On to the amps. Back when I originally designed these amps, some 10 years ago, the distortion analyser I had had a noise floor of 0.0013%. I achieved that fairly easily so moved on. I've just bought a PCB for a super-low distortion oscillator based on the Linear Tech "AN67" circuit, which uses some scary high-gain opamps wrapped around a resonator to generate an "unmeasurably" pure tone.
Now I've got a sniff of a world beyond 0.001%, I'm very keen to see how far I can go. Part of that's developing a whole new completely over-the-top amplifier (see previous posts), part of that is just playing with my existing amps to see what they're really capable of.
The process is straightforward. Put one together on a heatsink with a dummy load and thrown together power supply using bits from the garage, apply a stimulus from either my mac, my function generator, or my new oscillator, measure what's being put into the load with my CRO and spectrum analyser, tweak, repeat. It's a process that's quite involving.
Here's the amplifier in question. It's a little 50mm x 100mm PCB, implementing a modified version of Tilbrook's AEM6000:
The schematic for the small 100W flavour is below:
While it's connected to the tracking generator, I check the gain flatness across the audio band:
From here the measurements get increasingly challenging. First the noise floor of the amp. My spectrum analyser has a direct noise readout in nV/sqrtHz, which is incredibly cool. Pulling the amp input off the tracking generator, terminating it into 50 Ohms, and then winding the spectrum analyser all the way down to find the noise floor, we get this:
Next we start working our way down the THD rabbit hole. I started by using my macBook air, as it has a reasonably clean DAC. I used Audacity to generate a 1KHz tone, then played that at all the mac volume settings, both directly and via my amp output:
To go deeper, I really do need a better source. So I spent some time building the 1KHz low-distortion oscillator. Here's a photo:
I wondered what sort of improvement I could get by substituting faster VAS and second stage transistors. The MJE340 and 350 are bullet-proof, but they're pretty slow really. I pulled them out and substituted KSC3503D & KSA1381E, then made another plot: