depicted, still has the following small (practical) disadvantages:

• the (green) LED is always on, even during bypass
• to have an independent "effect on" indicator, still a second LED is needed

This can very easily be fixed.

Instead of using a bi-color LED (using the above nomenclature), a dual LED with a common cathode can be used to achieve the exact same results:

Download the COMPIND schematic V1.1 By downloading the file you agree to the Terms of Use.

• Nov. 29, 2024: first release