Graham Blyth: Designing Spirit Mixers

Paul White talks to Graham Blyth, Spirit's founder and design guru, about the company's product philosophy and its newest line, the Spirit PowerStation powered mixer.

Graham Blyth's past is steeped in the lore of British mixer design and dates back to the flared‑trouser days of the early '70s when he worked at Kelsey Morris, a company involved in PA and mixer console design and manufacture. When he started with the company, mixing electronics was in its infancy and Kelsey were engaged in building the console for the now‑legendary Isle of Wight pop concert. After learning much about audio electronics, and working on consoles for the likes of T‑Rex, Ten Years After and King Crimson, Graham had accumulated enough design knowledge from the sharp end of console use to design and build his own mixer — but he hadn't a clue how to go about marketing it! A friend put him in touch with Phil Dudderidge (now the owner of Focusrite), who was then at a company called RSD, and eventually Graham and Phil started Soundcraft.

Today, Soundcraft has a 21‑year history of building affordable consoles for both the PA and studio markets. However, since the company was acquired by Harman in 1987 there have been several changes. Most significantly, late in 1990 the 'Spirit by Soundcraft' line of low‑cost mixers was added to the company portfolio, attacking the lower‑priced mass market in a big way, backed up by sophisticated volume production techniques; I'm told that there are now over 120,000 Spirit users worldwide. I asked Graham, who has been responsible for Spirit designs from the start, how it was possible to achieve low selling prices without the need to cut corners in either mechanical or electronic design.

"I've never been one of those crazies who use four op‑amps where one will do, so it wasn't as difficult as you might imagine to implement elegant and cost‑effective design. I believe very strongly that good design doesn't have to be extravagant or expensive — it's just a question of common sense and refining techniques that you've picked up over the years.

"What has come about through Spirit, though, is a certain amount of revising of basic philosophies we've had at Soundcraft for a long time. For example, although the standard Soundcraft mic amp was very good, it was too complicated and took up too much space, and was therefore too expensive to use, so I had to find something simpler, but with the same performance levels. I spend a lot of time messing around with mic preamp circuits. The aim is to get as much gain range around the pot as possible, spread the gain range out, and to be able to hit the input with line‑level signals without having to use a pad switch. It's a process of continual refinement, as you'll see if you check out the UltraMic™ input preamp on the PowerStation.

"Meeting the price points is absolutely crucial, so we had to look seriously at how we designed the mechanical package. With Folio, for example, you get a carry handle, which also slopes the mixer so you can more easily use it."

Front End

"If we look at the mic amp on the new PowerStation powered mixer, the distortion at maximum gain is only around 0.002%, which is very good. This circuit required a pot with a custom taper, and out of that I've got something that goes from zero gain to almost 60dB in almost equal 10dB increments. The preamp incorporates an 18dB‑per‑octave high‑pass filter which comes in at 100Hz, so you have this tremendous front end, all around four transistors and one dual op‑amp.

"In the same way that we challenge ourselves, we also challenge our suppliers to improve what they do, because a lot of standard pots are awful — they have tracks that don't do anything for the first 30 degrees and last 30 degrees of rotation because it's cheaper for them to do it like that. But in an EQ, you really don't care whether the wiper shorts out properly to the track at either end, so you can run the track all the way across the entire range. It's a question of having the supplier design a complete set of pots and faders for us, with characteristics that suit the audio industry application."

You must carry a lot of weight with the component manufacturers, because it's only large mixer manufacturers who buy pots in such large quantities these days.

"Yes, that's true, and for the Spirit range, we've also had our own gold‑contact jack sockets and XLRs designed by Neutrik in Switzerland, to complete the set of custom parts we design for each mixer. We're always pushing the component designers for a better product design at a more competitive cost so that we can pass these savings onto our customers."

Once you've arrived at a suitable electronic design, it must be a major challenge designing the packaging so that it is mechanically rugged, cosmetically attractive, and inexpensive to build. For example, has the shift to single circuit board construction in the smaller mixers made life a lot easier?

"I think single boards make a difference in handling, but there comes a point where it becomes absurd to take it any further. I think the PowerStation is about as far as you can go in that direction, because above that, manufacturing tests become an expensive problem — far more expensive automatic test equipment is required because the number of test points has increased. It also creates difficulties in that the pots and switches are using up 'land space' on the circuit board that you want to put your electronic circuitry on — but at the same time this reduces the amount of internal wiring, which improves reliability.

"On the PowerStation, I wanted to get very good cutoff from the faders when they were in the down position, and I'm now getting better than 100dB, not by using electronic circuitry, but by using a design of fader that does it for me. Again this saves on component cost, space and complexity, but without compromising the performance. I also think it's important that all the inputs are electronically balanced."

Talking of balancing, how does your pseudo‑balanced, ground‑compensated connection system work?

"Ground compensation is a way in which a mixing console can drive a piece of unbalanced gear which is grounded (potentially a hum loop) and the hum will self‑cancel. However, when you're driving into a balanced input, a different approach is needed to produce the optimum result, and I'm now looking at voltage balancing for inputs, and impedance balancing for outputs. This came about as a result of a conversation with Clive Green at Cadac, and works on the clear logic that if you're running two cables through the air and each offers the same source impedance over a wide range of frequencies, then any interference will be impressed equally on both. Now, if my voltage‑balanced stage at the other end offers an input impedance of at least 10 times that of the output feeding it, the interference problem will go away. This means that any output onto a 3‑terminal plug can be impedance balanced. Many very expensive condensor microphones use this method — they don't have a differential ouput as such, but have what might be termed a neutral line that's exactly the same impedance as the live line. It's also compatible with both balanced and unbalanced wiring systems.

"A balanced input will only work properly if you wire the plug up correctly, and we suggest that when using unbalanced sources, such as keyboards, you connect the ground to the ring of the jack plug feeding the mixer. This will cancel out the hum voltage that might otherwise be developed along the screen of the connecting cable. This wiring system is explained in our manuals, and though it does mean making up special leads, it really is recommended — it does make a difference. If only other manufacturers would use proper 3‑terminal jack sockets for unbalanced outputs, and ground the ring, a standard stereo jack‑to‑jack lead would do fine."

Of course, good electronic design is one thing, but someone still has to turn the ideas into engineering reality, and this is where Martin Appleby, who is responsible for all Spirit's mechanical design, comes in. He explained that a vast amount of consultation goes on before a new project is embarked upon and it isn't unusual for more time to be spent fine‑tuning the idea than on actually putting that idea into production. In the case of the PowerStation, a number of working mock‑ups were built and then shipped to Spirit's key distributors around the world to be subjected to months of rigorous road‑testing before the final design was nailed down.

The Guru Factor

There's a lot of mystery surrounding equaliser circuits; what are your thoughts on the subject?

"Frankly, it's a lot of mystique. It's just experience — knowing what to do. The high and low controls are fairly conventional, though on some of the products, we do a bit of a trick with the treble which I won't divulge here — it gives the sound that extra transparency, especially when you have a sweep mid below it. It all comes down to the right choice of curves and the natural Q of my standard mid‑sweep circuit which I've been using for around 20 years. It seems to be about right as a general purpose EQ tool, but you also have to use the right pot so that you get a decent spread of frequencies, and not have them all crammed up at one end. I've seen some disgusting products where no attempt has been made to use the right components — and this is major league stuff, sold in large quantities!

"The other point that concerns me greatly is the unity gain point on the fader — because so many consoles get this vital design element wrong. Many smaller desks have shortish faders of around 60mm, with their unity gain point exactly halfway up. You almost never need to go above that point — you only need to go down. That leaves you with hardly any room — the tiniest of movements causes a big change in level. You can't mix on something like that; you can't mix on the +20dB you've got at the top, and if you do try, it means you've got your input trim set up incorrectly. It's the same with rotary aux sends that have unity gain in the centre position. You may never get into the second half of the scale, which, again, leaves you with very coarse control. If you do want to use all the range, you have to set the input trims at such a low level that the gain structure is compromised. The best place to use an aux send on a properly‑designed desk is about three‑quarters of the way up, or possibly a little more — just leave yourself a little room. On all Spirit mixers this isn't a problem, because their unity point is always‑three quarters of the way up the fader or around the pot's travel. This gives you more resolution exactly where you need it most."

Presumably that also produces the best signal‑to mix‑buss‑noise, especially on consoles with a large number of channels?

"That's true in theory but the circuitry is so quiet, now, that mix buss noise isn't a real concern except on something like a 32‑channel live console. However, good practice should be encouraged, by which I mean that users should be directed towards making the best use of gain structure."

Inside The Powerstation

Can you tell me more about the new PowerStation powered mixer? This seems to present a neat solution for small PA requirements, as it is compact, packs plenty of power and boasts an on‑board Lexicon reverb system.

"PowerStation has eight mic/line channels, plus two stereo channels, combined with stereo amplifiers rated at 300 watts per channel into 4 ohms (simultaneous, tone burst), and it also has an on‑board Lexicon reverb. We looked at products already on the market, and it was surprising to see how many came nowhere near their published power specifications.

"Powered mixers, until now, have been looked upon as the bargain basement part of the business — and that's being polite! Some of the product is the most appalling rubbish, but we have what I feel is a cracking good unit. It makes the powered mixer into a serious product, not a plaything.

"Powered mixers have certain conventions, one of which is that they all have dual graphic equalisers on the output. Often manufacturers market them as tools to eliminate feedback, which is misleading. Given the filter bandwidths that you have to use to get the correct operation on a 7‑band or 9‑band graphic, there's no way you can use those filters to notch out feedback — they're just too broad, and anyone who says differently is misleading you. You can reduce the boom of a hall or make other general changes, but you can't really do anything very precise. So we have to say that the graphic is really there to enhance or sort out the interface between the speakers and the room.

"With this in mind, I stuck my neck out and decided to restrict the range to plus or minus 6dB so that you can move the faders without going instantly into feedback — that way you get reasonable resolution on short faders. Putting a range of plus or minus 12 or 15dB on a 30mm fader, as some consoles do, is a recipe for disaster, especially for a user who hasn't really got to grips with it — and there's no way you need that amount of EQ range! Instead our graphic is a creative frequency shaper, and it sounds gorgeous — it really does enhance the sound of a decent pair of speakers.

"Another key point, something that I really believe is important, is that every input has a high‑pass filter. I've put one on everything I've ever done in the Spirit range, except Folio Lite, where there simply wasn't the room. If I had nothing else, I'd want a high‑pass filter, because it does so much. With the 100Hz high‑pass filter switched in, you can still use the LF control to create warmth, but without getting boominess. Also — and I wish every power amp had one of these switches on it — there's the subsonic filter which, in the case of the PowerStation, works at 40Hz, 18dB per octave. It means that you can do LF boost while completely taming those frequencies below the cabinet's cutoff point. Not many people think about this, but it's so easy to overdrive a bass speaker and produce coloration, or even destroy the speaker by banging it against the endstops all night."

With those points in mind, is there an argument, on PA consoles, for using a bell‑shaped bass control rather than a shelving one, to minimise the amount of really low‑frequency energy?

"Yes, there is, and you'll find that on the Spirit Live 3 and Live 4. However, it takes up more component space, so my feeling is that on our simpler consoles the high‑pass filter alone is the right thing to do, because it takes care of the problem without me having to design a more complicated bass EQ circuit."

I notice that the amplifier outputs are on terminals rather than Speakons. Do terminals comply with the new safety regulations?

Martin Appleby:"Those are shielded so that you can't actually get your fingers onto the metal parts. It also takes banana plugs, and in some of the countries we export to, it means the end user can get up and running without having to look for a special connector. One of our biggest markets now is China, where it isn't easy to get spare parts and accessories."

I've seen the power amp board, and it looks as though you've worked hard to make sure that it's cooled efficiently.

Graham Blyth: "It uses a good old wind tunnel, and the fan is probably stronger than is really needed. The bi‑polar power transistors are, in effect, bolted to a metal tunnel through which air is forced, and we designed in a nice touch, which is a music level sensor circuit, so that as soon as the music stops, the fan slows right down, reducing the background fan noise considerably. The air comes in through a grille fitted with a reticulated foam filter, which can be removed for cleaning."

That makes it ideal for things like acoustic guitar numbers, where you don't want to sound as though you're sharing the stage with a taxi‑ing Tiger Moth!

"Exactly. We also have a thermal cutoff failsafe, but you have to block up the grilles with coats and drive it flat out for a long time to get it to trip. And when you unblock the grilles, it's back on again in 30 seconds or so."

After poking and prodding the PowerStation as much as was decent, we went out onto the Spirit production line to take a few photographs, but Graham admitted that there were certain areas that he didn't want me to see, no doubt because of work being done on new and as yet un‑announced additions to the Spirit range. However, he had no reservations about admitting that Spirit are looking at areas other than mixers, most immediately studio monitoring, with the Absolute 2 monitors. It all adds up to a company which is one of the major success stories of the British music industry, proving that if the flesh is willing, the Spirit is Strong.