Arcam Logo subwoofer design concept
Design concept
The Arcam Logo subwoofer is designed to complement the Arcam Solo range of products. It offers stunning performance and an unobtrusive, stylish aesthetic. It is particularly well suited to use with the Muso speakers in surround sound or 2.1 stereo music systems.
Cabinet design
The Logo cabinet is manufactured from 18mm MDF with two stiffening braces inside the cabinet to reduce panel resonance. This delivers the minimum of cabinet coloration for accurate bass reproduction and maximum efficiency. The cabinet is further filled with acoustic wadding that reduces standing waves inside the cabinet by absorbing high frequency sound. In addition the wadding reduces the speed of sound inside the cabinet as sound passes more slowly though the wadding than it would through air. This increases the effective size of the box enhancing the low frequency response.
The electronics pack of crossover and controls is housed in a sealed die-cast aluminum box. This helps to isolate the electronics from the vibration in the enclosure and ensures that they are in an airtight box so that the cabinet remains sealed. The amplifier section is housed elsewhere in the cabinet (see later).
The Logo cabinet is a sealed box (or air suspension) style of enclosure. This design was chosen for the most accurate replay of bass with minimum group delay. High group delay means different frequencies taking different times to pass though the system. For example, with high group delay, the thud of a bass drum might take longer to be reproduced than the snap of a snare drum. This is clearly not desired!
The cabinet uses a downward firing drive unit making the Logo easy to accommodate in a home environment and also giving additional bass boost by using the floor to reduce the space the drive unit is radiating into.
Inside Logo cabinet (wadding removed for clarity). Shows:
- Dual cabinet braces
- Sealed die cast crossover enclosure
- High efficiency class D amplifier
Note use of foam to wrap cables going to drive unit this is to prevent any rattling at high volumes.
Drive unit
The bass drive unit in the Logo subwoofer is of 10" diameter with an extra long throw. Its long throw capabilities enable it to move more air and hence reproduce lower frequencies. It uses a large 2 inch voice coil which is cooled by vents beneath the spider (the part holding the voice coil in place) and by a vent through the magnetic pole piece. This venting creates extra cooling which allows the drive unit to work under heavy loads for extended periods of time without overheating.
The Logo drive unit uses three magnets; two are used to create a very high flux density inside the magnetic gap; this improves efficiency. The last one, a bucking magnet, is used to concentrate the field of the other two, further increasing flux density and reducing external fields. This means that the Logo can be used close to Cathode Ray Televisions or monitor screens.
The suspension of the drive unit is made to be extra stiff producing a woofer which is highly mechanically damped. This enables the woofer to remain in tight control of the bass signal even when playing loud. One downside of the extra stiffness is an early high frequency roll-off. However, active correction within the Logo crossover is used to overcome this.
Logo bass drive unit showing venting behind spider, large voice coil, long throw rubber surround and three magnet system.
Active bass alignment
The Logo subwoofer uses an active bass alignment circuit called a Linkwitz Transform. In technical terms this enables the acoustic alignment of the bass unit to be transformed into a different alignment. In the Logo the Bass alignment chosen is called a Bessel alignment. The advantage of the Bessel alignment is that the group delay of this type of filter has no peak in its response. This means that all the frequency components of the signal are replayed at the same time with no timing differences between them. The Bessel filter also has a quite gentle roll-off in the frequency domain. This makes it easier to integrate with the listening room. These characteristics also help the Logo be highly suited for music playback.
Crossover and control electronics
The Logo subwoofer has two different inputs. The first is a stereo input that is routed through the crossover and is designed to be used when the large, full-range speakers are being used. The second is a mono input which bypasses the crossover completely. This is for use with a surround sound system with a subwoofer (LFE) output. These two signals are mixed together so both can be attached at the same time if required.
Having independent inputs for left, right and LFE means the system can be set up to work with a surround sound system with a full analogue bypass mode without changing cables. This means that in stereo the left and right speakers can be fed with a signal that has not been through any digital conversion whilst the sub is still active. To do this the left and right outputs and the subwoofer output are all connected to the Logo inputs. When the surround sound processors crossover is used the signal from all the small speakers is transferred via the subwoofer output. If left and right are set to "small" their bass signal will also be transferred via the subwoofer connection; if they are set to large then the full-range signal will be transferred to the crossover via the left and right inputs.
When the system is switched to “direct” then the bass management in the surround sound processor will be shut off and the left and right speakers will be treated as large so the full range signal will be transferred to the crossover via the Logo’s left and right inputs. This allows a set-up with small front speakers where the surround sound processors crossover can be used for playback of film material at high level without risk to speakers. Then, using direct mode, the system can be switched to playing music at lower levels with the subwoofer extending the bass response of the left and right speakers.
The Logo crossover uses a fourth order filter to give a faster transition than most sub crossovers (which tend to be second order). This makes it ideally suited to work with main speakers that are ported and hence have a fourth order bass alignment. Using a higher order filter helps to reduce localization of the subwoofer as the higher frequency "leakage" of sound past the crossover region is attenuated twice as quickly as it would be in a second order system.
Internal picture of Logo input, crossover and Linkwitz transform circuits. Note use of high quality double sided fiberglass circuit board.
To improve the flexibility of the crossover it has been designed with frequency and Q controls. This allows the alignment of the crossover to be adjusted from a Q of 0.3 (a very soft roll-off) to a Q of 1.5 (a peaking filter). Most crossovers do not allow for adjustable Q and have a fixed Q of 0.707 (a Butterworth alignment) and can be very difficult to integrate with the main speakers. Arcam found in listening tests that being able to adjust the Q made integrating the speakers much easier. We found that to use Logo with its partner speaker the Muso the best setting tended to be a Q of 0.5 (a Bessel filter) with a slightly higher crossover frequency than the Muso’s roll off of around 90Hz this is to allow for the softer roll off this type of filter. However this does depend on the location of the Logo in relation to the Muso speakers and the room they are placed in.
Logo Amplifier
The amplifier used in the Logo subwoofer is a 200W class D amplifier using a switched mode power supply. This makes it very high efficiency so that the amplifier needs no external heat sinking and uses much less power than a conventional amplifier. It has an automatic signal sense system so that it can shut itself down when not in use, further reducing power usage.
Internal picture showing Class D amplifier stage.
The Class D amplifier in Logo is a switching design for high efficiency; however, it does not use digital circuitry and is an analogue controlled switching amplifier. The reason Arcam used this type of amplifier is that it has a very low latency. This is the time taken for a signal to pass through the amplifier. Digitally based switching amplifiers typically have quite high latency due to the DSP processing that delays the audio. This delay would cause the subwoofer’s output to arrive later than the signal from other the main speakers.
Summary
Arcam have used traditional high quality speaker design combined with advanced and innovative techniques to create a subwoofer capable of high quality music and film playback that is easy to integrate with the main speakers and easy to place in the listening room.
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