Upgrading the Servo-Bass Cable

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
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www.genesisloudspeakers.com
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

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Jun 22, 2010
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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)
Sep 6, 2010
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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.
 
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ca1ore

Well-Known Member
Aug 31, 2010
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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

Well-Known Member
Aug 31, 2010
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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 2, 2012
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San Antonio, TX
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! !
 
Last edited:

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
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www.genesisloudspeakers.com
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

Well-Known Member
Aug 31, 2010
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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

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Jun 2, 2012
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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)
Sep 6, 2010
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Seattle, WA
www.genesisloudspeakers.com
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
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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)
Sep 6, 2010
5,599
225
1,190
Seattle, WA
www.genesisloudspeakers.com
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.
 

colinhtucker

New Member
Feb 14, 2015
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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.
 

tommyb

New Member
Oct 22, 2020
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Hello,

First, let me introduce myself:
My name is Tom, I'm living in Switzerland, near Montreux city. I'm new on this forum, I didn't know it but it's full of very interesting information. My mother tongue is not English, so I apologize if I make mistakes while writing.

I know it's an old thread but I hope someone will still be able to answer me.

I just bought a pair of "Genesis VI" and the previous owner shortened the subwoofer cables to about 3 meters. However, I did not notice any oscillation. As suggested, I replaced the Speakon connectors on the amplifier and speaker sides. I also replaced the Speakon connectors on the cables with new ones.

Since then, the sound is not so good, the bass is like compressed or even muffled, it's like some kind of distortion in the sound I hear. And yet, I rewired everything properly.

I have no idea where it could have come from. I replace it with new and it's worse than before... :(

Anyway, I finally ordered 10m cables (2 x 10 m, unshielded, in 2x 2.5mm for the speakers + 2 x 10 meters shielded, microphone cable for the servo).

On the schematic above, there is a "drain wire for servo shield" which corresponds to the shield braid of my microphone cable visibly.

According to Gary: "+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"

But it doesn't say where to connect this "drain wire for servo shield."

Does anyone know what I should link it to please?

I thank in advance the person who will be able to answer my question. :cool:

Tom
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
5,599
225
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Seattle, WA
www.genesisloudspeakers.com
First, congratulations on your purchase of the Genesis VI.

If you have two separate runs of wire - speaker wire and microphone cable, you do not need to have the servo circuit shielded. I have also done away with connecting the drain wire as with my new servo-circuit, it is not needed. With the old servo-circuit, we attach the drain wire to the servo-bass amplifier chassis.
 

tommyb

New Member
Oct 22, 2020
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Hello Gary!

Thank you so much for your intervention and for taking the time to answer me. I really appreciate it !

The old cable, not original, of 3 meters was made of 2 separate cables. There were 2 braids shield, one for the speaker cable and another one for the servo cable. The 2 braids were soldered together at the negative pole of the speaker cable on both sides of the Speakon connectors.

My new 10m cable is also made of 2 separate cables. However, it does not have a shield braid for the speaker cable but a braid for the servo cable only. As for the 3 meter cable, thinking to do well, I first connected the shield braid of the servo cable with the one of the speaker (1+) at both ends and tried it like this....

I immediately had a very loud and very unpleasant noise (oscillation?) and so I turned off the amp right away. At that moment I thought I had destroyed my speakers. I checked my connections and found no error.

I unsoldered the servo cable shield to try again. I did not get a catastrophic loud noise but a "buzz" noise instead. I finally decided to solder the shield only at one end of the cable. I switched it on again and had that horrible loud noise again. I tried again with the same configuration but this time I took care to turn the volume down to the minimum on the bass amp and I didn't get any more unpleasant noise.

So now the braid is only plugged at one end, on the speaker side, and it seems to work like that. The sound seems better with my new cable but I can't explain why I have to turn the volume down to minimum before turning on the amp.

I've heard Genesis V's in the past and it seemed to me that the bass was phenomenal, much more in line with what I have now. Maybe I'm wrong but I'm afraid I forgot something important?

I also removed the rear bass drivers, both have no sensors, only 2 cables are connected. The one in front has 4 connectors but it looks like a loudspeaker with double coils, I don't see any accelerometer sensors. Do you think this is a normal configuration?

It's a bit long sorry :)

Thanks in advance for reading me.
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
5,599
225
1,190
Seattle, WA
www.genesisloudspeakers.com
Hi Tommy, sorry for the late reply.

Do not connect the braid to anything. If the capacitance between the braid and the signal (either feedback or speaker) is too high, you are going to get oscillation. That may be why you got loud ugly noises. As I mentioned above, the shield drain is connected to the amplifier CHASSIS only. Do not connect it to speaker negative. That would only work if your speaker negative is ground. If your power amplifier is fully balanced connecting the feedback shield to negative could cause destructive resonance.

The rear bass drivers on your speaker are slave. If the one in front has 4 connectors, that would be the active woofer. The accelerometer is under the dust-cap.
 

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