Power supply toroid in Mackie SRM450 PA loudspeakers

A friend brought one of these very common PA powered loudspeakers over to the workshop. An internal T4A mains fuse had blown and when I replaced it and re-powered there was a very loud 50hz hum for a few seconds and the fuse in the power cord blew. I assumed it was the audio path that was making the noise and I assumed that one one the probably faults was a short/open de-coupling capacitor that was letting 50hz through to the audio stage.

I found the schematic at Mackie-SRM450 actspk.zip (although this is for the rev.C of the board, but the PSU appears not to have changed).

So - I made sure both fuses were good and with Dave Jones's words "thou shall always check the rails" in my ears I disconnected the output of the large toroid from the two rectifiers/smoothing caps and tried again. I figured I'd make sure the o/p of the transformer was good (there is some de-coupling on the primary). However I got the same loud 50hz for a couple of seconds and one of the fuses failed. The toroid was also noticeably warmer than it had been! So - checking the DC resistance of the primary side showed it to be less than six ohms (so it was pulling an inrush of at least 40A!).

A quick flail around the web showed that the transformer is a known weak point of this design.

 The real bummer is that the transformer isn't stocked by Mackie (or indeed anyone else) and the folks at Save My Light only do a minimum order (ten pieces) from the Chinese factory that wind them when they have enough orders (and the chap there told me he sells an average of two a year).

So, do I just wait or pay him the thousand quid to have ten made?!

Time to keep my eye on eBay for a hopefully working second hand replacement?

UPDATE 16/01/2017: 

So after lots of flailing around the web and finding a few reclaimed ones for more than £150 I came across AJAudio on eBay (Alex Mathew sales@ajaudio.co.uk was super-helpful) and he sold me one for £99.
Job done.

For info - the failed toroid had around 5 ohms DC impedance across the primary whereas the replacement has 8.5 ohms.

It's been shaking the workshop with loud music all afternoon!

The Engineer's Bench podcast - "Top Tips; Audio"

Hugh and Phil talk about some tips and get-out-of-gaol-free cards with respect to broadcast audio.
Go to the website for a PDF of the notes.

https://youtu.be/QJO0Z4DuDFc


Find it on iTunes, vanilla RSS, YouTube or the show notes website.

Audio over cat6a cable?

My podcast-partner in crime, the mighty Hugh Waters asked me how often I run audio signals over twisted-pair data cable in media facility builds. Has has a customer who is eager to do it this way.

I’ve done it a few times and it’s fine with a few considerations.

1. Earthing is still important and since IT people have no idea about proper grounding it can be an issue. If it’s a cat6a install in a TV facility done by me I’d have no worries, but the average IT install might have issues. But as four twisted pairs with individual screens cat6a is ideal for audio. It’s the other parts of the facility I’d worry about,
2. RJ45s don’t have the same DC/LF performance as a good old B-gauge (or Bantam) connector; if the circuits are going to be patched often I’d be wary – mechanically they aren’t great next to traditional audio connectors, 
3. From a wiring perspective; how do you nicely terminate into XLRs from a piece of cat6a?
4. Track shuffling is hard,
5. AES – just fine. Cat6a has an impedance of ~100Ω per pair, ideal for twisted-pair AES. Same observations as 1, 2 & 3.

There you go. I did work at one facility where the engineer had got obsessed with structured cabling and did everything he could over cat6 – analogue video (via baluns), audio, RS422 etc. He liked the idea that you could patch an offline i/o with two RJ45s (stereo i/o on one and video i/o and remote on the other). It didn’t work well and I put in proper cabling and patching after a year.

Sometimes convenience blinds you to fitness for purpose.

AJA HD10AMA analogue audio pinouts

The HD10AMA is a great de/embedder with two HD/SDi outs; very useful gadget. Here is the D25 details.

Managing multiple identical sound devices in OS-X

I use Skype (although I may be looking for alternatives due to Microsoft's proved snooping - see here) and I like to have two sound devices so that the radio can keep playing through my speakers without me having to reach for volume control when I take a call. Also, when podcasting, I use the same laptop to run the presentation, keep the Skype call going and make the recording (that chews up three sound devices!). So along with the laptop's internal sound chip I have two cheap external USB dongles. Since they are identical they show themselves with the same name is all apps and invariably (especially if I've been away from my desk for a day and re-booted the OS without any of the USB devices attached) Skype picks up the wrong sound devices as default. It's trivial to change back but I always get it wrong ("..is the headset the first or second one"?!)

In Utilities is the Audio MIDI setup application (which I've never used before) where you can set "aggregated sound devices" - presumably to allow the same audio to play through several outputs? But - it allows you to create a proxy for a device and give it a sensible name.

So, I made new devices for the two USB sound dongles and gave them sensible names.

This now means that when I look at available sound devices in other apps (particularly Skype) I see things I can distinguish!

iTunes makes broken MP3 files now?!

I've always stuck with MP3 files for music because it is the only format that everything we've ever owned will play. Currently we've all got iPhones and iPods but the car machine is one of those no-name head-units that's a radio and a flash-memory player. In the past we've had an assortment of 'phones and no-brand MP3 players and so I think the choice I made back in the late nineties to start moving all my music to MP3 was valid.

People object to MP3s for one of two reasons;

• It's not an open format like OGG Vorbis - it's notionally "owned" by Technicolor. It is so ubiquitous that I suspect they'd have problems enforcing that.
• It's lossy, and not even the best example of a lossy codec.

As ever Wikipedia has a very comprehensive article. On the first point I think life is too short to get all religious about technology choices. In the case of documents formats - sure; send RTFs rather than DOCXs just for politeness (actually, both formats belong to Microsoft!). Not everyone has the right machine or can afford MS Office (oh, and NEVER send Open Office specific files!).

In terms of the lossy nature of MP3s I'd say that if you encode files well yourself it shouldn't matter for most people and most music. With very little effort you can get MP3s that are so close to the uncompressed data that came off the CD that you'll never know. Audiophiles (who still tolerate all the noise and 2nd order harmonics that come off vinyl - and don't get me started on the RIAA characteristic!) claim that no compression is good, but I suspect they do so for reasons of fashion or self-aggrandisement. The reason I say that is I have actually done the tests!

Back in 1999 I was involved in a project to transfer a large audio sound effects library to a server. The start of the project was to see how well compressed audio was suited to the task. Drives were small and expensive back then and so the success of the project rested one us using compression. So - we compressed several dozen bits of audio to 96, 128, 160, 192, 256kbits/sec;

• Spoken word - properly recorded in a high end audio booth for existing TV voice-overs, male & female.
• Music - a selection of acoustic, classical, rock etc
• Sound effects - from the BBC library, spot effects as well as longer ones (bird song etc)

So these were blind-played to the Oasis Television audio staff (at the time a good example of "golden ears" - people who have been trained to hear audio problems) in properly built audio suites (£10k speaker systems in acoustically dead rooms). So - not amateurs making judgements on sub-£1,000 domestic rigs, but a proper blind-test using professionals.

What we discovered was that past 256kBits/sec nobody could get reliably better than 50% correct - it was as good as if they were guessing which was compressed and which was uncompressed. This seems to fly in the face of current opinion that says that even 320kBit/sec is detectable on iPod earbuds (!) - and don't forget that the LAME and Fraunhofer compressors (reckoned to be the best) have been getting better over the last decade-and-a-half (particularly with respect to VBR encoding).

Lots of people also suggest that other codecs (particularly AAC and WMV) are now better than MP3; I've never been able to hear that when I've compared like-for-like (data rate, VBR vs CBR etc) and since MP3 is so ubiquitous it seems likely that manufacturers would have spent more dollars optimising it that any of those lesser used codecs?

So - I compress my music to 192kBits/sec using the LAME VBR setting and I rarely hear an artefact. There are a few albums I did back in 1999 that I've gone back to because modern compressors are so much better and the little MP3 player I had back then could only hold a complete CD at 128kBits! The cruel irony is now that I know what I'm listening for and can (just about!) afford decent speakers my hearing response is rolling off quite markedly. Pretty soon AM radio will sound good. I've also found that when I mix live music I drive the high-end a lot more than I used to and that must be the same effect.

There is another effect that people talk about - how tired you get listening to compressed audio - the brain doesn't like artefacts that you don't encounter in nature. I think this is true, but the people who make play of it tend to be vinyl & FM radio fans - both of which are covered in very unnatural artefacts.

So - to the point of the post; I discovered that a couple of CDs encoded by iTunes over the last year wouldn't play off a USB stick in the car. I had to re-encode them on the old AltoMP3 maker software I used to use. flailing around online seems to suggest it's the way Apple sticks artwork in.