An experiment in passive bi-amping

Discussion in 'Archived Threads 2001-2004' started by Michael R Price, Dec 31, 2002.

  1. Michael R Price

    Michael R Price Screenwriter

    Joined:
    Jul 22, 2001
    Messages:
    1,591
    Likes Received:
    0
    Now that I have my 15 watt Zen amp up and running, I tried a little experiment by also connecting my old Audiosource amplifier to the system. Why would anyone want to do that? To try passive bi-amping of course. The Zen's bass is frankly quite weak, and it does not drive my speakers very loud. With somewhere in the vicinity of 100 watts, as well as a more conventional solid state design, I believed that using the Amp One to drive my Kit281s' woofers would bring me an improvement in bass and midrange quality. This would also improve the treble by lowering the strain on the Zen caused by heavy bass currents (since it only saw the capacitive impedance of the tweeter crossover). Of course, this is not as optimal as active bi-amping, or putting an active crossover in front of the amps and removing the passive crossovers from the speakers. I thought I'd share my experiences, hope you don't mind the long post.
    My CD player has two sets of outputs, one of which is controlled by a volume knob on the front. That variable output was connected to the Zen amp, while the fixed output went to the old Audiosource (which has its own volume controls). I removed the jumpers on the back of my speakers and ran a 15 gauge DIY cable (a good one) from the Zen to the upper binding post, and 12 gauge copper wires from the Audiosource to the lower binding posts. The problem with this setup was that the tweeters and woofers had completely independent volume control, so it was very difficult to get just the right tonal balance. I found music reasonably normal sounding with the controls somewhere within a few decibels of each other. To get a more accurate match, I checked the voltages across the speaker terminals (upper vs. lower) while playing music, and adjusted the gains so that they became equal.
    How did it sound? In short, I got the great treble quality of the Zen and the much better bass quality of the Audiosource. Unfortunately, I also got the midrange of the Audiosource. [​IMG]
    Using the more powerful amplifier to drive woofers gave the sound much more punch and a richer, fuller sound (with the levels set correctly). I surmise this is due to the poor damping factor of the Zen design, but I'm not sure why else. Maybe the 23,400uf of capacitance in the Zen's power supply was not enough (although the Audiosource amp has 20,000uf itself). Ah, who knows. Of course, the treble sounded pretty much as it had before (very good).
    I found a curious effect: the sound did not seem as "loud" as it had before using only the old amp, given that I was listening to music above 95db at times. This was probably because the nasty distortions created by both amplifiers were reduced because they each had an easier load to drive. I think the speakers were producing a bit of distortion at that point but not enough to be offensive. Of course, active bi-amping could cause more of this kind of improvement by completely eliminating the unwanted signals at the amps' inputs. This leads to a tentative conclusion - bi-amping can significantly increase the undistorted output capability of a system with the same power amplifiers. Whether or not this improvement is more cost effective using multiple amplifiers as opposed to one large amplifier, I don't know. But bi-amping certainly allows one to realize the benefits of different amplifiers and save money, for example in using a small high-quality amp for tweeters and a larger "value" amp to provide bass punch, instead of buying one powerful and good-sounding amplifier.
    There's always some little problem that keeps us from being happy with a new setup. My speakers have crossovers at 2.2KHz, so the cheap old amplifier was also providing the music signal through the midrange. Its sound quality is really inferior to the Zen or any other good amplifier, the details are sort of glossed over and music doesn't seem as lively or coherent as before. The sound became more "flat" and the "floating in air" effect mostly disappeared. I got tired of listening to it, although it was pretty neat to get it working with the Zen at the same time. (I must have had the most complicated wiring arrangement of any plain 2-channel stereo system with one source. Wish I took a picture.)
    "You are not worthy of reproducing any frequency in place of the Penultimate Zen! Off with your head."
    Therein lies the problem with bi-amping and the reason we can't all get along with cheap dedicated amplifiers: you're still stuck with the sound of the two amplifiers you're using, regardless of the reduction in distortion realized. In other words, that big class AB woofer amplifier I'm building had better be good. [​IMG]
    Happy New Year to everyone. I'll probably be staying up until midnight...
     
  2. Dave Milne

    Dave Milne Supporting Actor

    Joined:
    Jul 2, 2001
    Messages:
    568
    Likes Received:
    0
    Thanks for your observations. Yes, passive bi-amping can be effective, although with totally different brands of amps you've got to check proper polarity. Some amps invert and some don't.

    I'm curious why you didn't use a y-cable rather than having to fiddle with volume matching the fixed and variable outputs.

    Since you're obviously adept at electronics (I assume you built your Zen) why not whip active crossovers and bypass the passive ones? You should be capable of even more output, but the sound may be better or worse, depending on what frequency- and phase-correcting features the original passive crossovers had. Definitely a fun experiment!
     
  3. Michael R Price

    Michael R Price Screenwriter

    Joined:
    Jul 22, 2001
    Messages:
    1,591
    Likes Received:
    0
    Oh, I am whipping up an active crossover which I will use to bi-amp using the Zen as HF amp and a 400 watt class AB amp (also DIY) for LF. I just haven't gotten around to it, once I finish the large amp I will bi-amp passively (or using a passive line-level filter if possible) until I finish an active filter. I am at the moment trying to decide on a way to do the active filters without op-amps... JFET followers and stuff. As a relative electronics newbie, I'm trying to learn.
    And I didn't have a y-cable. [​IMG]
    Thanks for pointing out that polarity may change - I think I had it correct (the Zen inverts and the other amp doesn't) but it's important to check because wrong polarity can mess up the sound (usually a big dip in response around the crossover).
     

Share This Page