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the Power Follower
original design by Andrea Ciuffoli - 1999 - 2005

After many years this project, published on my my website and on 3 magazines, is continuing to interest many persons in any country.

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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.

With this design we achieve:

The second feature is in the big concern in the power supply design.

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.

With a bias current of 3A you can get about 30 Wrms @ 6 Ohm load.

The power can be increased with the unique drawback of the dissipated power (heat)!

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.

Download this Excel table to customize your Power Follower (start download).

I have implemented this circuit in a standard rack with side heatsinks, whose dimensions are: 30x42x12 cm. I would suggest to increase its dimensions to 30x42x16 cm, or to choose for a different mechanical layout like a forced air flow tunnel.

Please note the pervasive ELNA Cerafine usage, wich I like a lot. I would say that the MKP/Paper in oil input cap, the input bypass (470 uF) and the output cap are significant over the sound quality. Concerning the others ROE, SPRAGUE, MALLORY o SIEMENS computer grade are OK. 

Concerning the output capacitor, it is important to understand that such a component is a need, and does not worsten the device quality. Again note that a dual rail with a push-pull output stage does NOT solve the problem! 

The presented topology, has NO gain (actually it looses something) and should be preceded by voltage gain stage, with an output swing not lower than 10Vrms and Rout < 1000Ohm.   

The input impedance is 47KOhm/1450pF and its resistive value can be adjusted by a pretty wide range, just using a different input resistance. 

The input capacitance, 1450pF, seems to be very high. Please note that a gain stage with a 1000 Ohm output resistance gives a > 100 KHz cutoff frequency.

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.

About wire; forget the teflon ! Use only stranded tinned copper wire with pvc insulator, and for loudspeakers use the same.

A very good quality stranded tinned copper wire could be bought directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.



It It is possible to wire the circuit without a PCB. In order to reduce the circuit complexity to be handled by this kind of implementation it is possible to leave the regulator, obviously increasing the Filter Capacitor from 3300 uF to a minimum of 22 mF.

Audio performances

This circuit do not introduce any alteration on the signal so is very important take care of driver stage/voltage amplifier.

If driven by a single ended vacuum tube preampl. with enough gain can give a sound better that a 300B amp.

Many person drive this amp. with E88CC in totem pole, E182CC/5842/6C45 in single ended or SRPP (Toptem pole) or  with the Nelson Pass pre using a single IRF610.