Home > Dalek Project > Sound System
Modified:22:24, 22 October 2013
DALEK PAGES
SOUND SYSTEM - (more pics to follow)

The sound system has a 4-channel mixer, then splits into 2 sections, bass and mid/treble. Each has its own amplifier. The Low pass filter removes mid/high frequencies and is amplified by a bridged TDA2030 amplifier, and the High pass removes the low frequencies and is amplfied using a single TDA2050 amplifier. The frequencies were sepearated in an attempt to remove distortion from speaker limitations.

Each of the following sections is detailed below:
  1. Mixer
  2. Low Pass Filter
  3. High Pass Filter
  4. 30W Power Amplifier
  5. 14W Power Amplifier
4 Channel Mixer
The mixer basically contains 4 non-inverting amplifiers based around IC1-4 op-amps, then the outputs are fed into a summing amplifier constructed by IC5. This mixes the channels together to form a final output. Each input A-D is connected to a potentiometer which sets the level of that particular channel. After this the input is fed to a non-inverting amplifier. 

Each amplifier has its gain set at different levels to accomodate various types of input signals. The gain of each input is as follows:

  • In A = 11x
  • In B = 5.5x
  • In C = 3x
  • In D = 2x

The gain of the summing amp is set to 47x giving the following overall gains and recommended inputs.

  • In A = 520x, 2mV
  • In B = 260x, 5mV
  • In C = 140x, 10mV
  • In D = 95x, 12mV

The main volume is from VR5 regulating the signal to the filters. The output signal was designed to be around 1v p-p.

The circuit runs from the audio supply of +/-12v to the connector on the left TB6. Inputs A-D are along the top and the output at the bottom right TB5. All potentiometers were wired off the board using single core screened cable.

The PCB measures
5.35" x 1.6"
(140mm x 61mm)

The mixer shown left, Not very flash since it's inside the Dalek, it's just functional. Left to right is channels A-D with 3.5mm mono jack inputs. Then the Master Volume. Above the master volume is a mute switch - as an after thought.

Input A was used for the Voice input and B for the MP3 lead - it should really be in D.

LOW PASS FILTER:
The Low Pass Filter is based around a Sallen-Key type filter set at 100Hz. In fact there are 2 identical filters in series to give a really sharp drop-off in response. In reality this wasn't necessary since the bass speaker can easily deal with frequencies up to 400-500Hz, but I wanted to experiment a bit.

TESTING:

Below are circuit simulation graphs of the response of the filters when tested with different frequency inputs. In reality, the filter removed a great deal of treble resulting in a rather upleasant base growl. However when combined with the high end, it sounds pretty good.


Test 1: 50Hz at 500mV

Test 2: 100Hz at 500mV

Test 1: 200Hz at 500mV
PCB:
Size = 2.95" x 1.5", 75mm x 38mm. Although there are ceramic capacitors shown, it is better to use resin dipped, or polystyrene/polypropolene types for better performance and less noise.
HIGH PASS FILTER:
The High Pass Filter is also based around a 2 Sallen-Key filters in series set at 250Hz.

TESTING:
Below are circuit simulation graphs of the response of the filters when tested with different frequency inputs. The circuit proved very effective removing pretty much all the bass frequncies for the high end drivers. There are 2 traces comparing input output signal. Blue = Input, Red = Output

Test 1: 125Hz at 500mV

Test 2: 250Hz at 500mV


Test 1: 500Hz at 500mV
PCB:
Size = 2.95" x 1.6", 75mm x 40mm.
30 WATT BRIDGE POWER AMPLIFIER:
This amplifier is for the bass driver. Since there is only a total of 24 volts available, I decided to use a bridge version of the TDA2030 IC amplifier. This will hopefully give plenty of effective voltage swing giving some good quality low distortion bass.

PCB:

size = 3.05" x 2.1", 78mm x 54mm

14 WATT POWER AMPLIFIER:
This amplifier is for the high end drivers which are only a few inches in diameter. The power of this amplifier does not need to have such a high output so a 14W version of the TDA2050 IC which has the potential to deliver 32W of RMS power, but at +/- 24v and into a 4 ohm load. I have only +/-12v into a 4 ohm load (2 x 8 ohm speakers in parallel) so the output is greatly reduced.
PCB:

size = 2.35" x 1.825", 60mm x 46mm

Written and Constructed by Phil Townshend 2009
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