This is a common issue indeed. AFAIK even the QA40x can't deliver enough juice for measuring the headroom. Long time ago, when I suggested higher output I almost got attacked by many members accusing me that I sacrifice THD and noisefloor for the sake of amplitude
I would love to build asome small amp device, which would balance and amplify the output of a signal generator but I don't know where to start... For the time being I've been using a step-up TX but the limitations are the fixed gain (+6db), the bandwidth and the linearity of course.
Yes, max output level is always limited. I have two interfaces that I chose specifically for their max output: Tascam US-2X2HR (+20dBu/7.746Vrms), and a Motu 4Pre, which tops at about 6.5Vrms. Neither, of course, is hot enough when testing pro audio devices, but should be fine for any consumer level thingy. Consumer interfaces usually don't perform will at 0dBFS, just FYI, you'll want to be a couple dB below that.
Of course, the QA401, as great as it is, sticks at 3.76Vrms (differential), +13.7dBu.
So...two things on that:
1. What do you really need to drive that hard? You need to know and define that first, because in practice, there isn't a lot, and as they stand, you can do 80% of everything you need with a US-2X2HR barefoot.
2. What would you be willing to give up for more output level? You will give up something, but it may not matter.
My answer to 1: Transformers. If I want to profile a line-level transformer, I'll need to hit it pretty hard. A 111C, for example...yeah, you really have to punch those.
My answer to 2: I'll give up a bit of noise floor and a tiny bit of distortion for hotter output. But that's also based on what you're planning to measure. Again, transformers, noise isn't often an issue, so getting more level into them with a tad more noise, isn't a problem. Distortion...well, everything I'd use to amp up the analyzer output has far lower distortion than any transformer, so that's not a problem either.
Now, my solution, and this is in progress... I have several cards I designed for broadcast audio use in the early 1980s. They're well made, and with a little modification, will work fine to carry the active parts. And the active parts they were made for were 990 opamps, which in 1980 were the bees kneez. The cards are already configured for them, and I have two new 990s set up for +/- 24V power, which lets them provide a maximum of 27.6Vrms/ +31dBu, differential. The card is set up for enough gain so the interface will push them to max output, or nearly, at 0dBFS. They'll drive 75 ohms, and will drive 150 ohms to 145KHz, with an EIN of -133dBu. And folks, that's good enough for anything I need. Pefect? Nope. AP? No. But until somebody agrees to pay me to make measurements with an AP setup, new, used, or ancient, this rig will be fine.
My solution is basically a cheat, because I had what I already had, I'm lazy, and I don't have as much free time in my old age as I thought I might. I got the new (and actually, improved!) 990s from John Hardy, along with a pair of his "output isolators", and a set of fresh sockets. Though honestly, my old 990s work too, though they are +/-15V designs.
. Thanks for triggering me!
