Text: Rob Squire

Thanks to the members of AARG (Australasian Audio Repairers Guild) for their insights.

Rob Squire runs Pro Harmonic

Text: Rob Squire

THE RESPONSE CURVE

The flipside of this story are two recent cases where enquiries about the price and availability of replacement parts disappeared down an apathetic plughole. One enquiry kicked off with a couple of polite emails to the local distributor to prompt a reply about a single part. These were eventually responded to with a request for the serial number of the unit. A couple of emails and a phone call later… nothing. Another month and a half of silence went by before I was finally contacted by the frustrated owner to see if I could make any further progress with it. Rather than beat my head against the same wall, I picked up the phone and called the support line for the USA-based manufacturer, who was happy to sell me the part I needed over the phone just by quoting my credit card. This experience was repeated again with a different product and distributor, where, despite emails and phone calls, the distributor never even responded to say that they either had or didn’t have the unique part I required to repair the unit, or that they would even sell it to me if they did. In this case I tracked down the previous distributor of the product line who happened to have the spare part I needed languishing on the shelf, and were pleased to offload it. All this detective work and arm-twisting is tiresome and ultimately adds to repair costs. Indeed, in both these cases I spent more time obtaining a part than it took to diagnose the original fault and install the new replacement… when I could finally get one!

SUPPORT – THE DEAL MAKER

There has been a lot of grizzling of late about the increase in folks purchasing products overseas, exasperated by the high Australian Dollar and the weakness of the US economy; all of it ultimately leading to ‘the downfall of local distributors and retailers’ they say. As a technician, let alone a customer, there’s no doubt about it; I need the local guys, but by the same token it seems to make sense to me that one thing they clearly have to offer that lifts them above the world wide pond of online stores, is local technical support. I also reckon that many distributors, with the help of the manufacturers they represent, could really lift their game in this department. Hell, if they really pushed the envelope it could be a deal-making promotional tool.

Then, of course, there’s the gear that’s old; so old that the original manufacturer doesn’t even exist anymore, or if it does, there’s no-one at the company who’s ever seen the product. There may not even be a faded copy of the original service manual on the company’s shelves, let alone spare parts. This scenario might seem strange, but it happens all the time. A case in point recently involved SSL in England where, after banging my head against the armrest in frustration at the litany of faults in a particular SSL 5000 console, I picked up the phone and called the home of SSL and asked if I could please talk to their most experienced console engineer. I felt a shudder down the phone line when I mentioned the words “5000 series” to the man at the other end of the line, and my own shudder upon hearing his carefully worded response: “we don’t have any documentation and we don’t have any spare parts.” When I persisted in asking a few technical questions about its operation I was met with a second, more unnerving insight: “it sounds to me like you know more about that model than anyone now working here.” This wasn’t the support I was hoping for, but still, the model is now 25 years old and taking this situation in my stride was what I’m paid to do.

CAPPING THE COSTS

I’ve had a number of old large consoles passed my way lately and I’m beginning to notice an interesting relationship between their secondhand purchase price and the expectations of the new owners about the costs involved in getting them installed and working properly – namely, that there isn’t one. It has never been cheap to install and maintain a large format mixing console and it certainly doesn’t get cheaper as they get older and more worn out. The harsh reality of this situation is borne out by the numbers of consoles being ratted out for parts, with input channel strips landing here on an almost daily basis to be racked up into standalone units. The cost of refurbishing a pair of channel strips, modifying them to work outside the console, putting them into a case with a power supply and connectors isn’t cheap but its digestible, and bang-for-buck can be a good proposition. Conversely, the cost of getting a neglected 30-year console up and running, installed and wired into the system can easily cost more than its secondhand price. Yet rarely do I see anyone budget for these costs or even research the condition of the console and its ability to be repaired, let alone the cost to do it. I talked specifically about capacitors in the last issue of AT and these are likely candidates for failure in a console, and just the replacement costs in this job alone are often comparable to the value of the console. And let’s not mention switches and potentiometers that inevitably become scratchy and intermittent – not the sort of thing you need sitting across your mix. These are often unique to the manufacturer and sometimes difficult (or impossible) to source.

Despite not running a studio and it being quite a few years since anyone paid me to pull a mix, I still get excited by audio equipment – old and new – so I understand how the heat of the moment can cloud sensible enquiry when the acquisition of a new widget arises. However, standing back and putting the smart hat on, it’s always worth running a few questions up the flagpole. For a new product, what do I stand to gain or lose by not buying it locally, and is part of this gain likely to be worthwhile after-sales support? Put the sales guy on the spot, or better still, pick up the phone and call the local distributor and ask about availability of spare parts, and where their authorised repairers are located. Get at least some sense of how well this new product will be supported. If you decide to send your dollars overseas be really clear from the outset that, by the book, for warranty support you’ll need to return the item overseas at your cost, and down the track when the warranty expires, there’s no guarantee the local distributors will provide replacement parts even though you’re paying them for the repair. Oh, and please, if you’re purchasing equipment from the USA, switch the unit over to 230VAC mains before you plug it into the wall and blow your shiny new purchase out the door. If you aren’t totally confident that your unit is set to 230VAC mains power, spend a few dollars and take it to a tech to check this out for you. There are mountains of dead USA purchases piling up in workshops around Australia resulting from goods set to 115VAC landing here and being plugged in willy-nilly by impetuous owners.

If your new purchase is old school, whether it’s a 52-channel console or a 1960s condenser microphone, if it costs anything more than weekend flash money, get a technical report on the unit. This will at least give you a feel for the real cost of bringing the device up to speed so that it does its job and benefits your setup, rather than misbehaves to the point of distraction.

PS: ON THE BENCH, LIVE!

I’ll be dragging a good slab of my workshop up to AT World at Integrate this year. The soldering iron will be on and I’ll be up to my elbows inside something interesting. Drop by and say hello while you’re there and feel free to quiz me on the state of your audio equipment. On Wednesday 31st at 12.30pm there will also be the ‘Tech Forum’ where I will join a bunch of techs much smarter than to chat about electrons, music and how to get the two playing happily together. Until then…

Rob Squire runs Pro Harmonic

Text: Rob Squire

THE ALUMINIUM ELECTROLYTIC

The most common capacitor type is the aluminium electrolytic capacitor. These caps are constructed from two conducting aluminium foils, one of which has an insulating oxide layer, with the foils separated by a spacer soaked in conductive liquid electrolyte. This is rolled up, placed in a cylindrical casing and fitted with two connection pins, or legs. This formula is cheap to manufacture and yields a cap with large amount of capacitance for its physical size. It’s polarised, meaning that any DC voltages applied across it must be orientated in accordance with the positive or negative markings on the body of the capacitor. The only other capacitor commonly available that cares about this DC orientation is the tantalum capacitor – other plastic and ceramic types aren’t concerned about their orientation with regard to DC voltages. Aluminium electrolytic and tantalum capacitors that have a reverse DC voltage applied to them will be destroyed by the experience (see pic, left), and if there’s enough power on hand at the time, often quite explosively.

The liquid electrolyte in this capacitor, while contributing to its cheap construction cost, ultimately leads to this type of capacitor’s downfall. Heat and time cause loss of the electrolyte: the cap ‘dries out’ – a familiar term to many – becoming less and less of a capacitor as time goes by. This leads to the daily grind of vintage audio repair and refurbishment: locating, testing and ultimately replacing dried out caps.

TO RECAP OR NOT RECAP

I’m often asked to speculate whether the caps in unsighted ‘Unit X’ need replacing. Sometimes I get the more direct request when a unit is sent in for service: “replace the caps while you’re at it can ya please Rob?”. I’ve poked a stick at hundreds of thousands of capacitors in my time, leading to some rules about capacitors and their replacement, but like all good rules, occasionally these get broken and at other times confounded by a new experience.

When an electrolytic ‘coupling cap’ in a signal path dries out two things happen to the audio passing through it: the overall level will be attenuated, and, in some instances, the low frequency response will be further depleted. These two symptoms of a degraded capacitor – loss of level and loss of low frequency response – are due to the cap developing a high resistance, and thus behaving more like a resistor, and the value of the capacitance decreasing. In any device that houses lots of caps in its signal path, such as a mixing console, this sort of degradation leads to strange variations in level and low frequency response from channel to channel, and output to output. From an operator’s point of view, a console in this state feels less precise and develops an aura of sonic uncertainty.

The thing that makes developing rules about replacing capacitors tricky is that different brands, and even different value caps within a single brand, degrade at vastly different rates. Even the physical position of a capacitor on a circuit board can affect its rate of degradation. Recently I was checking over caps on a very late model SSL 6000 console, and on one circuit card found three particular electrolytic caps of the same brand and value in each channel. In every case, across dozens of these channels, the same two capacitors tested fine while the third was badly degraded. The reason for this bizarre ‘consistent inconsistency’ only dawned on me after I positioned the channel strip vertically in its normal operating position. The degraded cap was located above half a dozen ICs that ran reasonably warm, whereas the two undegraded ones were located below these ICs. Simple convection (heat rising) had done its job on this one cap, the lower ones remaining just that much cooler and thus avoiding the most common killer of electrolytic caps – heat. This pattern was broadly spread across all the channel strips, where capacitors higher up on these large and deep circuit boards, had degraded more than those lower down.

TIME CAPS

Time yields hard won experience, they say. Standing back and surveying the myriad brands and values of capacitors used over the years reveals how some brands and values within brands have stood up to the stresses of heat and time better than others. A prime example of this is the blue axial capacitor, manufactured by Philips from the ’70s to ’90s. This was a capacitor used by the truckload by UK and European manufacturers of pro audio gear. Found inside the Neve 51 and V-Series console, which run very hot, these capacitors have proven to be a cap that degrades very badly, sometimes fatally in these environments. There wouldn’t be one of these consoles left working on the planet that hasn’t had all its capacitors replaced by now. Indeed, those that made the mistake of replacing the capacitors with the same brand the first time around have now had to replace them all over again. Ouch!

For capacitors housed in these hot environments there are now, mercifully, electrolytic capacitors that have a higher temperature rating, meaning that their expected lifetime at a given temperature is increased. These 105º caps, as they’re called, are not much more expensive that the more common 85º caps, and generally speaking there are very few good reasons not to use them. I will always use 105º caps wherever possible in any device that runs at a temperature much above ambient.

Of course I can already hear yelling from the back row… “What about the sound? Some capacitors sound better than others and I’ve been recommended these special audio capacitors that are made on the forge of Zeus!” Well yes… and no, and maybe.

Like many of these audiophile perspectives, there’s a touch of science and a mountain of mysticism involved here. Different types of capacitors used in different types of circuits can have an appreciable influence. Here a designer schooled in the science will be able to choose the right capacitor for the right job.

Certainly capacitors such as polystyrene and polypropylene have very advantageous properties in circuits such as filters and equalisers or in high power circuits such as passive speaker crossovers. But since these types of caps don’t degrade with heat like aluminium electrolytics they aren’t the usual candidates for replacement. Also, their comparatively large physical size for the same given capacitance makes them physically unsuitable to fit into circuit boards where electrolytics were previously installed. Thus the argument often revolves around the best electrolytic brand type for the job. Enter the snake oil salesmen.

My rule is simple: if there’s heat around use a 105º-specified capacitor of an established brand you can afford. If choices need to be made, a low ESR [see the box item for an explanation of this term] capacitor is generally more versatile, meaning it is suitable as either a coupling cap or decoupling cap and may mean you only have to get one type instead of two. When replacing the thousands of capacitors in a large console the fewer types you have to deal with the better.

DSITORTION SPECS

Back in 2002 Cyril Bateman published a series of articles in Electronics and Wireless World magazine that was an exhaustive scientific study on distortion behaviour in capacitors. The series kicked off with the design and construction of a distortion analyser to conduct the tests, as nothing was readily available at the time that could probe the minuscule levels of distortion found inside capacitors. Ultimately he developed a system that could measure distortion down to 0.00003% and proceeded to conduct over 2000 distortion tests on various types, values and brands of capacitor. The broad conclusion of his work was that polypropylene capacitors were the best performers, and you’ll now see these type of capacitors used in high-end loudspeaker crossovers and where smaller value high voltage caps are required. For example, Avalon Designs uses these types of capacitors in the signal path wherever DC blocking is required.

For the more common aluminium electrolytic capacitor, distortion performance wasn’t as good but nothing measured much worse than 0.005%. This puts distortion factors well down into the noise floor of most equipment and is comparable with distortion factors of other circuit components. The one electrolytic capacitor that stood out as having generally better performance was the bipolar electrolytic capacitor. This capacitor is essentially two capacitors placed back-to-back inside one case. It doesn’t care about the DC polarisation like other electrolytics. When musing over replacement capacitors this type can be worth pursuing; the drawbacks being that they’re physically larger, more expensive and harder to source. Some manufacturers also see the value of the bipolar electrolytic. For example, the SSL 6000 console mentioned earlier uses several of them, whereas earlier versions of the console used the more standard polarised electrolytic.

TANTALISING TANTALUMS

Tantalums are also a commonly found capacitor in audio equipment. They became all the rage in the 1970s due largely to their physically small size for a given capacity and excellent very high frequency performance. Despite also being a polarised electrolytic capacitor they are most often manufactured with a solid electrolyte and thus don’t suffer from ‘drying out’ like aluminium electolytics, which extends their service life considerably. However, tantalums are very sensitive to the application of reverse polarity DC and current surges and when they die they don’t go gracefully, usually failing as a dead short. Worse, since they’re often used as decoupling capacitors placed across power rails this dead short invariably leads to other parts failing down the line, the chain reaction in some cases being quite catastrophic. History has shown that it’s rarely a good idea to use tantalums as a decoupling capacitor, especially with aluminium electrolytics now available exhibiting equivalent high frequency characteristics. The other quite common failure mode of tantalums is to become ‘crackly’, and frustratingly, often intermittently so. I had a U47FET in the workshop recently that would spit out a brief crackle once or twice every hour giving me uselessly narrow windows of opportunity to locate the faulty part. In the end simply replacing every tantalum capacitor in the microphone was the only expedient solution. Many condenser microphones use tantalums, taking advantage of their small size, and these parts are probably the most common source of faulty mics – whether that manifests as a ‘dead’ mic or an intermittent crackle.

Tantalums are certainly one capacitor that raises debate about the contribution of capacitors to audio quality or euphonics. Various studies – including Cyril Bateman’s – demonstrate that tantalum capacitors do have significant distortion qualities, and given their tendency to fail either to a short circuit or an annoying crackle, it’s always tempting to replace them. However, one place where you’ll find these capacitors used extensively is in Neve products from the ’70s and there is little doubt that replacing all the tantalums in a Neve 1073 does alter its sonic qualities. Given this, capacitor replacement – especially changing the capacitor type – should never be undertaken without reference to the role the capacitor plays in the circuit. As a rule I tend to replace tantalums used as decoupling capacitors with low ESR aluminium electolytics to avoid the grief of a dead short tantalum down the track, but always replace tantalums in coupling circuits with fresh tantalums.

For those of you heating up your soldering irons for a DIY project or to refurbish that big second-hand console you recently bought for a song, certainly give thought to the types and brands of capacitors used. Good brands of high temperature, low ESR aluminium electrolytics will be suitable in 90% of situations. Before you fork out the dosh, understand the role the capacitor plays in the circuit first and replace like with like, unless you understand the ramifications of any changes you make.

Rob Squire runs Pro Harmonic

Text: Rob Squire

NOT AN iPAD

In essence a pad is simply a string of selected value resistors that provide a desired attenuation. There are a bunch of different ways of connecting resistors, of course, with each design meeting differing objectives and generating different outcomes.

Back in the day when professional audio systems were based around 600Ω input and output impedances, pads were designed to achieve a particular attenuation while consistently maintaining this 600Ω impedance. The two typical pads that met this objective were called ‘T’ pads or ‘Pi’ pads, with both having balanced and unbalanced versions. Calculation of the actual resistors required in these sorts of pads was (and remains) complex, where solving the twin goals of impedance and attenuation sometimes yields physically impossible resistor values.

Fortunately, most audio equipment today doesn’t provide – nor require – matched 600Ω input and output impedances and so pads on modern audio systems can be of a simpler type called ‘L’ pads (for unbalanced signals) or ‘U’ pads (for balanced signals). While we still need to consider the impedance of the pad and the devices connected to it, we’re free of the strict 600Ω requirement.

MICROPHONE PADS

Microphones typically have an output impedance somewhere around 200Ω while most preamps have an impedance of around 2000Ω. Even though the exact values found in the variety of real-world products vary around these figures, they’re sufficient to get us into the ballpark to begin designing a microphone pad.

THE U-PAD

In the U-pad illustration above, the value of R1 is always the same value as R2, the rule being that larger values of R1 and R2 and/or smaller values of R3 increase the amount of attenuation the pad provides. While there’s an infinite combination of resistor values that can meet a given attenuation we still want to keep the input and output impedance of the pad in the sensible range, and this constrains our choices.

Here’s a table of attenuation and suggested resistor values:

Note that the exact amount of attenuation will vary with the input impedance of the microphone preamp used and that some old-school preamps with lower input impedances will cause a greater amount of attenuation to occur than the table indicates. Some of the new-school preamps with a variable input impedance – which seems to be becoming the current fad – will actually create an amount of attenuation that varies with the input impedance dialed up. This could be a cool side effect of the design… or a pain in arse, depending on your circumstances.

PUTTING IT TOGETHER

There are a few manufacturers of XLR barrels around but one that’s particularly neat is the Neutrik barrel system. This adaptor fully disassembles into separate parts that screw back together to provide a robust XLR-to-XLR connector with sufficient room inside the barrel to incorporate the pad components.

In the picture above, the resistors are mounted directly off the XLR solder lugs with R3 soldered between Pins 2 and 3 of the male XLR, while R1 and R2 fly off Pins 2 and 3, and connect to their respective pins on the female XLR at the other end. Don’t forget to run a wire from Pin 1 of the male XLR to Pin 1 of the female XLR to maintain the ground connection.

As the barrel parts screw together it’s also a good idea to gently twist up the wires in the reverse direction a few turns before screwing the sections together. This enables the wires to unwind and avoid being over stretched or broken at the solder joints as the connector is assembled. Placing heatshink over the joints where the wires join the resistors is also a good idea.

LINE ’EM UP

This is all well and good for microphones of course, but what about line level signals? I’ve built a number of fixed pads for studios that required one set of line outputs to be reined in to match other sources. Here the same U-pad as for microphones will work but the overall impedance of the pad has been increased to suit broader range line-level sources, where there’s greater variation in the load different devices can drive without distortion.

THE VARIABLE PAD

A fully variable pad can transform the fixed pad into a new tool of exploration, useful not only between microphones and preamps but also across line outputs. This is particularly the case with some tube equipment that overloads gracefully, but while pushing the unit with hot level yields the sound you’re seeking, the output level becomes too unwieldy to deal with. Here the pad can be placed in between the output of the device you’re plying with hot levels and the next device in the chain. While there’s some compromise with the impedances involved, the solution in the above circuit works fine in 99% of situations. Here a dual-gang 1kΩ log pot is wired as a variable balanced attenuator with R2 setting the maximum amount of attenuation. With R2 being a 150Ω resistor the attenuation ranges from 0 to 25dB.

BUT WAIT, THERE’S MORE

While we still have these XLR barrel adapters open in our hands there are other uses for them well worth considering. Do you recall my reminder to join Pins 1 together in the XLR adapter of the microphone pad? If not, you may have already gone ahead and made it and forgotten to make this connection. No stress, inadvertently you’ve just made another handy tool: a ground-lift connector – where Pins 2 and 3 connect straight through but Pin 1 is left disconnected. When hunting down a ground loop hum, having one of these in your back pocket can be pretty handy. A phase reverse adapter can likewise be helpful, where the wiring between Pins 2 and 3 is swapped over from the male to the female XLR. I carry both of these types of adapters in my troubleshooting tool kit along with a 40dB pad, which allows me to take a line level signal and inject it into a preamp at a typical microphone input level. A word of warning though: if you do decide to wire these alternatives up, make damn sure you mark them clearly ‘Earth lifted!’ and ‘Phase reversed!’.

SHAPING YOUR SOUND

My dictionary gives a number of meanings for the word ‘pad’, but one I particularly like is: “a material used to protect something or give it shape, to clean or polish articles” [kind of like an editor – Ed.]. Armed with a range of pads of differing attenuations you can certainly shape, clean and polish your audio.

Rob Squire runs Pro Harmonic

Text: Rob Squire

HO-HUM

‘Hum’ is the bane of many a studio setup and, in the case of a new studio, is often the result of ground loops – as discussed back in Issue 64 where we looked at studio wiring. However, sometimes hum is a problem that has nothing to do with external wiring or ‘grounding’.

Anywhere that current flows, magnetic fields are created. If that current is alternating, as is the case with mains power, then a 50Hz (in Australia) or 60Hz (in the USA) field is generated. This field can induce a corresponding voltage in any electronic component that falls within that field. Mains power transformers generate quite strong fields, and generally the larger the transformer the greater the current flow and therefore field strength. Unfortunately, some parts used in audio devices pick up this field more efficiently than others. An extreme example that demonstrates this effect is the single-coil guitar pickup: take your Fender Strat and lay it down on the head of a Marshall stack. With a bit of gain dialled up on the amp the hum coming out of the speakers should be more than obvious.

Guitar pickups, inductors and audio transformers – which all contain coils of wire – are excellent ‘receivers’ of magnetic fields. Since mains power is a sine wave, the resulting hum from the Marshall will sound like a pure 50Hz note, just below G1.

Most audio equipment contains a mains transformer and considerable design effort is often required to ensure radiated hum doesn’t find its way into a device’s audio circuits. This is especially the case if the device uses audio transformers, inductors, physically large capacitors, or where high gain is employed, as in a mic preamp. Of course, the situation can – and does – arise where the mains transformer of one unit radiates into the sensitive components of an adjacent unit in the rack. There are a few ways to resolve this situation, the most obvious of them simply being to physically rearrange the position of the equipment – up and down and at different angles. Frustratingly, this is often the least practical method for studios since fixed 19-inch rack units make positioning things at odd angles all but impossible [See the Leave it There! box item about John Nowland of Broken Arrow Ranch, who positions his equipment to minimise noise first and foremost].

Returning to the demonstration with the guitar and amp head for a moment, the other thing that quickly becomes apparent is how much the hum level can drop by simply moving the guitar relatively small distances. Magnetic field strength decreases with the square of the distance so doubling the distance between the generator (mains transformer) and receiver (guitar pickup) will drop the hum level fourfold. Another not uncommon way hum is induced in a system is when devices with large mains transformers – i.e. power supplies or power amplifiers – are placed underneath a console. Most consoles have unbalanced summing buses that are not only unshielded, they span the width of the console and have high gain applied to them in the mix amplifiers – a perfect recipe for picking up radiated hum from equipment placed directly under the console.

ALL THE BUZZ

Buzz – as characterised by a low-frequency sound with many higher-order harmonics that make it far more perceptible – is generally the result of a failure in the power supply section of a device. Analogue or linear power supplies create a sawtooth-shaped waveform, which is regulated to produce a clean DC voltage for suppling power to the audio electronics. If this regulator fails the raw sawtooth waveform quickly makes its way into the audio circuits and finally the output of the device. This is a far more objectionable noise than hum or hiss, and typically the offending unit is soon pulled out of any audio system. In some cases where the designer hasn’t allowed for the widely varying Australian mains power voltage, a device can often ‘drop out of regulation’ if the mains voltage gets low enough. This results in a buzz that can be frustratingly intermittent; coming and going as the mains voltage varies at the power pole.

Another noise source that’s often described as a ‘buzz’ is interference from light dimmers and other mains power control systems. The switching supplies used in some fluorescent lights, computer power supplies and dimmers can often feed significant levels of higher-frequency garbage onto the power cables. If these run in proximity to audio cables the noise can be easily picked up. To prevent this, it’s always a good idea to keep some spacing between audio and power cables and to definitely avoid running them alongside each other for long distances. Instead, try to ensure that audio and power cable cross each other at right angles.

RUMBLE IN THE CAPSULE

Inside my workshop I have a small acoustic isolation box that I use to entomb mics while I’m trying to ascertain their noise floor. While this setup is more than effective at screening out the fan noise from the office PC and the birds in the gum tree outside the workshop window, the isolation isn’t sufficient to block low-frequency noise. It amazes me how readily most mics sitting inside this isolation box still manage to pick up rumble from traffic on the main road several blocks away. Having performed this test countless times, I can easily imagine the amount of very low-frequency noise that can build up in a multitrack recording done in most inner city studios. While the perception of this noise is weak because of the very low frequencies involved, cleaning up recordings with the careful use of high-pass filtering is a worthwhile process by removing this unwanted rumble. At the same time lowering the level of the flicker noise from mics and preamps, as we learnt earlier, increases in level as frequencies go lower.

MICROPHONIC TUBES

Tube-based equipment seems to be having some sort of sustained renaissance, and is increasingly finding its way into studios – of course other engineers never turned their backs on it to begin with. One characteristic of tubes is how much they can react when tapped, a bit like a spring reverb. But what’s not so obvious is the way they respond when exposed to loud acoustic sounds within their immediate surroundings. While electric guitar players are often experienced at knowing what happens when a tube becomes microphonic – when an amp starts singing and feeding back all on its own – a studio operator is often quite surprised to discover how much tubes can pick up acoustic sounds, amplifying and resonating with them in sympathy. It can be a life’s work finding a set of very low microphonic tubes – just ask Rick O’Neil – nevertheless if you start noticing a weird ringing or distortion, check that you haven’t just positioned your new tube preamp right under the main monitors. Loud sound and tubes only play together nicely when you’re pulling a screaming guitar solo!

As a teenager I’d spend hours sitting in front of the family’s old gramophone (yes it had tubes) playing Led Zeppelin records. My dad would walk in yelling: “what the hell is that noise?” I wish I’d known enough back then to say – as I lifted the needle off the record – “I think that’s shot noise in the first preamp tube!”

Rob Squire runs Pro Harmonic