Simple Ripple Eater PSU for the sx and kx-Amplifiers
A high quality double sided through hole posted and silk screened PCB is available for this power supply from Jim’s Audio here: Ripple Eater PSU for kx and six Amplifiers
Here is a very simple capacitor multiplier PSU aka ‘ripple eater’ for the sx or kx-Amplifiers. The LTspice simulated ripple rejection at 100 Hz is about 65dB and at 10 kHz it is 85 dB. These figures assume you have a 2.5A load and also include the on-board filter capacitors on the amplifier modules themselves (220uF per rail on the sx-Amp and 1000uF per rail on the kx-Amp per amplifier module).
Class A amplifiers draw heavy quiescent current from their power supplies, and this gives rise to significant amounts of mains related ripple that will affect the amplifiers noise performance. VFA amplifiers generally have better LF PSU noise rejection than CFA’s, but it can still be a problem with a class A VFA.
A further benefit of this supply is that the radiated mains noise from the PSU to amplifier module wiring is greatly reduced because you will be supplying close to DC into the amplifier module local reservoir capacitors, so the associated mains ripple charging currents are very low. Signal related currents will still be flowing in the wiring of course, so you have to pay careful attention to layout, how you dress the wiring, loop area minimization and common impedance coupling issues – you can read more about this in ‘How to Wire -up a Power Amplifier’. Nevertheless, this PSU goes a long way to making sure your finished amplifier can be as quiet as theoretically possible. When combined with the 50~60dB mains noise rejection of the actual amplifiers themselves, the noise levels on the amplifier output will be down as much as 100 dB which is an outstanding result by any measure. At HF, the performance will be even better, and that is important for the amplifier sound in the mid and high range.
The series pass transistors (8A MJE15032/33 devices) are designed to be mounted underneath the PCB with their tabs screwed to the amplifier chassis base using the appropriate thermally conductive insulator (same mounting technique as the main rectifier D4 as well but you do not need the insulating washer on the rectifier – only some thermal grease). Each pass transistor will dissipate about 10W in a stereo set-up delivering full output power into the loudspeaker load.
In normal operation, you will drop between 3 to 4 volts worst case across each pass transistor, so if you want an output of say +-27V, the loaded DC voltage into the PSU should be 31V. Note carefully as well, this power supply does NOT regulate the output – it will track the raw filtered DC voltage, but remove the ripple hence the ‘ripple eater’ name.
You can use this ripple eater power supply as is up to +-35V. You can also use it on amplifier with higher supply voltages up to +-50V. However, you must change the filter capacitors to 50 or 63V types (this is C1 and C6 in the schematic). In a class AB amplifier, you can use values that are about 3x LOWER than you would on a class A amplifier – so about 15 000 uF at 50 or 63V would be correct. You must also change C5 and C7 to 50 or 63 Volts. Finally, change R5 and R6 to 10k each.
Here is the component overlay for the PSU.