Subwoofer Phase Controller
Rod Elliott (ESP)
I must first explain at the very outset that this standard phase controller is usually not
very helpful, and can do more harm than good in many cases. The reason that I'm
publishing a circuit for a continuously variable phase controller is simply that I have
had many enquiries for one.
Having said that, these controller circuits are very common in subwoofers, and if
used properly can help to integrate a sub with the main system. Being able to vary
the phase can allow a sub to blend seamlessly with the main speakers, but it must be
understood that the phase will only ever be right at one frequency. This can cause
peaks and dips in the response to simply change their frequencies, without much
Before embarking on this project, please see The Subwoofer Conundrum and Phase
Control - Myth or Magic. These articles shed much light on the subject.
The circuit is completely conventional, and has been around almost forever in one
guise or another. Similar circuits were used in the valve (tube) era, so there is nothing
new about it. The circuit has already been published on these pages as a guitar
tremolo circuit, but fairly obviously, that is not suitable for this purpose.
The circuit uses one opamp, but a dual opamp is shown and recommended to ensure
that the source impedance is low. If driven from the output of a crossover (opamp
based), the impedance will be fine, and the first stage may be omitted.
It works by summing the two signals applied to the opamp's inputs, and the amplitude
remains unaffected - it is flat at all settings of the pot. Another name for this circuit is
an "all-pass" filter, because it passes all frequencies equally, affecting only the
The circuit can shift the phase from 180? (out of phase) through to 0?, but the useful
range is between about 160? and 20?. The 90? frequency is determined by the
F90 = 1 / (2 * * R * C)
where R and C are the resistor + pot (VR1 + R6) and the input capacitor (C1) to the
opamp. See below for the phase plot using 20% increments of the pot.
Figure 1 - Phase Controller Schematic
There is nothing special about the circuit, and a TL072 opamp will be more than
adequate for any subwoofer system. VR1 is the phase control pot, and should be
linear. No special precautions or close tolerance resistors or caps are needed. The
phase switch (open = normal, closed = inverting) needs some explanation, since the
phase can already be varied from 0? through to 180?, so the switch is redundant,
Not so as it turns out. Because of the way the phase control works, interactions from
crossovers and subwoofer placement may require that there is a full phase inversion
as well as phase shift. See Phase Control - Myth or Magic for more.
The response of the phase circuit is shown below. The graphs are shown for pot
rotations of 0 to 100%, at 20% intervals. The lowest frequency is obtained with the
maximum pot resistance and vice versa. The 90? phase shift frequencies are shown
in ascending order, from a low of 34Hz to a high of 92Hz. The range is easily
changed if necessary by changing the value of C1 - higher values will give lower
frequencies, and lower values will give higher frequencies. Any 90? frequency is
easily calculated using the formula above.
Figure 2 - Phase Response of Controller
Note that although the centre frequency is defined as the 90? phase shift frequency,
the phase is affected over a wide range. There will be noticeable phase disturbances
over ?2 octaves for a total span of 4 octaves, with actual disturbance extending for ?1
decade. This is the reason I don't like the circuit, as it creates a situation where the
actual phase is something of an unknown - you could be making things better at one
frequency, while making it worse at another. This is unlikely to cause major problems
with a subwoofer, but great care is needed to get it set up with the optimum phase
The circuit is easily made on a piece of Veroboard or similar. All resistors can be 5%
carbon, although metal film may give slightly less noise. Caps are standard polyester
(or ceramic for the opamp bypass caps) and should be rated at 50V.
The standard pinout for a dual opamp is shown on the left. If the
opamps are installed backwards, they will almost certainly fail,
so be careful.
The suggested TL072 opamps will be quite satisfactory for most
work, but if you prefer to use ultra low noise or wide bandwidth
devices, that choice is yours.
There are no special precautions that you need to take with construction. Use of a
clean power supply is highly recommended, having a maximum voltage of ?15V (e.g.
Project 05 or similar.
What does subwoofer phase mean? What does phase 180 mean on a subwoofer? A phase 180 is a setting that can be adjusted on some subwoofers in which the polarity of the driver is reversed, meaning that when the cone moves forward, it moves backward.
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