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Ranchu32
diyAudio Member

Join Date: Apr 2013
Quote:
 Originally Posted by AndrewT could you add some resistor Vdrops to your sch? Vr5&6, Vr18, Vr19, Vr3&4, Vr7, Vr12, Vr16&20, Vr37 and Vc2, Vc6
Andrew, refer attached. Thanks in advance for any suggestions you can offer.
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Ranchu32
diyAudio Member

Join Date: Apr 2013
Quote:
 Originally Posted by forr I think that with the current resistor values, the performances are acceptable (however the output DC is rarely as high) but not optimal towards the general blameless philosophy. R1 (currently 2.2 kOhm) should be of a value near to R9 (currently 1 kOhm). For DC balanced, non-inverting input current flows through R1+R2 = 49.2 kOhm as inverting input current flows through R33 = 47 kOhm. I would place R1 ahead of R2 at the expense of a loss (0.4 dB for 2.2 kOhm) of the input signal. In his book about power amplification, Self shows an increase of distortion with an increase of source impedance (6th edition, page 144), more than 6 dB between 50 Ohm and 2.2 kOhm at 1 kHz. For the needed voltage gain, 34.6 times, I would choose the following values : R1 = 10 kOhm R2 = 330 Ohm R33 = 10 kOhm R9 = 330 Ohm C6 = 680 µF or more C1 = 4.7, 10 µF or more (possibly bipolar)
I like this a lot - thanks.

I've had a look in my parts bins and have found some 1000u /16V caps that will fit the NFB cap pad on the board, and some 10uF bipolar Muse caps that I can substitute with the 1uF film cap.

Once I get to the bottom of the DC offset issue, I'll make these substitutions on my prototype board.

 31st March 2014, 09:55 AM #33 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Vc6=562mV and Vc2=557mV are close. The output offset should be 562-557 = 5mVdc But it is not. Base current of Q12 is zero. Q12 is turned off. Why? Q11 with 45uA of base current seems to be passing only 5mA and certainly <10mA That is also wrong. I would expect Vr16&20 to be around 6mV to 7mV for driver bias of 30mA. There is a small error in the current mirror, 96mV vs 100mV What about Vr3&4? __________________ regards Andrew T. Last edited by AndrewT; 31st March 2014 at 09:58 AM.
 31st March 2014, 10:36 AM #34 Ranchu32   diyAudio Member   Join Date: Apr 2013 Andrew, the drivers do seem to be passing some current. I have 1.12V at the base of Q11 and -1.24V at the base of Q12. The voltages at the emitters are 540mV and -667mV, respectively. There seems to be an issue with the Vbe causing a voltage offset. The Vbe transistor base is -641mV. Vdrop at R3 and R4 is 149mV and 146mV, respectively.
TonyTecson
diyAudio Moderator

Join Date: May 2003
Location: Maybunga, Pasig City
Quote:
 Originally Posted by Ranchu32 Gallery, thank you very much for your detailed explanation.

errr, name is Vallery....

Quote:
 Vdrop at R3 and R4 is 149mV and 146mV, respectively
this is very good...
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 31st March 2014, 10:44 AM #36 Ranchu32   diyAudio Member   Join Date: Apr 2013 Oopps sorry Gallery.... i mean Vallery :-)
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by Ranchu32 ..................Vdrop at R3 and R4 is 149mV and 146mV, respectively.
this small error can be removed by trimming.
It will reduce the 5mV of difference across the diff pair.
When the rest of the circuit is working, you can experiment with this trimming.
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regards Andrew T.

 31st March 2014, 10:54 AM #38 mcd99   diyAudio Member   Join Date: Mar 2008 Location: UK Those voltages at the driver bases look wrong to me. Am I wrong in saying you should have 26mV + 2x diode drop?
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by Ranchu32 Andrew, the drivers do seem to be passing some current. I have 1.12V at the base of Q11 and -1.24V at the base of Q12.
that indicates a Vbias of 2.36V. About right for a two stage EF output
Quote:
 The voltages at the emitters are 540mV and -667mV, respectively.
this LARGE difference in Vbe tells me the drivers are not working properly. BJTs are very predictable. 127mV of Vbe error is enormous.
Quote:
 There seems to be an issue with the Vbe causing a voltage offset. The Vbe transistor base is -641mV.
mark these new voltages on your sch and re-post.
__________________
regards Andrew T.

vzaichenko
diyAudio Member

Join Date: Jan 2010
Location: Moscow, Russia
Quote:
 Originally Posted by Ranchu32 Oopps sorry Gallery.... i mean Vallery :-)
No prob )) Some people even think I'm a lady... which is always fun when they eventually talk to me on the phone for the first time
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