More design details….and choices we made…

We designed our attenuator with reliability in mind using proven-in-use, high reliability components.

The attenuator uses low noise low signal relays which are controlled using our proprietary software. The attenuation step-up-step down sequence uses ‘make-before-break’ relay switch action

The relays we chose are also used in high reliability, high performance telecommunications systems. These relays have extremely predictable make, break and ‘bounce’ characteristics. The micro-computer carefully times relay operations to open and close the relays within specifications for consistent, noise free performance. Careful make-before-break operation is important so that the signal path is not opened before selecting the next attenuation step, otherwise loud clicks would be heard.

At each transition of the attenuation step (up or down) the software selects the next relay, holds the current step position until the stable next step has been achieved, then releases the current step position. In this way the noise created by switching in the next resistor in the attenuator chain is minimized.

When either mute or un-muting is selected, rather than jump to the targeted step relay, we step through all of the resistor chains selected relays. The net result is a more natural reduction or increase in volume, this sequence is extremely fast and in practice is faster than can be obtained using motorized potentiometers or switches.

Our experience and product evaluations have shown that when some systems change input sources sudden volume levels can occur, especially systems with standard switches. While this can not be eliminated, because each source is different, it can be minimized by providing continuous loads to the source. In our design, each source is loaded with the same impedance as the attenuator network. We have found that this minimizes the bounce in volume when a source is unloaded and then loaded with the attenuator network.

We have been asked why we selected 25K as the attenuator ladder impedance. We choose this as the best compromise between low noise and optimal loading, since most sources manage this load comfortably.

The digital control system is managed by a sophisticated micro-computer. Although physically small (<1cm sq), this computer controls all aspects of the attenuator including the IR detection system. The digital ground, power and signals are DC isolated from the source ground and signals. To reduce noise pick-up we decoupled the two grounds, signal and digital through a single small value capacitor. The signal paths have been hand-routed for optimum performance and low noise.

Although, perhaps controversial to some, we specifically chose metal oxide film surface mount resistors for our attenuator chain. These devices are stable, especially coupled with good quality PCBs (printed circuit boards). At low power levels, which are inherent in the attenuator, these rival all but wire-wound resistors for extremely low resistor noise. As importantly, with the small size of surface mount components, signal path lengths can be minimized, again for low noise. Our attenuator is built on a double sided PCB, with most of the digital control circuits on one side and the analog (signals) on the other, reducing noise.

We chose high reliability, sealed relays, rather than motorized switches, potentiometers or semiconductors for several reasons.

First, the mechanical reliability of these relays results in statistical failure rates of less than one in 50 million operations. This is difficult to match with mechanical switches or motorized devices.

 

Second, because the relays are sealed, their contact performance over time far surpasses open switches often used in switched attenuators.

 

Third, motorized potentiometers rely on mechanical contacts against resistive surfaces; these can degrade quickly over time, sometimes in less than 10,000 operations.

We seriously considered using semiconductor switches, but the capacitance of the devices (which are always connected) switching each of the attenuation resistors flavors and contours the sound. We have discarded this solution, at least for now...

The de-facto standard for HIFI and experimenters for IR control has been the Philips RC5 IR remote control standard. We chose this standard to make it easier for users to integrate into their existing systems.  The attenuator can be customized for different IR standards, since we design our own PCBs and software. To simplify use we chose to have the attenuator respond as though it were a TV. This can be custom modified to be a VCR, satellite control box or another device. Other custom standards for IR protocols can also be provided.  We chose to use the universal remote concept because it eliminates the need for a special controller, and allows most users to use either the remote provided or their own remote.

The functional operation of the attenuator can also be customized. For example, our feature of our volume up lock-out during mute can be changed. We evaluated the attenuator’s functionality with a wide range of users; however, we know some users would prefer their attenuator operate in a different manner.

The impedance of the attenuator and the specific step values (dB) can be customized.

You may have noticed that the steps are not equal. In our evaluation the ear is much more sensitive to sound changes when the sound level is relatively loud. At small and perhaps very small sound level changes larger steps can be made, with more subtle changes made at higher levels. In this way, the attenuator can provide more effective use of the ‘26’ steps and reduces the need for more discrete steps and lowers overall cost.

The attenuator has very simple, manual controls and visual indicators. We wanted to provide the ability to control the attenuator without the remote. Customers told us the value should be added to the attenuator’s performance and functionality, not in the displays, knobs or dials.

The start up sequence of the attenuator may look flashy, literally! The reason for this is actually subtle and mundane. To make sure the attenuator is functioning correctly and the LEDs are operational, the computer sequences through the LEDs during the initial power up sequence and its start up checks. After the power up check sequence the computer resets all relays and places the attenuator in MUTE, and sets the input to NONE and the volume to minimum. Since the default or off condition of the relays is N.O. (normally open) the attenuator has no input or tap off the attenuator chain unless specifically commanded. What does this mean? Well very simply, until the power is supplied and the computer has successfully initialized and started, no input or arbitrary volume level will occur. In some motorized systems and manual attenuators this can happen, especially after a power failure.

We provide a single LED for indication of volume level; this indicator is simple, yet effective. The relative brightness of the LED indicates attenuator volume level. There is not much point in indicating an actual volume level (attenuation). Depending on the main amplifier’s volume level, power and speaker efficiency can vary so widely that what could be a low level on one system may be overpowering on another, and we know a lot of users like to experiment with different systems and configurations. Our experience suggests that the volume level of the main amplifier be set for the maximum comfort/safety level with the attenuator at maximum.  Starting with a low level on the main amp and then progressively increasing the attenuator to maximum output level, adjusting the main amp during the ramp up of the attenuator.

We designed the IR system to be sensitive and selective.  This means the IR detector can be used in high ambient light at long distances. This is a trade-off for low sensitivity at short distances. Because the system is sensitive, the system may be unresponsive or error prone if the remote is used too closely to the attenuator. With most set-ups the attenuator is well out of arms’ reach, based on speaker and system-to-listener positioning. The remote can be ‘bounced’ off a wall or other surface, to improve performance if the attenuator is too close.

For the geeks, we used an 8 Bit Freescale micro-controller with all the code written in ‘C’. The internal architecture utilizes the I2C bus for peripheral control.

We specifically designed the attenuator with a separate power supply. This provides the lowest noise and greatest safety. It further separates the power system from the attenuator and reduces the attenuator’s foot print. This also facilitates the physical integration of the attenuator into an existing system, where the existing system can supply a simple 5VDC supply at a maximum of 100mA.