Damp

After Second Bass I decided to update my amp. My trusty 200W Class B amp does a great job as a practice amp when the 40W valve amp isn't going to be heard, but I felt there was "space" at the top for something else.
After a previous place of work got into financial difficulty, I found myself the proud owner of some industrial PSUs. These were destined for large Automated Test Equipment installations and were 800W units giving 48V at around 16A each. The idea of putting them to use in a bridge amplifier felt like my duty in these modern times of recycling...
The original intention was to design a Class D amp and put the tricky Class D modulator stuff inside an FPGA. I used an Altera Cyclone 4 device running at 200MHz, but the trade-off between simple PWM resolution and speed didn't look promising. I made a low level output stage and the results sounded (to my ears) like a badly tuned radio. I expect this is officially known as intermodulation distortion, but it was never going to cut the mustard.
To combat this fundamental problem the world of DSP promises much but when it comes down to details most is hidden away as proprietary information. The best chance of success was to use a sigma-delta topology which has similarities to SACD. After blowing a few sets of MOSFETs up I eventually had to resort to the tried and tested International Rectifier IRS2092 (now owned by Infineon). This may not be perfect, but with careful attention to layout (especially the dc offset protection monitor in my case!) and avoiding the usual grounding pitfalls it has performed admirably, albeit always making a weird weeeep noise when it is switched on.

The preamp has two separate inputs A and B, a compressor and a parametric EQ stage before the volume control. The controls were constrained by only having five excellent solid aluminium knobs that really needed to be used. The six controls are Gain A, Gain B, EQ frequency, EQ boost/cut, master volume and compressor level. Seeing as six controls and five knobs don't align properly, the compressor gain has to share the same knob as Gain B using a dual gang pot. Most of the time I use Channel A and on the rare occasions I need both channels, the compressor can be easily switched out if it misbehaves. The other switch mutes the output from the master volume which is handy at gigs to ensure no nasty noises build up between sets without having to twiddle anything else.

The Digital Amplifier working name was shortened to D-Amp and eventually just "damp". Maybe not as good as moist, but considerably better than fetid or stagnant.
I tried a resistive power test once it was built. I cobbled together some wirewound resistors to give 8ohms and then plunged them into water to keep them cool. Electricity - water - safety - blah blah - don't try it at home etc. I got to over 400W before the resistors burnt out! Before they let go, the amplifier was definitely happier at higher frequencies such as 1kHz and not so keen on 20Hz. This may be down to the amount of reservoir storage capacitance available, or the power supplies hitting their 20A current limits earlier than I'd hoped, but with real bass guitar signals it has so far belted out nothing but a good solid wall of sound. Or eaten watts and poohed bass as I have heard it put ;-)

The front is smoked perspex with a drilled aluminium panel behind. This allows the control fixings to be concealed, the (very noisy) PSU fans to be vented front and rear discreetly and the opportunity to put some eye-candy LEDs on the front around the master volume control. The ring shaped WS2812B LED board was sourced from a popular online auction site along with a simple controller. I was so pleased with these LEDs it made me ponder on another use for them...