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Yep. The Modulus architecture takes the LM3886 to a whole new level. Tom

I'd go for DF > 100. That's not hard to obtain in a headphone amp. Tom

Yep. DF = Zload/Zout. Tom

Yeah. I measure it for all my speaker amps: www.neurochrome.com Tom

You could do it by hacking the eval board. That's not going to be super fun unless you have a pair of hot tweezers, though. I forget if the resistors are 0603 or 0402 sized. If the latter you could entertain removing them by heating up both ends at the same time with a regular soldering iron...

BTW: I have a video where I show how to build the HP-22: Tom

The HP-22 has 10 dB gain (so 3.16x), though, you could pluck a couple of resistors and get unity gain. You can use discrete opamps if you wish as long as they're stable at whatever gain you choose. Tom

Transformer buzz can sometimes be caused by DC on the mains. A DC blocker can help. https://sound-au.com/articles/xfmr-dc.htm Tom

You are not comparing apples to apples here. The $249 Modulus-686 only had the surface mounted components populated. You had to buy about $100 worth of parts and solder up the rest to make it work. To equal the fully assembled module, you also needed about $30 worth of custom mounting hardware...

Dang! Nice work. Tom

Maybe... Personally I'd get the BK Precision AC generator. :) The ticks are high enough frequency that filtering might work, but you'll probably need a filter that works in the audio range and not the typical RFI line filters. I find it really hard to believe that anything on the AC line...

I would start by determining how that buzz gets in. Start by following the directions above. I don't know how you measured the mains voltage. If you used the ground clip on the scope probe or had long leads connecting to a transformer you may have picked up the interference through the scope...

Answered my own question: It looks like the USB-C is only used for power. The ADAU1701 is the DSP. Tom

I'm horrified by the number of videos I see where the ESD sensitive parts are stored in plain plastic boxes. Seriously?! If you're going to work with low-voltage ICs please follow reasonable antistatic practices. On a different note: Do you have any idea which USB receiver is used in this DAC...

There's quite a bit of loop area by the opamp in Bruno's layout as well. But, yeah... His layout is tighter than mine. I'm fine with that. :) Had I not allowed for gain switching by jumpers - a feature requested by my customers - I could have made the layout much tighter. The proper term for...

If you already have the SMPS1200A400 power supply and two Purifi 1ET400A amp modules, all you need is this: 2x Purifi 1ET400A / Hypex NC500 Input Buffer 1x SMPS Splitter You'll also need two wiring harnesses to connect from the SMPS Splitter to each of the two buffers and a 10-pin ribbon cable...

Those who are curious about a subjective review of the Neurochrome 1ET400A / Hypex NC500 Input Buffer versus the Purifi EVAL1 should have a look at this review in Positive Feedback: https://positive-feedback.com/audio-discourse/neurochrome-purifi-1et400a-hypex-nc500-input-buffer/ You're correct...

I bet the hum is worst when the volume pot is turned to the 2 o'clock (-6 dB) position. That's when its output impedance is the highest. Tom

Oh? Why is that? Does every discussion have to devolve into US vs THEM? I think there's room for more nuance like that. All I (and others) have said is that personal preferences play into the choice of audio gear. I've designed well-regarded tube amps and state-of-the-art solid state amps...

Actually, even-order harmonic distortion is supposed to add warmth. It can also add a sense of space. SETs generate mostly even-order distortion as they tend to clip harder in one direction than the other. Whether you believe it's OK for an amp to add warmth is another story - and a reflection...