Blencowe's bootstrapped DCCF cap value. - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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
Reply
 
Thread Tools Search this Thread
Old 16th January 2013, 04:11 PM   #1
diyAudio Member
 
Robert Kesh's Avatar
 
Join Date: Oct 2012
Default Blencowe's bootstrapped DCCF cap value.

Have been playing with this circuit

Click the image to open in full size.

from Merlin's page here The Valve Wizard

I can't figure out how to calculate the frequency response effect of the bootstrap cap. Any ideas? I'm using a variety of tubes and resistors. So am curious about this.

(pic reproduced without permission, but with much respect)
  Reply With Quote
Old 17th January 2013, 12:27 PM   #2
diyAudio Member
 
Robert Kesh's Avatar
 
Join Date: Oct 2012
No one?
  Reply With Quote
Old 17th January 2013, 04:42 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
It will probably add an LF shelf. Estimating the size of the shelf is fairly easy. Calculating it accurately is a bit more complicated.

To estimate:
At high frequencies when the bootstrap is effective you can assume the amplifier valve sees an infinite anode load, so gain is mu (assuming cathode is decoupled).
At low frequencies when the bootstrap is ineffective you can assume the anode sees the two resistors in series, so gain is mu x (R1+R2)/(Ra+R1+R2) where Ra is anode impedance (Rp for Americans).
So the size of the shelf is (R1+R2)/(Ra+R1+R2).

This estimate assumes the CF is ideal and/or unloaded. Neither assumption is true, but both are near enough true for many practical purposes. One thing to watch is that when a bootstrap is used it raises the output impedance of the CF by a ratio approximately given by (Ra+R2)/Ra.

The frequency where this shelf occurs will roughly be given by f = 1/( 2pi C1 R1 ).

Last edited by DF96; 17th January 2013 at 04:44 PM. Reason: expand
  Reply With Quote
Old 17th January 2013, 05:14 PM   #4
diyAudio Member
 
Robert Kesh's Avatar
 
Join Date: Oct 2012
Thanks. Since posting I did some simulation, and confirmed the shelf response.

One thing: As above f the load is effectively infinite, we have constant current, so isn't a cathode decoupling cap unnecessary? Because I was using no cap and low Ra valves with fairly high R1 and R2, I think this was the main gain advantage I was getting.

Edit: I just tested it with and without a coupling cap, there is some difference, but not much, and my bootstrap shelf was occurring at 70Hz, so below guitar frequencies.

In general though, the bootstrap seems to just sound good, even if I factor in the gain change by upping the input signal. Bootstrap mojo, I guess. Or maybe that worsening of impedance.

Last edited by Robert Kesh; 17th January 2013 at 05:38 PM.
  Reply With Quote
Old 17th January 2013, 07:46 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
With an infinite load you don't need a cathode decoupling cap, but then you can only apply an infinite load. A real CF is not an infinite load. Of course, at really low frequencies you lose both the decoupling and the bootstrapping so you can get two LF shelves which may or may not overlap.

The bootstrap seems to have a good reputation among people who have tried it. It is a close relative of the SRPP. Other things being equal the SRPP may be better, but the bootstrap circuit has the advantage that the CF part can be run at higher current for better driving capability.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Bootstrapped long tail Zian Tubes / Valves 5 24th May 2012 09:29 PM
Bootstrapped anode mashaffer Tubes / Valves 7 21st February 2012 11:16 PM
Bootstrapped ClassA - ? Bigun Solid State 24 28th August 2009 02:20 PM
Bootstrapped Op Amp davidallancole Solid State 5 14th May 2007 12:12 PM
What is bootstrapped? Optical Solid State 16 21st November 2002 12:22 AM


New To Site? Need Help?

All times are GMT. The time now is 08:57 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2