Class A preamp: please critique my circuit and give me helpful suggestions. - Page 9 - diyAudio
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Old 26th August 2012, 06:58 AM   #81
AndrewT is online now AndrewT  Scotland
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Yes, the input has to be scaled down as well, but you have already scaled it up by a factor of 3, so you can afford to scale it back by a factor of 10 to ~4k5.

The sum of the base currents into Q3 & 4.
yes reduce the impedance.
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Old 27th August 2012, 10:48 AM   #82
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Quote:
Originally Posted by AndrewT View Post
Yes, the input has to be scaled down as well, but you have already scaled it up by a factor of 3, so you can afford to scale it back by a factor of 10 to ~4k5.

The sum of the base currents into Q3 & 4.
yes reduce the impedance.
Sorry this took a couple of days. Is it better now?

Low frequency square performance is still adequate and high frequency square waves are completely stable even with C15 removed.
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Old 27th August 2012, 12:15 PM   #83
AndrewT is online now AndrewT  Scotland
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I would tap R24 into the right hand side of R15. If needed you can add a 100r onto the base of Q2.
That's your theoretical starting point for the build.
Now, go back and review the other advice given before I joined in.

Expect to make a number of small and maybe one or two big changes to component values once you start seeing the testing results.
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Last edited by AndrewT; 27th August 2012 at 12:20 PM.
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Old 27th August 2012, 12:23 PM   #84
AndrewT is online now AndrewT  Scotland
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Post68,
can you give the link to where you found that Dr. Cherry extract?
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Old 27th August 2012, 10:18 PM   #85
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Quote:
Originally Posted by AndrewT View Post
I would tap R24 into the right hand side of R15. If needed you can add a 100r onto the base of Q2.
That's your theoretical starting point for the build.
Now, go back and review the other advice given before I joined in.

Expect to make a number of small and maybe one or two big changes to component values once you start seeing the testing results.
Will do. Any tips as to what tests I should perform in order to see the results would be greatly appreciated.
So far I've been checking phase, freq response, DC offset, square wave, and FFT (although the noise floor of the oscilliscope is too high for the FFT to be of any real use).

Quote:
Originally Posted by AndrewT View Post
Post68,
can you give the link to where you found that Dr. Cherry extract?
Here you go: http://www.linearaudio.nl/linearaudi...ndfl%20amp.pdf
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Old 28th August 2012, 06:51 AM   #86
AndrewT is online now AndrewT  Scotland
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Start with sinewave and check the output shape compared to the input shape as you change both the amplitude and frequency. This is the easiet.
Then try triangle wave or sawtooth.
Finally squarewave.
Doing the squarwave last, allows you to know how not to overload the input.
Check that your sim correctly predicted the effect, or non effect, of adding a reactive load (the cable model).
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Old 1st March 2013, 10:01 AM   #87
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do not understand why such an amplifier is needed on discrete elements, with power + -15 V, just when everything is solved by means of the operational amplifier, especially for working on a load of 100 ohms at Taku load any OPA will operate in Class A
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