another excellent tut from gw1 @ austech
This is a variation of the audio mod for DM500 clones. It boosts the Dream audio up to the level of other set top boxes. The parts are readily available, cheap and easily assembled in a couple of hours provided you have reasonable soldering skill.
For this version I chose to desolder electrolytics rather than SMD resistors. The soldering is easier this way and you don't need the magnification tools.
The audio circuit in DM600 is very similar to genuine DM500, so the modification is almost the same as before. The only difference is I've removed the 470R current limiting resistors because they're no longer needed; the SMD resistors on the motherboard are used instead.
Some warnings to begin with
How it works
The DM600 uses a Wolfson WM8761 stereo DAC at a 48kHz sample rate. That part has a good digital low pass filter and noise shaper (something the cheaper DACs lack) and Dream include a supplementary third order Sallen-Key filter as suggested by Wolfson. The DAC's output swing and DC bias mean a rail-to-rail op-amp such as TLC2272 can manage with a single 5V rail. A final RC stage in the DM600 audio design provides current limiting and helps suppress any remaining high frequency noise (which Sallen-Key isn't good at).
This mod intercepts the audio signal at the output DC blocking capacitors C75 and C76. The LM833 op-amp I've chosen, after 2.54X gain and headroom allowance, needs around 9V as its supply. A 7809 regulator provides this. Ideally the +12V would be tapped from some point after L29, eg from C270. However it's better we avoid soldering on SMD capacitors if possible as the uneven heat might cause fractures. That's why I've suggested the cathode of D31 instead.
A divider network provides a clean +4.5V midrail operating point for the amplifiers, with DC blocking capacitors being necessary at the input and output. The receiver's RC output stage is preserved which is why no extra current limiting is required on the amplifier board; the 470R resistor found in the DM500 mod has been replaced by a wire link.
Construction
The veroboard layout is shown below. As you can see it's virtually identical to the compact one I posted on the DM500 thread, except the 470R resistors are gone.
Start by cutting the veroboard to size (14 x 9 holes) by scoring with a knife, snapping off on the bench, then cleaning with a file. It can't be much wider otherwise it won't fit between C130 and the hard disk.
Make the 10 track cuts using a drill bit, then inspect carefully to ensure fully cut.
I suggest soldering the three longest wire links first, then the IC, then the fourth wire link between IC pins 3 & 5 on the solder side of the board. Then fit the remaining components. Be sure to observe correct polarity of the electrolytic capacitors.
This is a variation of the audio mod for DM500 clones. It boosts the Dream audio up to the level of other set top boxes. The parts are readily available, cheap and easily assembled in a couple of hours provided you have reasonable soldering skill.
For this version I chose to desolder electrolytics rather than SMD resistors. The soldering is easier this way and you don't need the magnification tools.
The audio circuit in DM600 is very similar to genuine DM500, so the modification is almost the same as before. The only difference is I've removed the 470R current limiting resistors because they're no longer needed; the SMD resistors on the motherboard are used instead.
Some warnings to begin with
- This voids your warranty
- My DM600 motherboard is Version H (I imagine it will work with all versions but can't guarantee)
- Take static electricity precautions to avoid damage
- Unplug all cables from receiver before soldering on it
- Check workspace is clear before applying power to avoid shorts
- You need a temperature controlled iron with reasonably fine tip. If you're not confident about your soldering don't risk damaging your receiver - get someone else to help, or make do with an external amplifier.
How it works
The DM600 uses a Wolfson WM8761 stereo DAC at a 48kHz sample rate. That part has a good digital low pass filter and noise shaper (something the cheaper DACs lack) and Dream include a supplementary third order Sallen-Key filter as suggested by Wolfson. The DAC's output swing and DC bias mean a rail-to-rail op-amp such as TLC2272 can manage with a single 5V rail. A final RC stage in the DM600 audio design provides current limiting and helps suppress any remaining high frequency noise (which Sallen-Key isn't good at).
This mod intercepts the audio signal at the output DC blocking capacitors C75 and C76. The LM833 op-amp I've chosen, after 2.54X gain and headroom allowance, needs around 9V as its supply. A 7809 regulator provides this. Ideally the +12V would be tapped from some point after L29, eg from C270. However it's better we avoid soldering on SMD capacitors if possible as the uneven heat might cause fractures. That's why I've suggested the cathode of D31 instead.
A divider network provides a clean +4.5V midrail operating point for the amplifiers, with DC blocking capacitors being necessary at the input and output. The receiver's RC output stage is preserved which is why no extra current limiting is required on the amplifier board; the 470R resistor found in the DM500 mod has been replaced by a wire link.
Construction
The veroboard layout is shown below. As you can see it's virtually identical to the compact one I posted on the DM500 thread, except the 470R resistors are gone.
Start by cutting the veroboard to size (14 x 9 holes) by scoring with a knife, snapping off on the bench, then cleaning with a file. It can't be much wider otherwise it won't fit between C130 and the hard disk.
Make the 10 track cuts using a drill bit, then inspect carefully to ensure fully cut.
I suggest soldering the three longest wire links first, then the IC, then the fourth wire link between IC pins 3 & 5 on the solder side of the board. Then fit the remaining components. Be sure to observe correct polarity of the electrolytic capacitors.