Improving Bass/Mid-bass Coherence


WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
Seattle, WA
The large Genesis Technologies loudspeakers have an external servo-controlled amplifier for the woofers. The usual complaint about these speakers is that the bass is "slow", or that the bass is not integrated with the midrange - although there have also been reports of excellent coherence. The cause of these problems can be primarily attributed to the way that the servo-amp is hooked up.

The old Genesis mandated that the servo-amp that drives the woofer tower be driven by the preamplifier. The power amp driving the midrange/tweeters is also driven by the preamplifier.

There are a couple of problems that could arise:

1) if the two amplifiers have a different sound, it can be heard as a different character. This is especially true of some types of tube amplification used for the midrange/tweeters.

2) because the power amp drives the midrange/tweeter through a crossover, and the servo-amp (which also has a built-in crossover) drives the woofers, the difference in group delay could be heard as "slow bass" - the Genesis servo-amp has a phase adjustment, but the phase can only be delayed. Most times, as the crossover of the loudspeaker has a greater phase shift, it is the power amp that needs a delay.

The solution for this is to drive the servo-amp using the speaker-level output of the power amp. However, the servo-amp only has XLR and RCA inputs, and the high-voltage output would blow out the inputs. The following circuit can be used to attenuate and balance the speaker-level signal.

SBJ Circuit.jpg

Use a long shielded twisted pair interconnect (some capacitance and low inductance on the left side of the circuit is useful), terminate with bananas or spades, and hook this up to the speaker input binding posts of your Genesis speakers. I usually recommend the speaker input binding post instead of the amplifier speaker output binding post because if the speaker cable driving the midrange/tweeter has a character, you will want this to be picked up by the woofers too.

On the left side of the circuit, place as close to the XLRs as possible - we build it inside the XLRs when we sell this as a cable upgrade. XLR pin 1 is shield, and can be chassis grounded. Make sure R1 & R2 are well matched and sound good. I use a Vishay metal film.

While this will vastly improve the coherence and sound character between the midrange/tweeter and the woofers of the old Genesis speakers, it does not address the crossover inside the servo-amp and the crossover inside the speaker. To get that last "bit" of improvement, you'll have to buy a new pair of Genesis loudspeakers, or upgrade both the crossover and the servo-amp.

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