Hi Opacheco and others interested in comparing science with what we hear.
1. The Rload/Rload terminals are used to connect a resistor that the Radj2 pot uses to adjust for the cap's ESR (equivalent series resistance). I have been using a 1 kohm 1% metal film resistor there, and it seems to allow me to set the pot at a useful position in it's rotation. This pot doesn't really change things much when I adjust it though.
2. Keep in mind that this test does not produce a numeric value for DA. It does however allow you to compare one cap to another so in generic tests, you need to have a really good reference cap. I went onto Mouser and collected a range of capacitor values from the more trusted manufacturers like Wima, F&T, Solen, etc. choosing what are recognized as the better dielectric types (teflon and metalized polypropylene) and looked for voltage ratings that will work in tube amplifiers (200V to 630V). You'll find very few manufacurers actually list the DA.
The oscillator source needs to be a square wave that allows you to alter the duty cycle. The article suggests a level of 1Vpp to 10Vpp. I've been using about 2Vpp to 4Vpp. The article also recommends setting the wave's pulse width (tw) between 1ms to 20ms, and the wave's period (T) to about 5x to 10x the pulse width. I've been using a pulse width of 10 ms and a period of 35 ms. This is a little shorter of a period than it suggests, but I got pretty good results. A longer period gives more time for the residue to settle.
I Y-branch the signal generator to feed the tester, and also channel 1 of the o'scope. The output of the tester is sent to channel 2 of the o'scope. Notice that the bandwidth limiting capacitor C1 causes a slight rolloff of the leading edge of the source pulse. This keeps the artifacts below 100 kHz.
In the tests that I have done, I have set the gain switch to 100. I use a +/- 15V bench power supply.
I put the high quality reference cap on the Cref terminals, and then the cap to compare to this reference is attached to the Ctest terminals. As seen in my posted pictures, I use banana posts for all the connections except the 2 signal jacks which are bnc connectors,
My oscilloscope is set with channel 1 at 500mV/div, and the time base at 5ms/div. Trigger to channel 1. Channel 2's vertical sensitivity will vary as you adjust the trim pots. Try starting with about 2V or 5V/div on channel 2, and adjust the coarse pot to get channel 2's signal to drop. Then use the mid and finally the fine adjust pot. Now you can try adjusting the Radj2 pot to see if it makes any diference. You'll need to increase the sensitiity of channel 2 as you get it trimmed to it's minimum ampitude.
So far, the best I've seen are the Solen polypropylene caps and the worst I've seen are very old large Sprague caps pulled out of old guitar amps.
Despite this lengthy description of how to use this box, the tests go very quickly, and you can go through a stack of caps in very little time. I've got quite a collection of old Fender tweeds that I want to do some listening tests with, to compare the very good DA caps with the very crappy ones to see if I can hear a difference. One thing I should add though, is that I'm an active blues guitar player and I have become fond of using preamp tubes that are just a little microphonic because it give the amp a little resonance that I find pretty musical. To me, tubes that have no microphonics at all make a guitar amp sound a little dull. Of course tubes that have rampant microphonics I find unuseable. There is a nice in between spot though that really sounds great.
