Buffers are sometimes needed when high input impedance and low output impedance is required, but no voltage gain, like in filters or attenuators. J-fets and tubes have a great property other than good sound, namely negative gate/grid voltage in respects to source/cathode. With proper biasing, zero offset voltage from input to output can be had, eliminating the audiophile money-robber, the capacitor. Shown here are solid-state variants, but glass is equally suited.
In these circuits the lower J-fet is a constant-current source with zero volts between the gate and source resistor, if the top J-fet has equal parameters and the same source resistance, it will too have zero volts between in-out. Since J-fets never have equal parameters, the constant-current source should be biased with a pot which is adjusted for zero offset, shown in the right circuit. The circuit on the right has cascoded input and current-source transistors, greatly enhancing performance. The cascode transistors should have as high as possible Idss giving max possible drain voltage for the J-fets below them.
Here the input J-fet has no source resistor, it will therefore have an offset of a few positive volts at it's source output, this offset is adjusted with the pot at the lower J-fet's source till zero offset is at the power MOSFETs output. Use power MOSFETs with low Vgs voltage, the ones I use have a Vgs of about 1.5volts when biased at 1amp. This circuit has no voltage gain, but hooked directly to a CD-player will demonstrate how loud less than a watt really plays. It is a great tool for evaluating preamps or other sources since it does little to the signal. It may actually be used as a low power amplifier to listen to music in small rooms(like with the computer), it's a great headphone amp too. It's only peril is an offset drift due to temp variations, so some time must be spent finetuning both the idlecurrent of the output MOSFET and the offset at the jfet. Place the j-fets real close together, or better yet, glue them together and wrap some alu-foil around to keep them at equal temperature. The MOSFET I use happens to have about zero drift at approx 1.5amps, and the offset stays within +/-30mV. Nothing beats single-ended sound and simplicity.
The output power is adjusted with the pot at the lower MOSFET, anywhere from less than an amp for headphones to several amps to drive low impedance speakers, it's all up to the heatsinking and transformer rating. Sacrificing some simplicity, cascoded current-sources would be bitching.
The output stage above can be built in a day, no circuitboard needed with this few parts. The only hard part is the chassis and cooling of the power MOSFETs. Thick bottomed aluminum kettles are great for this project, you'll find one in the kitchen somewhere. Bolt the MOSFETs to the bottom and paint the caserole black for better heat dissipation and it's a great 'bitches brew'. If the kitchen chief (wive, mom, dad, girlfriend, or whoever rules those grounds) finds out, just blame me:)
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