Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

Here is a plot of the no load and 6-Ohm load THD vs. output level for the AHB2. As stated above, the 6-Ohm bridged mono load is equivalent to 3-Ohm stereo loads on each output simultaneously. Note that the 6-Ohm and no-load curves are virtually identical. Also note that the THD is always better than -120 dB relative to full output and at lower power approaches -140 dB relative to full output. If you do the math, you can calculate the SPL produced by the distortion signal given your speaker sensitivity. In all cases, the calculation will show that the distortion will be reproduced at a level that is always below 0 dB SPL at 1 meter from the speakers. This means that the distortion signal could not be heard in isolation in a quiet room. For this reason we can say that the AHB2 produces no audible distortion at any output level below the clip point.

In contrast, the JC1 (suggested by George) produces 0.15% THD at full output which is only 56 dB lower than the output signal. This means that the distortion produced by the JC1 will be reproduced at a level that is well above 0 dB SPL. The distortion produced by the JC1 should be audible.

However, in George's defense, I should point out that he is correct in stating that 100 W is plenty of power (20 dB above 1 W) with reasonable speaker sensitivity in a small room. Add 20 dB to the speaker efficiency to calculate your peak SPL. But keep in mine that this is peak and not average. With music, your SPL meter will typically read about 12 dB lower than the peak SPL due to the "crest factor" of the music.

Bridged mono increases the maximum achievable SPL by almost 6 dB. In some situations, this extra capability may be required. Unlike virtually all other amplifiers, there is no performance penalty when selecting the bridged mono mode on the AHB2, you just get a 6 dB increase in SPL (nearly 4 X power).

Here is a link to the THD plot for the AHB2

@tobes

It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room.
Obviously 380W is not required (or used by me) but 150-200W is useful with the low sensitivity SCM19’s if one wishes to explore the considerable max spl of which they are capable (108dB). ATC recommends 75-300W, though to be fair a single AHB2 drives the SCM19’s very well.
Since my equipment resides in a room corner I also wanted to exploit the placement flexibility of mono blocks which allows positioning behind the speakers and short speaker cables (which should also mitigate loss in damping factor from bridging - not that DF is an issue ;-)
In regard to your other snipe at me, go back and read the first line of my first response in this thread. My subsequent posts were then attempts to correct (apparently deliberate) misinformation. Something you apparently have no problem with.

Im not interested in getting into a fracus with anyone on here however you have stood fast and been a little in everyones face about your Bridged Benchmarks. You did not answer my questions about your listening level or power level. How do you know you need those watts?

There is no evidence that "It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room" Granted it is sealed, and thus rolls off at 12 dB/octave.

Here are the specs.

Drivers: HF ATC 25mm Neodymium soft dome, Mid/LF ATC 150mm SL
Matched Response: ±0.5dB
Frequency Response (-6dB): 54Hz-22kHz
Dispersion: ±80° Coherent Horizontal, ±10° Coherent Vertical
Sensitivity: 85dB @ 1W @ 1metre
Max SPL: 108dB
Recommended Power Amplifier: 75 to 300 Watts
Nominal Impedance: 8 Ohm
Crossover Frequency: 2.5kHz
Connectors: Binding Posts/4mm Plugs, bi-wire
Cabinet Dimensions (HxWxD): 438x265x300mm (grill adds 28mm to depth)
Weight: 17.8kg



If the speaker is already down 6dB at 54 Hz. (which agrees with the impedance curve) it is going to be down another 6 db or so at 40. The rolloff of sealed box speakers is 12 dB per octave in all cases. Its physics man. When bass is more than 6 dB down you are hearing more 2nd harmonic than the fudamental because our hearing slopes off rapidly down there.

Im an information junkie, Ive been correcting things here the whole time while you keep fussing with George about whether this tiny distortion changes or not. No one is going to hear this low level of distortion!

The entire discussion about this miniscule amount of distortion is a tempest in a teapot. THE OP JUST WANTED TO KNOW IF HIS IMPEDANCE DIP WAS GONNA BE A PROBLEM. Many of us advised him it would and further he might get more power with one amplifier in stereo and a lot safer.

Besides the "strawman" snipe, you haven’t been very nice to George. He is in the industry.While he and I don’t agree on everything, we disagree on many things, yet our public conversation here has remained civil and I thank him for that.

I am certain ATC makes a very fine speaker. I think their maximum recommended amplifier playing heavy music would but an end to the speaker. The powered version only has 35 watts on the tweeter, you have 350 watts which is an accident waiting to happen. Voice coils do melt and you have no protection against that at all. One full level HF or ultrasonic event and bye bye tweeters.

If 29 amps of current ever goes through your woofer voice coils for more than fraction of a second ohms law will become quite apparent. With the amplifiers unbridged you have a 4 times greater margin of safety.

And now a little story..... I like to brag sometimes too.

I was engaged to fix a Bryston amplifier that had been back to the factory twice under warranty. It had already destroyed 2 x B&W woofers at $500 each (plus service calls). When it came back it blew up 2 more because the problem was intermittant. On my bench it did not show up either but I added external fuse holders and left a handfull of spares. There were no speaker fuses before. I said, lets not blowup any more expensive woofers, 2 amp fuses are cheap, the 20 cent ones will do the job and protect the woofers. Only DC would be doing this and you can play quite loud with 2 amps fuses. I asked his kids how loud dad played, they said not loud :)

I told the customer to call me when he had blown up ALL the fuses. The day I returned to get the amp again the house was cold (Santa Barara cold is not very cold, like 60) The amp finally showed it was putting out 50 volts of DC. Now we had a problem that could be verified at last..

Because the Bryston was old and not easy to get to the part that was the problem we sent it back to Bryston in Canada with a full page letter telling the tech to simply cool the amp and he would finally see it act up where it had not before. It was in the last few months of its 20 year warranty and free is cheaper than me. He cooled the amp and ended up replacing the whole mother board.

I will close with this. Elaine LaVasseur, besides being a fine artist, is co-founder of HI FI Club. Their company installs complete home systems and home theaters for wealthy Montecito filmstars and directors. They did both for Robert Zemeckis.

She comes up with clever things to say and told the frustrated Bryston customer, who had now lost $2000 of woofers with no end in sight, "Having Roger come to fix your stereo is like having Picasso come to paint your house" :)



https://santamariatimes.com/calendar/community/elaine-levasseur-collaborative-printmaking-and-home-s...

In contrast, the JC1 (suggested by George) produces 0.15% THD at full output

What fool, sorry no, idiot! would run these massive Halo JC1 powerhouses at full output. Here are the real figures into 4ohms.
"Distortion was also acceptably low (fig.6), with the 1kHz difference component lying at -94dB (0.0015%). Note that these last two graphs were taken at 635W and 600W into 4 ohms!"

Also +1 for Rogers last post, and I’m also signing off this stupid merry go round.

Cheers George

Good luck geek, let us know how they work out. 

@ramtubes said:

Im not interested in getting into a fracus with anyone on here however you have stood fast and been a little in everyones face about your Bridged Benchmarks. You did not answer my questions about your listening level or power level. How do you know you need those watts?

Roger, with a calm and unbiased mind go back and re-read my posts above. My only interest was in correcting misinformation, which I believe I did in a polite manner - at least up to the 'Straw man' comment, where I admit I lost patience with the obtuse replies. Sometimes it's hard to gauge the mood of a poster. You have completely mis-read me ;-)

There is no evidence that "It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room" Granted it is sealed, and thus rolls off at 12 dB/octave.
.................If the speaker is already down 6dB at 54 Hz. (which agrees with the impedance curve) it is going to be down another 6 db or so at 40.

I debated replying to this, since of been accused of excessive self interest - but I like to correct misinformation.
Here is a quote from SCM19 lab report George referenced above:
"Based on this figure, SCM19's bandwidth ranges between 40Hz and 23.2kHz (using the standard -6dB cut-off points) and between 52Hz and 21.6kHz for the standard -3dB cut-off points"
If you're interested in reading the whole review, here is the link:
http://www.avmentor.net/reviews/2016/atc_scm19_2.shtml

Im an information junkie, Ive been correcting things here the whole time while you keep fussing with George about whether this tiny distortion changes or not. No one is going to hear this low level of distortion! 

I agree that the AHB2's distortion is so low it matters little whether it doubles or not into low impedances (it doesn't at any rated load). 
However the inference by George - and this is important to the OP's question - is that the AHB2 will sound worse when bridged. This may be true with other amps but is NOT true with the AHB2.
George goes on to infer stability issues which are also baseless.

THE OP JUST WANTED TO KNOW IF HIS IMPEDANCE DIP WAS GONNA BE A PROBLEM. Many of us advised him it would and further he might get more power with one amplifier in stereo and a lot safer.

What do you base this on? Experience with other completely different designs?
Benchmark's John Siau says it won't be an issue - but what would he know right?
FWIW - and I've repeated this many times - my first line in response to this thread was "George I agree with you that with efficient low impedance speakers the OP may likely be better off with a single AHB2".

Besides the "strawman" snipe, you haven’t been very nice to George. He is in the industry.While he and I don’t agree on everything, we disagree on many things, yet our public conversation here has remained civil and I thank him for that.

As I said above I lost my patience with the 'straw man' comment. Otherwise I've been polite and careful about what I wrote. Read my comments again without bias.

I am certain ATC makes a very fine speaker. I think their maximum recommended amplifier playing heavy music would but an end to the speaker. The powered version only has 35 watts on the tweeter, you have 350 watts which is an accident waiting to happen. Voice coils do melt and you have no protection against that at all. One full level HF or ultrasonic event and bye bye tweeters.

Not sure why you are taking me to task with this, it has nothing to do with the original thread. I don't think you know much about ATC and their drivers. The tweeter comment is obvious, but what's the relevance?
Well designed, stable, high power amplifiers are only a risk if they are used injudiciously.