Re: Negative feedback: cheap engineering, sure, but is it cheerful?



Patrick Turner wrote:

> Sander deWaal wrote:
>
> > Pooh Bear <rabbitsfriendsandrelations@xxxxxxxxxxx> said:
> >
> > >> > Some folks claim that hybrid amps offer the worst of both worlds; others,
> > >> > the best. Let me know if I can help.
> >
> > I make hybrids.
> > E288CC, single or a pair, in SRPP as a driver, several pairs of
> > 2SK135/2SJ50 as outputs.
> > No loop feedback, just a DC servo around it.
>
> Is it a class A mosfet source follower output stage with a total of 6 fets?
>
> It makes a nice output stage, a transconductance transformer if you like,
> and the beauty of multiple parallel mosfets is that their Gm of about 0.9A/V
> becomes 2.7 A/V with 3 top fets or 3 bottom fets,
> but altogether in class A the Gm = 5.4 A/V, and since open gain A
> = Gm x RL, and RL = say 8 ohms, then A = about 40,
> bit this is reduced to less than unity because of the follower connection
> so the amount of series voltage local NFB = approx 32 dB.
>
> In class A there is no need for any other NFB providing you can keep
> the linearity of the driver tubes while providing speaker type voltages.
>
> The down side of mosfets is their gate to source capacitance,
> and with 6 fets all in class A it amounts to about 400pF per fet, 2,400 pF total
> but the
> source follower connection reduces this capacitance dramatically
> to probably around 200 pF, which should not roll off the
> HF to any great extent if the Ra of the driver triodes is low.

Interesting.

One of my designs used no less than 12 Hitachi lateral mosfets with an ever higher
*monster* transconductance.

The gate input capacitance was dealt with by driving the fets from a class A emitter
follower stage.

2 highly linear voltage gain stages ( using local feedback ) meant that modest global
NFB could be used.

This was the amp that could produce 350W into 8 ohms with 0.0007% THD ( -103dB ) @ 1kHz

It could drive 2 ohm loads too !

Graham

.

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