The L|A Autoranger
After a lot of to-ing and fro-ing I decided that my soundcard needed a front end that would give even an Audio Precision credit! Well, that was easy. Now designing, building and testing it - that was far more work (and far more money!). But after more than a year, I am now at the stage that I have a fully functioning prototype, power supply, software and all.
Status and documentation
January 2017: I have submitted two complete auto-rangers to trusted persons for beta-testing. A also have produced a preliminary User Guide which can be downloaded here to get a feel of how the unit operates. It also has some pictures of the physiacl unit, but be aware that the final PCB layout may still change; for instance, some parts that show as SMD will be changed to Through Hole. But the overall setup is pretty well fixed.
What does it do?
Most of us use a soundcard and ARTA or similar software to test our amps and whatnot. But the soundcard has a 'sweet spot' at around 1V input level; go over that and the soundcard distortion goes up or it even can get damaged. So there is always a need for some rigged-up attenuator. irrtating! It also makes repeatability much more difficult, not to speak of varations in frequency response due to that makeshift attenuator.
So, what the Autoranger (AR) does is take the signal from the amp you want to measure, and size it to around 1V for your soundcard. Automagically.
What's the setup?
The AR is a separate small box (see pic) with a single-ended and balanced input that accepts the signal to be tested, and a single ended and balanced output to your soundcard. Simple and easy. But people don't like black boxes, so there's a small display that tells you what's going on: it shows the signal level going into the soundcard, it shows the signal level going into the AR, and the attenuation or gain that was applied to get these values. Then there's a pair of LEDs showing when the AR gets too large (more than 150VRM) or too small (less than 100mVRMS) an input to get it to 1V: the Overrange (red) and Underrange (green) indicators.
Although the AR runs fully automatic, you can manipulate it through a trio of pushbuttons. Firstly, there are two pusbuttons to select either the single ended or the balanced input. Internal processing is a bit different for the two modes so selection allows optimal results in either mode.
Secondly, if for instance you want to do some kind of level sweep, you can set the auto levelling at the highest level you expect and then press HOLD. This locks the attenuation or gain at the present setting until you release it with another press of HOLD.
Finally, one more manipulation: if you are in HOLD mode, the input select pushbuttons take on a new role: by pressing one or the other, you can force the AR into a flat -20dB or -40dB setting, or return to the initial locked setting. I have tried to think of all circumstances for use and this gives you additional options (it is also used for calibration by the way).
The AR accepts anything between 100V RMS (140V peak; yeah that's some amp you have there) and 150mV RMS. And as noted, puts it out levelled at around 1V RMS. The range goes from -40dB to +16dB in 4dB steps.
The frequecy response is flat within +/ 0.2dB out to 100kHz so it is flat even with a 196kHz soundcard.
Distorton is down to -120dB, also better than almost anything out there.
Input impedance is 100k in all attenuation and gain settings, within 1%. Input capacity is TBD but probably around 30pF in all settings.
I measured (adjusted) CMRR at better than 60dB out to 50kHz but will re-measure it because it is almost too good to be true...
And the power supply...?
Good that you asked! I have develoed the power supply with the notion that it should be totally separate from the mains. Too many of thee setups are plagued with hum, niose and line harmonics that somehow always seem to sneak in through the mains. The PSU for the AR runs on nominal 5VDC input. It can run on batteries from 3V to 10V, but the idea is to run it from a USB charger or a Power Bank (PB); it has a standard USB type B connector on the back). Of course, with a plug-in USB charger it is not strictly separated from the mains (although better than with a 'normal' power supply), but the PB option is ideal. You can dedicate a small PB to it and have it constantly on the charger, and for sensitive measurements (or for portabilty) you just unplug it from the charger. The power draw is small enough for many hours of operation even with a 2000mA PB*.
*That power supply is really a problem solver and you can also use it for anything that needs both a very low noise +/-15DC supply and a 6V, 5V or 3.3V 'digital' supply. I call it the SilentSwitcher.