When is a buffer stage required?

[chipco3434]

I have built a couple of NIGC's from BrianGT's kits and some speakers. But, I have no idea why I would need a buffer stage.

Help me understand this.

[cunningham]

I am not familiar with this amp circuit but a buffer usually refers to impeadence matching. Perhaps the input inpeadence of this circuit is lower than what a typical pre-amp circuit outputs, and some type of emitter folower type circuit is needed to lower the impeadence. Other than this...

[janneman]

I agree with Cunningham. A buffer is also required to avoid capacitive loading of a source with high output impedance by the cable used to connect it to the power amp. Some of these 'special' cables have a relatively high capacitance per unit length so might exerbate the problem. (One good reason to avoid the 'special' cables. The other could be the outrageous price ).

With sources like CD players or SS preamps this very seldom is a problem. A buffer may be required with tube-output CD players or preamps, or when you are driving poweramps with relatively low input impedance (less than a few k or so).

It is my understanding that sometimes people use buffers to change the character of their sound system. That is of course a valid reason, but the buffer is not required for technical reasons in such a case.

Jan Didden

[robert_chien]

A preamp is usually a voltage feedback amplifier, which is very sensative to capacitive loading. Square wave response will show very significant overshoot. I guess this is why cables make differences. A buffer does not have overall volatge feedback path, and usually has more graceful response to high capacitive loading. Inserting a buffer stage thus will usually make the response more close to what you want.

[janneman]

I guess you refer to some 'naked' VF opamps., driven by laboratory grade high-speed square-wave generators.

A competently designed preamp or source, IC or discrete, has no problems with anything but insanely large cables capacitances, and certainly no overshoot. I don't know of any musical instruments that give off square waves with very fast rise/fall times that cause ringing. Even if they exist, the source (analog or [filtered] CD) will not pass those fast edges anyway.
A buffer is totally superfluous here.

Jan Didden

[robert_chien]

I think Jan is right that there is no square wave in music. Whereas I would think that the square wave response is a sign to indicate that the transient response has become worse, because the phase margin has decreased. If a preamp shows apparent overshoot for a square wave input, it is doubtful that it is in a good condition driving music signals. Inserting a buffer stage alleviates this doubt. And of course a buffer stage is not necessary if the pre amp is designed to drive capacitive load up to some possible value, say 2000pF.