Thiel Owners

usound - Thank you for your thoughts. I share your concern about the power ratings, especially into low impedance loads like the (maddening) Thiels. To learn more about the discrepancy between the AHB-2 non double-down performance vs. my intensive auditioning experience with it, I engaged John Siau in conversation. Among the lessons Iearned was Benchmark’s eccentric power measurement protocol which goes like this:

They rate the amp at 0.0003% THD+N into all loads. In other words if it doesn’t shut down, that running spec is met with no allowance for additional distortion. The amp exceeds that 3 zeros spec at the power limits you cite (which obviously decrease into decreasing impedances.) But, if the traditional 1% THD+N spec were used, it would double-down as we want and as I (among others) experience it doing. It sounds and acts like it is doing what John says it is, and BM is very conscientious about its claims. Their internal tests go to 1 ohm continuous and their customer service tells me they are viable to 1/2 ohm resistive loads. All Thiel models are extremely resistive via Zobel networks on all drivers - which push the amp limitation from distortion-limited toward heat-limited. I have shut down the AHB-2 via overheating, but it required louder levels than my comfort zone. Admittedly, my installation maximizes radiation and convection cooling. When John evaluated the load graphs of Thiel models considering phase and impedance, he recommended stereo-only except for the CS7/7.2 = stereo or mono. Due to the BM feed-forward distortion reduction the only advantage of bridged mono is 4dB greater headroom before shut-down, no increased distortion as in all other amps.

I recognize that these claims run counter to everything we know about amp behavior. Have you seen any second-party lab tests using the traditional 1% distortion limits? I would love to see those results. Note that I (among others) have requested a higher-output AHB-2, but John is firm that ’it isn’t necessary’. Perhaps from BM’s perspective of primarily pro market and inability to meet demand, he has a point.

Note I am not arguing that better and more suitable amps aren’t out there. I imagine they are. My personal situation is needing an amp that drives the loads while telling the truth. The AHB-2 does that for me, at a price I can afford.

Here's the AudioScience Review

I recognize that these claims run counter to everything we know about amp behavior.

Ha! Exactly.

What are we going to believe --spec abstractions or our lying ears?

@tomthiel, While Benchmark’s class leading distortion specs are truly impressive, their use of non-standard standards is curious. One has wonder if the difference between the vanishing low distortion measurement standards of the Benchmark compared to the 1% standard measurements is more academic than of practical use to the consumer. I’d be more impressed if Benchmark would spec in writing their power delivery into load with a reduced distortion criterion, than to avoid addressing whether it can deliver the goods for my application. While I don’t doubt your reporting what you’ve been told, the cynic in me doesn’t buy what they aren’t willing to put in writing.

It has bee quite obvious that Stereophile doesn’t measure amps into 2 Ohms unless the manufacturer specs their amps into 2 Ohms. And even then they often only do the 2 Ohm measurements from one channel, even when a stereo amp shares power supplies between the 2 channels. One might question whether or not Stereophile is more concerned with avoiding embarrassment to potential advertisers than providing useful guidance to its subscribers? And if manufactures spec their gear in such a way as to avoid the exposure?

That the Benchmark uses what appears to be a rugged protection circuitry is nice in that it prevents an over stretched Benchmark from causing further damage down the line. Some manufacturers go out of their way by over building their gear to eschew protection circuitry because they believe the use of which has negative sonic characteristics. To be fair the Benchmark’s exemplary noise and distortion measurements might put that argument to rest. Still, I wouldn’t want my automobile or even my washing machine to shut down because it was overstressed, and given the choice after sitting down for a nice recreational listening session, I’d prefer to be able to avoid protection saving shutting down altogether if possible.

The Benchmark comes in at the minimum suggested 100 Watts per channel power rating at 8 Ohms for many Thiels, and struggles to keep up below that 8 Ohm rating, there is no extra power to start with to offer some wiggle room below. A Benchmark could be generously compared to an 80 Watt per channel amplifier that could truly double down. And as we know how the sensitivity decreases with impedance, I don’t think that’s an unfair comparison. The above is regard to it’s stereo performance. In bridged mono mode the Benchmark might be excellent for insensitive speakers with a high impedance load, but as spec’d doesn’t appear at all appropriate for low impedance speakers. The price for the Benchmark for it’s performance within it’s limitations is favorable, but for the similar money there  are other options that might be better suited for some Thiels.

t

@unsound - thank you again. I hear you and am eager to hear about other options. I came to Benchmark via recommendations from high-end recording folks, tried it and like it. I can't explain how or why it outperforms its expectations.

I did connect a dot or two around the power spec idiosyncrasy. Due to its feed-forward error correction topology, the amp simply can't function beyond the limits of that brick wall. It can't be rated at 1% distortion because it can't produce power with 1% distortion.