Saturday 2 November 2019

It's Alive! The Linear Audio Autoranger MK II...

And the kit is now alive just in time for Halloween the other night. :-)

For those still wondering what this is, you can check out Jan Didden's video of the Autoranger MK I from 2017 and his demo of how it's used:

As you can see from the video, this device basically serves as a single-channel-at-a-time but 2 inputs (1 unbalanced and 1 balanced input) front-end to a measurement ADC (these days, for me it's the RME ADI-2 Pro FS). While the ADI-2 is already very flexible with the ability to handle a wide range of input and output levels, allowing it to optimize dynamic range (settings for +4dBu/1.23Vrms, +10dBu/3.46Vrms, +19dBu/6.90Vrms, and +24dBu/12.28Vrms depending on single-ended and balanced operation), the switching of input levels would still need to be manual. Furthermore, for measurements of devices that produce a range of output levels like amplifiers where we might want to measure the quality of a signal from say 0.28V (20mW across 4Ω) all the way to 20+V (100+W across 4Ω load), the Autoranger would provide an essential level of convenience and protection without fiddling with input levels and using fixed or manual voltage dividers.

As per the video, one can just set the Autoranger to a specific nominal voltage level (choice of ~0.4Vrms or ~1.0Vrms) and with whatever signal presented on the input, the device will determine a gain or attenuation to try to achieve the nominal voltage on the output side. Both single-ended and balanced output always active.

You can see the button on top of the BNC/XLR inputs to determine whether it's the single-ended or balanced being analyzed. This device has the ability of adding up to +18dB gain, or attenuate down to -42dB. The other essential convenience is the "Hold" button to stop automatic action and allow manual switching throughout the range.

As you can see from the post a few weeks back, it comes in a "quarter kit" with enclosure, a SilentSwitcher USB-fed power supply, the preprogrammed microcontroller, and the PCBs to build from.

No need to rush with building the kit; in total I spent about 4 evenings taking my time to solder the pieces in place. There are a few surface-mount capacitors to solder on so make sure you have decent eyes, relatively steady hands, and fine tweezers. :-) The rest of the components are done with easier through-hole soldering. As usual, make sure to double check on some components like the electrolytic capacitors, LEDs, diodes, trimmers that need to be inserted with proper polarity or correct orientation.

Note J2. Put a jumper here for the display color you want. Also there's a bit of fiddling that needs to be done to get the contrast trimmer right.

Linear Audio Autoranger MK II, front view. Notice the SilentSwitcher power supply attached to the rear of the case.

Linear Audio Autoranger MK II. Rear/inside view.
All done. Here's my "audiophile hobbyist's AP" set-up ready for amplifier measurements:

On the table is the "System" running a loopback of a signal played from the computer using foobar, output through the DAC into the passive attenuator, then sent to the Autoranger input. The Autoranger output is then fed back to the RME for ADC conversion to digital for measurements. The Microsoft Surface 3 Pro is used for analysis and data capture.

The RME ADI-2 Pro FS is essentially the "heart" of the system. It's capable of producing clean analogue output and excellent capture of analogue input for analysis (as we've seen in the past). I've attached an inexpensive but quiet passive attenuator to the System - the Nobsound NS-05P (~US$60). The reason this is here is for measuring power amps like monoblocks where I can use this as my "standard" signal attenuator when needed. For integrated amps where there's already volume control, I'll just leave this at 100% to pass the full signal through.

To the left we have my Rigol DS1104Z Plus oscilloscope for looking at square waves when needed, it can also help with looking into the far ultrasonic spectrum.

On the right, we have some power packs; specifically the Talentcell 6000mAh 12V pack and a generic 5V USB pack to power the RME ADC/DAC and the Autoranger respectively. Other than the oscilloscope on the table, the System is run off batteries to keep noise as low as possible.

In front we have a potential "test subject" :-). That's the little Class D Yeeco TI TPA3116D2-based amplifier that I mentioned back in late June. I'm not showing the various BNC cables and the large 4Ω dummy loads I'll be using here.

Before we start actually measuring devices, we of course need to get some baseline "calibration" and "verification" data to make sure the System works and appreciate some potential limitations. We'll have a look at noise levels, distortion levels, and stuff like that next time when we run this through a loopback test to look at the resolution available.

A few final notes for those wanting to build one of these Autorangers:

1. Remember to pick up a few standard 0.25W resistors including 1kΩ (see R1-R12 on the control board, and R48-49 on attenuation board) not listed in the Mouser bill of materials. Also, you'll need some DIP sockets for the ICs like your opamps (PDIP-16 for the ULN2003A, CDIP-14 for AD536AJ, 2x PDIP-8 for NE5534 and OPA2134).

2. You might need to pick up some 5mm nylon spacers and M2 nuts/bolts worked for me when mounting the control board with the LCD panel (see the Assembly Guide). There's also a short 2x14-pin cable you need to make plus a few Dupont 2/3-pin connectors you'll need to construct for the power supply feeding the boards.

3. For those who haven't worked with surface mount soldering, remember that fine tweezers, thin 0.3mm solder or thereabouts are important. Also, make sure you have a fine tip for the soldering iron (I just used an inexpensive soldering station like this). I made sure to do the surface mount caps first since they're flush to the board before stuffing the larger components that can get in the way.


Finally got around this week to listening to The Beatles' Abbey Road Blu-Ray multichannel disk:

Nice to see that this disk provides both DTS-HDMA and Dolby TrueHD Atmos presentations. The DTS-HDMA is reported as 24/96 while the Atmos mix is 24/48. No worries though, because this is an old analogue recording, I've happily downsampled the 24/96 DTS-HDMA to 5.1 16/48 FLAC for my music server; no special resolution here demanding 24-bits or >48kHz IMO.

The Atmos mix sounds good. Conveys a bit more space - for example the bells and nature sounds at the end of "You Never Give Me Your Money". Overall tastefully done without unnecessary placements of instruments and such to accentuate the multichannel technology.

As usual, have a wonderful week and hope you're all enjoying the music! Wow. It's November already... How quickly the years go by.

1 comment:

  1. Very cool Arch! I had my eye on Jan's Autoranger for a while, but like you say, how quickly the years go by. Looking forward to seeing the results of a loopback test. I am sure it will be more than enough resolution for your testing purposes. Will be interested in seeing the test results of that little class D amp :)

    Keep up the great writings!