Switchpanel/Speaker Emulator

The switchpanel is a unit which I have designed and build for my audio setup. It basically serves three purposes. It serves as a power distribution box, as an amplifier routing unit and as a speaker switching/emulator unit. Below is a photo of the switchpanel. The left hand side contains the power switches, while the right hand side contains the signal section.

The power distribution section switches the outputs on in four stages. In the first stage all signal processing functions in the unit itself is switched on. Secondly the external signal processing units are switched on. (These includes the mixing desk, CD players, etc.) The PC is then switched on. The sepearte stage for the PC is just too keep the inrush current of the PC seperate from the inrush currents of the other units. Lastly power to the power amplifiers is switched on. When the unit is switched off, everything is switched off in reverse order than switch on. Since the power amplifiers is always the last to switch on and the first to switch off, audio pops are elliminated.

The signal section of the switchpanel consists of an amplifier signal selector/distributer, a 7-band EQ and a speaker emulator. Below is a close-up of the signal section:

The unit has been designed to route the signals to two power amplifiers. The signal router has a two stereo inputs coming from the mixer, and two stereo outputs going to the power amplifiers. The two switches at the right selects which signal source is send to an amplifier output. The two large knobs controls the signal level to the amplifiers. I have had this problem that by connecting my 180w r.m.s. per channel directly to the output of the Stereo Master of my mixing desk that I was only using about the lower 20% of the range of the Stereo Master control of the mixing desk. With the additional volume control on the switch panel I am now able to further attenuate the signal level to the power amplifier, thus extending the usable range of the control on the mixing desk. Another purpose of the two volume controls is to trim the volume levels of the two amplifiers to be equal, which is handy if one wants to switch between amplifiers to evaluate the sound difference between the two. The Mute switch at the bottom left is used to mute both amplifier channels.

The equaliser section is a 7-band equaliser using rotary pots instead of sliders. It can either be assigned to the speaker emulator, Amplifier channel A, or Amplifier channel B. A toggle switch switches the EQ in or out, while a 3 position rotary switch selects into which signal chain the EQ is inserted.

The speaker selector takes two speaker inputs and routes them to 2 speaker outputs. It is possible to connect two speakers in parallel on one amplifier, but I have designed the unit in such a way that it is not possible to accidentally connect the two amplifier outputs together. The B  speaker may either be routed to an external B speaker, or it may be routed to the internal speaker emulator.

The speaker emulator consists of two parts, the impedance matching part and the signal processing part. The purpose of the impedance matching part is to emulate the load a real speaker would present to the amplifier output. The signal processing part is to filter and attenuate the signal tapped from the amplifier output so that the signal is at line level. The signal processing consists of a first order high-pass and low-pass filters followed by a 2nd order high-pass filter. The speaker emulator has 5 knobs (refer to the close-up of the signal processing part of the switchpanel.) The left-most knob selects the resonant frequency of the speaker. This resonant frequency determines the impedance peak of the speaker, as well as the cut-off frequency of the second order hi-pass filter. The second knob controls the Q (sharpness) of the second-order high-pass filter. The H-PASS knob determines the cut-off frequency of the first-order high-pass filter. The L-PASS knob determines the cut-off frequency of the low-pass filter. The LEVEL knob controls the signal level send to the speaker emulator signal output on the back of the switch panel.
 

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