2019, Chapter 15:
...There was, however, another part we’d been playing with, the relatively new OPA1688 from TI. The OPA1688 could take the +/-12V easily—it was rated for +/-18V. It didn’t have quite the current output capability of the LMH6643, but it certainly measured a lot better.
Jason,
I'm pleased to see these products released, I hope they do well for you. I was also pleased to see you (rightfully so) bragging about the quality of the opamps you used in Hel. And you did it right in front of a bunch of self-admitted audiophiles!!
I decided to start studying just what an operational amplifier is a few days ago, and some of the answers I found were surprising: The "operational" part of the name refers to the amp operating mathematical functions, such as addition, subtraction, division, integration, etc... Originally designed as computing modules, c. 1930 or so, and after a decade or so of development the name "Operational Amplifier" was coined and a set of parameters for the perfect opamp were set down; Infinite input impedance, zero output impedance, infinite gain, and perfect linearity of output to inputs (this last one would have EE's working around transconductance droop almost 100 years ago...). They also must force both inputs to match each other through the use of feedback, which as a practical matter will result in clipping at the rail voltages when driven too hard, unless the designer didn't think that was a priority and didn't bother to address the exception conditions, in which case they might invert the output and probably release magic smoke. Whew!! That is just the beginning of an incredibly complex decision making process for someone designing circuits with opamps. Or not. The application notes for these particular opamps show a circuit that uses a +/- 5v supply and can sink 20mw at 30 ohms!! That should do it, right?
All of that to say that the OPA168x opamps are a seriously compromised design compared to the ideal, but those compromises and the high voltage rails are what makes them ideal for this application as a headphone amp (well, four each in series for each channel in this application). And of course this is about as non-standard of an implementation of these chips as you could do and still drive headphones with them. I love it when a venture into unexplored territory works out, and I am never disappointed by your designs; Always something new and different from Schiit!
Hel directly addresses one of my task-critical use cases, IP Telephony, but I needed it three years ago. So I fell back on what little I knew about sound production, bought a couple of cheap-ass mics, decided that the worst problem I had was a noisy environment, and bought a dbx 166S channel strip to process the input. Proper gain staging, compression, limiting, de-essing, expansion, mild eq, and suddenly I sounded good on the phone. I'm using a Focusrite Solo for I/O, and a Chromebook for the call software. Windows 7 couldn't stay connected with the Solo, or any of my other dacs, so I'm taking advantage of a Linux kernel in a user friendly environment in order to make my work flow more consistent. The biggest issue with all that junk is the balanced connections. Yeah, I said it, balanced sucks because the cables are big and heavy and expensive, the connectors take up a lot more real estate, and at least in the instant matter, NOT NEEDED. My production mic at the moment is a cardioid pattern face mic. It will tolerate a 12v bias, but really only needs a coupla few volts, so I feed an inline adapter with 48v phantom from the 166s which drops the bias voltage and buffers the output from the mic to get to 4v pp for the balanced connection. The big advantage of this setup is that all the pieces can be quickly configured to do something else if need be. Oh yeah, it doesn't pick up hum at all, either. Did I mention music? Modi Multibit -> Magni 2 Uber -> k702. Those components really cast a long shadow over the Focusrite Solo (2nd gen) which I really can't stand to listen to music through.
The major payoff with Hel is the utter simplicity of the setup. All single ended, cheap cabling, and industry standard consumer class microphones. And Hel will fit in my computer bag so that when I am inevitably called on to work while I'm on vacation in some far-flung corner of the planet again I will have all my tools with me. And, I'm willing to bet that it will sound better than the Solo (not hard to do, my original Fulla The Dongle is still way better, scratchy pot and all).
The only thing left to ask for is the much discussed digital preamp... I really need 32 bit floating-point dsp for the filters I want to run... I can get that for free, but I have to use a DAW running in Linux to get there. Talk about not user friendly, and the last thing I need is yet another computer in my setup.... And, as the banner on the advertisement above says at the moment, "KB EAR, Kiss Your Ear, Be Your Favorite." Words to live by...