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Rackmounting Mixer Channel Strips

Is It Worth Your Time And Expense? By Sam Inglis
Published February 2020

Rackmounting Mixer Channel Strips

We all love vintage mixer EQs and preamps, but it takes a lot of work to rack them up for use in today's studios. Is it worth your time and expense?

Some people base their buying decisions on a carefully considered assessment of their needs. I usually base mine on whether something looks like a bargain. This can be fun, but it's possibly not the most effective way to put together a studio! I've certainly made a few bad choices over the years, and it's especially easy to get carried away in the bargain-spotting stakes with things that need work to make them work.

"Just rack these up and you'll have X channels of vintage preamplification and EQ!", the sellers say, and it's a tempting prospect. From the '70s onward, most professional analogue consoles have had channel strips with preamps, several bands of EQ, and sometimes dynamics. Those consoles cost tens or hundreds of thousands back in the day, and represented the pinnacle of British, European and American engineering. If you can pick up one or two of the important bits for a few hundred quid, surely that's a better option than paying a couple of thousand for a new high-end channel-strip-in-a-rack?

Having recently dived down this rabbit hole myself, I can report that it's not as simple as it sounds. Sellers are always keen to tell you about the audio Nirvana that awaits once you've "just racked up" your modules, but they tend to gloss over what's involved in actually doing this. After all, if it was that easy, the sellers would do it themselves

When Is A Mic Preamp Not A Mic Preamp?

One thing to be aware of is that not all mixer modules are quite what they seem. For example, circuits are sometimes misrepresented as mic preamps when that was never their original role. The classic cases are the Neve 1272 and the Telefunken V72, both of which were originally line amplifiers. These particular units can successfully be modified for use as mic preamps, and often are, but the same doesn't apply to every line amp, headphone amp, talkback amp or other little grey box. Input stages from reel-to-reel recorders are sometimes resold as 'preamps', too, though they may need a lot of work and extra circuitry to fulfil this function, and once you start looking into modules made for broadcast, you get into a maze of almost-identical model numbers that can be pretty hard to navigate. It's also not unknown for unscrupulous dealers to sell modules missing vital components such as input or output transformers.

Modules from older professional mixing consoles usually plugged into multi-pin connectors, often mounted on a  backplane like this. Make sure you can find the right connectors, and that you know which pin carries which signal!Modules from older professional mixing consoles usually plugged into multi-pin connectors, often mounted on a backplane like this. Make sure you can find the right connectors, and that you know which pin carries which signal!Even if your modules really are true 'channel strips' with a mic preamp and EQ and so on, they will have output stages designed to feed the mixer's internal bus architecture. These may not be capable of delivering a line-level output when the module is used stand-alone, and if not, extra circuitry will be necessary. Finally, don't forget that mixer modules usually talk to the rest of the console via multi-pin edge connectors; these come in a variety of shapes and sizes, and mating examples can be hard to track down if they're not supplied.

Then there's the question of condition. Most second-hand mixer modules will either have spent a hard life in a working console, or a long time gathering dust in a box. Neither is exactly ideal for maintaining optimum performance. If the seller promises that the modules have been "serviced", press them to find out exactly what was done. Have worn-out pots, noisy switches and leaky electrolytic capacitors been replaced? Have the modules been calibrated to the correct specifications? Do the meters and lights work correctly? What sort of noise and THD measurements can they report?

If you suspect that the seller's "servicing" amounts to little more than a squirt of Electrolube and a wipe down with a damp cloth, or if the modules are "untested but removed from a working console", they'll need the attentions of a good technician. Assuming you can find someone with the necessary skills, which is not trivial, expect to pay an hourly rate of between £25 and £40 or more for this, plus parts. The technician's job will be easier if schematics and other technical documents are available. Without them, fault-finding in complex modules can be seriously difficult, to the point where the whole project will quickly become uneconomic.

Since you can buy new items of rackmounted audio equipment for £100 or less, you might think that racking an existing piece of kit would be even more affordable. Unfortunately, it doesn't work like that.

Another factor that can make servicing and racking uneconomic is availability of parts. Some components, especially obscure integrated circuits, are extremely hard to find, expensive, or both. Nothing in the modules I bought was made of pure unobtanium, but we were astonished at how hard it was to locate something as apparently basic as centre-tapped potentiometers with a 4mm shaft. In the event we had to get them specially made — with a lead time of 16 weeks!

Still, once you've received your modules, checked that they are complete, and had them gone over by a specialist, you're out of the woods and standing in front of the Pearly Gates of audio, right? Well, no. At this point your modules may be technically in working order, but they are not useful, because you still haven't "just racked them up".

Racky Races

Since you can buy new items of rackmounted audio equipment for £100 or less, you might think that racking an existing piece of kit would be even more affordable. Unfortunately, it doesn't work like that. The low cost of off-the-shelf rackmount units is made possible by highly automated mass production, economies of scale and cheap labour, none of which are available to you or me. Nor will most mixer modules fit into 500-series or other generic chassis; quite a few won't even fit into a 19-inch rack without being cut down. What's more, every type of module has its own powering requirements, needing to be fed the right positive and negative rail voltages plus +48V for phantom power, and often other voltages as well. Powering is even more fun with valve modules, and you may also need to consider ventilation and so forth.

When it comes to rackmounting old modules, much of the metalwork and electronics will need to be custom-made.When it comes to rackmounting old modules, much of the metalwork and electronics will need to be custom-made.

There may be more that needs to be done to make the modules function properly outside of the mixer they used to call home, too. For example, they may not have been intended to produce a balanced output, or they may be stuffed full of complex routing or automation functionality that is redundant in a rack environment and needs to be removed from the circuit. To be really useful in a modern studio context, many vintage modules will also require either an additional output gain stage or an attenuator, and some will need a phantom power supply adding too.

If you plan to sell your modules on, they'll need to look as good as they sound, so check how your tech plans to handle panel engraving, paintwork and so on.If you plan to sell your modules on, they'll need to look as good as they sound, so check how your tech plans to handle panel engraving, paintwork and so on.It is actually possible to buy off-the-shelf rack enclosures with integrated PSUs for a few classic modules, but these are manufactured in small quantities and don't come cheap. For almost anything else, you're going to need custom metalwork and/or woodwork, plus suitable power supplies, cabling, switches and connectors. Creating a custom housing for mixer modules takes a fair amount of time and effort even for an experienced metalworker; the modules themselves need to be mounted securely, holes need to be cut for switches, lights and sockets, power supplies may need to be shielded, and so on. And if you ever plan to sell on your modules, don't neglect the cosmetic aspect of this process. The home-made aesthetic of Dymo labels on bare metal appeals to some, but making your units look like professional studio tools is likely to involve costly engraving and painting.

How Much Will It Cost?

I contacted several different technicians when I was getting my modules racked, and costs were quite variable. Restoration work for the big London studios and dealers is handled by a handful of specialists. These people are, unsurprisingly, at the pricier end of the scale, and have full diaries — but they do this sort of thing day-in, day-out, will almost certainly have worked on similar modules before, and have a reputation to maintain. It would be a false economy to opt instead for someone without much experience of working on pro audio gear.

For a 3U rack housing two modules, with PSU, audio connectors, phantom power switches and additional output attenuators and transformers, the quotes I received ranged from roughly £450 to £800. My modules also required fairly extensive servicing, with lots of replacement pots and other components. I was lucky to find an excellent technician locally who was more affordable than the London experts, but even so, the total cost per two-module rack worked out at at least £700 once servicing, parts and labour were factored in. Given that this figure doesn't include the purchase price of the modules themselves, it's easy to see how something that looks a bargain at first glance can turn out to be anything but.

Depending on what modules you buy, then, "just racking them up" might represent good value on a personal level. It might give you a unique piece of gear that inspires you to make great recordings, and it might work out cheaper than buying a new channel strip unit of similar quality. But unless you can do it all yourself, it will never actually be cheap — and there is a real risk that you will spend a lot of money creating something that only you will value. Although there will always be a healthy demand for rackmounted Neve 1073s, few other high-quality mixer channels have the same hype around them. That obscure mixer you discovered in an abandoned telephone exchange might have the best preamps ever, but that doesn't mean it'll be easy to find a buyer when you come to move them on.

As for me, would I do it again? In a heartbeat. But I'll never be one of those people who bases their buying decisions on a carefully considered assessment of their needs

What Modules?

Modules from the discrete Neve consoles of the '70s, like these 2073 EQs and 1272 mic preamps at San Francisco's Hyde Street Studios, will always be desirable.Modules from the discrete Neve consoles of the '70s, like these 2073 EQs and 1272 mic preamps at San Francisco's Hyde Street Studios, will always be desirable.Photo: Hannes Bieger

Mixer manufacturers have come and gone with alarming frequency over the years, and it's impossible to give more than the barest outline when it comes to suggesting whose modules might be good candidates for racking up. To offer a ridiculously general overview, if you can get your hands on any genuinely professional studio mic amps, equalisers or compressors from the 1960s or earlier, there's a good chance that they will be worthwhile. Be aware, though, that there isn't much proper studio gear from that era: most of what you'll see for sale comes from consumer or PA equipment.

The 1970s was the golden age of the discrete solid-state mixing console, and early Neve modules are the quintessential candidates for rackmounting. Some early IC-based designs from the late 1970s are also very well regarded. By the mid-'80s, however, large-format studio consoles majored on features rather than 'mojo'; channels from these desks are not sought-after from a sonic point of view, and tend to be very complex.

The bread and butter of most professional mixer manufacturers was broadcasting rather than recording studios. Consoles made for radio and TV were usually built to very high standards and can offer excellent quality, but they don't have the sex appeal of studio consoles, so don't expect Neve resale value from a rackmounted Glensound channel strip. What you might want to look out for, though, are small portable broadcast mixers from manufacturers such as Calrec or Studer. These are sometimes available at fairly affordable prices and often have direct outputs on every channel, meaning they are usable as front ends for recording as-is, with no need for expensive rackmounting.

It's also worth noting that although API's 500 series is now the de facto standard for modular audio processors, plenty of other systems have come and gone over the years. You won't find the same wide range of modules available in a Scamp or Rebis rack, but if you're looking for basic mic preamps, EQs, compressors and so on, they can offer cost-effective alternatives.

Modular Mystery Tour

Buying old equipment with a view to restoring it is always a gamble, and as such, the golden rule is never to spend money you can't afford to lose. There will be times when you have to write off your stake, but there will also be times when you are taken down unexpected paths.

One of my mystery modules with the outer casing removed. The Helios name is just visible in the rectangular cutout at the lower left.One of my mystery modules with the outer casing removed. The Helios name is just visible in the rectangular cutout at the lower left.My racking project started, inevitably, with an eBay listing. It was long on photos and short on information, but I recognised the name Tweed Audio as a well-regarded British manufacturer from the '70s, and decided to take a risk. A week or so later I took delivery of some battered cardboard boxes, and the scale of the project I was about to embark on began to sink in!

What the seller was able to tell me was that the modules had come from the Master Control Room console in Capital Radio's old London studio. I also found former Tweed engineer Peter Gillespie's helpful website about the history of that company, which shows several photos of the desk in question. Installed in 1981, this was an unusual and highly advanced console for its time, with much of the audio processing happening in remote racks.

The modules arrived with two large folders of documentation that included wiring diagrams, layout plans and schematics for almost every part of the mixer, including the 'outside source equaliser' modules that were now mine. Mysteriously, though, there seemed to be no information at all about the input channels I'd bought. These were nice-looking things with mic preamp, compressor and EQ, and had undoubtedly come from the same console — but they bore no maker's logo, and looked nothing like any Tweed Audio gear. In search of clues, I eventually removed the outer panels from one of the modules to examine the printed circuit boards. And there, to my surprise, was etched the name of a completely different mixer manufacturer!

This made things a lot more interesting. Designed by Olympic Studios engineer Richard 'Dick' Swettenham, Helios desks had equipped such legendary facilities as Island Studios, The Rolling Stones Mobile and 10CC's Strawberry Studios, and had recorded some of the greatest albums of all time. They are also rare, with only around 50 desks known to have been made. So how had Helios input modules ended up as part of a Tweed Audio radio mixer?

The story is still frustratingly incomplete, but I've been able to fill in some of the gaps. It seems Helios were hired to refit the Capital Radio studios in 1979, but ceased trading before they could finish the equipment they were building. Capital then acquired three empty console frames that had been constructed, along with many other parts, with a view to seeing what could be salvaged. It's not clear whether the frames or any of the other parts were ever used, but these modules were either complete, or close enough that Capital's own engineers were able to finish them. Capital then contracted Tweed Audio to build a new Master Control Room console that incorporated the Helios input modules.

There was no 'off the shelf' configuration for Helios consoles, which were all custom-made to suit the needs of the client, and these modules — quite possibly the last ever made at Helios's Teddington factory — are interestingly different from earlier designs. The mic preamp with its Sowter 3374 transformer is familiar, but the EQ, though clearly based on the same active design used in Helios consoles from the mid-'70s on, is configured differently, with the outer bands being shelving rather than peaking. The real surprise, though, is the compressor. As far as I know, these are the only Helios input channels to have one built in, and whereas the compressor circuits in other Helios consoles were licensed from Audio & Design, these use a dbx 202 VCA (as found in the classic SSL circuit). With no threshold or make-up gain, their primary purpose was presumably to protect the transmission chain against overloads, but they seem equally happy in more creative roles!

Published February 2020