High-Power Buffers

Local Feedback Buffers
    The local feedback buffer has no global feedback loop, relying instead on degenerative feedback at each active device's output to keep the output in line with input voltage. Yes, this is the same mechanism used in a cathode follower, which keeps both its output distortion low and its output impedance low. (Solid-state devices offer the added advantage of allowing a symmetrical topology not possible with tubes, as there is no P-channel version of a triode.)
     Whereas the global feedback buffer is usually run in lean Class-AB mode, in order to reduce power consumption, the local feedback buffer is usually run in beefy Class-A mode, as the higher currents both enhance the linearity and extend the frequency bandwidth.
   IC examples of both types are readily found in the National Semiconductors catalog. The LH4004, LH4006, LM102 and LM310 are of the global feedback type and the LH0002, LH0033, LH0066, and LH4001 are of the local feedback type. (Linear Technology also makes some excellent buffers.) Examining the schematics for these buffers is a good source for gaining insight into the designing of a buffer, as the schematics are often reduced to idealized versions that are much clearer than their actual implementation.

MJ Stereo Technic
    The wonderful Japanese audio magazine, MJ Stereo Technic also known as Audio Technology MJ, has for the last two decades run articles that featured high-power buffer circuits. Usually, the circuits look like they are just the last half of a conventional amplifier, but a few have been more interesting. The aim of these buffers is to provide no gain, but sufficient current gain to drive loudspeakers.
    Surely, the logic is compelling: tubes cleanly provide voltage gain, but are current limited; solid-state devices provide huge current gains, but are not as linear at voltage amplification. (A gross oversimplification, but essentially correct.) So why not use each only for its best use? In other words, why not have a hybrid system?

      Although buffers might be new to many audiophiles, they are a central part of the analog electrical engineering practice. In short, a high-power buffer is a special type of power amplifier: it relays its input voltage to its output un-amplified and it delivers the needed current increase into the load. In other words, like a conventional amplifier, the buffer can deliver power into a load, but the buffer does not add any voltage gain; instead, it only provides the required current gain. For example, a good tube-based line stage can usually put out at least 30 to 50 volts of peak output swing, which into an 8-ohm load would equal 50 to 150 watts of power, which if the line stage could deliver 4 to 6 amps of current, it would deliver that much power.
      Buffers come in two basic styles: the global feedback buffer and the local feedback buffer.

Global Feedback Buffers
     The global feedback version is simply a conventional power amplifier with all its gain being returned to its input stage. A typical power amplifier has a voltage divider in the form of a feedback loop, bridging its output to its input, which defines the gain of the final gain of the amplifier. The greater the voltage division, the greater the final gain.
    What is happening here is that the feedback mechanism within the amplifier strives to keep the voltage divider's center voltage in line with the amplifier's input voltage. The greater the voltage division, the greater must be the output voltage to allow the same matching of amplifier's input and feedback voltages. If, on the other hand, we eliminate the voltage divider and return all of the output voltage to the negative input, the amplifier's feedback mechanism will force the output voltage to fall until it matches the input voltage. In other words, we have turned an amplifier into a buffer. (Not all power amplifiers, however, can be converted into buffers, as not all amplifiers are unity gain stable.)


www.tubecad.com   Copyright © 2002 GlassWare   All Rights Reserved