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Old 31st March 2014, 05:00 AM   #31
Ranchu32 is offline Ranchu32  Australia
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Quote:
Originally Posted by AndrewT View Post
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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Old 31st March 2014, 05:38 AM   #32
Ranchu32 is offline Ranchu32  Australia
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Originally Posted by forr View Post
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 F or more
C1 = 4.7, 10 F 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.
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Old 31st March 2014, 09:55 AM   #33
AndrewT is offline AndrewT  Scotland
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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?
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Last edited by AndrewT; 31st March 2014 at 09:58 AM.
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Old 31st March 2014, 10:36 AM   #34
Ranchu32 is offline Ranchu32  Australia
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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.
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Old 31st March 2014, 10:41 AM   #35
TonyTecson is offline TonyTecson  Philippines
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Originally Posted by Ranchu32 View Post
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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Old 31st March 2014, 10:44 AM   #36
Ranchu32 is offline Ranchu32  Australia
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Oopps sorry Gallery.... i mean Vallery :-)
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Old 31st March 2014, 10:51 AM   #37
AndrewT is offline AndrewT  Scotland
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Originally Posted by Ranchu32 View Post
..................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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Old 31st March 2014, 10:54 AM   #38
mcd99 is offline mcd99  United Kingdom
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Those voltages at the driver bases look wrong to me. Am I wrong in saying you should have 26mV + 2x diode drop?
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Old 31st March 2014, 10:54 AM   #39
AndrewT is offline AndrewT  Scotland
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Originally Posted by Ranchu32 View Post
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.
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Old 31st March 2014, 10:57 AM   #40
vzaichenko is offline vzaichenko  Russian Federation
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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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