[sdiy] Tubes in Synths [ was : Harry's Nightmare! ]

[René Schmitz]

Hi Jürgen and Ben!

At 16:52 08.11.01 +0100, jhaible at t-online.de wrote:
>At first glance, just build a phase splitter with one tube
>to drive a LDR / Capacitor combination per stage.
>On second glance, this passive load will vary in a wide
>impedance range, and this will interact with the tube's
>output resistance @ anode and @ cathode. It's difficult
>to build an all pass network with truely constant amplitude
>gain this way. So I guess it's quite easy to build a
>resonant phaser (with one dominating peak), but difficult
>to build a notch phaser with deep notches.

Typical problem of this simple phase splitter - unequal output resistances.
This 
is also of concern in push-pull tube amps, where at high frequencies the amp 
isn't truely push-pull anymore, because of the different phase shift. 

However there are phase splitters that manage to have a low output
resistance at 
both outputs. The circuit I have in mind here is the phase splitter of the 
wiliamson amp. Basically a simple phase splitter that is preceded by a
cathode 
follower. Ok, you'd need two triodes per stage then. Many stages would be
expensive.
Yet, I think I'll try that one day.

>I'm not a tube expert at all. This is just where I stopped to
>follow the tube path when I built a univibe-style
>(transistor phase splitter) phaser some years ago.
>(That was the Neptune Phaser, and it's all transistor in the
>end.)

I'm a bit surprised here, because this circuit has also unequal output 
impedances. The emitter output is having current feedback, while the 
collector has not. (Similar situation as for a triode, although the 
impedances are lower.)

Cheers,
 René
-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159