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Two channel amplifiers with bass management and room correction!

by ct_acoustic_frontier July 06, 2022

What's so good about a two channel amplifier with bass management and room correction?

Recently I have become aware of a new class of product: two channel amplifiers WITH bass management AND room correction. Why is this cool? Well until this new class of products came out the only way to do this type of thing was with a home theater receiver or a separate preamplifier such as a DEQX or TacT and a power amplifier. Two channel systems benefit just as much from subwoofers and equalization as home theater systems. Many two channel speakers, even high end ones, cannot reproduce low bass below say 30-40Hz with low distortion. A JL Audio F112 can play down to 19Hz at a -3dB point! Using subwoofers also allows for cunning room mode cancellation and avoidance of speaker boundary interference. As Classe say in their press release:
"A properly implemented subwoofer (or two) is at least as beneficial to stereo playback systems as it is to home theaters. Placing the Left and Right speakers for best stereo imaging rarely delivers ultimate bass response at the listening position, no matter how well those speakers can reproduce low frequencies. One or more subwoofers avoid this compromise... EQ enables further tweaking of the low frequency modes to achieve the best overall system performance."

Contender 1: The Harmon/Kardon HK990.

This integrated amplifier boasts 150W per channel and has some really cool features: bass management, dual subwoofer outputs, digital and analog inputs and Harman's EzSet/EQ II's room-equalization and system-calibration technology. Retail is $2,399. More information on the Harman site. Harman/Kardon HK990 front view The bass management can be setup to crossover to the subs at any frequency between 40Hz and 200Hz. Bass is summed into mono below the chosen crossover frequency, so you can't create stereo subs, where each sub supports only one channel. Crucially Harman's correction algorithms can be setup to apply to the low frequencies (sub channels only), the low and mid frequencies up to 1000Hz or the full frequency spectrum. The room correction algorithm used to optimize the bass response is Harman's BassQ, which is the same stuff used in their standalone JBL Synthesis BassQ processor. Harman's mid and high frequency correction actually combines a cunning farfield measurement (for correction of frequencies between the sub crossover and 1000Hz) and nearfield measurement (for correction of frequencies above 1000Hz). At Acoustic Frontiers we are strong believers in the beneficial power of room correction based on farfield measurements but we also maintain that it should only be used to deal with bass issues below about about 300Hz. In an ideal world you would only use room correction below about 100Hz and use passive acoustic bass traps to deal with the 100Hz to 300Hz range.  

Harman/Kardon HK-990 top view

Above about 300Hz EQ is correcting speaker response and for this you need to measure nearfield, so they are nearly but not quite there in our opinion. They are trying to correct for reflection from boundaries in their midrange correction but, in our mind, they should not even attempt to do this. Far better and more effective is to use passive acoustic treatments to deal with reflections. To trick the Harman algorithm what you should do if you want to try out their speaker correction is to do ignore Harman's instructions to do the midrange correction measurement farfield. You should do both the midrange and high end measurement nearfield for best results. For more about our opinions on room correction read our detailed primer on room correction and the two articles explaining our functional criteria for a room correction products here and here. The amp also picked up a really good review in the UK magazine Hi-Fi Choice a while back where it was lauded for its high current delivery.

Contender 2: The Classe CP-800.

The second product that I am aware of in this category-of-two is the Classe CP-800. Retail is $5,000.

Classe CP-800

The CP-800 has an asynchronous USB port, for connection to a music server. Aside - we love music servers and have been using them nearly exclusively for music playback for 7 years now, way before the mainstream audio community picked up on them. We even DIY'd our own USB to SPDIF converter back in the days before you could buy them off the shelf! It has bass management, although I am not sure if it allows for stereo subs or sums the bass in both channels to mono and what crossover choices there are. The room correction capability provided is in the form of 5 bands of user programmable parametric EQ. Whilst you can get VERY good results with parametric EQ you need to have some form of a acoustic measurement system to tell you where to put the filters. We have sold quite a few XTZ Room Analyzers to Classe SSP-800 users. The XTZ unit measures your room and automatically suggests EQ filters. What's more it has a simulation mode to allow you to see what the response would look like with different combinations of filter frequencies, bandwidths and Qs. Two very cool products, I hope you will agree! What do you think of inclusion of things like bass management and room correction into audiophile two channel products? Do you know of any other products that do a similar thing? Let me know via the comments below!



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Nyal Mellor, Founder, Acoustic Frontiers

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