Follower design



Keywords: follower design, single ended 813 amplifiergm70 845 2a3 300b
Description: It seems a silly and common circuit but it has got great differences from any other similar design, that’s why this is unique. We have a typical source follower (as an emitter follower but

It seems a silly and common circuit but it has got great differences from any other similar design, that’s why this is unique.

We have a typical source follower (as an emitter follower but with a Mosfet) working in pure class A with a current generator.

Please note that this circuit works only in pure class A, so it requires enough bias current for the requested output power.

In my design the first feature making a big difference in the sonic results is the negative power supply and the signal ground tied to the drain of source follower.

The discoupling of the signal current from power supply because the current generator has a huge impedance (an open circuit for the signal, in theory)

The constant current flow from the power supply also during the music peaks that prevent any stress of the power supply.

The simmetric distribution of the power dissipated under static and dynamic conditions both on the current generator and the source follower, so I have used the same type of devices.

Onto a capacitive-input power supply, where there is a diode bridge connected to a big capacitor, very high current peak are generated on the diodes. Those peaks generate residuals all over the amplifier power band.

So, this circuit adopt a little capacitor after the bridge (only 3300uF for an amplifier working with 3A bias).

This capacitor is followed by a MOSFET voltage regulator, featuring a very low frequency CR filter (220uF/100Kohm) which is based on the Virtual Battery Operation, invented by Technics (something good can be found in the consumer world, too).

The 500 Ohm precision trimmer should be set to balance the output fuse voltage to 1/2 of the regulator output.

This single trim could be done after the switch-on, because it just optimizes the output swing to the maximum available.

The 2 x 18V 1W zener are optional and these can be used to prevent crash of the mosfet when it receive bad input signal like a vacuum stage startup (these can decrease the sonic performances).

The quiescent current is set by the resistence on current generator source pin, and can be changed with the simple formula Iq = 0.7 / R where 0.7 is the BD139 Vbe.

I have implemented on-board a connection for a switch to modify the bias current (2 values) because, since sometimes I listen at music with a low spl in this way I can save power dissipation.

The power supply transformer can be used with different voltages and currents, from 24V to 33V but with 250 VA min. per channel otherwise you have to reduce the bias current.

In fact this is a single ended class A, therefore the theoretical efficiency is 25%, with 100 W dissipated power, we have only 25 W available to the load.

The output device, IRFP150, can be substituted by their TO3 equivalents or by other similar MOSFETs like IRF250, IRFP250, IRF240, IRFP240, with a minimal impact.




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