Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

@djones51 thanks but the axes of the plot they have are not readable to me. Does the impedence of the speaker ever dip below 4 ohm. This is not clear to me.

https://benchmarkmedia.com/blogs/reviews/ahb2-driving-pmc-ib2s-studio-monitors

@djones51 I am preparing to ask this question. Right now I am trying the speaker manufacturer / dealer to get me answer to the question "what the lowest impedance the speakers can dip to". 

Every speaker manufacturer should publish impedance vs frequency graph or just state the lowest impedance. Only very few do that, I don't get why.

I did get one response from John @ Benchmark regarding what the tested output @ 4 Ohm in bridged mode is. 

@twoleftears I am glad you mentioned this, I did also hear about this but no concrete info anywhere regarding this. I will try to find out. Thanks for point this out.

Is the raise of Noise floor from -112 dB to -109.2 dB really bad?, Seems like this is quite better than many implementations by wide margin at least 6 to 9 dB below, or did I get this wrong?.

@twoleftears I did read the article by pooraudiophile.com and did reach out to Benchmark a while ago and asked about possible tested numbers till 4 Ohm when bridged. Here is the response I have recieved:

OUTPUT VOLTAGE INTO VARIOUS LOAD IMPEDANCES

< 0.0003 % THD+N at the following output voltages and load impedances, 20 Hz to 20 kHz

· 29.03 dBV, 31.25 dBu, 28.28 Vrms into 8 Ohms, both channels driven

· 28.92 dBV, 31.14 dBu, 27.93 Vrms into 6 Ohms, both channels driven

· 28.81 dBV, 31.03 dBu, 27.57 Vrms into 4 Ohms, both channels driven

· 28.57 dBV, 30.79 dBu, 26.83 Vrms into 3 Ohms, both channels driven

· 27.14 dBV, 29.36 dBu, 22.76 Vrms into 2 Ohms, both channels driven

· 35.05 dBV, 37.27 dBu, 56.57 Vrms into 16 Ohms, bridged mono

· 34.83 dBV, 37.05 dBu, 55.14 Vrms into 8 Ohms, bridged mono

· 34.59 dBV, 36.81 dBu, 53.67 Vrms into 6 Ohms, bridged mono

· 33.16 dBV, 35.38 dBu, 45.52 Vrms into 4 Ohms, bridged mono

Use dBV to calculate the peak SPL from your speaker/amplifier combination. Use the following formula: Amplifier output voltage in dBV + speaker sensitivity at 2.83V - 9 dB. Example: (29.03 dBV at 8 Ohms) + (90 dB SPL @ 2.83V 1m) - 9 dB = 110 dB SPL at 1 meter

AHB2 OUTPUT VOLTAGE AND OUTPUT POWER

Mode, Impedance, Noise Voltage,   Maximum Output Voltage, Watts

                             dB relative to 2.83 Vrms, dB relative to 2.83 Vrms

Stereo 8-Ohms -112 dB 20 dB 100

Stereo 4-Ohms -112 dB 19.8 dB 190

Mono 8-Ohms -109.2 dB 25.8 dB 380

Mono 4-Ohms -109.2 dB 24.1 dB 518

Word has come through that the speaker can only dip to 3.8 Ohm. So given that Benchmark AHB2 is stable till 3 Ohm. 

Now the question I have is that the amp is rated at 18A per channel unbridged. Once bridged what does it put out?. I think there is a way to do the math from above numbers, when I plug it in I get 11A for 4 Ohm number and that does not sound right. 

Can anyone help?.

@georgehifi I cannot bi-amp since Legacy Aeris has the low frequency section already powered.

The reviews from Douglas, pooraudiophile.com and measurements from stereophile all encouraged me to try this amp out. I appreciate all their work and comments. 

Now I just have to decide if I need to buy two amps or just live with one. Does bridging bump up the 18A per channel limit or keep it the same. Non bridged amp can send 18A per channel so I am guessing bridged version can put more than 18A to a single channel. How do I calculate this number?.

@georgehifi Benchmark has no peer in controlling distortion , zero cross over distortion but without class A hassle. I will get answers regarding damping factor and total current. I am buying them used btw.

@kijanki thank you so much pointing fhat out. Will make sure to newer revisions. 

@georgehifi look at the following from Benchmark AHB2 manual page 22.

Signal to Noise Ratio
132 dB A-Weighted, Stereo Mode
135 dB A-Weighted, Bridged Mono

Distortion
THD+N
1 kHz, 80 kHz LPF, at full rated output into any rated load 
   < -118 dB (< 0.00013%) - Stereo Mode
   < -118 dB (< 0.00013%) - Mono Mode
THD
1 kHz, 20 kHz LPF, at full rated output into any rated load 
   < -119 dB (< 0.00011%) - Stereo Mode
   < -120 dB (< 0.00010%) - Mono Mode

This only leaves damping factor. 

Any thoughts?.

@kijanki never mind the website does mention 
"MAXIMUM AUDIO OUTPUT CURRENT

• 29 A peak, per channel, both channels driven"

@georgehifi This article https://benchmarkmedia.com/blogs/application_notes/ahb2-driving-pmc-mb2s-studio-monitors also talks about second iteration of using ABH2 as mono blocks. 

Seems like outstanding question is just the damping factor everything else looks excellent to me. I will ask this question to Benchmark and see what they say.

Benchmark does seem to perform quite well in bridged mode, in-fact lot of thought went into making it work in that mode. That is why they never made it put out a lot of watts in stereo mode. Even at $6k and putting out 500w per channel at non existent distortion and quite good voltage peak 64.4(beats the requirement I have which is 54) is a deal compared to what is in the market today. Please feel to correct me if I am wrong.

Only Bryston 4B3 comes close at this price range and it suffers from cross over distortion that benchmark does not, if I understand it right.

@kijanki My amps manual says the same i.e 29A peak into 1 Ohm. Is this per channel I am assuming?. Most amp state this number per channel if I understand that right.

@onhwy61 I only have experience in running it in stereo mode. I am looking and specs and reviews from others to consider the pros and cons.

@d2girls from what I heard the feed forward error correction is THX. I am not sure if they designed the amplification and power stage.

@twoleftears Is that sarcasm or a fact. Sorry sometimes I don't get sarcasm.

@georgehifi Legacy Aeris is spec'd for 500w. I am just considering starting with a setup that does it all when dealer comes to calibrate. If I consider the cost of extra Benchmark it is 10% of my system and I can always sell it or re-purpose it.

The challenge is find 500w of cleanest power that does well in rest of the parameters (current, voltage and damping etc) and optimized for price. 

To my limited knowledge even after bridging Benchmark seems to out perform its peers. Correct me if I am wrong. I am happy to learn more.

@twoleftears  thanks :)

@georgehifi thanks. Consider this thread closed.

I would like to thank John Siau a.k.a @gearbuilder for taking the time to provide detailed explanation and clarification. I appreciate the technical detail.

I have answers to my questions, apart from "damping factor" being halved I really do not see anything I am loosing by trying two AHB2 with each in bridged mono mode. My speakers can only dip to 3.8 Ohm minimum.

Sure two AHB2s may be overkill for my speakers but I will audition in both bridged and non-bridged mode now that I have all the information. Thanks to everyone.

@ramtubes lol noted, it did cross my mind to get preamp and DAC from Benchmark but I am constrained by using Wavelet. My core concern was stability at or below 4 Ohm bridged. My questions are answered. I am in the process of getting a second amp. Thanks.

All issues with AHB2 are settled! as far as I am concerned. Let us not continue this thread and discuss things not relevant to technical issues of AHB2.

I will report back if I try them in bridged mode and feel like it. Thanks everyone. 

@ramtubes, Impedance minimum for top section of Aeris is only 3.8 Ohm (this information is from the source). So it seems AHB2 is fine with that.

Which amplifier would you think can be a better option than Benchmark AHB2 given that I have gotten myself one amplifier for $2100?. What do you think in technical terms another amplifier can be better than Benchmark and at what price and size?. I am looking for 150 watts with no distortion per channel easy, which means no crossover distortion or anything else. Just because I have spent some money on a speaker does not mean I am going to weight that and buy an expensive amplifier. If I could find a $2k Legacy Aeris I would have purchased that instead. I want a great amplifier optimized for cost.

Please let me know I have been looking for a while. I just cannot audition 10 amplifiers to compare and select which sounds good in my room and with the speakers I have. I have to look at the specs, build quality, technical innovation and what generally people think. Anything that comes close to this in my opinion is Hypex NC500 based modules which again can be obtained for $2k approximately. I will consider if you can point me few of them.

General rule of thumb is fine but at the end I am constrained by my experience and capacity to audition amps and I like to re-verify past ideas. Technology changes and things change. If someone says X amp sounds good then I have no context, I will go read the specs, read the forums and reviews and move on.

@georgehifi With Legacy Aeris what you are suggesting is not possible.

@ramtubes Agree with you that great specs on a single tone at some frequency or even frequency sweep tests does not really come close to complexity of music. But at this point that is all I have.  Good point about complexity yes it is more complex but the amp has been out there for a bit. And people who have it never seem to complain why it failed or how it faulted. Still I see these (amplifiers) as simple things compared to stuff I am aware of. These days manufacturing and design has come a long way with computer simulation etc. Testing techniques etc have come a long way. I am more concerned with someone in their backyard building a complex amp more than anything else.

Agree with you that good specs might not translate to good sound. But I cannot go on some one saying some monster class A/B or class A sounded great and since it is big and consumes a lot of power I have to go for it.

I have a class A SET tube amp and I am going to compare with this amp along with class D (Hypex NC500) before making my mind up on few things. I am in the bucket of people who think all good amp sound the same.

At the end can an amp just amplify the source faithfully and get out of the way. Can it just get out of the way!!. This is what I am going for and I want it at lowest price possible. 

Suggest me few amps better than AHB2 at this price and I will consider them, I am only looking for a great amp that do justice to my speakers. I don't have any past baggage or any biases.

If I understand correctly Legacy Wavelet when used with Aeris at build time is configured to correct/adjust for the small errors among how different drivers work and also corrects for room also in the domain of time. I do like the idea of using dsp to correct these errors. I think from what I have heard in person it works well. No speaker can make the room go away!.

Buying passing Wavelet and the internal crossover is not an option, I think they add quite an amount of value. 

As I said before in an another thread, if I could redo this it will be an all active speaker. The more expensive ATC look quite good to me. I want active crossovers and internal amplification. No need for this amp matching speaker merry go around.

@ramtubes The tube amps you guys make look quite good.

@hifidream I will try one amp first and two later. Thanks , I prefer neutral components too. Will report back for sure.