Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

This is John Siau, VP and Director of Engineering for Benchmark Media Systems.

I would like to clear up some erroneous information in this thread. The AHB2 is a unique design, so some of the usual assumptions do not apply.

1) The peak output current of the AHB2 is 29 A not 18 A. Many of the posts above are based upon the 18 A number which is incorrect.

2) The SNR of the AHB2 improves by 3 dB in Mono Mode (bridged mono). The signal is 6 dB higher but the noise is only 3 dB higher, producing a 3 dB improvement in SNR when running bridged mono. The low-level noise produced by the two channels of the amplifier is uncorrelated white noise and this is why the noise only increases by 3 dB when using the two outputs differentially.

3) The THD is virtually identical when comparing stereo mode to bridged mono mode. This is achieved through the use of the feed-forward error correction.  Every other power amplifier will show a substantial increase in distortion when bridged.

4) All references to "Mono Mode" or "Bridged Mono" on our website or in the AHB2 manual refer to the bridged mono mode of the amplifier.

5) The THD produced by the AHB2 does not increase as the impedance decreases. The THD into 2 Ohms, 4 Ohms, 8 Ohms and no load are virtually identical.  Again this is a unique characteristic that is achieved through the use of feed-forward error correction.

6) The AHB2 is not just stable into low-impedance loads, it stays clean when driving low impedances. The THD does not change with loading.

7) We do not specify the output power at 1% THD because, unlike other amplifiers, the THD does not gradually rise as the output level increases. The amplifier maintains 0.0003% THD+N, into any rated load, until the clip point is reached.

8) The rated "continuous average output power" of the AHB2 is not specified below 3 Ohms stereo or 6 Ohms mono because of the FTC regulations (16 CFR Part 432) that require continuous operation at 1/3 rated power for one hour. This is a long-term thermal limitation when playing sinusoidal test signals and has little significance when playing music. It should not be taken as indication that lower impedances cannot be driven cleanly.

9) The AHB2 has a high damping factor and this allows excellent performance in bridged mono. The damping factor is 350 into 8 Ohms stereo and 175 into 8 Ohms bridged mono.

The AHB2 will deliver THD+N < 0.0003% at full output voltage into various loads is shown in the following table:

< 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

Bottom line, the AHB2 is well suited for bridged mono operation into 4-Ohm nominal impedances and the performance is virtually identical to stereo mode except that the power is nearly 4 X higher. Dips in the speaker impedance curve are not a problem and the AHB2 drives these cleanly.

The SNR actually improves by 3 dB when running bridged mono, and the THD is virtually unchanged. We almost always demonstrate the AHB2 in bridged mono mode at trade shows and have done so with many different hi-fi and pro loudspeakers. In most cases, these demonstrations have used speakers with a 4-Ohm nominal input impedance.

The following link will take you to a series of application notes that discuss the performance of the AHB2 in more detail. They include a plot of THD vs output level under various load conditions:

https://benchmarkmedia.com/blogs/application_notes/tagged/ahb2

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

There are many good comments in this thread but there is also much erroneous information. In most cases, this erroneous information is based on the assumption that the AHB2 behaves like most power amplifiers. The statements made would be true of most power amplifiers, but do not apply to the AHB2.

The AHB2 does not act like a traditional power amplifier because it has a very unique design. It has tightly regulated power supply rails, a patented feed-forward error correction system, and an offline protection system that is not in the signal path.

Unlike virtually all other power amplifiers, the THD produced by the AHB2 does not increase under load. The 8-ohm and 4-ohm THD vs. Power curves are absolutely identical. Even when loaded with 2 ohms, the THD is virtually unchanged. To the best of my knowledge, no other power amplifier can do this, or even come close to doing this. And, no amplifier delivers lower THD or lower noise at any price. The AHB2 holds the record for lowest THD and lowest noise. It is also important to understand that this performance is not just achieved at one particular load impedance, test frequency, or operating level. It is achieved into any rated load at any power up to full output power, and any test frequency between 20 Hz and 20 kHz.

The red clip lights on the AHB2 are driven by a circuit that measures the THD. If it exceeds 1% the lights will turn on. The AHB2 delivers an astonishing 0.00011% THD, 1 kHz at full rated power into any rated load.  The transition from 0.00011% to 1% is abrupt, but it occurs above the rated output power. This means that the AHB2 stays clean, even when you drive it almost to the rails. Ordinary amplifiers cannot do this. With most amplifiers, THD gradually increases with output level and with increasing current load. With the AHB2, THD is not a function of output current.

The impedance load ratings for the AHB2 are based upon long-term thermal limits (prolonged operation into the specified load impedance). This is not a limitation on the allowable minimum impedance of the loudspeaker. It is a rating for the nominal impedance of the loudspeaker. If the impedance drops to 1 or 2 ohms at certain frequencies or is presented with a difficult phase angle, this is not a problem for the AHB2. Feed-forward correction is stable into any load. Feedback is not stable into any load. The AHB2 uses a large amount of feed-forward correction and a minimal amount of feedback. This allows stability into virtually any load. Match your speakers to the AHB2 on the basis of nominal impedance not minimum impedance. The impedance minimums are not a concern.