Direct Servo Subwoofers

January 5, 2019
 by Paul McGowan

5 comments on “Direct Servo Subwoofers”

  1. Paul, The two observations I have about subwoofers at the moment are 1) why are no servo-controlled subwoofers included in the top recommended subwoofer lists of the major audiophile publications? and 2) why do so many subwoofer manufacturers think their frequency response for music is adequate if they can reach 20Hz? They must not get any exposure to pipe organ music which requires realistic sound pressure levels at 16Hz. For those audiophiles who have never heard (felt) 16Hz pedal notes in their listening areas, they have no idea what they are missing. Additionally, you never mention the downsides of a servo-controlled subwoofer. There must be some. Additional complexity? Over correction distortion? More failure modes? Higher cost? And how much better is a servo-controlled subwoofer compared to the current best subwoofers like the JL Audio f212v2, JL Audio Fathom f113v2, the MartinLogan BalancedForce 212, or the SVS SB-16 Ultra?

    1. I think the answer to your first question is that there are very few servo subs made and so the ones that get reviewed aren’t servos. The technology is not trivial to implement. I have spent a great deal of my life narrowing down the best ways to do it.

      The downside to servos is their difficulty of implementation, their requirement that the amplifier never clips (if it clips the servo goes nuts), and the enormous amount of amp power they demand to do their job.

    2. Hi SoundWarp, it’s a 32 ft organ pipe you must be referring to when you talk about 16 Hz. Those organs are impressive indeed. But to get to that level in your living room is a different story. You are talking about a “little” 12” driver performing the work of a serious amount of air supply being fed into a hardwood (mostly oak as these lengths of pipe are not made of tin anymore) square box of approx 1 ft x 1 ft x 16 ft in a space (as big as a cathedral) that allows those reproduced wavelengths to travel so far that they are not reflected by any wall prior to reaching the complete wave length, which is 64 ft (approx 19,5 m). Surely that must require a lot of power and control on that bass driver to reproduce and even then you will miss the accustic part of that listening room. I prefer to actually attend those organ concerts. As you described right you actually feel it in combination of hearing that audible part. This is one of those factors that will keep reproduction of life music perhaps close, but never real. Although reproduction is good enough in case it starts memories passing by when you close my eyes. So whether it reaches that 16 Hz or not, a 16 ft pipe organ sound is impressive, hack even a 8 ft organ, and good enough to ge those memories going. For the rest, visit a Church, you get free of that live music, often twice a week, and might also learn that high end audio is relative for your needs in life.

  2. Wow- Never type a long comment on here with an iphone not plugged in. Lost the whole thing so I will try to recreate.

    I have had some good time spent in front of servo subs with accelerometer technology as well as some others.

    Servo exposure #1-
    We had a stereo store in town that had the Infinity RS-1’s on display for years. Wow. Talk about a drooling 14 year old. I couldn’t seem to mow enough lawns to afford those. As I understand and from what Paul said as well, these were not accelerometer technology (Circa 1982).

    Servo exposure #2-
    Infinity Epsilons. These were for a customer of mine. They had built-in Single unamplified 12″ servo drivers, one per speaker. As memory serves, my favorite sounding speaker that I have had the pleasure of listening to. Ever. I was using an inexpensive Yamaha 5 disc carousel with no outboard DAC, inexpensive Parasound preamp and amplifiers (setup required 4 channels so I had 205 WPC @ 8 ohms x4). I can only imagine how good these would have sounded if I had different equipment upstream, although the synergy of the Parasound gear with the Epsilons was stunning. The Epsilons became my Holy Grail, and while I have found some amazing combinations with servo subwoofers I don’t believe that I have been able to surpass the A-Z sound that I achieved using the Epsilons on their own. In my small room at the time (guessing 12’x18′), the only potential issue that I witnessed with the Epsilons was max spl might have been limited to around 107spl. Being a power junkie, I found myself wanting a little more on occasion but would still love to have a pair of these. Based on used prices being high, parts pretty much unavailable as far as I know new or used, I am unwilling to take the chance. Despite the high used price tag, If parts were available I would own a pair. My Holy Grail.

    Servo exposure #3-
    I purchased an old Infinity 212 Servo sub. Huge output. Great subwoofer. I was disappointed that I was later told by more than one source that this model as well as some other Infinity model(s) did not have the accelerometer. Despite, it is a wonderful piece and one I never plan to get rid of. Not sure what if any servo technology this Infinity product has built in? Paul?

    Servo exposure #4-
    I purchased a Rythmik Kit with a GR Research 12″ driver. In fairness, I put this in a small sealed enclosure that was slightly smaller than what was specified (because I already had one made on hand). I was told that I wouldn’t get the performance from it unless I made the box the exact size called for. I refused to believe that making the enclosure slightly bigger would give me the performance that I was looking for, but it was still a very nice sub. Personally, from my experience I prefer the accelerometer technology by a large margin. No comparison.

    Servo exposure #5-
    I was very fortunate and found not one, but two Genesis G-928 servo subs used for sale within 100 miles or so from home. Done. Best subs I have ever owned. Honestly, unless my mains are capable of pushing higher spl than most of what I have had, I just use one of them and it seems to do everything I want it to do. I have blended these with Infinity RS2.5’s, Carver AL-3’s, Vandersteen 2’s, BG Radia LA-600’s, Martin Logan Monolith’s, and they work extremely well with everything. Whatever Paul is putting together with the PS speaker lineup- I am confident that he already has the low frequency servo drivers all figured out. Getting the rest of the speaker to do everything as right as possible has got to be one of the biggest challenges I can imagine to best some of the best speakers on the market today and from the past. I am really looking forward to hearing the results!

    I still don’t feel that I have found anything that I like better from bottom to top than the Infinity Epsilons. Every track played through them had detail I had never heard and smooth as silk- absolutely nothing hard or edgy. These speakers somewhat ruined me. As I understand, these were designed by Cary Christy after Arnie Nudell had left Infinity. I liked everything I have heard that Arnie designed as well, but unfortunately never owned or even heard the IRS’s, IRS Beta’s, Gamma’s or Deltas.

    I had fun recalling and writing this, and hope it helps generate some excitement in all those awaiting the arrival of PS Audio’s AN speaker lineup.

  3. For the control loop (i.e. servo) to work correctly, the accelerometer has to have significantly lower non-linear distortion than the LF driver in the cabinet. If this isn’t the case, the servo will add distortion. Is there any information about accelerometer linearity?

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