Speakers have a rated impedance. Most are rated at 4, 6 or 8 ohms. There’s a catch word in the spec that says “nominal.” That means average. Speakers, in fact, can vary dramatically in their impedance, given different frequencies.
This matters because all amplifiers are happy with 8 ohms. Most are OK with 4 ohms. However some amps have switches to throw on the back for a 4 ohm impedance because they’re not stable enough into that load. This means it’s vital to pay attention to what amp you have, if your speakers present a difficult (sub 4 ohms) load at some frequencies.
What gets dicey is when a speaker’s impedance falls below 4 ohms. Most moderately priced amps, and many expensive amps, simply don’t like 1-2 ohms. Their power goes well below their rating into these impedances. Further, it’s very possible to damage your amp if you try to drive a 1-2 ohm load at high volumes for any length of time.
Bryston is among the very few companies that actually does an impedance graph on its speakers so you can see that it will be safe with your amp. Here’s a graph of Bryston’s A3 ($3290 per pair). A3 is rated as a 6 ohm speaker. When you study the graph you can see it starts at about 5.5 ohms at 20Hz and goes up with frequencies. Please note that none of the impedance is below 5.5, which means your amp will have NO PROBLEM driving this load. Bryston’s rating is VERY conservative given this benign load.
Let’s compare Bryston to some of the other guys. Well, wait. Who else publishes an impedance curve- where they’ve actually run the test? Very few competitors do. All most companies do is tell you the speaker is 4 or 8 ohms. That doesn’t begin to tell the story. Let’s do some digging.
In March 2024 British hi-fi news, there is an actual TEST that shows my point. On p70 they start a review of the Sonus faber Lumina V Amator, $3000 per pair, so it’s apples to apples with Bryston’s A3. SF rates it as a 4 ohm speaker. But is it?
As you read the fine print of the TEST that was taken (p73) some interesting results come to light. SF ran 2.8 ohms at 104 Hz, right in the bass where you need more power. The impedance falls to 1.15 ohms at 85Hz, and 1.7-2 ohms at 15 kHz in the high end. The very fine print says with a whisper that these speakers “demand a very capable amplifier.”
Let’s have a look at the Kef LS-50 Meta, reviewed in Stereophile, Dec 2020. It runs $1500 per pair and is made in China. While Kef rates it at 8 ohms, actual tests show it to average at 4. They measured 1.66 ohms at 135 Hz, and 1.7 ohms at 660 Hz.
Now, both Sonus faber and Kef make good sounding speakers. But what these impedance TESTS show us, is that you have to be much more mindful of which amp you choose, to run SF or Kef, than Bryston.
Since A3’s impedance is mostly at 8 ohms or more, virtually any amp on the market will be safe in driving them. Amplifiers have different sounds, so you’ll be rewarded for buying a better amp than lesser amp- but at least the impedance CAPABILITY of the amp isn’t in question.
If you choose to buy SF or Kef, you’d better be sure your amp is stable driving an ohm or two, because that’s what your amp will be seeing at some frequencies. If not, you’ll be getting a fraction of the amp’s rated power, or worse, could be running that amp into a brick wall. It’s the advertising department that creates the sales brochure.