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Old 8th August 2012, 12:39 PM   #41
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by Joachim Gerhard View Post
[...]Can you post it here ?[...]
OK here it is...balanced, so one leg can be eliminated if one wants to.


Quote:
Originally Posted by Joachim Gerhard View Post
[...]The way i see the smms73 circuit is how he deals with the base currents. Q7 and Q8 pick up the base currents on the emitters and feed a portion in on the other side ( polarity ).
This is a kind of base current feedback.
Thank you, now I have to think about this, later...
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Old 8th August 2012, 12:54 PM   #42
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Thanks for posting. I have seen that circuit some years ago. I build a prototype that way that worked. It is a nice way to avoid DC offset into the cartridge.
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Old 8th August 2012, 02:32 PM   #43
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by Joachim Gerhard View Post
By the way i find your circuit fascinating, i have not seen it exactly that way and i have seen many. I find it innovative whatever others say. I am just working to make it low noise into low impedances.
Donīt care about the thread jackers, that happens all the time when you enter an open forum.

thanks Joachim

Fell free to make the changes you want.
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Old 8th August 2012, 02:39 PM   #44
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Quote:
Originally Posted by Joachim Gerhard View Post
Thanks for posting. I have seen that circuit some years ago. I build a prototype that way that worked. It is a nice way to avoid DC offset into the cartridge.
Dr. Leach discussed using a floating power supply in order to avoid a coupling cap on the input.
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Old 8th August 2012, 02:41 PM   #45
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Quote:
Originally Posted by zinsula View Post
OK here it is...balanced, so one leg can be eliminated if one wants to.



Thank you, now I have to think about this, later...
One thing I don't understand is why the second stage is also a common base arrangement, unless I misunderstand the circuit. Is this considered some kind of cascode?
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Old 8th August 2012, 02:56 PM   #46
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The second stage is a folded cascode with base current feedback. At least that is how i look at it. Normally the input stage is not common base when a folded cascode is used but it works as well.
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Old 8th August 2012, 02:58 PM   #47
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by zinsula View Post
It's a folded cascode stage. Some even replace the ccs (I2 in this simplified schematic) with a resistor, as it forms a ccs anyway.

See here what I had in mind some years ago...you may also see how you can bias the input transistors.

It mirrors the input current to the high impedance node (collector).
I still do not really get what the transistors Q7 and 8 are doing special other than providing a low impedance biasing for the folded cascode transistors Q3 + 6 (kinda darlington folded cascode).
Can you enlighten me?
Yes, Is a of folded cascode.

the folded cascode have already low distortion, the key for a even lower distortion is to use very high impedance ccs (a resistor is a bad choice) , and to take care of the distortion caused by the base currents.

As Joachim said, q7 and q8 compensate for the distortion of the base currentes , by going to get current to the opposite side.
hope you understand, Its very difficult for me to explain better, as english is not my natural language (i have some help for google traducter ).
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Old 8th August 2012, 03:27 PM   #48
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Yes, that CCSs help to get the gain up. When you then degenerate or feed back the distortion is lower because the open loop gain is higher. I think your circuit is very clever.
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Old 8th August 2012, 03:31 PM   #49
smms73 is offline smms73  Portugal
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a more complete version with a capacitor in the base of the input transistors as Joachim suggested.
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Old 8th August 2012, 03:46 PM   #50
smms73 is offline smms73  Portugal
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with a op amp in the input to lower the input impedance.
I don't mind use op amp , as long as the load impedance is very high ,as in this case.

PS: is no need for the base resistors r18 and r27 and is better to put a 1 ohm resistor between input node and the source, for preventing that capacitors in the source degrade the phase margin of the op amp.
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