This thread is very interesting to me because I have seen so many other opinions that seemed to make it very complicated to bi-amp.
Active crossover, bypass the internal speaker crossover, set the crossover frequency above the speaker crossover frequency, etc, etc.
Now here comes Mr Von Schweikert. Certainly someone we all feel is an authoritative source of knowledge and he promotes the easiest of solutions to implement. Now, I don't need to stress over buying yet another piece of equipment (the crossover), and I don't have to mess with the internals of the speaker and I don't have to figure out if the crossover frequency I've chose is correct.
All I have to do is run the amps to the speakers and if the amps have different sensitivities, pop in a resistor. What could be easier?
Am I missing something?
If someone get the resistor value formula, please post it here.
Proud VR4.5 owner
I am gratified to see so many audiophiles interested in trying a "biamp" configuration, even though it is not easy to achieve. In my original post, I alluded that amplifier matching could be accomplished by using resistors to attenuate the louder of two amplifiers that do not have matching SPL levels with the same input volume. I wish to expand on this topic, with the following additional information, below.
First, depending on the actual gain of the amplifier's input and output stages, the most sensitive amplifier (as rated by "input sensitivity" in mV) may
be the loudest amplifier in a "passive bi-amp" configuration. However, as we don't know the actual gain configuration of these amplifiers, the closest input sensitivity value would be the "best guess" at this time. It is actually best to use a solid state bass amp with a higher gain and input sensitivity, and use an outboard attenuator to reduce this amplifier's output, see more about this idea below.
Calculating the additional resistance to be added to the amplifier with the loudest SPL is straight forward, but requires that we know the "gain" of the amplifier design. If you have been following this topic at the LAB section, it has been noted that the gain can be measured by using a SPL meter and pink noise, then using the formulas to calculate the additional resistor required. My thanks to JoshK and jeffreybehr for their input, see their latest posts at THE LAB.
In my experience, it is easy to accomplish the level matching by using an external attenuator to find the correct resistor value, then installing that value into the input jacks of the amplifier. However, for best results, the amplifier designer should be consulted, as adding a resistor inside the amplifier may void the warranty, while using the outboard attenuator will not.
Channel Islands Audio, one of the Audio Circle Members, offers the VPC-3, which can be inserted in the signal path before the amplifier that is playing the loudest. Use Pink Noise and a RTA (or your ears) to match the levels of bass-to-midrange/treble. Using music to balance the two amplifiers is difficult, since you will need to play a wide variety of tunes to get a feel for the balance, while using Pink Noise is far easier.
I have found that using the attenuator for the bass amplifier works the best, since the very slight sonic "veiling" is not readily apparent in the bass, but is more audible in the upper midrange/treble area. This is not due solely to the potentiometer in the attenuation box, but also the extra connections and necessary interconnects. In the bass range, the added "veil" is not audible.
If you have balanced inputs and outputs, you can make an adapter to convert RCA to XLR when using the Channel Islands attenuator; contact Michael Percy at www.percyaudio.com
for further info.
Albert Von Schweikert