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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.

"The similarities between Rho and my front-loaded designs are:
  • Two-way design for optimum coherence even at short listening distances. Easy to drive (impedance 7 +/- 1 ohm from 100Hz up, above 7 ohms with low phase angle below) and for use with low-power SE tube amplifiers. Even if the 90dB sensitivity figure seems low, it can be used to good effect with an SE 2A3 amplifier. A good SE 300B amp can give wonderful results in larger rooms. If, however, you want to use it with a more current-source-like amp (Pass First Watt, SE tetrodes like some Shindo amps), you can order it with optimum impedance correction at a little higher cost. This can be retrofitted for better bass control.
  • Constant directivity behavior. A broad overlap of tweeter and bass speaker in the crossover region (about 3 octaves) allows me to use the mid/woofer up to nearly 7kHz while the very gentle roll-off (less than 6dB/oct acoustical) starts at about 1kHz. From here the tweeter blends in with the same low acoustical slope and corrects the power response of the bass, which starts to beam above 1kHz.
  • Good time-domain behavior thanks to the low-order acoustical filter slopes and time alignment. The geometrical time alignment is obviously a bit unusual: It does not align the initial time response peaks of the drivers as is usually done by placing the drivers in the same vertical plane. This would result in destructive interference at lower frequencies within the broad crossover bandwidth and therefore produce strong side lobes (destroying the easy room-integration properties of the speaker) and reduce the impression of an 'open' sounding loudspeaker with 'effortless dynamics'. The downside is that the speakers should be adjusted for optimum vertical listening axis, e.g. with pink noise and a good set of ears (there are 8mm threads for spikes in the front while in the back I prefer a single piece of hardwood with proper height).

"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.

"By the way, internal wiring and binding posts will soon be upgraded accordingly and a nice decorative ring placed over the woofer flange for better(?) aesthetics as per customer demand. This will raise the recommended list price to €2.650/pr.

"The differences to my (much bigger) front-loaded horn systems are:

  • Wide dispersion design (less detailed sound).
  • Lower efficiency and lower macro dynamics.
  • Higher distortion (the nonlinear distortion of the best horn systems is actually lower than that of many amplifiers on real music).
  • Uses a two-chamber back-loaded basshorn/reflex cabinet over a wide range to provide proper bass extension for freestanding environments. Small-bandwidth reflex ports with low tuning frequencies in relatively small enclosures result in large group delay and high phase angles, neither of which is ideally suited for low-power amplifiers. The wide bandwidth horn opening keeps the phase angle below 20 degrees and supports the response of the speaker in the lower midrange without the help of a wide baffle around it. This needs to be designed very carefully in order not to radiate strong resonances from within the cabinet. Proper two-chamber design and damping material takes care of this. Another advantage of this cabinet is that it balances the air flow in the low bass region in a nice way. Bass reflex ports have lower radiation impedances on their outward side than towards the cabinet. This results in non-linear port and cone movement. Adjustment of inside/outside opening area relations produces dramatically lower LF distortion than a simple reflex design with the same cabinet and driver size would.

"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)."

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.

"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.

"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.

"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.

"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."

Sidebar: More tech talk by David Haigner: You know the effect of producing a tone by blowing across the opening of a bottle. For larger bottles and longer necks, the tone lowers in frequency. That's the Helmholtz resonator in action (abbreviated: HR). Here's an easy physical comparison to our bass reflex speaker box. The volume inside the port pipe (bottle neck) is a hydraulic mass. The main volume of the bottle presents the compliance (spring). Now set the hydraulic mass into vibration and it will resonate like any spring-suspended mass... continue for two pages