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Tech talk. As we shall see, the Rho is a very well-performing loudspeaker in the Zu Druid / WLM Diva monitor vein, with more unassisted LF bandwidth than either, more treble extension than the former and a lower price than the latter. Understanding some background will counter predictable knee-jerk reactions based on appearance and price: "The Rho is aimed at those who like low-power amplifiers but do not have the room or money to buy huge horn systems; or simply don't like the direct sound of horns. I like 8-inch / 1-inch combos so I started by trying to match the Audio Note AN-E specs: 92.5dB/2.83V, minimum impedance 4 ohms (for AN-E measurements, see Stereophile's results here). But I do think most amps sound better when they need not drive a low impedance so I settled for the same efficiency (per watt) of 90dB but at a higher impedance of 7 ohms. |
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"The similarities between Rho and my front-loaded designs are:
"Though I am just beginning to understand the influence of very good material engineering on cables (after some "wow" impressions with Kondo gear while doing some initial experiments for a big custom Goto speaker), I do believe in good crossover components and therefore the cost of those parts in the Rho are much higher than that of the main driver. Of course, no electrolytics are inside. |
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"Please note: While this sounds all very technical and is indeed the result of two years of measurements and simulations, I finally base all my designs on long iterative listening sessions. Besides that, my goal will always be to make listener- and space-friendly loudspeakers. 3 watts are enough to power my speakers to mostly satisfying levels. Thinking global while producing local is meant to be helpful in terms of both global warming and local economics. "Looking like arrow heads split in two, the 'bass tuning devices' fit into the port opening, pushed back to the rear with the smooth side along the inner cabinet sides so that the half-arrow ends point towards the listener. These parts are optional at no additional cost and their use should be explained (if they are needed at all) by the dealer who demonstrates the speaker to the customer. They can help to adapt the speaker to the output resistance of SE amplifiers. The simulation results for a freestanding Rho on the floor (no sidewall influence) shows the effect. If you connect the Rho to a good voltage source like a high-feedback solid state amplifier, the bass response at 2.83 volt looks like the black curve (0.4 ohms output resistance). Assuming a moderate output impedance of 2 ohms, you get the red curve, the application of two devices per speaker results into the blue line. Sometimes this can also be helpful to tune the speaker in problematic acoustical environment. As mentioned before, higher output impedances (above the 3-4 ohms of typical 300B SE-amps) should be taken care of by more sophisticated means, a modification we offer at the cost of the additional parts (100 Euro per pair)." |
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I next requested a brief Primer 101 on the differences between ported alignments, tuned pipes, transmission lines and rear-hornloading because the market favors so many hybrids between them that confusion is common. "A short answer to your question about bass cabinets may be the following. Bass reflex usually radiates the smallest frequency bandwidth (abbreviated: BW) usually tuned around the lower cutoff of the woofer. Small BW equals longer time domain ringing and a small, undamped reflex port (pipe) coupled to the backside of the driver via a large volume. "The same goes for a tuned pipe, with usually less ringing if damping material is applied in the pipe. Smaller coupling volume gives larger BW and needs a longer pipe for similar LF cutoff. A true TL should have infinite length and damps the SPL at its port down to zero, being completely filled with absorbing material for zero coupling volume. The behavior is identical to a closed cabinet of infinite volume, providing perfect separation of the driver's front/back radiation (to prevent phase cancellation, the basic rationale for bass enclosures). This gives the best time response. |
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"A back-loaded horn looks and works like a quarter wave TL without the damping but increasing cross-sectional area along its length. This generates maximum BW + SPL but heavy time domain ringing if you don't make it large or accept a high LF cutoff. Also the 'port' output tends to destructively interfere with the driver's front wave above about 100Hz in many designs. Practical examples show that you can easily blend one design into another, however. |
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"Imagine a reflex cabinet with a small volume and a long, slightly damped port - this is close to what modern 'TLs' look like. This combines a quarter wave resonance of the port ('line') with the Helmholtz resonance of the reflex enclosure for maximum SPL from of a given set of cabinet volume and driver parameters. It's always a trade-off between time and frequency response. The Rho was not invented by me but simply combines the parameters a little differently. The hornlike opening enhances BW and SPL while the large two-chamber coupling volume with damping material provides (this was the hard part of the design) the necessary low-pass characteristics and phase behavior of the port output to avoid cancellation with the front wave. It works similar to a reflex alignment in the way that you can tune it to a lower LF cutoff by blocking part of the narrow 'horn throat' with those wooden inserts. |
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"A nearly constant impedance crossover can be achieved if the overlap is wide enough so that the low-pass filter's rising impedance is compensated for by the high-pass' decreasing impedance - nearly. In fact, I had to add just a little bit of impedance correction to achieve the result. And of course this will only work if the filter slopes are symmetrical and if the bass needs no further compensation (of usually rising response far below the x/o frequency, mostly because of the so-called baffle step. The Rho does not need this because of the assistance of the horn opening). After some experiments I arrived at the opinion that we need to do impedance compensation with the same quality parts as the rest of the x/o - which has a price. But then it seems to work well, with no nasty side effects like lower dynamics or reduced resolution as a result of the network's higher parts count. |
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"Actually just today I tried a modification of the Alpha horn's impedance correction. This speaker seemed to be more picky about amplifier LF control than my other models, working well with some and less with other amps of similar design. And I simply could not figure out why. So I started to believe the less-than-scientific talk that big speakers need bigger amps but could not really accept it. The impedance was 7 +/-1 ohm over the whole range from 20Hz up. This should give perfect bass response with any amp (103dB sensitivity)! Now I worked out a way to better couple the correction circuit to the driver's reactance and finally, after years, the problem is cured! We did this at Reno's place, a/b-ing the changes on a cheap Triode Corporation 300B amp a few hours ago. Now the Alpha is ready for 45 and 2A3 SE amps. So I do believe in high-quality impedance correction. It provides a short circuit for the speaker's reactance, 'protecting' the amplifier from this unwanted signal. And, of course, it presents a constant load to high-Zout amps to avoid frequency-dependant voltage divider effects."
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