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Old 1st January 2012, 10:36 PM   #21
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I have a ClarityCap on the RCA jack inputs and the NFB capacitor is 10uF to limit the bass to my speakers. I don't think that the decoupling capacitors have anything to do with it, the amp is not oscillating. I did have a problem with the amp oscillating at first until I changed the VAS transistor from BD13916 (HFe 100-250) to KSC3503D (HFe 60-120), do you think that the HFE classes of the transistors makes a difference.

The voltages at A is 35.4V, B is 34.1V, C is 12V, D is -35.4V, E is 34.2V

Last edited by benproiii; 1st January 2012 at 10:40 PM.
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Old 1st January 2012, 10:42 PM   #22
kevinkr is offline kevinkr  United States
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Originally Posted by benproiii View Post
I think there is something wrong with the amp.

I connected the amp to a function generator and sent a 1KHz sine wave into it.
I connected an AC meter and oscilloscope to the amp output with NO LOAD and the max volts the amp will make before clipping is 14.9VAC on the meter with 700mv into it.

This is about 27Wrms into 8 ohms and 54W into 4 ohms, and is about 21Vpk which seems rather low given the supply rail.

Yes the hfe class of the transistor could make a difference..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 1st January 2012, 10:46 PM   #23
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would agree there...and for the given supply rail's you can use 100uf's there's no need for them to be 470uf's...
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Old 1st January 2012, 10:52 PM   #24
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I get all my transistors from fairchild and they let you decide the hfe grades can someone tell me what hfe grades should be used for this circuit. here are the data sheets.
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Old 1st January 2012, 11:18 PM   #25
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have you connected the power to BD139/140 from the correct side of R5/R14
Replace/check Bootstrap cap, and if connected to correct side of R20.
Is clipping symmetrical. ?
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Old 1st January 2012, 11:29 PM   #26
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Sorry, just saw the voltage readings...If these are under load conditions, i presume thats negative for(E is 34.2V) if so all fine. are you using scope to measure supply?
Dont worry about the cap values..the only cap thats going to affect clipping is the bootstrap C20. unless you have very thin wiring in the power lines
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Old 1st January 2012, 11:38 PM   #27
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Clipping is symmetrical, I ment -34.2V, The circuit is build correctly because the PCB was designed using the same EAGLE file. Do you think that the low hfe of the vas causes the voltage not to be able to swing that high. Under heavy load the voltage of both rails only sags 4 volts.
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Old 1st January 2012, 11:40 PM   #28
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Since DX usually only specifies standard (unclassified Hfe) semiconductors, it seems pointless to be more specific than to say "midrange" Hfe. Using highest grade as the displayed HX design shows will, as we find, run into excessive gain and oscillation problems whilst absolute low end Hfe parts may restrict gain too much. You should have no problems with BD139/40-10 for any event.

The fact that you report a single, fixed maximum output voltage suggests something quite different to gain limitations however. There is an error here somewhere, whether the parts, board and schematic are fine or not.

Stereo has not been discussed. Is the second amp constructed and tested yet for comparison? If so, verify that the response is the same in both channels. Then you can be certain what problems are more likely. It should be possible to scope the VAS voltages at R15 and compare similar points at the bootstrap cap. and driver bases. Let's see if there are better clues available.
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Old 1st January 2012, 11:57 PM   #29
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you have plenty of gain in the VAS already. lack of gain will cause lack of output, but clipping wont happen due to lack of hfe.
Are you sure its clipping ?/
the overall gain should give you about 19V for 700mV input. 15V isnt way off the mark here.
if you want more overall gain, reduce R20/R27 ratio.
Scope the base of T5. if its small clean waveform then amp is behaving properly.
If theres big spikes on it, compare them to output to see where the problem is.
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Old 2nd January 2012, 12:00 AM   #30
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The amp is actually completly finished and has been running for months now and I did not notice the restriction in voltage until i hooked up my new SR71's which are less sensitive. The left and right channels of the amp both clip at 15V
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