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Old 24th February 2010, 08:12 AM   #1
GEirin is offline GEirin  Argentina
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Default Switchoff speed capacitor

Hi all.
Which is design criterion for to determinate the value of the capacitor?
Self Blameless amp: 220R // 1uF
Ostripper FA2 amp: 100R // .68uF
Ostripper FA2S amp: 100R // .02uF
DestroyerX DX Blame ES amp: 220R // 220nF
MikeB Symasym5 amp: 33R // 100nF
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GEirin
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Old 24th February 2010, 09:50 AM   #2
AndrewT is offline AndrewT  Scotland
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the resistor determines the bias current flowing through the driver stage.

The capacitor can be approximated by trial and error and listening.
I have seen [0r82+1uF]//R
There must have been a lot of trail and error to find that combination.
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Old 24th February 2010, 10:21 AM   #3
GEirin is offline GEirin  Argentina
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Thanks Andrew

RC time constant and cutoff frequency

Self Blameless amp: 220R // 1uF --- 220 useg, 723Hz ---
Ostripper FA2 amp: 100R // .68uF--- 150useg, 1061Hz ---
Ostripper FA2S amp: 100R // .02uF --- 68useg, 2340Hz ---
DestroyerX DX Blame ES amp: 220R // 220nF--- 48,4useg, 3288Hz ---
MikeB Symasym5 amp: 33R // 100nF --- 33useg, 4822Hz ---

Other amplifier
Harman Kardon AVR745 --- 820useg, 194KHz ---
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Old 24th February 2010, 10:36 AM   #4
PMA is offline PMA  Europe
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bipolar (bjt) transistor families for audio power output stages
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Old 24th February 2010, 10:52 AM   #5
GEirin is offline GEirin  Argentina
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sorry for mistake

RC time constant and cutoff frequency

Self Blameless amp: 220R // 1uF --- 220 useg, 723Hz ---
Ostripper FA2 amp: 100R // .68uF--- 68useg, 2340Hz ---
Ostripper FA2S amp: 100R // .02uF --- 2useg, 79577Hz ---
DestroyerX DX Blame ES amp: 220R // 220nF--- 48,4useg, 3288Hz ---
MikeB Symasym5 amp: 33R // 100nF --- 33useg, 4822Hz ---

Other amplifier
Harman Kardon AVR745 --- 820useg, 194Hz ---
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Old 25th February 2010, 11:33 AM   #6
Bonsai is offline Bonsai  Taiwan
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I use a triple output stage. I am using MJE15033/32's as the drivers. These have a 15 Ohm resistor connected emitter to emitter, and they then feed into the output devices. Originally, I had no base stoppers and therefore had stability problems in the output stage. Now I have 3.3Ohm base stoppers - no instability. So, my feeling is, if you run the driver stage hot and in class A with a very low emitter to emitter resistor (like 15 Ohms), and you use fast output devices, you can get away without a base charge suck-out capacitor.
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Old 25th February 2010, 12:02 PM   #7
Eva is offline Eva  Spain
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There are more impedances into play than just the bias resistor (base input resistance and capacitance actually), and stored charge in the B-E junction, so time constants are much shorter.

The whole subject is difficult to understand without having done some experimentation with bipolar transistors used as switches, where base charge storage and removal phenomena become quite evident.
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Old 24th April 2011, 11:34 AM   #8
GEirin is offline GEirin  Argentina
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Hi all.
Here two types of speed switchoff capacitors.
First, the traditional R / / C. TC = RxC.
The second, Roender amplifier is more complex, with damping resistor + C. Here, how to calculate the TC?
Thanks.
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File Type: jpg Switchoff speed cap OS.JPG (10.5 KB, 123 views)
File Type: jpg Switchoff speed cap Roender.JPG (14.3 KB, 123 views)
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Old 24th April 2011, 01:15 PM   #9
Dave S is offline Dave S  United Kingdom
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I agree iwth Prof Leach. It is not neccessary if the driver stages are fully run in class A.

It also sounds better without the cap in the 2 amps I tried (including the DX Blame).***

*** Opinion alert!
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Old 24th April 2011, 06:14 PM   #10
GEirin is offline GEirin  Argentina
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Dave S, thank you very much for your replies. But I need to know how to calculate network TC: (R + C) / / R.
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