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Old 30th December 2011, 08:20 PM   #1
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Default Differential pair questions

In the attached schematic,
1)How does the circuit, in its static(idle) state, ensure IcQ1=IcQ2? does it have something to do with the feedback resistor?
2)If R6 and R3 were calculated wrongly, could this cause DC offset at output of amplifier, even though Q1 & Q2 are matched?
3) Do R2 and R9 have to be matched to minimise offset?
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Old 30th December 2011, 08:22 PM   #2
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Here it is!
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File Type: pdf Sinclair Split2.pdf (88.0 KB, 136 views)
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Old 30th December 2011, 09:03 PM   #3
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yes, if R3 and R6 are not properly proortional it will cause the differential input stage to go unbalance.
This should also cause an unbalance at the output thus creating DC offset.

This is my first impression looking at the circuit but I have never implemented it so maybe someone here will be able to give you a more solid answer.

Why are you using a single ended output stage. Do you have any benefit over the standard push-pull topology?
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Old 30th December 2011, 09:07 PM   #4
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Another consideration aside looking at the schematic: the 47uF capacitor on the Feedback is used there to create an extra zero so that your overall DC gain drops down thus minimizing thermal issue and DC drifting at the output.
It is surely better than a servo control in certain aspects but its sound is going to be pretty bad and lack of live music sound.
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Old 30th December 2011, 09:08 PM   #5
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Another consideration aside looking at the schematic: the 47uF capacitor on the Feedback is used there to create an extra zero so that your overall DC gain drops down thus minimizing thermal issue and DC drifting at the output.
It is surely better than a servo control in certain aspects but its sound is going to be pretty bad and lack of live music.

I would stay aeay from it if I were you, the topology looks a bit old and over-compensated.
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Old 31st December 2011, 01:07 AM   #6
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Look at any ss amplifier circuit from this 1970s-80s period. (Naim etc) They often have this same (R6) asymmetric resistance to balance collector currents because of loading by the the next amplifier transistor stage.

That was the plan. but it doesn't work properly and balance is not truly established so balanced Voltage Amplifier stages were developed to load both collectors and even more ambitious and complex stages have evolved to overcome these theoretical problems or just notional ideas of symmetry.

However, some of the most famous brands and some models still use this basic schema. and are still in demand. In which case, owners are probably not so convinced by modern technology as by a particular sound quality.
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Old 31st December 2011, 09:28 PM   #7
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I would be surprised to find an old topology like this on a modern hi-end audio equipment.
Asymmetry of this design makes me wonder why anybody would still want to use it and sincerely don't see it matchin any hi0end standard of hi-resolution, natural sound and so on.
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Old 1st January 2012, 02:51 AM   #8
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Then consider yourself well surprised. - For just one manufacturer, Naim's circuits haven't changed since 1978!! Their prices at least are high enough to call "high-end".
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Old 1st January 2012, 02:56 AM   #9
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I believe you....then....I don't have else to say other then....what a shame!!!
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Old 1st January 2012, 04:26 AM   #10
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Regarding question #3: yes, assuming C1 is used, and/or the preceeding source has a series capacitor. This equalizes base voltages due to base currents.
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