Here's something from Albert Von Schweikert posted
on the Von Schweikert board at Audio Circle. It may answer some questions, or raise new ones. In any case, the full discussion can be found here:
http://www.audiocircle.com/circles/index.php?topic=59529.0
From Albert Von Schweikert:
Hello Fellow Audiophiles,
Over the past 32 years, I've used bi-amping as a method to achieve more realistic sound quality. A decent, high powered solid state amplifier will give you explosive and tight bass power, while a smaller tube amp will give you the "air" and image float you are seeking from your sound system. Here are a few tips:
1. The bass amp does not have to necessarily be expensive, but make sure it has a large transformer and power supply to ensure "tight" bass. All of our speaker designs employ 4-ohm woofer systems, with a dip down to 3.5 ohms, so make sure the amp is stable into a 4 ohm load. Power requirements depend on your room size and how loud you want to go. I suggest 200-300 watts per channel if you have a large room and want an explosive dynamic range.
2. The tube amplifier can have as little as 20 watts per channel in a small room, but in a large room, you may want to have 50-100 watts per channel to avoid clipping at high volume levels. Due to the distribution of power over the frequency range of 20Hz to 20kHz, you can use a much smaller amplifier on the midrange and tweeter module. Use the 8-ohm taps on the tube amp, but feel free to experiment - it won't hurt anything to try other taps.
3. The "secret" to achieve "killer" sound quality is to ensure that the amplifiers have exactly the same input sensitivity. The amplifier with the lowest numerical rating, i.e. 100mV is much more sensitive than an amplifier with a rating of 500mV - the higher the number, the lower the sensitivity. You'll need to reduce the higher sensitivity by using a series input resistor. This can be installed inside your amplifier, directly at the RCA female jack leading to the input stage. If this sounds a little scary, then build an adaptor to house the resistor outside the amp. If you contact Michael Percy at www.percyaudio.com, he will advise you on what you'll need to get from him. Usually, a female RCA jack, a male RCA plug, a high quality metal film resistor, some plastic sleeving, and a short piece of high quality hookup wire is all that is required. Basically, you're inserting the resistor between the "hot" connection from the female RCA jack to the male RCA plug, and then using hookup wire to connect the ground leads. The entire assembly can be only a few inches long and installed into a plastic or Teflon sleeve to prevent short circuits. This adaptor is inserted into the signal path between your preamp and the most sensitive amplifier. As you can infer from this description, the adaptor is used in between your interconnect and the input jack of the amplifier.
Michael Percy sells these parts for less than $100 for everything you'll need, and he'll also help you select the proper value of resistor to match the sensitivities. Although this simple technique may sound like a "Micky Mouse" setup to "sophisticated" engineers that design chip OP AMPS and sell electronic crossovers to the PA industry, it is a very "pure" form of passive matching that will stomp the crap out of any electronic crossover I have ever tried.
4. Brands and Models: basically, put your money into the tube amp, since even inexpensive solid state amplifiers have stiff power supplies and enough quality to drive woofers with high speed transient response, high volume levels without clipping, and tight bass. Twenty years ago, Adcom built a 200-watt amplifier for around a thousand bucks new, and half that price used. Today, there are many expensive amplifiers that sound great, but you're paying for an expensive chassis, face plate, branding (advertising), and so forth. Instead, look for an amp with a large transformer and high quality power supply.
Tube amplifier choices abound, at all price ranges. Some of the best values are from China if you are on a budget, but large American tube amps from long established companies will be a good investment, with good resale value and the ease of repairs. Don't worry about the availability of tubes, they're going to be around for another 50 years or more due to the ever-expanding market for tube amplifiers.
Happy Listening from Albert Von Schweikert
