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Old 20th November 2002, 10:26 AM   #11
Optical is offline Optical  New Zealand
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So for example, on the esp website under project 30, the mixing desk, he mentions that the balanced input circuit would be better if it were 'bootstrapped'.. how would this circuit be modified?

would i be right in thinking from previous posts that at the input, where the 100k resistor goes to ground, could be connected to an attenuated output of the circuit?

btw thanks for all the advice and knowledge guys

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Old 20th November 2002, 10:29 AM   #12
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site here for those who are interested..
http://sound.westhost.com/project30a.htm
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Old 20th November 2002, 11:14 AM   #13
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Quote:
Originally posted by Optical
So for example, on the esp website under project 30, the mixing desk, he mentions that the balanced input circuit would be better if it were 'bootstrapped'.. how would this circuit be modified?

would i be right in thinking from previous posts that at the input, where the 100k resistor goes to ground, could be connected to an attenuated output of the circuit?

btw thanks for all the advice and knowledge guys

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Well, it would, except in this case there is a hitch (assuming the + and - input are equal in amplitude but opposite in phase). Because of the balanced inputs, the signal at the top of the 100k is already (drum roll) ... zero! So there is no point in making zero what already is zero. There is only DC through the 100k, to provide a return path for the input bias current.

Now, if you use this circuit single ended, with one of the inpuits grounded, then yes it would work as you say.

One thing to watch out for: if you put the bottom on a signal that is larger than the input, the circuit will start to generate its own input without external help. Such circuits are commonly known as "oscillators". (In fact, bootstrapping is a form of positive feedback. The gain around this loop must always be less than 1). The max bootstrap voltage would be exactly the input voltage, which would make the resistor appear infinite. In practice it will be very difficult to exactly define that point. But if you would use a bootstrap level of 0.9 input voltage, you are on the save side and the resistor will appear 10 times as large.

And, oh, make sure the bootstrap signal is in phase with the input signal. Otherwise - you guessed it - the resistor appears much lower than it really is. Can you tell me why?

Jan Didden
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Old 20th November 2002, 11:37 AM   #14
Optical is offline Optical  New Zealand
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destructive interference..?

hmm, what would you then recommend doing to improve this circuit? is bootstrapping not an option here?
if so, is there a common circuit that would work with balanced inputs?
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Old 20th November 2002, 02:59 PM   #15
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Default Bootstrap

Well, I assume that with "improvement" you mean the input impedance? The differential input impedance consists of R2 & R3, increasing them would help, but I haven't really analysed this circuit. Probably one could separate R2 & R3 and connect their cold ends to a suitably attenuated version at the output of the first set of opamps.

But, note that this discussion started about "what is bootstrapping". I don't want to imply that it is always a good thing or even recommended. But in some cases it can be a quick & dirty way to increase input impedance.

The case with the bootstrap capacitor mentioned above by Hugh Dean to linearize the collector load in a Vas stage was widely used, until someone introduced the current source loading of a Vas collector and that then became the trend.

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Old 20th November 2002, 10:35 PM   #16
Optical is offline Optical  New Zealand
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thats given me plenty of ideas
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Old 21st November 2002, 12:22 AM   #17
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Default Nothing exceeds like excess.

Quote:
Originally posted by janneman
The case with the bootstrap capacitor mentioned above by Hugh Dean to linearize the collector load in a Vas stage was widely used, until someone introduced the current source loading of a Vas collector and that then became the trend.

Jan Didden
Of course, you can go quite silly like me in my class A amp somewhere else on Diyaudio, and use a (voltage) bootstrapped current source. All part of the DIY ethos actually.
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