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Old 25th November 2012, 11:57 AM   #21
AndrewT is offline AndrewT  Scotland
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R1 is too high, not enough damping. Try 1r0 to 10r instead of 22r.
R11 is a variable resistor. Try 500r or 1000r
R13 is too small, try 100r to 220r.
RF attenuating resistor is missing. Try 1k in series with input cap C1.
The other half of the input signal connects to the bottom of C2, bottom of R2, bottom of C4, top of R9.
Add a trimmer to R14. This sets the currents through R5 & R4 exactly equal.
Then, adjust either R2 or R16 to set the output offset to zero mVdc.

Otherwise a good circuit to learn how an amplifier works.
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Old 25th November 2012, 12:50 PM   #22
maouna is offline maouna  Greece
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Thanks for the advice Andrew.Ι had in mind AKSA and a RCA amplifier design of 1970 when designing this schematic.Thats why the lack of RF attenuating resistor at the input and the hi value of R1 which indeed is quite high.R11 and R13 where used in LTspice to find the optimum quiscent current.They will be replaced by a trimmer.I have not included resistors at the bases of drivers and output transistors and i think i should. My aim is to build an amp not with an ultra low distortion and flat sound,but one with warm sound.

Click the image to open in full size.

Last edited by maouna; 25th November 2012 at 12:52 PM.
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Old 25th November 2012, 02:55 PM   #23
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by maouna View Post
Thanks for the advice Andrew.Ι had in mind AKSA and a RCA amplifier design of 1970 when designing this schematic.
Good choice ! ... but adding the rf filter is still a good idea for using the amp in a modern environment.

see also: RCA 1972 Basic amplifier MODS
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Old 25th November 2012, 03:40 PM   #24
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Default Nice amplifier.... this kind of amplifier sounds very good

I like it Maouna

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Old 25th November 2012, 04:05 PM   #25
maouna is offline maouna  Greece
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By the way what happened to Ostripper's site? http://67.248.209.21/D%3A/WEBSITE/
There were many good amplifier schematics and their simulation files there.
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Old 25th November 2012, 05:18 PM   #26
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You can do way better than that old RCA circuit. I built one back in the '70s as my first amp project. It worked well, but we've moved on! Here's a circuit I did some time ago using similar devices to what you have, based on the circuits and ideas in Doug Self's book. The circuit will do sub 0.001% THD over most of the audio range and is generally well behaved. The grounds were drawn the way they were so I could experiment with what to tie where. LCG stands for low current ground, HGC for high current ground. You can eliminate the opamp on the front end if you have no need for differential inputs. C12 is probably a bit heavy, so reduce it for better bandwidth and less filtering. Transistor types are not terribly fussy. There is no protection, so don't short it out! It should make a great headphone or small speaker amp.
Attached Files
File Type: pdf LOWTHD.pdf (17.5 KB, 75 views)
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Last edited by Conrad Hoffman; 25th November 2012 at 05:23 PM.
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Old 25th November 2012, 05:52 PM   #27
maouna is offline maouna  Greece
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I dont know. 2-pole compensation combined with sziklai output seems a bit risky... Furthermore i think that the simpler the circuit is,the warmer is the sound despite of the THD figures.Maybe someone more experienced can comment on this.

Last edited by maouna; 25th November 2012 at 05:56 PM.
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Old 25th November 2012, 09:04 PM   #28
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The circuit works as well with single pole compensation but I figured what the heck, let's try it. It wasn't all that troublesome and it did improve performance slightly. Warm, cold, sterile, accurate, tight , loose, I dunno, but the signals that come out exactly match the signals that went in.
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Old 26th November 2012, 10:01 AM   #29
maouna is offline maouna  Greece
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I want to ask something.There many darlington pairs out there whether they are in a single package or made descrete. How we choose the values of these 2 resistors (R1-R2) when it comes to power amps? They have different values in many datasheets even if they are of the same model but different brand. Click the image to open in full size.

Last edited by maouna; 26th November 2012 at 10:03 AM.
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Old 26th November 2012, 11:57 AM   #30
AndrewT is offline AndrewT  Scotland
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use discrete transistors and choose the resistors to get good performance from each transistor.

Too high a value of R2 reduces the driver transistor current to below it's optimum and the Ft falls off the cliff. Go to an even higher resistor value and the hFE then falls off the cliff.
That's the basic problem of the integrated Darlingtons. They are there to reduce production costs and only work well over a very small range of Ic value. That optimum Ic value rarely suits Audio Power Amplifiers.
Go discrete.
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