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Old 7th December 2010, 12:06 PM   #11
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Join Date: May 2009
Quote:
Originally Posted by random007 View Post
10K is more reasonable for R3/R9?
it would reduce the offset, as bonsai pointed out.

Of course if you had a source that can't drive 10k then the circuit is pointless.

What is the input impedance of your gain clone. if it is somewhere around the 10k your source should have no problem driving it, again as bonsai pointed out.

So if that's the case (your sources beeing able to drive the GC directly, and the ones you mention very likely are able) then there is no technical reason to add this stage.
Quote:
Do you recommend changing R5?
Only if your going to drive loads with significantly less impedance than 10k like 500 Ohm, because then your buffer would clip at levels of around 2.5Vpeak (see my first post for the math) which is roughly the level of CD-Player outputs (correct me if I'm wrong here).
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Old 7th December 2010, 04:57 PM   #12
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Join Date: Aug 2010
Location: Adelaide
Quote:
Originally Posted by godfrey View Post
Try get the BC547B (or C). They have better Hfe (i.e. current gain) than the BC547A. See the datasheet here for how the grading works.
Good point. They're actually quite hard to find.

Quote:
Originally Posted by Ian Finch View Post
If you haven't already visited this popular site, do - often!
Elliott Sound Products - The Audio Pages (Main Index)
Project 37 (The DOZ preamp) might look a little familiar in its topology and good to learn from.
I am, indeed, familiar with the site and have used many of the designs (verbatim or as inspiration) in other circuits. The higher complexity of the Elliot circuit makes me worry this Dungeon design is going to have issues. I'll build the Dungeon buffer anyway and do some tests - should be interesting.

Does one omit the feedback resistor in the ESP design for 0db gain? I'm familiar with NFB in op-amp ICs but have much to learn about fundamental transistor and amplifier theory. Am looking into doing an EE course at the TAFE (apologies to foreign readers: TAFE = partly government funded vocationally oriented polytechnic-like tertiary education organisation).

Quote:
Originally Posted by krachkiste View Post
it would reduce the offset, as bonsai pointed out.

Of course if you had a source that can't drive 10k then the circuit is pointless.

What is the input impedance of your gain clone. if it is somewhere around the 10k your source should have no problem driving it, again as bonsai pointed out.

So if that's the case (your sources beeing able to drive the GC directly, and the ones you mention very likely are able) then there is no technical reason to add this stage.

Only if your going to drive loads with significantly less impedance than 10k like 500 Ohm, because then your buffer would clip at levels of around 2.5Vpeak (see my first post for the math) which is roughly the level of CD-Player outputs (correct me if I'm wrong here).
Thanks. Sorry for the stupid questions: I should know more about this subject than I do!
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Old 7th December 2010, 08:10 PM   #13
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Join Date: Apr 2010
Location: Coffs Harbour
Quote:
Originally Posted by random007 View Post
The higher complexity of the Elliot circuit makes me worry this Dungeon design is going to have issues. I'll build the Dungeon buffer anyway and do some tests - should be interesting.
Does one omit the feedback resistor in the ESP design for 0db gain? I'm familiar with NFB in op-amp ICs but have much to learn about fundamental transistor and amplifier theory.
No need to apologise for noobiness. We are all noobs at some time and are just fortunate to now have DIYAudio to help speed up the learning process.

The only complexity of the ESP preamp circuit is due to inclusions like quiescent current adjustment and some linearising circuitry, including a little feedback. In practice, you'll need to do some things to your buffer to establish acceptable operating conditions and, since they will be a critical part, the power supply(s) too.
Note that just removing the feedback resistor R5 will disturb the DC conditions of the input transistor and lead to some problems you won't need.
Feedback also determines the gain of the amplifier by the ratio of the voltage divider formed by R5+R4, so the appropriate change is to increase R4 which, due to the cap., only affects AC gain. You won't be able to remove all gain and the reduction of feedback will result in increased distortion. However, this is a good testbed for checking the amp theory out and show you the problems ahead for such simple buffer designs.

Having a scope, even a 16bit, 11kHz bandwidth crappy soundcard one will make things very interesting with the right free software. At the output levels here, you should have no overload issues but use some protection such as 2 series pairs of IN4148 diodes wired in opposite direction across the analog input terminals anyway. A blown input is a blown card so don't try it with on-board cards if you can help it! Make sure that you can derive a 1kHz test tone that works in real time while the device is being measured or you will need to buy or build a very low distortion oscillator.

The ESP tutorial on amplifier operation will bring the theoretical issues to your attention but a good simple text on amps, even opamps, (Check Amazon, Fishpond etc.) is the go. By all means play with the simple buffer circuit, but use the opportunity to learn something that will give you much wider understanding.
Personally, I think the net is absolutely great for cherry-picking facts, papers and docs. here and there and the 'wow factor' of stunning graphics etc.
For formal learning, there is nothing like a good book and a more comfortable place than squinting at poorly scanned documents on your computer's VDU.
__________________
regards

Last edited by Ian Finch; 7th December 2010 at 08:38 PM.
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Old 10th December 2010, 06:37 AM   #14
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Join Date: Aug 2010
Location: Adelaide
Quote:
Originally Posted by Ian Finch View Post
No need to apologise for noobiness. We are all noobs at some time and are just fortunate to now have DIYAudio to help speed up the learning process.

The only complexity of the ESP preamp circuit is due to inclusions like quiescent current adjustment and some linearising circuitry, including a little feedback. In practice, you'll need to do some things to your buffer to establish acceptable operating conditions and, since they will be a critical part, the power supply(s) too.
Note that just removing the feedback resistor R5 will disturb the DC conditions of the input transistor and lead to some problems you won't need.
Feedback also determines the gain of the amplifier by the ratio of the voltage divider formed by R5+R4, so the appropriate change is to increase R4 which, due to the cap., only affects AC gain. You won't be able to remove all gain and the reduction of feedback will result in increased distortion. However, this is a good testbed for checking the amp theory out and show you the problems ahead for such simple buffer designs.

Having a scope, even a 16bit, 11kHz bandwidth crappy soundcard one will make things very interesting with the right free software. At the output levels here, you should have no overload issues but use some protection such as 2 series pairs of IN4148 diodes wired in opposite direction across the analog input terminals anyway. A blown input is a blown card so don't try it with on-board cards if you can help it! Make sure that you can derive a 1kHz test tone that works in real time while the device is being measured or you will need to buy or build a very low distortion oscillator.

The ESP tutorial on amplifier operation will bring the theoretical issues to your attention but a good simple text on amps, even opamps, (Check Amazon, Fishpond etc.) is the go. By all means play with the simple buffer circuit, but use the opportunity to learn something that will give you much wider understanding.
Personally, I think the net is absolutely great for cherry-picking facts, papers and docs. here and there and the 'wow factor' of stunning graphics etc.
For formal learning, there is nothing like a good book and a more comfortable place than squinting at poorly scanned documents on your computer's VDU.
Thanks for all that and thanks so much for your (and everyone's) help.

(Incidentally, I have a CRO, rather old and definitely uncalibrated but it works - rescued off the top of someone's curbside hard rubbish dump. )
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