Audio Interoperability

It’s kind of nice that unlike video, audio equipment is largely interoperable even when individual components are from different decades.  My main audio setup was a good example of this – I’m using speakers from the 80s (Magnepan MG-IIIs), a power amplifier from the 90s (Musical Design D-140i), a pre-amp (Emotiva USP-1) and one source (Logitech Squeezebox) from the 2000s, and a second source (Chromecast) from last year.  By comparison, video went from RF/coax to composite to S-video to component to HDMI in that timeframe, with HDMI itself going through 1.0, 1.1, 1.2, 1.3, 1.3a, 1.3b, 1.3c, 1.4, 1.4a, 1.4b, and now 2.0 revisions – connecting equipment separated by a decade is basically a non-starter.

However, even audio changes with the times, and today I finally made a change that I would have made a year ago if I was less lazy – because getting things to work closer to the way I’d like actually took some custom elements. Here’s what my setup looks like!

Source:  Chromecast + ViewHD

Once upon a time, it was necessary to have multiple sources.  There were legacy sources – AM/FM tuners, LP players, and cassette tapes.  Even today, CD players can remain relevant if you have a large collection of music on CD, plus despite being 25 years old, good CDs sound as good as any digital format.  Of course, digital sources now dominate and are vastly more convenient; my Squeezebox was the primary player for my entire collection of ripped CDs.

However, with subscription services like Google Play Music and Spotify, the very idea of music ownership seems to be coming to an end.  I subscribe to Play All Access, and since Play allows me to upload my existing collection – which does actually include esoteric game soundtracks that aren’t available directly via Play – I truly no longer need any other sources.  The main shortcoming of Play Music was that it was difficult to connect into my audio system.

Fortunately, that changed with Chromecast (though as a reminder, I work on this area, so I’m very biased!).  While Chromecast lacks audio outputs, an audio converter like the ViewHD model above ($40) solves this quite easily.  If you don’t need SPDIF output or EDID controls, there are devices as low as $20 that seem like they’d work equally well.  If I was more worried about the quality of the DAC inside the ViewHD box, I’d use the SPDIF output to connect to a separate DAC – but I’m pretty sure my kids have permanently damaged my hearing anyways.

Output:  Magnepan MG-III + Musical Design D-140i


I’m using the same speakers for music that I bought the year I finished school – the Magnepan MG-IIIs.  They’re 6 foot tall ribbon speakers, and while they’re not well suited to home theatre, they’re still plenty good enough for my ears on music content (though positioning them optimally is hard these days, and the kids like hiding behind them while murdering my wires).  I bought these speakers used in the 90s, from a friend’s uncle who purchased them new in the 80s.  They’ve aged much better than the original Nintendo Entertainment System or Sinclair ZX 48K spectrum sitting in boxes nearby from the same era!

The amp, pictured above, is a Musical Design D-140i that I picked up, used, in the late 90s, and have used since.  It’s a Class A amp.  If you’re not familiar with amp terminology, that might sound like a compliment, but it actually refers to the design of the amp.  Perhaps the most pronounced characteristic of Class A amps – now largely obsolete and replaced by newer designs – is their inefficiency.  That’s certainly the case here – if the amp is powered up, it sucks a ton of power and gets very warm even if it’s not making a sound.

This fact has been my main dilemma with my current setup.  I can’t leave it turned on, and the only switch on the amp is the big mechanical one you see on the face plate.  The amp sits nicely inside a cabinet, but I needed to open it up and turn things on every time I played music.  It was very easy to forget that it was on after I stopped casting, in which case, a few days later I’d have both a flaming inferno in the cabinet as well as a higher power bill for the month.

Integration: Custom Ugly Box

Remember I said above that I was lazy? It’s because the solution to my Class A power amp problem was actually simple – all I needed was an automated way to turn the amp on whenever music was playing.  Complex home automation solutions where I could use some other app to power outlets on and off via some app were out of the question, because my family would never use this; I needed the solution that added no complexity over just pressing the Cast icon in Google Play Music.

Sadly, there were no ready-made solutions to this problem, despite that it seems simple.  My power amp wasn’t new enough to have 12v triggers, and 12v triggers would still require me to turn on a preamp.  Some newer amps have auto-sensing, but that would mean replacing the D-140i which continued to have good quality and worked very well – plus very few amps actually have auto-sensing.  There were current-sensing power bars triggered off another device, but Chromecast draws very small amounts of power, with little difference between idle and active states.

In the end, I had to build the custom box pictured above, which takes a single audio input, and uses it to switch an AC circuit.  It should be possible to make a box like this for $10, but being lazy I spent vastly more than this.  If I have time, I will try and make a $10 version someday, but that would require brushing up on basics around filters, op-amps, and timers.  So what’s inside the box, and what did it cost?

  • RDL ST-ACR2.  This is an audio controller relay, and is the main component in the system.  It cost a whopping $96!  It basically uses an audio signal to control a modest relay, with adjustable sensitivity, and a switching delay up to 50 seconds.  The delay was key – there were cheaper circuits, but you don’t want your amp power cycling between songs!
  • TDK-Lambda LS25-24.  This is a 24v DC power supply, which cost $17.50, but was necessitated by the fact that the ST-ACR2 requires 24v power.  Like the ST-ACR2, the LS25-24 has nice screw-based terminals that saves a bit of soldering.
  • SSR-25DA.  This is a 25A relay, which costs only $5, and has a trigger voltage of 3-24VDC.  While the relay in the ST-ACR2 claims to be rated for 10A, power amps have a notoriously high current draw when they’re first powered on – I blew the auto-switching circuit in my TA-E9000ES because I didn’t realize this.  So I just use the ST-ACR2 to trigger this higher rated relay – plus if this relay blows, it’s $5 to replace instead of $96.  This particular relay also has screw terminals; the first one I got was PCB mount, and worked until too much shuffling of components inside the box caused one pin to break from metal fatigue :(.
  • Hammond 1591E.  This is the black box you see above, which cost $7.  Not much more to say about it, other than that my skills with a Dremel (to cut holes out) are severely lacking!

All told, this was a $125 box – more than what four Chromecasts go for Amazon at the moment.  That’s the price of forgetting my electronics!  Wiring everything up was surprisingly easy and more or less worked the first time.  One mistake I made was not connecting the outer terminal of the audio input to both the negative and ground input pins on the ST-ACR2, which caused a loud hum on the affected input channel, but that was easy to remedy.  Now, my amp powers up whenever I cast music, and is switched off as soon as the music is over!

“Preamp”: Emotiva Control Freak

There was one last piece to the puzzle, and that’s the pre-amp.  My current setup was based on the Emotiva USP-1, which was a nice and not terribly expensive 2-channel analog pre-amp with great build quality.  It’s far more modern and efficient than the D-140i, but I would have had to leave it on all the time, because when you first apply AC power, it goes into standby mode regardless of its previous state. Alternatively, I had an older Halfer analog pre-amp, but it had always been flaky in one channel and I’d been unable to repair it.

However, a preamp has a pretty basic set of functions – input switching, impedance matching, and volume control. When you’ve got just one source, as was the case here, you can scratch input switching. Impedance matching might be important especially with a range of sources, but I was betting that a relatively modern device like the ViewHD, despite being cheap, would have reasonable output impedance.  So that just leaves volume control!

While some passive preamps seem to think they can still get away with a premium price of over $1000, Emotiva’s approach was far more sensible – a big, high quality potentiometer for $50, with decent RCA unbalanced audio interconnects.  With this between my Chromecast/ViewHD and power amp, I have an easy and inexpensive master volume knob – and can use Chromecast’s digital volume control when I’m feeling lazy.

In a perfect world, volume commands sent to Chromecast would turn this knob, but that’s asking the impossible.  In the future, a volume-enabled ViewHD with HDMI CEC based volume control – and the matching feature on Chromecast – would give me what I truly want.  But until then, I have the big, smooth volume knob above.  I just need to make sure to keep it hidden from the kids – because if it was ever turned to maximum, something would break.  My ears, at the very least!

One Comment

  • Michael

    Hi Mark,

    I came across your Blog after hours of online search for a ‘Custom Ugly Box’ just like the one you built here. The only difference in my case is that it’d be for Europe (220V). I am ok with very basic electrics and can do the odd soldering job, but I am by no means very experienced. I therefore wanted to ask you, if you could send me a wiring diagram of how you wired up the RDL ST-ACR2, TDK-Lambda LS25-24 and the SSR-25DA? I understand most of it is self-explanatory, however, especially in regards to the SSR-25DA I’m just not certain I understand and would really appreciate your help (else a simple photo of the guts of the ‘Custom Ugly Box’ would already be good too). Also, if there’s anything in particular I’d have to take into account with this design in regards to the 220V here in Europe, it’d be great if you could let me know, just to be on the safe side (the SSR-25DA seems to be ok in this regards, right?).

    BTW, I think you did an awesome job with this ‘Custom Ugly Box’ and I’ll make sure to call mine by the same name: The CUB!

    Many thanks for the great blog entry, all the best,


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