[AudioCats]

Why is this design less suitable (comparing to the resistor loaded) for capacitive load?

I thought the upper tube in a SRPP design forms a constant current source, (as long as the output current is not higher than the bias current), it should act the same as any CCS, no?

 

[bidoux]

does not matter anymore

 

[nikongod]

Quote:

Originally Posted by AudioCats /img/forum/go_quote.gif Why is this design less suitable (comparing to the resistor loaded) for capacitive load?

The advantage of SRPP as a circuit element is that it provides low output impedance AND gain in a single bottle. compared to a cathode follower which has gain of ~1 but low output impedance this is a significant advantage to cost. Compared to a simple grounded cathode amplifier stage driving the load off of the plate the output impedance of either (SRPP or various cathode followers) is of course much lower assuming similar tubes... Whether lower output impedance is actually an advantage for electrostatic headphones is debatable.

The first part comes from the biggest problem with SRPP.
SRPP is load sensative. this is a problem for any headphone amp really, but in the case of an electrostatic where the load impedance varies from infinite at DC to relatively much lower at 20K this could be a problem. If you want SRPP to provide gain as well as output current you quickly enter a delicate balancing act between the size of the "middle" resistor and how much current you can put through it with your voltage rails. things get tricky fast.

Problem 2 comes as a unique surprise to anyone trying to use any type of follower to drive large voltage swings: The Vhk of the tube must be respected, but this can be very difficult as you can imagine.
Quote:

I thought the upper tube in a SRPP design forms a constant current source, (as long as the output current is not higher than the bias current), it should act the same as any CCS, no?

The "top tube" acts like a CCS if the load is driven off of the plate of the bottom tube. If the load is driven off of the cathode of the top tube (SRPP) the top tube behaves more like a cathode follower. This is a problem because the current through the top tube varies with load current and may starve the bottom tube of current. This is of course class-B operation.

If you are interested in reading more about why SRPP sucks compared to a white cathode follower for a headphone amp or how it works in general check out the broskie/tubecad articles on the subject. He also has a couple electrostatic amplifier scheamtics posted around his site that could be cool.

 

[FrankCooter]

Great post, nikongod!

 

[El_Doug]

thanks Ari, that was extremely informative!


now I want to hear an A-10 even more

 

[AudioCats]

Thanks for the great info