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Old 15th January 2013, 08:30 PM   #21
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Quote:
Originally Posted by Mike H View Post
Yep sounds familiar ....
Explain it! :-)

Hi
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Old 16th January 2013, 04:35 PM   #22
Mike H is offline Mike H  United Kingdom
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Murphy's Law?
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Old 16th January 2013, 04:43 PM   #23
Mike H is offline Mike H  United Kingdom
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As a matter of interest which pair were deciding to act like an SCR?
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Old 16th January 2013, 09:09 PM   #24
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Quote:
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As a matter of interest which pair were deciding to act like an SCR?
As reported in the first schematic posted at the start of this thread, QGN5 with QFLOAT1: collector of one transistor is wried to the base of the other in a riing fashion.

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Old 17th January 2013, 12:38 PM   #25
Mike H is offline Mike H  United Kingdom
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Oh right OK.
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Old 19th January 2013, 06:52 PM   #26
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Circuit rebuild and retested; after adding a start resistor and a compensantion for cutting off an otherwise huge (and harmful) bandwidth. Now seem work fine.

Click the image to open in full size.

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Old 24th January 2013, 06:50 PM   #27
Struth is offline Struth  Canada
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Hi Guys

With everything bootstrapped, it seems like the main goal is to reduce input capacitance and increase input impedance. R-IN seriously degrades the latter and could be much higher if you needed a really high z-in.

Otherwise, Doug Self showed how to achieve high z-in using just BJTs and liberal bootstrapping in his "Small Signal Audio Design" book. Bootstrapping has been around since tube days. Noise can sometimes be compromised using these methods and distortion more often is.

There are simpler ways to have a good signal buffer with low distortion and reasonable signal amplitude handling. But... like you say, sometimes you just have to try something for the sake of trying.

Have fun
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Old 25th January 2013, 10:17 PM   #28
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Hi Guys

With everything bootstrapped, it seems like the main goal is to reduce input capacitance and increase input impedance. R-IN seriously degrades the latter and could be much higher if you needed a really high z-in. r
Thanks for your considerations but the goal is not having an high input-z for itself (although the real INNER input Z is above 10 Megaohm: R-In is just for locking to ground DC the input coupling capacitor and may be chosen of any value below a couple of Megaohm).

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There are simpler ways to have a good signal buffer with low distortion and reasonable signal amplitude handling. But... like you say, sometimes you just have to try something for the sake of trying
Exactly! :-) One "psychological" goal is to show how a circuit can become complex if someone want to achieve without feedback the (near) same result obtained WITH feedback - that however is not really absent! :-) Transistor used are of the cheapest and common type but however the circuit is neatly more expensive of a simple voltage follower implemend with any low cost good audio operational amplifier... ;-).

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Last edited by Piercarlo; 25th January 2013 at 10:29 PM.
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Old 27th January 2013, 12:00 AM   #29
dimitri is offline dimitri  United States
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Enclosed circuit was used in condenser microphones for many years
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Old 27th January 2013, 12:13 AM   #30
dimitri is offline dimitri  United States
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This circuit can be found in almost every mixer from Ma*kie
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