
Home  Forums  Rules  Articles  diyAudio Store  Blogs  Gallery  Wiki  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
26th March 2008, 05:38 AM  #1 
diyAudio Member
Join Date: Apr 2002
Location: Belleville, IL.

Current to overcome 6.5 uuF input capacitance?
How much current would be needed to overcome a input capacitance of 6.5 uuF in order not to roll off the highs below say 30KHz?
__________________
Gavin 
26th March 2008, 06:26 AM  #2 
diyAudio Member
Join Date: Jun 2005
Location: San Diego, CA

To properly answer that question you would need to provide two more specs....
What is the source impedance..ie, what is the output impedance of where the signal is comming from ....also if it has output caps..provide those values... Also you need to know the input impedance that is associated with this cap... Is there a input tube grid at this input also??? If so the miller cap needs to be accounted for as well... All these R's and C's need to properly put togather into a equivelent circuit model..then you can properly calculate the frequency response.... The current you are refering to is proportional to the source output impedance..... Chris 
26th March 2008, 06:54 AM  #3 
diyAudio Member
Join Date: Jan 2002
Location: nowhere

The source impedance does play a role in that it provides a voltage divider, but it has nothing to do with his question does it?
The current does however depend on the amount of voltage you need to slew at 30kHz. So if it at the output stage where you can have more than 100volts with low mu tubes, or at the input where you likely have less than 1volt at teh input you will have different currents (obviously). Isn't the formula something like iC=(2*Pi*f*Amplitude)? But remember the capacitance is not just the datasheet capacitance, but the stage's gain multiplied with the capacitance. The output caps have nothing to do with this either. 
26th March 2008, 06:59 AM  #4 
diyAudio Member
Join Date: Jan 2002
Location: nowhere

Hmmm...it's been ages since I bothered to calculate such, so not sure I got this right, probably should have kept my mouth shut.
Think it is iC=(2*Pi*f*Amplitude*C) Btw, uuF is today called pF. 
26th March 2008, 09:18 AM  #5 
diyAudio Member
Join Date: Jun 2005
Location: San Diego, CA

The slew rate limiting equation (5 time constants) when fully derived is equal to the 3dB POLE x 3.3 , I did the proof many years ago...
Lets say you want 20kHz to be free and clear from the slew rate limiting distortion...or it will be the boundary.... By putting the 3dB high frequency pole at 3.3 x 20kHZ , at 66kHz...puts 20kHz in the clear.... The only reason I wanted to know the output caps was for determining the low frequency rolloff... The ouput impedance plays a major role in the high frequency rolloff... It is directly proportional to output current drive.... For the same given output amplitude...the lower Z has to source more current... For proper calculation you really need to include the interconnects, since these have inductance and capacitance per foot that can be modeled as a lumped parameter Transmission Line... Chris 
26th March 2008, 10:22 AM  #6 
diyAudio Member
Join Date: Jan 2002
Location: nowhere

Yes u are right. Its been a while since I last contemplated the issue. (It was interesting to me when designing with MOSFETs and after switching to tubes I havent worried about it).
The formula I mentioned for figuring the needed current for the given slew rate, is just a variation to the formula for capacitor impedance. Impedances and capacitances go into making low/high cutoffs so it just makes into a different view  same problem. Ohms law works either which way it is approached... So basically he needs to know the max source impedance at 3.3*30kHz = 99kHz? Also find te actual capacitance, is there gain etc.. 
26th March 2008, 12:36 PM  #7 
diyAudio Member
Join Date: Apr 2002
Location: Belleville, IL.

Well the source is a Cambridge audio 540P MM phono pramp. That doesn't really help does it? Here are the only specs they publish:
http://www.cambridgeaudio.com/specif...0preamplifier I suppose I could write them a email and ask for a little more info. The tube is a 6AS7.
__________________
Gavin 
26th March 2008, 12:51 PM  #8 
diyAudio Member
Join Date: Jan 2002
Location: nowhere

So u are coupling a 6AS7 to a 39dB gained phonostage huh?
At least u wont be bothering the neighbours Should sound great thru headphones perhaps. 
26th March 2008, 12:53 PM  #9 
diyAudio Member
Join Date: Jan 2002
Location: nowhere

Seriously G. U need not worry about the 6.5pF when u also have cables interconnects and such that totally dominate there.
What is it u are trying to achieve? 
26th March 2008, 12:59 PM  #10  
diyAudio Member
Join Date: Apr 2002
Location: Belleville, IL.

Quote:
__________________
Gavin 

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Input capacitance  scottw  Chip Amps  0  3rd January 2009 06:43 PM 
AT95E/SUT Input capacitance  pengboon  Analogue Source  0  23rd June 2007 04:10 AM 
MOSFET input capacitance  xpro  Solid State  20  26th April 2007 01:18 PM 
About the input capacitance of the MOSFET  billy  Pass Labs  11  26th February 2003 09:14 PM 
MOSFET input capacitance  DarkOne  Solid State  6  7th February 2002 06:28 AM 
New To Site?  Need Help? 