audio – Richard Mudhar

piezo contact microphone preamp for plug-in-power

This is for a piezo contact mic that is powered off a field recorder’s plug-in-power supply. That is for small recorders like Olympus’s LS10, LS-5, LS14 series, which are typically unbalanced audio inputs, stereo on a 3.5mm jack, not for a P48 phantom power from a mixer or pro recorder¹. You also get an opportunity to drink beer.

Batteries are a pain in field recording. One battery to rule ’em all is best in my book, the fewer things with a battery required to make a system work the better. The recorder generally has to have a battery, and Sony agreed with that many years ago, back in the days of Minidisc, and came up with plug in power (PiP). to eliminate the battery in the microphone. An electret mic doesn’t need much power so battery life is weeks not hours, but it has a switch you often forgot to switch off.

Plug-in-power

Plug in power is about 3-5v supplied through a series 6.8kΩ resistor, with the output developed across the 6.8kΩ resistor. It so happens that electret microphones were coming in at that time, and typically had a FET to buffer the high source impedance of the capacitive electret mic, an audio source through a few tens of pF. The 6k8 resistor is the drain resistor of the electret mic. You can read more about plug in power here.

Ideally a contact mic would work with this just like an electret mic, after all the problem and the solution are the same – a high source impedance ² requiring a FET to buffer it.

there’s not much power available in plug-in-power

The trouble is that both the FET piezo amplifier and the opamp version need a 9V battery, which is a pain in the backside. An Olympus LS10 needs two AA cells and will run hours, and then the piezo contact mic wants a box with a 9V battery all to itself. That’s not good, and you tie yourself up in wiring.

The opamp version will never run on plug in power. The basic FET buffer is optimised for 9V, so it won’t run off PiP either.

The design

Initially I tried this with R2 omitted (replaced with a short) but there is not enough current to get the drain off the 0V rail. That’s fair enough – the IDSS is specified as 2-20mA and the most current you are going to get from a 3V supply through a 6k8 resistor is a little bit shy of 0.5mA. I bought a job lot of 2n3819s and experimenting with the value of R2 showed 470Ω put the drain voltage about 1.7V, with the source voltage about 0.5V, dropping about 1.2V across the transistor and 1.3 across the 6k8 drain resistor, which is good enough. Surprisingly, though they were from Ebay, nominally Fairchild parts, all five I tried biased about right with 470Ω up to 680Ω. Ordinarily you’d bypass R2 with a capacitor to maximise gain, but you are rarely short of signal level with a piezo mic, so I omitted that, it will slightly reduce gain, and also limit the current slightly if the piezo puts too much through the gate/source junction if I drop it. I have decided to take a chance with this omitting the diodes of the original FET version, because they’re hard to fit in. I expect peaks to be lower because I have mechanically damped the mic a lot. Time will tell if I get to regret that and the mic goes noisy or fails in service if I drop it once too often.

Your optimal value of R2 may be different due to the spread in FET Vgs- put a multimeter on the drain and aim for about half the open-circuit voltage³ ±10%. Mine is chosen purely for the supply through the left hand channel. If you parallel the inputs you may need a different value. I have hopes of using two to record stereo, else for mono paralleling the inputs makes for easier monitoring in the field.

Plug-in-power contact mic construction

First open a bottle of beer, trying to preserve the shape of the crown seal rather than bend it in the middle cavalierly. Obviously you can’t leave an open bottle of beer in the lab, so drink it. A 2cm piezo disc fits in a typical bottle cap, as shown

Now pick out any plastic seal in the bottle cap, and then clear the residue. I used a Dremel and the face of a small stone attachment to get this

You are going to solder to that surface, it will be the ground, and the cap provides shielding on that side, the brass disc shields the other side. This is a good time to drill a 3mm hole in the side of a cap, I used a small modelling pillar drill. If you don’t have a pillar drill you are probably going to give blood and turn the air blue with cursing. So better file a U shaped slot with a round needle file instead 😉 A hole locates the cable better in the later glue stage, but it’s not essential.

Now assemble the amplifier Manhattan style over the ground plane of the bottle cap, which also takes the shield of the cable. Use relatively thin cable for the output signal, because else handling noise will drive you bonkers. Some cable protection against the sharp metal would be wise, heatshrink in my case. Solder the piezo to the relevant place. You don’t want too much of the red and black cable going to the piezo element because you will have to lose this in a load of glue. I had too much.

Now would seem a good time to test this with your recorder, handling the piezo should be easily audible.

I wanted to keep a continuous solid interface to both sides of the piezo. Air is quite high impedance mechanically, and a contact mic detects signal in low-impedance solids. The sharp resonance of piezo discs is because they are a solid in air on both sides, and I wanted to damp this and match the disc to the solid medium. I used glue for that.This takes out a lot of the dreadful 2kHz honk in the sound, which is the natural resonance of the disc in air. In an ideal world you would put the contact mic in a solid medium with the same impedance as the ceramic either side to pick up the longitudinal vibrations, which would get rid of that resonance, but getting rid of the piezo-air interface on both sides helps a lot, even if epoxy and hot-melt glue aren’t a great match for the ceramic – they’re much better than air! The cap and glue mass-load the rear of the disc somewhat, which usually improves the sound.

In the final device, you want the disc to sit slightly proud of the bottle cap so the brass disc is in contact with the sound source, but the bottle cap isn’t. For that you can use hot melt glue all the way, though probably in two stages.

I wanted to glue a neodymium magnet to the other side of the piezo disc. Many things that are interesting with a contact mic are steel surfaces, and a magnet saves you trying to clamp or stick things to such a surface.

I discovered the hard way that hot melt glue is above the Curie point for neodymium magnets, and the loss of magnetism is permanent, turning it into an expensive metal disc. So I used epoxy resin to stick the magnet to the back of the piezo disc. I also got to resolder the red lead to the friable silvering on the back – best done quickly.

It pays to manipulate the magnet with non-ferrous tools, the best way to hold it in place is put it on the disc over a steel surface covered by masking tape. The next stage is glue this to the cold hot-melt glue encapsulated amplifier in the cap – the depth of the glue plus the magnet should set the brass disc proud of the cap. I also had to lose all the red and black wire in the gap and fill this with epoxy. I did this glue operation is two stages, because otherwise it was going to be a very messy job.

first the magnet and disk are glued to the hot melt glue surface

This stage is used to prove the brass disc is clear of the wires and the bottle cap. Only just in the case of the wires.

The second stage infills the gap between the disc and the cap, by mixing the glue on a piece of paper and using a fold in the paper to pour the epoxy into the gap.

You will get to scrape some epoxy off the disc. Aim to make nothing stand proud of the brass disc, which will then be held on steel surfaces with minimal gap.

Field testing – what does it sound like?

This is the mic attached to galvanized farm gate, the excitation is entirely the wind. I could not hear this sound standing next to the gate, it was a strong breeze, not a hurricane. I found it was very easy to overload, because you can’t really hear those low frequencies and metering isn’t fast enough for the transients. I record the main output on the LH channel, and put a series 15k resistor to the RH channel, which gives me about 10dB attenuation, but also more noise. I had to take this recording from the RH channel because I made a mess of the recording level.

Experimenting with stereo

I made another one of these, to see what stereo sounds like in a contact mic. Using a Y cable that connected the tips of two TRS jacks to tip and ring on another, I can combine these. This was on a farm gate, tapping the gate to test the sound

this was recorded on the opposite sides of the middle vertical strut of the fence below

this was recorded on adjacent struts

and this final one was recorded as shown below

You can hear a bit of the wind impacting the cables, perhaps I will have to clamp them to the gate to get rid of that effect. The speed of sound in steel at nearly 6km/s is much higher than the speed of sound in air at 330m/s o although the spacing of the third sample (pictured) would be outrageously wide for a spaced pair of microphones in air, the stereo effect is clearer for me on that than on the first two.

typically presented on three-pin XLR, or five-pin for stereo. ↩
The piezo disc doesn’t need anywhere near as high a source impedance as the electret. 1MΩ is fine for the piezo, the electret wants nearer 1GΩ ↩
That’s o/c without the FET present, typically 3-5V on a PiP input ↩

Piezo contact mics on low voltage power supplies

A few years ago I did a couple of piezo contact mic amplifier designs, because people often moan that these things sound tinny and crap. There’s a wrong way and a right way to use these – they want to work into a high impedance. Using Piezo Contact Mics Right sets you right. Trouble is these use a 9V battery, and it seems world + dog want to use 5V, because that’s what they had. Time was when power supplies were +/- 15V for analogue and 5V for digital, but that’s a different story for later.

So what can you do with your piezo contact mic at 5V then?

Not much. If you are looking for low signal level performance an emitter follower biased at an output of 2V would work well, but if you only have 5V available it’s likely you are trying to digitise this signal and bung it in an Arduino or something. In that case, think laterally. Toss the power supply. I developed those amps because as a field recordist I wanted to hear faint signals from the contact mic. You know, like the whispering in the rails as a distant train approaches, though you need to avoid the Fredzania Thompson ending.

These days people would look at you funny if you attach a box with wires to the underneath of the rails. Don’t try this at home and all that.

Turns out many people want to use their contact mics on an instrument, or drum pad, or generally something they bash seven bells out of. Life is a lot easier for you. As established in Using Contact mics right, you want an input resistance of about 330kΩ so the bass doesn’t roll off with the typical series tens of nF capacitance of the sensor. 330kΩ is a damn sight more than your typical plug-in-power audio recording doohickey, which usually feeds the electret mic power from 3V via about 6.8kΩ. I measured my Olympus LS-14 and even the line input is 10k.

So stick the 330kΩ resistor in series with the input. Even writing that makes me cringe, because it will lose a hell of a lot of signal level, making a potential divider with the input resistance – for a 6k8 input you’ll take a loss of 33dB. That translates into a direct worsening of your noise figure by that much, that’s a lot of performance to throw away¹. OTOH it works perfectly well down to 1.8V, it’ll be OK down to 0V as it doesn’t use power 😉

how much signal do you get from a piezo contact mic?

Let’s take a look at the sort of signal level you get from a piezo disc sensor. I got one on the bench and fed it into a DrDAQ signal capture device and Picoscope Continue reading “Piezo contact mics on low voltage power supplies”

International Dawn Chorus Day – getting bird sounds indoors without opening the window

International Dawn Chorus Day 2020 is somewhat overshadowed by the coronavirus pandemic for its human listeners. The birds probably appreciate getting some peace! The Wildlife Sound Recording Society was after getting a live broadcast of this from as many members as possible. They proposed two methods of live broadcast, their preferred option using Mixlr and a more gonzo alternative using locusonus.

Mixlr seems all about tablets and mobile phones. If a project’s got a mobile phone in it I’m not interested. I loathe smartphones – jack of all trades and master of none. They don’t do stereo¹, FFS… Mixlr is Cloud. I don’t do Cloud, particularly if it comes with a subscription. It’s bad enough when Cloud goes AWOL and you’ve put effort into the platform for free. Pay for the privilege? Nope.

So I passed on that and went to locusonus, who are doing this under the Reveil soundcamp moniker. Locusonus is funded by the French State, bless their arty dirigisme – just look at their publications. And sponsors

Reminds me of reading about musique concrete as a kid in the 1970s, IRCAM and all that, while I was piddling about with a hand-me-down Stellavox tape recorder. Mad, but inspirational. I’ll hitch a ride on French exceptionalism.

I’m lucky in that way back when I bought a Cirrus Logic sound card for a Raspberry Pi. Or perhaps unlucky in another way – I never found a good use for it till now, as the software drivers were a whole load of hurt. By the time they got incorporated into Raspbian, the card was end-of-lifed so you can’t buy them any more. That’s Linux for you. Free as in beer but slow to integrate hardware. If you are doing this from scratch, either use a cheap audio adapter with mono audio or something like the Behringer UCA202 USB audio card – stereo line in and works great with the Pi, right out of the box.

Despite fiddling with the CirrusLogic on and off I came to the conclusion a timed bird sound recorder is better done with a Dribox and a real audio recorder and a timer. However, a Pi and the CL card is perfect for locusonus. Perfect enough, indeed, that downloading the relevant Pi SD card image, blowing it onto a SD card and firing it up on ethernet gave me an instant win², using a set of OKMII binaural mics into the line in port with the bias enabled. I was able to hear myself, albeit at a low level, but the locusonus software lets you ram the Cirrus programmable gain amp up to +30dB and max digital gain. Sure, it’s noisy, but showed the principle.

Now all I need is an outdoor microphone Continue reading “International Dawn Chorus Day – getting bird sounds indoors without opening the window”

Olympus LS-14 clock fix

When I bought an Olympus remote control to reverse-engineer the Olympus remote control code, I got a Olympus LS-14 for £30. I’ve had a LS-10 for donkey’s years, these are reliable. However, the RS-30 is £60 and there was a LS-14 with RS-30 on ebay for £90. The LS-14 is a nice machine, key things that it does better than the LS-10 are a pre-roll buffer and the sound is a little bit more natural from the mics, where the LS-10 could venture on the tinny side.

It can also sample the first 12 seconds of sound to set recording level, should you lack the smarts to be able to learn how to set record level yourself 😉 No automatic recording level setting has ever been made that works properly, although the smart recording feature will work fine for a band running a sound check in the first 12 seconds, it leaves record level alone after the initial sound check. The feature is about as useful as a chocolate teapot on unscripted field recordings and wildlife

The clock on my LS-14 wouldn’t keep time. Indeed, after a couple of days I’d turn it on and it would invite me to set date and time. That’s easy enough to do, but irksome to do regularly, and usually means the lithium clock backup battery is shot.

Manufacturers do clock backup in various ways – some use a lithium primary cell, on the principle that planned obsolescence means you should have changed the device out in three years. You throw it away and buy a new one. Others use a supercapacitor, charged from when the device is switched on. That works well, these have a service life of about 10 years, you can buy replacements on ebay. Some use a rechargeable lithium cell, again these have a ten-year plus lifetime.

Nobody else seems to moan about the clock needing resetting on the LS14, so Google was not my friend in this case. I suspect mine had been in storage for a while. There was something about the seller getting this for his daughter to use at uni and it didn’t work out. Perhaps he left it in the garage or a shed in the damp. There’s not much else made of mild steel in the LS-14 to compare.

Dismantling the LS-14.

Take the three screws out the back. You do not need to remove the screws on the microphone housing. To get at the battery sadly you will need to take out the little board at the very back. It has a plug on connection to the main board, use your intelligence and ease this off. I had to unsolder one of the main battery connections. You will be presented with the main board, and the offending article can be seen here.

You can see that the battery is rusty as hell. Couldn’t decide whether the chemical splurge was the flux from the solder or some battery catastrophe. It was stiff like flux, whereas battery ooze usually wipes off.

The battery looks like bad news, particularly as I have not managed to determine the type or a replacement number. Probing with a multimeter showed it is charged to about 3V when the unit is powered.

Since I didn’t know what sort of part this was – rechargeable battery of supercap, I had a go at it with a fibreglass pencil to clean off the rust. I’ve had this experience with watch batteries – if you have one leak in a watch you have got to get all of the gunk out of the battery compartment, else you will find replacement watch batteries only last a few months before dying. That means either a special type of watchmaker’s putty, which is the right way to do the job, or q-tips and isopropyl alcohol. This is a less critical case than a watch. I don’t mind resetting the clock after a few months of non-use, it is resetting it after less than a week that hacks me off. I figured all that rust was possibly giving a high-resistance leak across the battery shortening battery life.

Luckily for me, cleaning all the rust off this was a win, the clock keeps good time and lasts over a couple of weeks still ready to be used again. So if you have trouble with the clock needing resetting in your Olympus LS-14, give cleaning the battery a go. I needed a replacement for my trusty LS10 which has been claimed by another family member, so fixing this means I have a useful upgrade now.

Raspberry Pi Zero audio recording with the AudioInjector hat

Just when I thought the remote Olympus recorder is the way to go here, along comes a promising new Pi solution for remote recording – this looks to be low cost and small. What more could a fellow want?

Decent instructions and specs, for a start 😉 Australia seems to have a vibrant electronics tinkering community, and Matt Flax of audio-injector has come up with a dinky little recording sound card suitable for the Pi Zero, without the sort of stupendous kernel-compiling hurt associated with the now discontinued, Wolfson/Cirrus sound card. Matt even used Cirrus tech under the hood, kudos to him for making it work in the Pi environment- I guess the Pi has advanced in standardising add-on gizmos too.

You can buy the AudioInjector Zero from Australia, only to discover postage is about as much as the sound card, so Google helped me discover that you can get it in the UK from Amazon, who drop-ship it at a much more acceptable price of £12.50 delivered free if the total order is > £20. So I go get one.

The Pi Zero sound card – tiny. Look ma, zero connectors!

Nice. No GPIO connectors, though you get a nice bunch of extra audio connectors to make this connect to phono jacks. How does that work, then? Continue reading “Raspberry Pi Zero audio recording with the AudioInjector hat”

Cirrus Logic audio card for the Raspberry Pi revisited

There is more green space and trees around me where I am now, with many more garden birds, though no sparrows¹, and occasionally some tawny owls in the night.

Tawny owls, recorded ME66 handheld

So thoughts turn to a garden recording gizmo again. I have enough power, and a network connection to a shed, a oddly wind-sheltered location and many trees nearby. For short term recordings in the field I am still in favour of the timed field recorder approach, but for the garden where I have power and data the Pi still scores. You don’t have to fiddle with it, it’s entirely remote controlled. Many years ago I had a PC in the garage which was the music server, I used a piece of software called loop recorder on that. This cat fight is one of my favourite urban recordings from that time.

although I was really trying to record a hedge full of sparrows. A loop recorder lets you go back and catch things like that, and the microphone would be much closer to the area where the owls are.

So I thought I’d revisit this Cirrus Logic audio card, particularly as a case for a Pi with this mounted was being sold off cheap for £5

The Cirrus is the only audio card for the Raspberry Pi that lets you record sound with it, as opposed to the legion of DAC cards for the Pi. You can, of course, use a USB sound card instead, though that precludes using a Model A if you want to use wifi and have the lowest power.

The good news is that a hero hacker, Matthias Reichl, has sorted out the drivers, it’s now a RPI-update rather than patching kernels and esoteric crap.

The bad news is that the manufacturer discontinued the card 🙁 Having said that, it still seems to be available for about £60 if you work hard enough, GIYF. That’s dear – a Behringer UCA202 is a good Pi compatible USB sound card for about £24, line level input. The Cirrus Logic card offers a bit more sensitivity and on board mic bias. Continue reading “Cirrus Logic audio card for the Raspberry Pi revisited”

Olympus LS-10 remote control success

I’d experimented with the wired remote for the Olympus LS- series recorders before. I have an Olympus LS-10 and an LS-14, and previous experiments showed I could make this work in principle. There’s a big gap between making it work on the bench and getting it to work in the field, however. This is the next step of boxing it up and making it stand alone.

16F628 PIC is fitted into the space of two batteries in a 4-way battery box, giving me a small box with battery holder and on/off switch. A 32kHz watch crystal gives an easy integer divide down to seconds and then hours, and reduces the power drawn by the PIC and lets me drop down to 2V Vcc and stay in spec over the industrial temperature range.

Either my LS10 is knackered or it never was compatible with Olympus’s wireless remote, it doesn’t provide 3.3V power on the plug tip, so I have to power the PIC 16F628 from two NiMH cells, which means I am short of headroom for 3.3V because there’s a 0.9V difference. I’d expect the PIC to drag the remote control line, which rests at 3.3V down to ~ 3V (2.4V VCC + 0.6V input protection diode drop)

I used a diode for the stop command pulling to ground, which still works with that diode drop, so the drive circuit is

RA4 is an open-drain connection, I figured I would chance the forward-biasing of the input protection diodes via the 100k. It works fine, at least at room temp – a 100ms pull to gnd via RA4 starts the recording, and then a 100ms pull to ground of RA2 stops the recording. Pins are switched to hi-Z inputs when not active. I guess the 3V3 from the LS10 has to go through two diode drops now to get to the 2.4V rail (diode shown and the input protection diode), and this is enough to let it float OK.

I got it to start the recorder at 4am, which is too early, but recording for two hours got me this recording at about 5:30 am of the local birds. I hear Great tit, Robin, Blackbird, some sort of gull, Wren, Woodpigeon, Crow, in that lot.

Using a 3.5mm socket as a workaround for the fiddly 4-pole 2.5mm jack plug – it’s a lot easier to wire a socket than a 4-pole plug, and I got a 4-pole 3.5mm jack to 4-pole 2.5mm jack cable from Ebay. Wiring the 4-way socket is dead easy now, and saves having a flying lead from the box.

In search of microphone weatherproofing ideas

I need to now find a way to get a reasonably weatherproof microphone. Looking at how B&K do this in the manual for the UA1404 the way to go is to use a small raincover just over the mic capsule

Their mention of birds makes me thing this is very close to a mesh nut feeder – I could put horticultural fleece around the mesh and use the top cap as a rain guard. Another option is to go minimalist, recess an omni electret capsule in something like a plastic bottle cap. I’d have thought that the cavity of the raincover would cause dreadful resonances, but if it is say 2cm diameter that would be a wavelength of 330/.02 ~ 16kHz – perhaps theirs is 0.5cm keeping this down to ¼ wavelength. Where this would score is it’s small, and electret mic capsules are cheap so I could afford to lose some. I can take the line that I’ll omit the big foam guard and use a piece of horticultural fleece across the cap, this makes a reasonable wind baffle, and I’m not going to get a good recording if the wind is over 5 mph anyway because of the hiss of the wind in the trees even if I were to keep wind blast out of the mics.

I am thinking of using a small Dribox to rig the recorder and timer, and sample some birdsong from other places. A pair of AAs run the timer for at least three days and the power drain of the LS10 on standby is also low, probably good for a couple of days, but I don’t have more than four hours of recording time on the LS10, it is 2Gb. So I can live with that – the Dribox has enough room for a bigger battery if that starts to look necessary.

Skytec PRO 600 PA Amplifier repaired but bad design can’t be fixed

tl;dr – to fix the problem throw the Skytec Pro 600 away and buy something better before the Skytec blows your speakers again. Don’t buy Skytec, and if you have it throw it away before it fails on you.

Skytec is cheap rubbish made in China for kids who are wannabe DJs but have little money. This is not quality – I had to repair this amplifier because of a fundamental defect in the engineering design. These are fine for background music, say in a pub. They’ll go reasonably loud in a modest party setting ,say 30 people, but it’s rough, and it’s nasty. You’ll save on the amp and pay in bass drive units if you DJ with this at any scale 😉 And get a limiter if you can, but if you can afford that you won’t be down at the Skytec end of the market.

I made the dumb mistake of buying one of these used from Cash Converters for £30 a while back. I bought it purely on price, I wanted something basic for parties of about 30-50 people. I knew nothing about PA, but I figured a hifi amp wouldn’t cut it for that sort of usage. What I hadn’t anticipated was people shift junk onto the PA market with design defects that were solved in the 1970s. They don’t even need any new parts, just put the Vbe multiplier on the heatsink rather than on the circuit board.

Skytec 600s are sold as 600W and the manual claims 600W output. They are absolutely away with the fairies on that, to the extent they should be done under the Trade Descriptions Act. I guess they hide behind the fact they don’t say RMS power, so they probably mean peak power, though that’s still only 280W. I measured 80V p-p, which is about 28Vrms. Run that into 4Ω and you’ll get V²/R≈200W. Do that for any length of time and it will blow because of the inadequate heatsinking and bad thermal design.

about 80Vp-p (I am using 10x probes) into 6ohms, 130W per channel. Don’t do it for too long, though

HiFi tower talk glowingly about the MOSFETs

Skytec’s PA-600 gives you the extra power you need with exceptional bass. All sound components are co-ordinated carefully and captivate their longevity. The modern MOSFET transistors and extra large power transformers give great sound and dynamics. The high build quality makes it the ideal amplifier for tours and gigging. For use on stages, for DJs, monitoring, parties and conferences.

but there ain’t no MOSFETs in this, simply a pair of paralleled bipolar junction transistors in the complementary pair output stage, 2SA1941 and 2SC5198. Toshiba described the transistors as suitable for 75W amps, you have two in parallel so 150W tops, okay times two for stereo = 300W. The toroidal transformer isn’t over 600W, I’d guess 200W from the size.

It worked OK for me for a couple of years, but then I let someone use it unsupervised for live music. Which brings me to the first warning

Do NOT use the Skytec 600 for live music unless you are aware of the risks you are taking!

I wasn’t, there, and the result was a blown output stage and blown woofer. It only cost me £11 to service the amp and £50 to change out the woofer, so I am now down £91, and I still have a junk amplifier, though it works now. Now that I know the ghastly horror of the circuit design I am not sure I have the balls to use it again, but at least it works as it was meant to originally 😉

Why not for live music then?

After all, the promotional blurb says this:

The high build quality makes it the ideal amplifier for tours and gigging. For use on stages, for DJs, monitoring, parties and conferences.

so what’s the problems then? Dynamic range – live music has a higher peak to mean ratio than recorded music. You end up pushing the bugger harder, so unless you limit the live source in the mix you’ll clip the output. At least that’s what I assume happened, I wasn’t there when It failed 😉 The Skytec is fine for prerecorded music, but the basic problem is that this amp has zero protection for the speakers or the output stage. Worse still, the VBE multiplier that biases the output stage isn’t thermally coupled to the heatsink on the output stage. Let’s hear it from Rod Elliott why this sucks

Thermal Stability

It can be seen that in the Darlington configuration, there are two emitter-base junctions for each output device. Since each has its own thermal characteristic (a fall of about 2mV per degree C), the combination can be difficult to make thermally stable. In addition, the gain of transistors often increases as they get hotter, thus compounding the problem. The bias ‘servo’, typically a transistor Vbe multiplier, must be mounted on the heatsink to ensure good thermal equilibrium with the output devices, and in some cases can still barely manage to maintain thermal stability.

If stability is not maintained, the amplifier may be subject to thermal runaway, where after a certain output device temperature is reached, the continued fall of Vbe causes even more quiescent current to flow, causing the temperature to rise further, and so on. A point is reached where the power dissipated is so high that the output transistors fail – often with catastrophic results to the remainder of the circuit and/or the attached loudspeakers.

I got to find that out the hard way. I’ve actually managed to do a fair number of parties with this fine, but I was always careful to keep the bouncing LEDs of the output display under control by controlling the master gain.

How does the Skytec PRO600 do thermal stability?

On a wing and a prayer.

PC case fan blowing on the internal heatsink

They run a PC case fan 100% of the time onto the main heatsink, sucking air out of the case, inflow is through the front. There’s no margin for error – although I didn’t trace the circuit it’s a complementary pair of paralleled output transistors driven by a driver (effectively making a Darlington output) so you got four VBE drops reducing with temperature at 2mV/deg C, asking for thermal runaway. There’s no fight against that with the VBE multiplier because it’s not thermally coupled. Get the die temp of those output devices hot enough, say 40C above ambient and you have 40*2*2 = 160mV less bias than you started with (the drivers are conveniently mounted on the heatsink to make sure their VBE drops too). This is designed for thermal runaway and the only thing standing between you and a blown output stage is the hope the heatsink and the fan keep the temperature rise down. You can get a little bit of an idea of the architecture from this thread and this PDF of a similar noname PA amp which gives a rough idea of the architecture on the output

Schematic someone has traced out of a similar piece of junk. In my case the four o/p transistors are 2x 2SA1941 (BG7,8) and 2x 2SC5198 (BG 9,10). BG4 is the offending Vbe multiplier that isn’t on the heatsink.

How to fix a Skytec 600 blown output stage

Change the 2SA1941 and 2SC5198 transistors 😉 I buzzed these through with a DVM on diode setting and found them all short, traced back to the drivers expecting them to have gone but they were OK, traced back a further stage of BJTs but they were OK too. The 5A fuse saved the other passive components.

It’s quite repair-friendly – unscrew the three screws on the base holding the heatsink, unplug all the connectors after taking a photo to remember where they go back. Lift the PA module out, snip the duff transistor legs to save the PCB while desoldering the pins one at a time.

snipping the duff transistor legs lets you unsolder the legs one at a time, saving the PCB from overheating.

I powered up the repaired stage on a 30-0-30V bench power supply set to limit at 100mA, I know it’s meant to work off 60-0-60V but I got a signal through and confirmed it wasn’t still duff, before getting it onto the main supply. I also compared the quiescent current (10mA at 30-0-30V) with the good side, which was the same, so I figured the VBE multiplier was still set about right. Easy win for about £11 in parts. In fact one of the old output transistors was still okay, presumably saved by it’s parallel buddy shorting across it, but I’m not chancing it.

I also went round and tightened the output transistors a tad. It’s easy to overdo this, but the still- working side was about finger-tight like the failed side. I wonder if this also led to the early demise. You just can’t risk the transistor die heating up to any great amount with this design.

Having fixed it I started to test it looking for why it blew. I got a couple of 50W 6Ω wirewound resistors. These are sold on Ebay to people doing LED upgrades to their lights, to put in parallel with the LEDs and draw 24W so the automotive CAN bus filament blown detector doesn’t keep going off. I figured 6Ω is a nice compromise between typical 8Ω and 4Ω speaker loads; real speakers present complex loads anyway. It was the cheapest way of getting a power resistor up to the job. I then dunk the resistors in a pan of water.

since I don’t have a heatsink/fan combo up to dissipating 300W. I know electricity and water don’t really mix, but I figure the water isn’t going to shunt my 6 ohms too much. Worth heatshrinking the ends of the resistors though 😉 The reason I used a pan is because the failure mode of these type of power resistors is to violently eject the ceramic slug out the end. So a Pyrex dish or a jam jar isn’t really desirable.

Running both channels full tilt at 130Wpc for two minutes the transistors get up to about 50C at the hottest part of the plastic case. If fairness to the amp I’ve been able to fill a rented Scout hall with music without ever taking it up that high even on peaks, so I ran it for five minutes at 33 watts per channel (~40V p-p). And got the transistor cases up to 110C. The manufacturer’s spec for the junction temperature is 150C peak. If you thrash this like that for a long time I guess the heatsink/case fan combo is hopelessly inadequate, and it blows.

Sadly I battle tested the inadequacy of the design a second time. Five minutes after running the second test, after I had brought the signal down to 0, I was greeted with this, telling me the right hand channel has gone DC, presumably thermal runaway again.

While I know how to repair this, I don’t know how to fix it to make it fit for purpose because of the fact the Vbe multiplier isn’t on the heatsink. It’s probably true that my needs don’t push it that hard, but an amplifier that blows after running a steady 33W for five minutes isn’t something I’m going to risk ever using, so it’s time to scrap it.

Skytec Pro600 – Avoid. Just say no.

Olympus LS10 and LS14 DIY wired remote control

tl;dr – the schematic

Olympus LS10 (and LS14) wired remote schematic

A new approach to a timed recorder

For the last year or so I’ve been trying to make an timed start recorder using a Raspberry Pi and the Wolfson/Cirrus audio card. I was able to make it work, but never eliminate some rattiness in terms of overruns on record – I confess I couldn’t hear them, but it didn’t give me a good feeling. Then I added up the costs –

£25 – Cirrus Audio card
£27 – Raspberry Pi B+£10 – case and odds and sods to make it work
£20 – PCB, time and bits to make a preamp to get from mic to line level

so I’m looking at £80 to get off the ground, and that gives me a seriously power-hungry SD audio recorder, although I can use a timer to save the power drain for active service.

Alternatively, if I could crack the remote control for them I could go on ebay and get a secondhand Olympus LS10, or one of the similar models (LS-5, LS-11, LS-12, LS-14) and use my own LS10 to start with. I can feed a mic straight into the LS10, no extra preamp required and the audio spec is good.

Reverse engineering the Olympus remote control protocol

This cost me £90 on ebay, and it turned out I didn’t need it. You get the info for free, but then I got a natty nearly new LS-14 with an RS30 remote control, so I’m not too unhappy. Unfortunately the RS30 doesn’t work with my Olympus LS10, don’t know why. I’d have been hacked off if I’d just got the RS30¹. Works a treat with the LS14 it came with, on their own a RS30 seems to go for £50, so I got an okay deal.

Google first – I owe dashanna of the naturerecordists’ list for inspiration, I vaguely recall seeing that post go through on the list. Their solution is this

1610_dashanna

The connector is an evil little 2.5mm four-pole jack, and these are a bear to solder

nasty connectors to solder, though easier when you realise you only need t wire to two parts. You can pick up 3.3V on the tip, which may be of use... — nasty connectors to solder, though easier when you realise you only need to wire two parts. You can pick up 3.3V on the tip, which may be of use…

I can’t help wondering if life would be easier using a three-pole jack, since only sleeve and ring are needed. Now I didn’t like that battery in dashanna’s version – I mean who the heck would make a wired remote for a machine offering you a 3.3V supply on the tip of the plug and demand you go fit a battery in your remote? It’s just not a clean engineering solution at all. But apparently it works.

So I rigged the cable in series with the RS30 and sniffed the signals. Of the TRRS the tip had 3.3V, the second ring seemed open circuit, the first ring had the wanted signal and the sleeve was ground. Presumably the IR receiver and LED driver are powered off the 3.3V on tip. The signal on the first ring rests high at 3.3V.

Record is this funny little signal, 100ms at about 1.5V followed by a low

Stop is this signal, pull to ground for 100ms

In practice you can ignore the second pulse. For all I know it could be an ack back to the receiver to light the LED. I tried using a couple of diodes to pull the signal down to 1.2V but that didn’t initialise record. I then figured this is one of those analogue resistor chain remotes, so I look for what resistor would give me ~1.5V. Turns out if you replace the 1.5V battery in dashanna’s schematic with 100k you get about 1.5V and the recorder starts recording. You don’t need the second pulse at all, and the debouncing seems to be done in the recorder, it takes a little while, up to about half a second to start recording. I guess that means inside the recorder there’s a 100k resistor to the 3.3V rail in series with the first ring.

That works with both the LS 10 and the new LS14, although the RS30 only works with the LS14. So now all I need do is mod the timer to pull down a couple of pins, one through 100k. If I make the stop command the open-drain pin to ring and the rec command a normal pin resting High via 100k to ring, and pull the relevant pin down for 100ms I should be good to go.

I’ve just got onto the Olympus RS30 website and if you scroll through the models that is compatible with it includes the LS-3, LS-5, LS-11, LS-12, LS14, LS-20M, LS100 so perhaps my LS10 was never compatible with it and Olympus have changed their mind since writing the LS10 manual which says on p65 “Exclusive remote control RS30W (scheduled for Spring 2008)” ↩

Timed Audio Field Recorder with a Raspberry Pi Cirrus Logic Audio Card

The problem is still the same as it was this time last year – the birds get up before I do in the Spring and I can only be one place at a time. Automatic recording devices let me scout locations in parallel.

A timed field recorder needs to be cheap, because somebody might nick it, it needs to be weather-resistant because it’ll be stuck outside, and it needs to be low-power, because 13A mains sockets are rare outside. Oh and it needs to be standalone, and not part of some cloud, because mobile Internet is ratty and expensive.

tl;dr the hardware performance is good but software support is dire

You can make this work but it isn’t fun at all. If you can use something like a USB stereo audio in board then do that rather than use this Cirrus Logic Audio Card, particularly if you have mains power available. I like the Behringer UCA202 and it works with the Pi.

A Raspberry Pi and A Wolfson audio card sort of fitted the bill, but the Wolfson Audio card is no more. I say sort of, because I’m still looking at about £70 for a Pi¹, the audio card and enough odds and sods to power it. You can buy a Zoom H1 for about £80, although there’s still a bit more cost in powering it for long times, keeping the water out and making up some gizmo to pretend to be you pressing the big rec record button early in the morning.

But with the Pi I get to drive the recorder via cron and ssh, and transfer the files via the internet or mobile data in some places. Even if I don’t get a case, though they are to be had for the Pi/CL Audio card combination…

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