Compression
Indicator For The Engineer's
Thumb "Above Threshold" Indicator Adapted For Single Supply From A THAT Co. Design last update: Nov. 29, 2024 Copyright 2024 by H.
Gragger. All Rights Reserved. All information
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MAIN PAGE>MUSIC STUFF>COMPIND Index Playing With The Engineer's Thumb Prior Art THAT Solution Update History Back To Index Playing With The Engineer's Thumb 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. 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.
Back To Index Prior Art The solution Merlin came up with was
brilliantly simple. It did not require huge
modifications of his unit, and it was cheap: Unfortunately, this type of visual feedback
is not very effective, because
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
(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. THAT Solution 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.
Assume that we have a single LED that turns
on when a "below threshold" condition is reached.
All
of THAT's above suggestions will yield the
same indicator functionally, but the last one
seemed like the one to go for.
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:
The circuit from THAT still works exactly as before, but this one has the double benefit of working as an "effect on" indicator into the bargain, eliminating the need for a second LED on the front panel, and without any compromises. The common cathode is wired to the bypass switch in a typical 3p2t wiring scheme (a typical variant is shown in the schematic for convenience).
Upon activation of the effect by hitting
the "bypass" switch, the green LED will light
(after maybe some short settling of the gain
cell).
Turn-on current is mitigated by the R/C time constant, which is optional but recommended. This is another trick out of Jack Orman's box of tricks, LED Popping - Stop the LED Indicator Noise, and does not interfere with the above design. Indeed, not pop is noticeable. Note that the circuits from THAT as shown are using a symmetric supply, whereas the COMPIND schematic is adapted for a single supply. Its input is connected to the emitter of the transistor on the Engineer's Thumb, which works better than to the output of the preceding OPA directly. The input of the comparator (not any different to an OPA) exhibits an extremely high impedance and thus does is not "seen" by the other circuitry. For set-up turn the pot until the LED turns red in idle mode. Carefully turn the trim pot into the other direction until the LED suddenly switches color. The LED should now faithfully reflect the state of compressor activity. Done. This add-on is, despite its simplicity, quite helpful for dialing in the compressor parameters and has zero interference with the rest of the circuitry. Back To Index Update History
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