The Advantages of using SS Amplifiers for Bi-/Tri-Amping

[Chris A]

Looking at the Active Bi-Amping/Tri-Amping FAQ, it was stated:

 

Quote

 

4. [bi-/Tri-amping] provides the ability to use less expensive amplifier designs for each driver.

 

This thread will provide insights and listening experiences on using solid state (SS) amplifiers for bi-amping and tri-amping, and the reasons for the disproportionate increase in sound quality (SQ).  Let's start with the most obvious reason:

 

1) The reduction in the load that each amplifier drives. 

 

For example, it has been noted the there is an average power split between bi-amped loudspeakers based on the crossover frequency chosen, with the 50-50 power split frequency close to the 350 Hz band.  If you bi-amp with a crossover point around 350 Hz, then you can expect to need only half the rated output power for each bi-amping amplifier. 

 

This is a real advantage for the low-wattage amplifier crowd, including tube/valve amplifiers and chip amps.  By bi-amping, even sub-watt amplifiers can drive their loudspeakers to higher SPLs without accompanying distortions produced by each amplifier.  Since these lower power amplifiers often cost much less than higher power amplifiers, the economic advantage of bi- or tri-amping becomes more attractive.

 

If Tri-amping, the audible spectrum (i.e., 20-20,000 Hz) can be broken into three parts, with each amplifier requiring, on average 1/3 the output power to produce the same loudspeaker output SPLs.

 

2) The elimination of amplifiers driving unnecessary resistive and reactive loads that passive crossovers introduce.

 

This usually isn't talked about by the "old school", especially by those that otherwise "distrust" active crossovers, since it isn't pleasant to talk about when defending passive crossovers. . It isn't a secret that real-world acoustic drivers all have resistive and reactive characteristics.  What's not discussed is how much more resistance and reactance is added by the passive crossovers.  In some cases, the added resistance and reactance is greater than the drivers themselves.  If you look at the older passive crossover designs used by Klipsch on it's Heritage loudspeakers (particularly the Khorn), you will see impedance swings from 4.2 to 43 ohms on the frequency vs. input impedance curve:

 

Direct-coupled amplifiers are amplifiers that do not require an output transformer to couple to the loudspeakers without presenting a higher amplifier output impedance to the loudspeakers than the loudspeaker's input impedance. Solid state amplifiers are typically direct coupled amplifiers--with noted exceptions.

 

SS amplifiers lose many of their advantages when driving loudspeakers with high levels of load reactance, of which unnecessary contributions are generated by passive crossovers. Direct coupled amplifiers "like" resistive loads.  Most of the output power stage design differences between typical Class AB and Class D SS amplifier designs lie in how the amplifiers are designed to handle complex and non-linear reactive loads.  By minimizing load reactance by direct coupling loudspeaker drivers without using passive networks, the amplifiers themselves can do a better job of faithfully reproducing the music via the loudspeaker.  Driving highly reactive loads contributes negatively to the resulting sound.

 

Additionally, the amplifiers in a bi-amp or tri-amp configuration are not driving any parasitic resistive loads that simply dump a significant fraction of amplifier output to heat: all the amplifier's output is going directly to the drivers themselves - direct coupled.

 

3) Reduction of modulation distortion (FM and AM), transient intermodulation distortion (TIM, i.e., "slew rate" distortion), and zero-crossing distortion via separating low frequency/high amplitude signals from high frequency/low amplitude signals.

 

Much of the discussion centering on amplifier "classes" (A, B, AB, C, D, etc.) is tightly coupled to non-harmonic distortion types, including modulation distortion, transient modulation distortion (TIM, otherwise called "slew rate" distortion, loudspeaker impedance-coupled frequency response distortion, zero-crossing distortion (i.e., higher-order harmonic distortion) and clipping distortion (higher-order transient harmonic distortion).  The objective of using the different types of amplifier classes or architectures is the avoidance of these type of distortions, not the introduction of any them into the signal path.

 

By separating low frequency/high amplitude signals from higher frequency/lower amplitude, better choices for amplifiers of differing classes and strengths/capabilities can be used.  For instance, amplifiers of very high slew rate (low TIM), low intermodulation via fewer gain stages and lower levels of feedback can be direct coupled to the HF drivers that typically exhibit capacitive reactance , while high output current/voltage low output impedance amplifiers can be direct coupled to the LF drivers, drivers that often exhibit large relative amounts of inductance reactance. 

 

Additionally, it is well known that LF drivers are usually much less efficient than HF drivers, such that output gain matching can be accomplished by the amplifier gain controls or the active crossover without resorting to increasing impedance or resistance in the signal path to pad down the HF drivers. 

 

This last point (reduction of non-harmonic distortion levels) using bi-/tri-amped amplifiers of differing types that are individually more optimal for driving the their respective loads at lower overall non-harmonic distortion levels -- is the greatest single advantage that is achieved via active bi-/tri-amping. 

 

This aspect of bi-/tri-amping is also the one that receives the least visibility and understanding, and that likely makes the largest difference in the sound quality improvements over typical passive crossover - single amplifier/loudspeaker configurations.

 

Chris

[Chris A]

The upshot of all this is that SS amplifiers, once they are chosen for their strengths to the load that they are driving, and those loads are split into LF/high amplitude and HF/lower amplitude sections, the resulting sound is clean, powerful, natural, and detailed, much more than using passive crossovers and single amplifier/loudspeaker. 

 

I've found that the differences between even high quality SS amplifiers and lower quality/lower cost amplifiers becomes much narrower and that the overall SQ improves greatly over "traditional" passives and no bi-/tri-amping arrangements.

 

YMMV.

 

Chris

Edited August 30, 2015 by Chris A

[Chris A]

Reserved.

[Khornukopia]

 

This is a solid looking amp. More power than most of us need for our bi-amped woofers, but interesting to see and hear a product like this at the audio show.

 

[NBPK402]

Great info... I am currently building a 16 channel IcePower amp with the front 3 channels being bi-amped with 50wpc to the horns, and 125wpc going to the woofers. I won't be able to test out how it works for a few weeks though as we are still ordering needed parts for the assembly. I wish I had had the new amp before I sold my JBL 2360A/Q-pies...now i will just be able to see how the bi-amp setup sounds with the new Synergy style speaker, but no reference to other amp combos.