Re: Gain equations : ref frequency response



Ruud Broens wrote:

> "Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
> news:43A66DC8.F67BF88B@xxxxxxxxxxxxxx
> : As a consequence of the issues raised in the recent debates, I'm
> : amazed that there seems to be widespread assumption here that the gain
> : of a tube output stage ( SET *or* P-P ) is simply derived from a stock
> : equation.
> :
> : In the gain equation the plate load determines the upper term.
> :
> : Where the ( refelcted ) load has a variable impedance ( such as a
> : loudspeaker ) then this term is also variable.
> :
> : Hence in the real world - it's impossible for such a circuit
> : configuration to have a flat frequency response. From first
> : principles.
> :
> : A flat frequency response can only exist into a pure resitive load.
> : Speakers are *not* pure resistive by any means.
> :
> : The response can be *improved* by negative feedback but the response
> : errors can never be eliminated.
> :
> : This only applies to circuits with the load in the plate circuit.
> : Loads in the cathode have no such problem by reason of the circuit
> : operation ( some understanding of electronic theory is presumed ).
> :
> : Graham
> :
> Graham, if you reallly want to discuss something here,
> start with dispensing with the debating trade technique,
> then learn how to write coherently :-)
>
> Rudy
>
> o, btw, we know Rp is a dynamic, current dependent impedance
> with ul or triodes in pp, that is of no concern as it balances out
> and as long as that stays well below reflected impedance it is
> close enough to a 'perfect voltage driven loudspeaker'.
>
> with se you can make the change in Rp small by using a high
> current bias, then using a transformer that will give a reflected
> impedance well above Rp-avg., no problems, either
>
> so what was ya gonna say ?

Depends on your view of 'close enough'.

Yes - at the expense of total power output you can adjust the turns ratio of
the output transformer so as to make the output Z appear lower but you're
into a game of diminishing returns since you can't realistically get the
output Z of a non-nfb SET to a figure that's genuinely not going to give
freq response errors. Only a topology with a naturally low Z can do that..

It's a fundamental flaw of the topology. You can tinker with it to your
heart's content but it won't go way.

Why not consider my "new approach to the SET" ? Fixes the problem at source.

Graham


.

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