E. Reservoir Capacitors and Ripple

The main question we all ask — how big should my reservoir capacitors be? Professor Ken Kuhn wrote a great pdf document for his students.

I consider this document essential reading. Also check out his fantastic web site.

Some audiophiles get carried away with ‘requiring” ultra low ripple at high power. From Ken’s article, consider aiming for medium to low ripple and use the smallest capacitance that will achieve that goal. Higher capacitance increase surge current as Ken mentioned in his article. More capacitor = more dollars too!

Ripple is a nuanced number that’s conditional and contextual. DC supply ripple increases in tandem with PA stage current draw. So ripple could be quoted with no power supply load, with the amp at 1/2 power, full power and so on. For a clean jazz guitar amplifier, the best case seems to be with no applied test signal and the worst case occurs when you apply the maximum test signal before the amplifier sine wave distorts. This is often quoted as the maximum signal drive where amplifier THD is under 1%.

On the web, you’ll find a number of write ups and videos how to calculate percent ripple, or, the reservoir capacitor value you need to get a certain percentage ripple. Sometimes they lack precision and seem a bit theoretical. We amp builders need a practical, measurement-based way to evaluate ripple. I’ll show some simple oscilloscope measurements and apply Professor Kuhn’s formula below. It seems to work OK. It’s also fun to actually view the ripple on your DC rails with various reservoir capacitor values and power supply loads. Visceral stuff.

Above — The formula taken from Professor Kuhn’s document. To get data to calculate, you’ll need a power supply, a signal generator, a PA stage, a dummy load and a oscilloscope or DSO. I’ve already shown how to measure rail DC with a ‘scope DC coupled to your rail earlier in Figure 1.

Again, I’ll only show the positive rail.

I place additional ripple filtration capacitors on both my pre-amplifier and PA boards. We’ll only consider the PA board. As shown in my power supply schematic, my main reservoir caps are 6800 µF. On my PA board lie additional capacitors and on my test 23 W power amplifier, I’ve got a 2200 µF /50 volt electrolytic capacitor on each of the DC supply rails. So per rail, that’s a total of 9000 µF.

Above — DSO screen capture with an AC coupled 10X probe on the positive rail with no applied signal drive. This would be the “best case ripple”, but seems totally unrealistic as the PA is drawing only 16 – 20 mA quiescent PA bias current from the power supply.

Above — A DSO capture of the AC signal on my positive DC rail using an AC-coupled 10X probe. The V peak-peak = 660 mV. This is ‘worst case’ ripple as my PA is drawing maximum current; a heavy load for the DC power supply.

Using Ken’s formula, let’s calculate the percent ripple from the data we’ve gleaned.

Ripple = 100 * RMS Ripple Voltage / Average DC Voltage

Your DSO may calculate and display the RMS ripple voltage, however, let’s assume you’ve got a 25 year old oscilloscope with no math functions. We might simply just estimate RMS ripple voltage by taking the measured V peak-peak value and dividing it by 3. We bring the ‘worst case’ DC voltage from the earlier DC measurement of the positive PA rail.

Ripple = 100 * (0.66 / 3) / 23.1 = 0.952%.

On a popular (but unnamed) jazz guitar amp I recently measured, the AC ripple signal was around 2 V peak-peak with maximum clean drive signal into a 8 Ω resistive load. So I did OK.

This simple method allows you to measure and crudely calculate ripple percentage to make comparisons with different capacitors in real time. So my 9000 µF of C in a low-power jazz guitar amp ranks as ultra-low ripple at ‘worst case’. Of course, if you go for a 4 Ω load, it could be a game changer. More on that in Part 2 of this series.

Above — My 2 main reservoir capacitors. Installing them proved a little time consuming. I built my power supply using Ugly Construction on 2-side copper clad board. Since you can only solder their short, thick leads on the bottom side of the main circuit board, I had to improvise. I cut Cu islands for the DC rails and O volt pathways on the top side.

I also had to cut islands on the bottom side of the Cu board. I connected the positive, negative and 0V top islands to their respective capacitor leads below by joining the top and bottom carved islands with via wires. I drilled 5 via holes for each lead and soldered 20 gauge solid copper wire to join each appropriate top and bottom island. I’m used to doing this to provide a low impedance ground in UHF circuits, so it’s not a big deal for me. After final testing with an ohmmeter, the 6800 µF caps were installed. I did the soldering with my 80W iron.

I installed the filter capacitors by drilling 1 hole for each cap lead into each of appropriate carved islands and then soldered each capacitor lead to the bottom copper board.