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Thread: Passive Bi-amping

  1. #1
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    Passive Bi-amping

    I first heard the term “passive bi-amping” a couple of years ago on an audio forum. I was responding wrongly to a question about bi-amping because the concept of “passive” bi-amping was new to me. When I found out what was meant, I was a little perplexed as to why anyone would do it! Seems like a marketing ploy to me… Since the subject keeps appearing, I thought it worthwhile to express my thoughts on the matter.

    Bi-amping to me has always implied a line-level crossover between the preamp and power amplifiers. That way, the power amplifiers are used only in a limited frequency range, and their outputs applied to the appropriate speaker driver with no (or limited) passive crossover components. Eliminating the speaker system’s passive crossover and connecting the amplifiers directly to the drivers can improve the damping factor and allows much more flexibility in crossover design. Of course, not all of us are equipped to redesign our speaker’s crossovers. When I used bi- (tri-, or more-way) amplified systems I generally followed the manufacturers’ recommendations.

    Many pro systems are designed for this but fewer home systems. Furthermore, I have noticed not all systems treat those terminals on the back the same way. In fact, my limited experience with a handful of systems indicates the consumer speaker’s high and low terminals rarely bypass the internal crossover; they just connect to the low- and high-pass filter sections, not directly to the drivers. For most consumers, this is probably good, but in my mind defeats the purpose of using multiple amps and electronic (passive or active) crossovers. Of course, with passive bi-amping it makes sense, since there needs to be filtering somewhere before the drivers.

    Passive bi-amping does not utilize line-level crossovers; instead, two power amps are driven full-range into each speaker’s high and low terminals. The crossover (technically now split into a LPF and HPF) inside the speaker provides the filtering (“splitting”) of signals appropriate to each driver. The amps thus still provide a full-range signal, but the speaker’s crossover rejects the out-of-band signals to each driver.

    A few key points:

    1. Each amplifier requires the same voltage drive as a single amplifier since they each have the same signal. There is no voltage headroom benefit.
    2. Because the load (speaker) is essentially an “open” in the unused frequency band, less current output is required from each amp. For example, the bass amp drives the woofer, but there is essentially no high-frequency current since the crossover “blocks” the HF energy. The opposite is true for the HF amp; the voltage swing is the same as a single amp, but there is almost no LF current so the net power per amp is less. Of course, this could potentially cause stability issues with the amp. Higher-order crossover networks may load the out-of-band frequencies to reduce their input into the drivers, increasing power dissipation the amps (which are driven full-range in a passive bi-amp system).
    3. There is no net system power increase at the speakers assuming the amps have the same voltage rails (e.g. inside an AVR or multichannel amplifier with the same power voltage rails to all amps). If you had a 100 W amp before, passive bi-amping does not give you 200 W to the speaker. You have split the load into two frequency bands, but the net power is the same to the speaker. That is, 100 W to the lows and 100 W to the highs is the same as having a 100 W amp that covers the entire frequency range. It is not the same as driving the speaker with a 200 W amplifier; to increase the power, you need to increase the voltage rails.
    4. In fact, there is more power lost, since the amps are not 100% efficient. That is, it actually takes more energy from the power supply to passively bi-amp than if you used a single amp. This is also true for active bi-amping, but in that case we can choose lower-power amps for the highs (which rarely need the same power as the lows) and realize net power savings. That does not happen with (typical) passive bi-amping.
    5. There is no damping factor improvement over a single amp since the speaker crossovers are still in-circuit. One of the benefits of active bi-amping is direct connection from amp to driver, providing better driver control; this is not true in passive bi-amping. The lossy, passive high-power internal speaker crossovers must remain in place else out-of-band energy wil be applied to the drivers (causing at least distortion and quite likely destruction). This defeats one of the main reasons for bi-amping.
    6. There is no longer electrical interaction among drivers with passive (or active) bi-amping. (There may still be mechanical coupling if the drivers are not isolated from each other.) That is, if the woofer starts to distort the input signal through electromechanical forces, it no longer modulates the HF amp’s output. One plus for bi-amping, active or passive.
    7. If the amps share a power supply, as do most AVRs and many (most?) multichannel amps, then modulation between high and low amps can still occur through the power supply. This can also happen with active bi-amping, although separate amps are the norm in the pro world. At least when I have done it…
    8. There may be some distortion reduction since power output is lessened in the amps. I suspect this is not significant, but it should happen due to the lower current draw. The catch is that the voltage swing of each amp is unchanged, so any distortion related to voltage swing is not changed. Only distortion components depending on output current may be reduced. That is design-dependent, but since most amps are primarily voltage-mode amps, I suspect any distortion reduction is small.
    9. You have two amps now so presumably noise is a little higher since you have two uncorrelated noise sources. At the speaker outputs I suspect it’s a wash since only a reduced frequency band gets through the drivers to hear.
    10. Thermally it is a loss since no amp is 100% efficient. There is always a little “waste” power that gets turned into heat, both standing bias current (especially if not class D amps) and losses through the components in the amp. Thus passive bi-amping will cause your AVR/amp to run hotter than if using a single amp (assuming unused channels). It is worth noting that amplifiers are typically most efficient at maximum output; the HF amp is probably loafing most of the time and thus wasting power and generating heat.
    So, there are some potential benefits, but I suspect they are inaudible (I have not tried passive bi-amping so cannot say). And a lot of drawbacks. The major benefit is mostly mental, IMO; users can now use their “extra” amp channels. Whether this benefits anyone other than the electric company I cannot say, but I strongly suspect not…

    I did simulate a passively-bi-amped system just for grins. I used a simple first-order (single LC) crossover at 1 kHz and modeled the speakers with ideal 8-ohm resistors. I assumed 80 V rails (theoretical rails for a 100-W amplifier) and drove a relatively small signal into the speakers (1 Vpk at 100 Hz, 0.1 Vpk at 10 kHz). In the schematic below, the top is the single-amp system, and the bottom the passively bi-amped system. There is an input stage at left, combining the 100 Hzand 10 kHz signals to drive the amps, E1 – E3. (SPICE purists will note I did not need the dependent sources for this, but it makes the picture easier to follow.) I ran the simulation for 400 ms to allow the RMS power calculations to reach their final values.


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    The top plot shows the input signal; the “fuzz” is the 10 kHz signal riding on the 100 Hz tone. The second plot shows all the speaker voltages; note the woofer and tweeter signals are identical for the two systems as desired. Since I used only a first-order network, there is a little modulation of the tweeter by the low-frequency signal and vice-versa.

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    The third plot from the top shows the amplifiers’ RMS output powers. The single amp (P1) outputs 63.12 mW; the bi-amp system’s amps output 62.5 mW (P2, woofer) and 8.84 mW (P3, tweeter). Note that superposition does not apply with power, and P1 does not equal P2+P3. The extra power? Lost, wasted…

    Looking at the power actually delivered to the drivers, the woofer (P1W, P2W) receives 61.89 mW and the tweeter (P1T, P2T) 1.239 mW for both systems (identical as expected). The power dissipation of the amplifiers, assuming 80 Vpp power rails, is 7.11 W for the single amp (Pd1), 7.04 W for the woofer amp (Pd2), and 1.00 W for the tweeter amp (Pd3). As expected, using two amplifiers costs a little power. This also highlights the inefficiency of amplifiers with relatively low output power. Unfortunately for our power-mad egos, average output is more often in the 100 mW to few Watt range than anything like 100 W.

    FWIWFM - Don
    Last edited by DonH50; 11-27-2011 at 07:17 PM. Reason: Spelling, added power labels to text; bit more about crossovers
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

  2. #2
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    From my experience it makes sense as something that will achieve "better" sound then just the single amp. Most power supplies are a long way from being perfect, and most amplifying circuits are susceptible to power supply modulation. If the latter were not true, then passive bi-amping would make little difference but it obviously does for a lot of people, so it's one reasonable way of getting around the some of the deficiencies of their equipment ...

    Frank

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    Unless the amps have separate supplies, supply modulation is still a problem with passive bi-amping... For me, "obviously does for a lot of people" is very subjective; I have no way of proving or disproving any of that, not enough sampling.
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

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    I think in the majority of multi-amp systems there would always be a level of local bypassing in every amplifier module. My experience is that high frequency glitches in supplies do the most damage, and of course a capacitor in exactly the right position in a circuit is worth 10 further away in the main power supply bypassing, as far as high frequencies are concerned. Many amplifiers have great PSRR, so long as you don't check how they perform at those nasty upper frequencies!

    Frank

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    Site Founder And Administrator amirm's Avatar
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    Great article as always Don. Like you, I had a double take when I first heard about passive bi-amping. I like that the distortion from one amp doesn't bleed into other as it is easy to create harshness induced by the lack of drive for the woofer. But lots of other benefits are left on the table as you mention.
    Amir
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    i first heard of passive bi-amping from Richard Vandersteen, about the time bi-wiring, tri-wiring, etc was becoming popular in British speakers (late '80s?). back in the day, another adaptation i recall from steve McCormack when the Mod Squad ws still around, was to use two stereo amps with a y-adapter at the input effectively creating a mono amp for each speaker but with their separate channels driving the low and mid-high frequencies separetly - a configuration Richard recommended on the Vandersteen model 4 which was his flagship at the time. He was one of the first US designers to completey separate crossover circuits for each driver.

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    Senior Member wgscott's Avatar
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    Would there be any benefit to active bi-amping (i.e., with an active crossover) while leaving the passive crossovers in the speakers in place?

    It seems that is, in essence, what one does with some of the velodyne subs that have speaker level input and output.

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    Don it would be great if, some time, you could do a breakdown like this for active biamped systems. I have come across some explanation of the benefits I clearly hear, but none as clear and methodical as the above.

    Tim
    In high-end audio, you can't even fight an opinion with the facts.

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    Funny, being totally unfamiliar with the pro-world, bi-amping is using 2 amps to drive a passive speaker for me. When reading an article by a pro about bi-amping it always take some time before I realize this is about using an active cross-over.

    Likewise the phrase “passive bi-amping” is new to me. But very apt.

    Any thought on a typical audiophile issue as vertical or horizontal (passive) be-amping?

    5. There is no damping factor improvement over a single amp since the speaker crossovers are still in-circuit. One of the benefits of active bi-amping is direct connection from amp to driver, providing better driver control; this is not true in passive bi-amping.
    Might it be that there is a slight improvement because driving 1 speaker over a low or high pass filter is a less complex load for the amp than driving 2 speakers over a high + low pass filter

  10. #10
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    Quote Originally Posted by wgscott View Post
    Would there be any benefit to active bi-amping (i.e., with an active crossover) while leaving the passive crossovers in the speakers in place?

    It seems that is, in essence, what one does with some of the velodyne subs that have speaker level input and output.
    You don't get the fine driver control (higher damping factor), and have to worry about interaction among active and passive crossovers, but the other benefits apply as in the opening post. If there is an additional passive crossover in the sub, you may gain some by not sending lower frequencies to the mains than they can handle (there is typically no roll-off on the bottom end in the speaker itself other than acoustic/mechanical).
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

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