Input Selector
Neurochrome
The Input Selector is a relay-based input selector which supports the selection of one of up to six stereo inputs. It can be used as a passive preamp by adding a volume control, such as the Selector Switch & Volume Control. An active preamp, or a preamp with gain, can easily be implemented with this Input Selector by adding my Universal Buffer. This will result in a preamp with six stereo inputs and both balanced and unbalanced outputs. Balanced inputs can be implemented with the Universal Buffer as well.
The control input to the Input Selector is compatible with 3.3 – 5.0 V CMOS logic levels. This makes the Input Selector easy to interface to the many popular micro controller circuits, such as the Arduino.
The 45 × 112 mm gold-plated PCB is made in Canada.
The features of the Input Selector are listed below.
- Relay-based input selector with up to six stereo inputs.
- Can be built with fewer inputs if desired.
- High channel separation and excellent off attenuation.
- On-board 5 V regulator powers the relays.
- Gold plated relays, designed specifically for small-signal use.
- Connection to volume control by 10-pin, gold-plated connector.
- 3.3 – 5.0 V compatible logic control inputs for audio input selection.
- Except for one SOIC-14 IC, all components are through-hole components.
- Gold plated PCB. Made in Canada.
The parts budget for the Input Selector is just over $25.
The specifications of the Input Selector are tabulated below.
| Parameter | Value | Notes |
|---|---|---|
| Number of Stereo Inputs | 6 | |
| Power Supply Voltage | 7.5 – 30 V | |
| Power Supply Current | 40 mA | |
| Input Impedance |
10 kΩ |
With recommended volume pot. (Note 1) |
| Max. Output Impedance |
5 kΩ |
With recommended volume pot. (Note 1) |
| Channel Separation | 111 dB | 1 kHz, incl. volume control |
| Off Attenuation | > 113 dB | 1 kHz, incl. volume control |
| Assembled Dimensions | 45 × 112 × 23 mm | |
| Assembled Weight | 50 g |
|
| Note 1: The input impedance of a passive preamp is equal to the resistance of the volume pot. The maximum output impedance of such a preamp is equal to half the resistance of the volume pot. | ||
The amplitude response and gain flatness of the input selector is shown below. As you would expect from a passive preamp, the gain is completely flat and slightly below unity due to the load of the volume pot on the signal source.
More importantly, the left-to-right channel crosstalk is very low as shown below. Note that the crosstalk is the inverse of the channel separation. Thus, the channel separation of the Input Selector is very high.
In addition to providing good separation between left and right channel, the Input Selector must also have good isolation between inputs. Otherwise the signal from an unselected source can leak through to the output. The measurement below shows the leakage from an unselected channel. As you can see, the leakage is very, very low. In fact, it is below the noise floor of the APx525 audio analyzer.
