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Sep 04, 2016, 11:54 PM
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Final assembly

It's rare for anything to work when transferred to the final assembly. In this case, the sound was still perfect, but thermal management was gone. The 5V regulator was pegged at 93C. It took a major rework to add a heatsink to it. It's adequate for normal apartment use. The regulators & STA540 stay in the 50C range. For 20Hz sine waves at high volume, fuggedaboutit. The 12V regulator instantly jumps to 100C & shuts down.

There were definite compromises for cost. It would have more power with a PC power supply, but need a lot of space. A 19V laptop brick with linear regulator was the most compact. A lot of wires & heatsinks are flapping in the breeze. It's unsuitable for a vehicle.

Adjusting treble is a buster. The probe points for R1, R2, R3, R4 are shown. R1, R2 determine the treble. They're both 40k when the pot is centered. R1 or R2 is 62k when the pot is fully deflected. The resistances can be calculated by applying parallel resistor equations to the Baxandall schematic.

Bass can be adjusted visually, but the bass pots turn in opposite directions. Probe points for bass resistors are R3, R4. The EQ could be broken out to a front panel extension, but it isn't changed often enough.

The 12V regulator is adjusted with R5 & its output is measured at the cap terminals. Voltage must not exceed 16V or the caps will explode.

The output of the DAC is attenuated with R6, R7 before going to the front panel volume control. The audio waveform can be probed at TP1, TP2. A reasonable volume range has a peak to peak V of 2.

The microcontroller sets yet another attenuation in the CS4227. This has to be quite high to fit in the rail to rail voltage of the butterworth filter. When the CS4227 initializes, its output goes from 0 to 2.5V, making a pop sound. If the volume is at full during power on, the pop will destroy the speakers. It's another cost saving compromise.

Considering how much space it took just to use an IC amplifier to power speakers from optical input, it makes you wonder how much space would be required to amplify 6 channels with a scratch built discrete amplifier. The DAC supports 6 channels, although the caps have been taken out. Don't think there's anyone in the world who actually built a discrete 6 channel amplifier from scratch. It would take an entire rack, like what old IMAX theaters used to have on display.
Last edited by Jack Crossfire; Sep 05, 2016 at 12:19 AM.
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