Amplifier Input low pass filter issue ?

[Veysel]

Hi all
I want to use 1k and 100pf for input of amp to make it a fast amp.but it needs to increase to 330pf value for not burning the zobel when touching input by finger.Is it possible to use 100pf again but with a more powerful zobel circuit? Will it burn speaker ? Thanks

 

[sajti]

Zobel burning usually means oscillation. If You will use more powerful Zobel, that will not help against the oscillation, and yes, it can be dangerous for Your speaker.
Sajti

 

[Veysel]

Zobel burning usually means oscillation. If You will use more powerful Zobel, that will not help against the oscillation, and yes, it can be dangerous for Your speaker.
Sajti

hi ty for reply
but related to input filter.
this is luxman amplifier not may design.

 

[Mark Tillotson]

It may be that your amp is unstable for certain source impedances - presumably its fine open-circuit and with a low-impedance source, but the finger represents a highish reactive impedance.


I suppose it is possible you are injecting RF directly at high level by touching the amp, but that would imply you are close to an active transmitter.

 

[Veysel]

hi Tillotson , is this normal ?

 

[Mark Tillotson]

I wouldn't expect there to be that much high power RF in a normal environment - the 100pF should be taming it anyway. Instability can be due to the input impedance being "seen" by the feedback point through the input device capacitances - it indicates the amp as a whole is too close to oscillation for comfort. Adding more input capacitance might help for this.


You know that there's no need for a fast amp to have a fast input? The high bandwidth in an amp allows more negative feedback to be available at high audio frequencies, its not so ultrasonic inputs can be amplified.