Roll off frequency.

Katapum · 24th April 2007, 05:35 PM

Hi all,

I´d like to find Jadis roll off frequency.
I think main formula is f=1/(2*pi*C*R). What are R and C values or what to do to find them?

I appreciate your help.

Katapum

Tweeker · 24th April 2007, 06:44 PM

R is in ohms, C is in Farads. To convert microfarads (uF) to farads, divide by 1 million.

This page is handy for determining the cutoff (-3dB) of an RC circuit.

This wont necessarily give you the amps actual limits, the OPT comes into play here too.

Tweeker · 24th April 2007, 07:02 PM

HF rolloff will be determined by OPT or whenever the pair of 12AX7 sharing .8ma runs out of steam trying to drive 3 6550s in UL. I wouldnt expect it to be very high.

Katapum · 24th April 2007, 07:28 PM

For now I´m only concern about two ½ 12AX7 and 3 6550. So how can I exactly find C and R?

Tweeker · 24th April 2007, 07:35 PM

The dominant roll off will be .47uF and 474k.

474k is a very high grid leak for what appear to be fixed bias 6550s.

Katapum · 24th April 2007, 07:40 PM

And Miller capacitance doesn’t play any roll?

Tweeker · 24th April 2007, 07:57 PM

The .47uF and 477k is for LF roll off. Miller capacitance enters into HF and is much more complicated to calculate. It is often worse than it appears as the 12AX7s must supply the current to charge this capacitance, the 12AX7s output impedance is not the only factor. Calculating Miller capacitance for UL op is involved.

Designing Common-Cathode Triode Amplifiers.

Katapum · 24th April 2007, 08:05 PM

From another thread:

Quote:

Posted by fdegrove

One caveat, the amp is seriously limited in high frequency response by its driver stage, it's starts to roll off gently from ~ 16KHz.

So how can I find 16kHz value?

PCC88 · 24th April 2007, 09:49 PM

This page details the effects and calcs for Miller Effect

http://www.aikenamps.com/MillerCapacitance.html

24th April 2007, 06:44 PM	#2
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	R is in ohms, C is in Farads. To convert microfarads (uF) to farads, divide by 1 million. This page is handy for determining the cutoff (-3dB) of an RC circuit. This wont necessarily give you the amps actual limits, the OPT comes into play here too. __________________ Be sure your foil hat has a good low impedance ground.

24th April 2007, 07:02 PM	#3
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	HF rolloff will be determined by OPT or whenever the pair of 12AX7 sharing .8ma runs out of steam trying to drive 3 6550s in UL. I wouldnt expect it to be very high. __________________ Be sure your foil hat has a good low impedance ground.

24th April 2007, 07:35 PM	#5
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	The dominant roll off will be .47uF and 474k. 474k is a very high grid leak for what appear to be fixed bias 6550s. __________________ Be sure your foil hat has a good low impedance ground.

24th April 2007, 07:57 PM	#7
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	The .47uF and 477k is for LF roll off. Miller capacitance enters into HF and is much more complicated to calculate. It is often worse than it appears as the 12AX7s must supply the current to charge this capacitance, the 12AX7s output impedance is not the only factor. Calculating Miller capacitance for UL op is involved. Designing Common-Cathode Triode Amplifiers. __________________ Be sure your foil hat has a good low impedance ground.

24th April 2007, 09:49 PM	#9
PCC88 diyAudio Member Join Date: Apr 2004 Location: South East	This page details the effects and calcs for Miller Effect http://www.aikenamps.com/MillerCapacitance.html __________________ the northern seas are cold, but they're our own.

24th April 2007, 07:28 PM	#4
Katapum diyAudio Member Join Date: Mar 2003 Location: Europe	For now I´m only concern about two ½ 12AX7 and 3 6550. So how can I exactly find C and R?

24th April 2007, 07:40 PM	#6
Katapum diyAudio Member Join Date: Mar 2003 Location: Europe	And Miller capacitance doesn’t play any roll?

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