[JN] SRPP deconstructed (was: (no subject))

Christian Rintelen christian at rintelen.ch
Mon Aug 17 13:23:49 CDT 2009

it seems that the opinions on SRPP vary quite a bit... brosky writes in 
his article

> "Today, the prevailing view (a new view) is that the SRPP is an SE 
> circuit made up of a grounded-cathode amplifier that is loaded by a 
> cathode follower that also functions as a constant current source; (a 
> nice trick: providing
> a low and an infinite impedance at once)"
which i don't think is true.

i'm not using dissimilar tubes for SRPP, just one twin triode for L and 
R (or + and - phase in case of balanced drive) for the bottom and 
another one with an elevated heater supply for the top. one of the 
reasons why SRPP tubes don't last very long is the huge voltage 
difference (heater-cathode).

yes, the bottom tube appears to have the gain. but the top tube is in 
the signal path as well - and (imho) not as a cathode follower, but 
something between (to quote another joenetter):

> its
> like a feed-forward active load with an rP that varies with signal.
> the load decides if it works "pp" or "quasi-current source". it is not
> constant current, nor is it completely pp. with a very high impedance
> load, it can approach "mushy current sourced triode", but with the
> right load it alternately sources and sinks current into the load from
> both tubes... (pp) this is when the distortion is lowest (cancelling).
and since i'm using different tubes, this gives me the opportunity to 
use the good ones where they influence the sound more and the cheaper 
ones where it doesn't count that much...


bear wrote:
>> joes,
>> which of the two tubes in a SRPP configuration has more influence on the
>> sound - the bottom or the top? and why so?
> (bottom one appears to have the gain...?)
>> thx for hints, ?
> Hmmm... I always assumed you want the two tubes to be of the same type so
> that there is maximum linearity top to bottom...
> but it brings up some interesting ideas about it, some of which I'll give
> some thought to... deliberately making them different...
> Broskie's site has a pretty complete discection of the circuit... he may
> have covered the dissimilar top and bottom tube.
>            _-_-bear
> _-_-bear

Konzept   und    Text:     Christian   Rintelen
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