Upgrading the Servo-Bass Cable

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
#1
The Genesis servo-controlled bass system is a pretty easy one to understand. A little accelerometer is mounted on the voice-coil of the woofer. A circuit on the preamplifier/crossover senses the signal generated by the accelerometer and compares it to the input signal. A differential circuit then generates the difference and this is injected back into the amplification system. In effect, it is a negative feedback loop that goes all the way from the woofer to the input of the amplifier.

There are many debates to say that feedback is bad, but correctly applied, feedback fixes a host of problems. For example, a woofer has weight - and hence inertia and momentum. The servo-system is a "brute force" approach - when the servo-circuit senses that due to inertia the woofer cone is not accelerating as fast as it should be to track the incoming signal, it pushes more power at the woofer. And, if it senses that due to momentum the woofer is overshooting the signal, it pulls the woofer back. The effect is "infinite" damping.

What has all this got to do with the Servo-bass cable?

As the accelerometer is in the woofer, and the amplifier is outboard, the servo-bass cable is inside the feedback loop, and it is the interface for both the woofer driving circuit as well as the accelerometer sensing circuit.

The original Genesis Technologies servo-bass cable used two twisted pairs of 14awg wire for the two different signals. Since the accelerometer signal is very low current, very low voltage (almost like a phono cartridge), and the woofer signal is high current high voltage, magnetic coupling between the two twisted pairs created crosstalk that contaminated the accelerometer signal. Also, the old servo-feedback circuit oscillated when the cable was too short. 10m of 14awg speaker cable to run multiple 12" or 8" woofers was too skinny a cable.

The next version was developed by Audioquest - it was a bit better with two 12awg twisted pairs. However, the magnetic coupling between the two signals was still there, and they still needed 10m of cable to reduce the risk of oscillation in the servo system between the woofer and the amplifier.

Enter Genesis Advanced Technologies. We looked at the servo-bass cable from first principles.

Cross-section detail.JPG

There were two different signals that the cable had to carry - the woofer driving signal from the amplifier to the woofer, and the feedback signal going back to the amplifier. Obviously both signals had to be treated differently.

Since the woofer signal is a high current signal (necessary to drive up to four 8" woofers per channel), we used two 12awg copper conductors for the current. Since the signal would be purely in the bass, and any high frequencies would be noise, we used soft polyurethane as a jacket (soft and high capacitance). The soft polyurethane also help to control any resonance or vibration in the cable.

To carry the low-voltage low-current feedback signal back, we used an 18awg interconnect cable. Again, since the signal is only in the bass, high inductance is a benefit as high frequencies can be filtered by the cable.

To reduce the magnetic coupling between the two signals, we put a shield between the two. But what worked even better to reduce magnetic coupling was to have opposite twists on the inside feedback signal and the outside woofer signal. Geometry is hence important, and we needed spacers to hold everything together.

The result - better bass and shorter cables! No more need for 10m cables to prevent oscillation.
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
3,524
3
38
Monument, CO
#2
Interesting... Why not a large-gauge high-current pair for the driver, and low-current well-shielded (foil) RCA cable for the feedback? Or, if you are making your own cables and only want to run one, you could embed an RCA signal cable in the speaker cable. Magnetic coupling would still be an issue; perhaps a reverse twist as you have now or cross-helical wrap or something like that to help cancel magnetic coupling?

Short cables causing oscillation means the feedback circuit is not properly compensated. An additional. or slightly larger, feedback (integration) capacitor should take care of that.
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
#3
Interesting... Why not a large-gauge high-current pair for the driver, and low-current well-shielded (foil) RCA cable for the feedback? Or, if you are making your own cables and only want to run one, you could embed an RCA signal cable in the speaker cable. Magnetic coupling would still be an issue; perhaps a reverse twist as you have now or cross-helical wrap or something like that to help cancel magnetic coupling?
It was the WAF - it's tough enough having these huge cables (minimum of 2 channels per woofer tower), let alone separate runs of speaker cables and feedback signal cable. That was what I did to try to get the cables to be as skinny as possible. I'm not a big fan of fire-hose sized cables.

Short cables causing oscillation means the feedback circuit is not properly compensated. An additional. or slightly larger, feedback (integration) capacitor should take care of that.
Yes, that was one of the things I did.
 

ca1ore

New Member
Aug 31, 2010
52
0
0
#4
Reviving a sleepy thread ..... I've always been a bit skeptical of 'upgrades' as I dont think they always offer adequate value; however, upgrading my old Audioquest cable for these newer servo-bass cables was most certainly not the case. Perhaps the best upgrade $$ I've spent in a while. Not just sound improvements, but getting rid of the massive coils of extra cable has saved me a few bruised toes. Highly recommended :)
 

ca1ore

New Member
Aug 31, 2010
52
0
0
#6
Hi Ca1ore,
Where did you get the upgraded cable and how much was it? Thanks !
Hmmm, don't recall for sure. Lyric in New York, I think. It has been probably 5 years at least. I recall paying about $1,500 for the 2 meter length, and then reselling the long Audioquest sets for $300 or so. One of the few times that changing cables in my system resulted in a clearly apparent improvement (yes, I am a cable skeptic, mostly).
 

Odysseus

New Member
Jun 3, 2012
24
0
0
San Antonio, TX
#8
Thanks Ca1ore and Gary for your replies! Gary, I would never ask this to another manufacturer of high end audio equipment, but you seem like a very cool guy who genuinely cares about his customers and audio fans in general: I can't spent $3,500 in cables, is there any advice you can give me to make some decent DIY cables to use with Genesis 350se? (they didn't have the original cables when I got them for Christmas last month) I don't mind running two sets of cables (for the woofers and the servo), but I would prefer not to have 25-30 feet runs of cable. THANKS! !
 
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garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
#9
Odysseus, what a fabulous Christmas present you got!!

To make your own servo cables, you'll need some Neutrik NL4FX connectors, speaker cable and interconnect cable.

On the Neutrik connectors, the terminals labelled +1 and -1 are used for the voice coil and you would use speaker cable for this. The terminals +2 and -2 are used for the servo-feedback circuit and you would use interconnect cable for this.

The accelerometer has a resonance of 1kHz, and an impedance of 35k ohms. The input to the servo-feedback circuit has an input impedance of 75k ohms. With this, you do not want to use high-capacitance coax interconnect. Use something with shielded twisted pair, otherwise you might pick up huge hum.

The woofer circuit has an impedance of 2 ohms, so you'll need at least 2 x 14awg or even better 2 x 12awg. An example might be Mogami W3104.

Unfortunately, the bass quality would depend on the quality of both the speaker cable and the interconnect cable, plus one additional complication. Get it wrong and if the system oscillates, the result could destroy both your woofers as well as your servo-bass amplifier if you don't get to yank the power cord in time. When you try this out, at least for a while, make sure that the power cord of the servo-bass amplifier is close at hand. Sometimes when it goes into oscillation, the remote control won't turn it off properly.
 

ca1ore

New Member
Aug 31, 2010
52
0
0
#10
Wow! Lyric must have been more than 7 years ago. They haven't been a dealer for a long time. Current retail on a 4-channel 2m set is $3,500.
OK, fine, you made me go look (I'm OC, have kept a record of everythng I have bought, where, when and how much since 1974). It was from Lyric, but in mid-2006 and $2,200.
 

Odysseus

New Member
Jun 3, 2012
24
0
0
San Antonio, TX
#11
Great information, Gary, thank you! !! Another question; do both woofer cable and servo signal cable need to be of the same length? If not, which one is the one that should be very long to avoid oscillation?

I also noticed that both servo signal cables in the 350se are soldered to a single terminal in the woofer (the terminals that come from the accelerometer inside the dust cap) why would that be? I was expecting one cable to be soldered to one terminal and another to the other terminal.

Thanks again, you ARE a pretty cool guy =)
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
#12
Great information, Gary, thank you! !! Another question; do both woofer cable and servo signal cable need to be of the same length? If not, which one is the one that should be very long to avoid oscillation?
You can also add a 0.047uF film cap across the servo-feedback input - then both cables can be as short as you like. However, the older cables oscillating was also caused by the wrong design of the cables. Built as I have suggested, you should have no problems as long as they are at least 3m long.

I also noticed that both servo signal cables in the 350se are soldered to a single terminal in the woofer (the terminals that come from the accelerometer inside the dust cap) why would that be? I was expecting one cable to be soldered to one terminal and another to the other terminal.
What?? That's wrong. In the G350SE, the 4 woofers are in two parallel pairs. Only one woofer in each pair is "active" - our terminology for accelerometer controlled. The other woofer is also active, but slaved in parallel to the feedback woofer. Inside, there should be two pairs of wires - the blue/white should be the servo-feedback and the red/black should be the woofer-signal.

Thanks again, you ARE a pretty cool guy =)
Just glad to help. With the time and effort to source the right cables and test and build them, you might end up costing about the same even with decent quality cables - but as a DIY guy, you'll have more fun!
 

RLRB

New Member
Mar 7, 2013
4
0
0
#13
Hi Gary,
i have genesis Vi speakers,
i shorten the cable to 3 m aprox and the bass is more fast, i mean the old audioquest cable sr 1404.
please explain clearly how to put the film cap, between + and - feedback cables? i am no a technician
wich awg for signal cable, you mention braided and shielded but wich awg? for 2 or 3 meters? Running in paralel (braided and shielded) inside the same nylon jackets would be a bad idea? the AQ is shielded with mylar
would a ferrite prevent oscillation?
i think this issue is with bigger speakers than genesis v or vi, i mean more watt than the amp provided with genesis vi? perhaps? in some discs i hear some bass artifacts i suppose due to recording.
Another thing, can i leave connected both rca and xlr in servo amp, of course one of them (the connected to home cinema off)
And the last one, what about replacing caps?

thanks, best regards, Raúl


Art Audio Mestro monos
AudioResearch Ls 15
Classe cdp 102
Harmonic cyberlight P2A
Silver braided XLR IC
Transparent musiclink xlr to bass
Kimber tweaked with core cotton to speakers
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
#14
Hi Raul - to add a cap, solder it between poles 2+ and 2- inside the amplifier where the servo-bass cables connect.

However, the G-VI has a different servo amp that I haven't ever seen or taken apart, so it might not work and might react different with your speakers. I'm not sure what you can do. Best would be to have a competent local technician look at it.
 
Feb 14, 2015
7
0
0
#15
Does this mean that the original servo cables can have the Neutrik connectors replaced? I sense from your comment that it is doable and a DIY project. Being a technical incompetent, can a shop that would do this be recommended? Seems tragic to set an amateur like me loose on such a nice cable.
 
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