JLH gets regulated (temporarily)...

Over the weekend, I tried to get modify some of the high efficiency DC/DC converter boards to output 26V for the JLH, by replacing most of the passive components with different values suggested by TI's design software, but not a great deal of joy was had - while they will happily do 26V into no load, the under voltage lockout/ramping doesn't appear to be right as they only output around 3.3V when connected up to the amp... ah well.

So why not try plugging into the linear bench supply?  If memory serves (I built it a few years ago), this is LT1084 based, so capable of decent grunt with the die cast aluminium case acting as a heatsink.  The outputs have been configured for a voltage of around 33V.

First results are promising with an 8 ohm load - modulation is now down to almost nothing, and the noise floor is much flatter... but there's a lot of high order harmonics present that weren't seen before in the unregulated supply.  Let's try reintroducing that hand-wound 2mH 0.03 ohm choke between the supply and the JLH... hey presto, much cleaner!

(Note: signals are still being normalised to 0dBFS)

It's important to remember that most linear regulator ICs are not good at suppressing higher frequency noise... that said, it is surprising just how big an effect the choke is having.  The performance is now very impressive indeed for a simple circuit, and fully satisfies a basic objective criteria for a "clean watt".

Let's go further and try it with a 15 ohm load again, this time comparing to the measurement reference DAC being used as a source in these tests.  The DAC is based around an old WM8740 evaluation board whose heart has been replaced by an AK4396 in the name of better objective measurements... here's what it looks like inside...

All the mains circuitry in the picture is purely for charging - when in use, the design is disconnected from the mains and the DAC runs fully off battery power, with digital input coming via. optical for electrical isolation - this eliminates the possibility of annoying ground loops at the input end.

The measurement reference DAC is driven with a 24-bit 96kHz test signal, captured by a modified LynxTWO-B board with AK5394 A/D conversion... this is as high as the board will take in its current form via. S/PDIF, and appears to give good results... while it is certainly possible to achieve lower distortion than this (the simplest means being high order filtering around the test tones), the results are plenty good enough for a basic check point.

Putting to one side the second and third order harmonic, the JLH closely mirrors the distortion and noise of the input signal... in fact, if anything, you could say that the JLH appears to have "cleaned" the signal up a bit - this suggests that the output of the DAC probably would benefit from a little bit more filtering.

This is pretty good performance and suggests I should build up some linear regulators as soon as possible for the JLH so I can go back and listen to them again!  While a discrete regulator would be interesting to play with, I've ordered some LT1083 to do an initial first run with, as the design seems to work so well with even a traditional series regulator.