Quad S-2 loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Quad S-2's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses. My estimate of the Quad's sensitivity was 86.9dB(B)/2.83V/m, confirming the specified figure of 87dB. Fig.1 shows how the impedance and electrical phase vary with frequency. Though the impedance has a minimum value of 4.17 ohms at 190Hz and there's a combination of 5.8 ohms and –37° phase angle at 117Hz, the S-2 is a relatively easy load for amplifiers to drive.

The traces in fig.1 are free from the small discontinuities that would suggest the presence of panel resonances. However, when I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I did find a strong mode at 500Hz on the sidewalls (fig.2), with several lower-level modes present on the top panel. I note that Ken Micallef didn't comment on there being any midrange congestion, so it's possible this behavior measures worse than it sounds.


Fig.1 Quad S-2, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).


Fig.2 Quad S-2, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The crossover between the woofer and the ribbon tweeter appears to be set at 3.3kHz, with fairly steep high- and low-pass filter slopes (fig.3). The woofer alignment is slightly underdamped, with the port on the rear panel (blue trace) tuned to 38Hz, the frequency of the expected notch in the woofer's output (red). The port rolls off cleanly above its passband, with no significant resonant peaks in its midrange output. Fig.4 shows how the outputs of the individual drive-units sum in the farfield, with the response averaged across a 30° horizontal window centered on the tweeter axis. The rise in output in the upper bass will be due in part to the nearfield measurement technique, but the S-2's woofer alignment is somewhat underdamped. As KM writes, "bass was juicy, rich, practically tumescent." Ahem!


Fig.3 Quad S-2, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer (red) and port (blue), respectively plotted in the ratios of the square roots of their radiating areas below 350Hz and 600Hz.


Fig.4 Quad S-2, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses plotted below 300Hz.

Higher in frequency, the Quad's farfield response is impressively flat, with a very slight rising trend evident between 1 and 12kHz. The horizontal off-axis behavior is also well controlled (fig.5), the ribbon tweeter maintaining its top-octave output up to a reasonable 15° to the speaker's sides. The ribbon tweeter is quite directional in the vertical plane (fig.6), and a suckout in the crossover region develops more than 5° above and 10° below the tweeter axis. For the optimal balance, the Quad S-2 needs to be used on stands that place the listener's ears level with the tweeter.


Fig.5 Quad S-2, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.


Fig.6 Quad S-2, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

In the time domain, the speaker's step response on the tweeter axis (fig.7) indicates that both drive-units are connected in positive acoustic polarity. Although the tweeter's output leads that of the woofer, their outputs meld relatively well, even if this graph does suggest that the optimal blend occurs just below the tweeter axis. The cumulative spectral-decay plot on the tweeter axis (fig.8) is superbly clean, which correlates with KM's having found that the S-2's treble offered "purity, clarity, and extension."


Fig.7 Quad S-2, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).


Fig.8 Quad S-2, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

I have known Peter Comeau, director of acoustic design for IAG, since his days at Heybrook in the 1970s. The Quad S-2 is a worthy addition to his loudspeaker portfolio.— John Atkinson

US distributor: MoFi Distribution
1811 W. Bryn Mawr Avenue
Chicago, IL 60660
(312) 738-5025

Axiom05's picture

"Although the tweeter's output leads that of the woofer, their outputs meld relatively well, even if this graph does suggest that the optimal blend occurs just below the tweeter axis."

From the graph, how do you determine that the optimal blend is just below the tweeter axis? Thanks!

John Atkinson's picture
Axiom05 wrote:
From the graph, how do you determine that the optimal blend is just below the tweeter axis?

That very slight discontinuity in the step response just below the time axis at approximately 3.8ms suggests that the woofer's output needs to be moved forward in time a little. This could be achieved by moving the listener's ears down or the speakers up an inch or so.

John Atkinson
Editor, Stereophile

Axiom05's picture

I see that this same discontinuity is seen between the tweeter and the midrange of the B&W 800 Diamond and the 802 D3. This is consistent with the fact that the intended listening axis is just below the tweeter axis and why these speakers usually need a little forward tilt to sound best.

supamark's picture

those are some unusually dimensioned rooms... your building must be very very old.

ken mac's picture

No, really.

supamark's picture

Old houses/buildings always have weird dimensions. Everything is a "one-off" design. Hope (for your sake) it's rent controlled lol, that's become some mighty expensive real estate. Visited Manhatten before Guiliani was mayor, was a fun and interesting place (that sadly no longer exists).

Charles E Flynn's picture

From page 3 at http://www.quad-hifi.co.uk/upload/files/manual/20150817100845_23.pdf :

Quad crossover networks separate the treble and bass networks into two distinct sections, each connected by a dedicated pair of terminal posts. The enables the treble and bass components to be separately driven for optimum performance.

ken mac's picture

I would've loved to bi-wire these monitors for the review, but I have no bi-wire cables. Something to consider, for sure...

Charles E Flynn's picture

AudioQuest has an interesting set of recommendations regarding bi-wiring at http://www.audioquest.com/resource_tools/downloads/literature/learning_modules/Understanding-BiWiring.pdf .

They make two potentially strategy-altering points that I have not seen widely discussed :

"Is BiWiring so important that you should spend twice as much on speaker cable?

BiWiring is actually a way to get higher performance for the same expenditure. The BiWiring question is not about how much money to spend, but how to maximize performance and value."

Later, the article has a diagram showing how a speaker that is capable of being bi-wired should be wired if it is not being bi-wired. The diagram is contrary to what many speaker makers show in their instructions. I asked a local dealer about this diagram, and he replied that "Everybody at the trade shows knows about this and wires their speakers that way." The relevant section is titled "Using Full Range Cables On BiWire Capable Speakers".

spacehound's picture

That well-known purveyor of snake oil sold via pseudo-scientific total gibberish, just wants to sell you twice as much of its overpriced lengths of wire.

"Full range cables"?

At the frequencies that even the best speakers are capable of even a straightened coathanger is "full range" and will supply far more current than any speaker will ever need.

Ortofan's picture

... now:

Charles E Flynn's picture

Thanks for taking the trouble to find the current link for the Audioquest article about bi-wiring. I think it is worth saving the pdf for future reference.

DougM's picture

Seems to be an awesome little speaker for $999. I like the way many more manufacturers are using ribbon or AMT type tweeters these days. And, the IAG family of companies is really making some great affordable stuff, another example being the Diamond series from Wharfedale. I have the older Diamond 9.1s, and they sound great. And you gotta love a reviewer who uses Chet for some of his test discs. Great Taste!

ken mac's picture

I had the 10.2, absolutely wonderful speaker. Chet is the best!

Charles E Flynn's picture


ken mac's picture


mrkaic's picture

I have pair of slightly cheaper Quad S-1's and I drive them with a Quad VA-One. One gnarly combo, warmly recommended.

spacehound's picture

There are so many of these around at all sorts of prices that the speaker industry will soon be able to offer every hifi enthusiast in the world a different one.

Long-time listener's picture

"Both recordings presented nonfussy senses of immediacy and intimacy."

Senses? Excuse me, but hearing and sight would be two different *senses.* But since the immediacy and intimacy you speak of both came to you via your hearing, why not write, as everyone else does, "a wonderful sense of immediacy and intimacy." Forest here, not trees.

gbougard's picture

These speakers look yummy as hell, yet after reading your review, I don't know if those are for me
I produce Jamaican music with Sly & Robbie, one of the world's HEAVIEST rhythm sections, and love to listen to Reggae, Dancehall Hip Hop and Funk.
All these musics are bass and drum-heavy and the music you use to evaluate the speakers is not.
So do you think you could pop a record like DUBRISING by Sly & Robbie and crank it all the way to 11 and report back to me with your impression on how these speakers behaved when subjected to real drum and bass and heavy beats.

iListen's picture

Which integrated would you pair with these?
Quads own? or the Sphinx?

toofastdad's picture

I just ordered a pair of Quad S2 speakers, do you think that pairing them with a NAD 3020 will have favourable results?

toofastdad's picture

Using the above combo Sly & Robbie sound amazing, the bass is very warm and rich — great speakers, I'm really pleased with my purchase. I just ordered a Musical Fidelity M2Si amp which I guess will make these babies sing. Joe Cocker's Sheffield Steel sounds really good (Sly & Robbie on drums and bass).

Oldoiler 1963's picture

You probably already have them and are enjoying them by now. I use a 20w Cary amp and the sound is amazing.