Shure 701 Pro Master

“What on earth…!?” you might be thinking when stumbling across this article, and even more so if you have been following this blog and perhaps thought you had me pegged as a straightforward audiophile who just happened to be into vintage gear. What on earth is a pair of Shure 701 Pro Master (that was obviously built for public address) to do on an audio blog that is dedicated to the enhancement of domestic listening pleasures? Well, to be absolutely honest, I was a little surprised by this acquisition myself. I guess, some little voice had whispered to me that these anno 1979 PA speakers would be worth the effort of exploration, simply because they would yield new experiences on many levels.

For one thing, the Pro Masters featured a 15-inch paper-cone woofer that was similar to some highly relevant Tannoy and Altec classics. From experience, I had learnt that paper was a decent enough construction material to build natural sounding dynamic drivers. On the other hand, more often than not, I had been warned by my audiophile friends that 15-inch drivers would be far more difficult to integrate with the room, as their greater dimensions could more easily excite room modes that would in turn limit placement choices. I had therefore thought it best to first experiment with speaker placement on the affordable level of the Shure Pro Masters before finally taking the plunge on their more expensive domestic cousins.

In addition to the new bass experience, the Shures featured a radial horn driver with adjustable angle that would offer fresh insights into horn-loaded designs as they were found on many renowned Hi-Fi and studio loudspeakers built by Altec Lansing, JBL, TAD, Electro Voice, Fostex, among others. Horn designs offered the benefit of high sensitivity output even at the low voltages produced by small tube amplifiers. In fact, the Pro Masters’ 102dB in sound pressure (measured at one watt power input and at one meter distance from the speakers) could mostly be attributed to the horns of the high frequency drivers. While the Shures’ horn principle may not have been the most delicate on the market back in the 1970s, it would allow me to learn more about the sonic characteristics of horns in terms of minimum distance, sweet spot, room modes and reflections, and other parameters which would be difficult to judge without first-hand experience.

For the pair of 1979 Pro Masters presented here, I ended up driving all the way up to the infamous Marburg “Hinterland”. Their previous owner Micheal was an ageing audiophile and collector of loudspeakers himself and in the advertisement had described the speakers as being fully functional. Hence, I was quite optimistic when I arrived. I found the speakers set up in a furnished garden hut, in which Micheal had compiled his third or fourth system. A quick listening tests showed them to play music. However, neither the positioning on a table nor the room itself allowed for a deeper tonal analysis. There was pretty much a lack of everything, and so I decided to trust his verdict that they were still fully intact. We carried the Pro Masters to my car and spent the next few minutes listening to his current basement horn system.

Back at home, Sabina helped me carry the heavy speakers up our winding stairwell. I quickly found a spot for them, and we ended up parking them there for a few weeks to finish some other projects. When I finally came back to the Shures, I quickly set them up on two stubby stepladders. I connected them to our Dynavox VR-70 tube amplifier which was fed from our Marantz CD-17 and Cambridge DAC on Restek V1 combo. Having been fully broken in, the Dynavox VR-70 (still with original Chinese tubes) was capable of producing a tonally and spatially accurate musical image. The big question was, if the amp would be able to offer the same dexterity with the Shure Pro Masters connected instead of our EPI 500 loudspeakers. Having limited experience with PA speakers myself, I really had no idea what to expect next, but I was prepared to keep an open mind.

Our upstairs listening room was a little special in terms of dimensions. It was almost 13 meters deep, 10 meters wide, and over 4 meters high in its centre. There were no two walls running in parallel. With all this space available, it may come as a surprise that the only area reserved for listening was a unilateral triangle of about 2 meters in length with a thick piece of carpet laid out at its centre. Somehow the juxtaposition of short listening distance and the massive delay of the room made it relatively easy for the brain to filter out reflections from the music source. Hence, this was usually not a poor choice of venue for testing loudspeakers. However, when setting the Shure PA horn-loaded speakers in motion for the first time, I realised that listening at this short range would be a real challenge.

The horn’s design principle naturally caused high levels of compression at short range that could easily overstrain the ears. Luckily, some previous owner had added Visaton LC57 attenuators to dial back the horns. This worked quite well, indeed. I then started experimenting with the horns’ dispersion angles and adjusted them from formerly 120 to now 60 degrees. At short range, this made the treble sound less bloated and more musically effortless. It was possible that this observation would have been different at greater distance from the speakers or other room dimensions. To my utter surprise and delight, the Shures’ horn drivers did not only sound large and impressive but were also tonally sensitive and dimensionally accurate. The Pro Masters were capable of natural transients and even tonal nuance despite delivering the output to entertain an event.

A lot has been said (and written) on the discrepancy between the relatively small size of horns in relation to their 15-inch woofer companions, as found on many professional audio and domestic designs. This naturally takes us to the subject of bass performance. It seems that on PA loudspeakers, 15-inch drivers with rippled cone surrounds were used as 'midrange' chassis that were also capable of extending into the upper bass frequencies. This was a significant finding when it came to the expectations one might have towards such designs. Coming from domestic speakers, I would have suspected a 15-inch driver to act as a subwoofer that would only with great difficulty reach the higher crossover frequencies when coupling to the horn. However, listening to the Pro Masters perform, I quickly understood that I would have to dial down my expectations in terms of bass extension. 

The Shures played an excellent treble and even created a believable midrange, perhaps as one might expect from listening to a home concert, and yet bass was more hinted than it was fully executed. This was also the moment when I started to feel confined by my up-close listening position. I wanted to have a solid back wall behind me to more fully appreciate the slapping of the large cones. In order to find out more about the Shures’ tonal balance, I would need to give them an audition in our main listening room in which my listening distance was at five meters, with the back wall about 90cm behind me. To make this happen without getting in trouble with my wife for aesthetic reasons, some dedicated stands would need to be built first. I drew up a quick design and went to buy some wood and paint from a local DIY store. On my way home with my bundle of carpenter board and solid wood beams, another question came to mind: What made these horns sound so great, and what was the crossover design for the woofer?

Back at home, I decided to first open the speakers to find out what was inside. The large woofers were held in place by eight screws that were professionally countered from the inside by threaded drive-in nuts. This would make woofer replacements an easy task, and I could already see why. As it turned out, one of the woofers had a crack in the cone and had already been taped from the inside. The crack had hardly been visible from the outside, and I had a strong sensation that Michael the pensioner and loudspeaker collector from Marburg's Hinterland had in fact sold me a damaged loudspeaker without telling me about it. In my years in Hi-Fi, I have come to understand that there is a generation of people who have learned to twist the truth in any way they like, even to the point where they start to believe it themselves.

Luckily for me, the damage did not render the bass driver useless. With a fresh strip of black tape in place, the driver could still be trusted to perform for a number of years. And, to my surprise, I saw that the woofer was attached straight to the terminal without passing any kind of crossover. While this would leave it up to the driver's cone and its spider's mass and tension to reduce unwanted high frequencies, it also lessened the likelihood of phase issues, as long as the drivers were positioned correctly to begin with. I was strangely reminded of the Orbid Sound Pluto's (kit) design, which seemed to go by the same method, even though their much smaller bass drivers had great difficulty in losing the higher frequencies and therefore contributed to a squeaking midrange.

Following the inspection of the woofer (seemingly a 15" Eminence-type with the typical square magnet and three magnet screws), I unmounted the Shure's horn driver and noticed that a number of audiophile improvements of the crossover and mounting had been made in comparison to the original design. The capacitors had been replaced with modern audiophile foil caps. In addition, a variable Monacor DSP-1 resistor had been installed to protect the horn from damage in the event of unintentional power spikes. The horn was no longer screwed onto the cabinet's rear wall. It was instead placed on a wooden support with foam cushion. This would minimise some of the back wall resonances and prevent these from afflicting the horn's diaphragm.

Some prior owner had also attached self-adhesive bitumen mats to the interior walls; however, they had already become loose in many places and had begun to slide down the sides. I decided to take them out completely in order to once again reach that warm, vintage, cabinet sound of a resonating wooden box. If the original intension had been to reduce cabinet resonances, some simple internal struts would probably have done a better job anyway. I decided to hold on to the bitumen mats just in case I would change my mind upon hearing the speakers perform.

Building the stands for the Shures was a simple enough task. We started out from two 16mm carpenter boards that would form the bottom plates of the stands (32mm in thickness) and to this attached three 360mm beams of 80x55mm in strength in a triangular position. We then screwed the top plate onto the beams, again made from two sheets of 16mm carpenter board. I was hoping that this construction would allow for more freedom on the side of the speakers which would in turn result in a more pleasant tonal representation. Three spikes would isolate the stands towards the floor. And, on the side of the speakers, I would replace the four original plastic feet with three rubber cushions made for professional music gear. I had already noticed that three supporting points often produced a better defined coupling between materials and was therefore easier to execute than four spikes or cushions on which the weight distribution could be less than ideal.

When I set up the speakers in our main listening room just a few days later, I was actually feeling hopeful that the combination of greater listening distance and better stands would also lead to superior imaging, and that the boxy shape of the room would do well to accentuate bass response. To my surprise, however, the Shures did not sound any more bass-heavy than they had done before. Unless they were used for public address, where they would be called upon to transport vocals over a great distance, these speakers needed a subwoofer in order to capture all aspects of natural instruments. My next task would be to find a sensible sub to pair with the speakers, and thus I found myself crawling even deeper down the rabbit hole. In order not to get lost on the way, I decided to see an expert on professional audio solutions, hoping that he would aid me with my difficulties.

As I found out from running a few web searches, there was an audio enthusiast by the name of Rainer Weimann who operated a part-time sales office southeast of Darmstadt in which he sold affordable audio gear to small-time local musicians, entertainers, and organisers of events. Over the years, he had established a reputation as distributer for the Chinese brand Dragon Audio. Founded in Shenzhen in 2004, Dragon Audio had grown into a major OEM producer of audio gear with over 1,400 staff working in 14 factories. I was specifically interested in an active Hi-Fi subwoofer made by DA that Rainer had listed on an auction site. The other product I had seen on sale in Reiner's shop was an active 15-inch Vonyx PA subwoofer. Vonyx was another audio company that had made a name for itself serving the lower end of the market. Ideally, it would be possible for me to listen to the two subs in direct comparison.

Upon arriving at 'ProSchall', as Rainer's shop had been named, I was given a warm welcome and invited inside. I immediately recognised some of the products I had seen offered online. The two subwoofers were standing side by side, right next to some even larger specimen. Rainer explained that the 15-inch Vonyx PA subwoofer was designed to power public venues and would sound best when driven at high levels of sound pressure. It had a rigid suspension that could handle high volume music program but would sound flat and overly dry when played at living room volume. The Dragon Audio woofer, on the other hand, had been designed to be played in a household environment. It had a softer suspension that would allow the driver to operate accurately even when played at low volume.

Seeing that I was not yet fully convinced, he quickly connected the 12-inch DA woofer for me. The sound it produced was instantly familiar from listening to Hi-Fi gear. I could imagine that this would play well with many types of speakers. Rainer then connected the 15-inch Vonyx woofer, and immediately the room sounded too small for the hard and dry bass of the woofer. The music could be physically felt in the room more than it could be heard, and it was easily conceivable that the neighbours would derive greater benefits from the output of this woofer than we could in the listening room. It was at that moment that I could quite literally feel the difference between Hi-Fi and PA speakers. Either of them would perform like a fish out of water if it was employed outside of its intended venue. I thanked Reiner for the wonderful experience and, with a deeper understanding of the matter (and one Dragon Audio subwoofer called 'The Art of Sound' in the trunk), I made my journey back to Frankfurt.

From Reiner, I had also learned that professionals often preferred to have the vocal section separate from the bass drivers, because this would reduce the load on the amplifiers, decrease the likelihood of damage to the delicate top-end, and allow for the perfect number of bass drivers in relation to the size and sonic characteristics of the venue. This made sense of course, considering how long it took me to integrate simple 3-way speaker systems in our listening rooms. PA speakers could be used for a whole range of venues, from outside gigs on which massive amounts of bass were needed, to boxed-in basements in which low frequency waves were accumulated from being reflected back from the walls. In our living room, the 12-inch DA woofer turned out to be more than sufficient for filling in the Shures' missing bass. For the first time, the tonal balance was as one would expect from speakers of this size. As you might imagine, the following movie night turned out to be a real treat for the whole family, with the Pro Masters effortlessly rising to breath-taking volume at an instant and the subwoofer giving the sound effects their palpable bass dimension.

< Watts, Sound Pressure, Decibels | Shure 701 in Public Application >

Specifications

• Type: full-range public address speaker
• Principle: 2-way, front-ported bass reflex
• High frequency section: radial horn driver
• Low frequency section: dynamic 15-inch woofer
• Special features: 60° / 120° adjustable horn
• Power handling: 150 watts RMS / 55V peak
• Sound pressure level: 102 dB (at 1W / 1m)
• Frequency response: 50 Hz to 15,000 Hz
• Crossover frequency: 2,000 Hz, 18dB
• Nominal impedance: 8 Ohms
• Horizontal sound distribution: 60° or 120°
• Vertical sound distribution: 90°
• Operating temperature: -7°C to 43°C
• Cabinet: 15.9 mm multiply wood, vinyl coated
• Original weight: 26,4 kg per speaker
• Dimensions: (H) 700mm; (W) 585mm; (D) 405 mm
• Country of manufacture: USA
• Year: 1979

Updates

• Terminals for bananas and spades
• Visaton horn volume attenuator
• Audiophile horn crossover
• Monacor DSP-1 resistor