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Old 22nd September 2012, 11:48 AM   #1101
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The new input from Keantoken worked just like. . .
Quote:
Originally Posted by Elvee View Post
That said, a darlington input like this one will raise the input impedance and eliminate the bias current modulation effects, practically without drawback.
. . . it worked just like he said it would. It grafts onto Standard Circlophone (post 1 on this thread) seamlessly. The main benefit is compatibility with lossy sources and volume knobs, without having to build a preamp or buffer. The side benefit is unlocked future development due to generally improved tolerances. And, board layout with this input can be easily jumpered back to Standard Circlophone if desired, just by reconnecting IN+ and IN- per the standard schematic. Just one board can do both options. Terranigma, its your turn.
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Old 22nd September 2012, 06:20 PM   #1102
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Quote:
Originally Posted by danielwritesbac View Post
. . . it worked just like he said it would. It grafts onto Standard Circlophone (post 1 on this thread) seamlessly. The main benefit is compatibility with lossy sources and volume knobs, without having to build a preamp or buffer. The side benefit is unlocked future development due to generally improved tolerances. And, board layout with this input can be easily jumpered back to Standard Circlophone if desired, just by reconnecting IN+ and IN- per the standard schematic. Just one board can do both options. Terranigma, its your turn.
Good news as usual.. Since that I'm making changes on layout manually, I need some time for prepare something usable.

Last edited by terranigma; 22nd September 2012 at 06:24 PM.
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Old 22nd September 2012, 07:12 PM   #1103
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Happy to hear it.
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Old 29th September 2012, 09:11 PM   #1104
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Quote:
Originally Posted by terranigma View Post
Good news as usual.. Since that I'm making changes on layout manually, I need some time for prepare something usable.
Well, I just got an epic workout and sore fingers from making a new schematic.
Could you check it out and see if I've implemented the new input system right?
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Old 29th September 2012, 09:33 PM   #1105
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DC feedback. Interesting. I predict you ill have a falling output voltage ith temperature, since the LTP base impedances are't matched. It ill probably be small because R20 is small but it ill be there. Offset ill also change ith rail voltage, but I think this ill also be small. I see the compensation scheme has not changed.
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Old 30th September 2012, 02:26 AM   #1106
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Quote:
Originally Posted by keantoken
DC feedback. Interesting. I predict you will have a falling output voltage with temperature, since the LTP base impedances are't matched. It will probably be small because R20 is small but it will be there. Offset will also change with rail voltage, but I think this will also be small. I see the compensation scheme has not changed.
One possible option for the similar gain setting AND similar base impedances is:
R20, 12K
R19, 330R
R17, 12K
R16, 330R
Any problems with doing this?
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Last edited by danielwritesbac; 30th September 2012 at 02:31 AM.
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Old 30th September 2012, 02:36 AM   #1107
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R16 shorts out R17; R20 is not shorted because C10 blocks the ay. By removing the feedback DC blocking cap you imbalanced the LTP input impedances.
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Old 1st October 2012, 07:07 PM   #1108
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Okay, howabout this?
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Old 2nd October 2012, 12:02 AM   #1109
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Q15's base sees 12k to ground. Q14's base sees 390R to ground. Traditionally this is solved by putting a cap in series ith R16. Furthermore, I've realized the offset may be much orse. 1mV offset ill be amplified by your gain, to be 40mV or something. ithout base impedance matching or some sort of compensation your offset may ander all over. Multiplying R28/R29 by 4 might not even help, unless the input stage is matched.

PS: Miller action blos up C4 to 830pF. So 820pF for C5 may be orth experimenting ith to see if the balance brings ne sonic qualities.

I don't kno the capacitance of zeners offhand, but the input zeners may be a significant source of nonlinear capacitance. Putting them in parallel ith 1N4148 or BAT54 in series ith them individually ill reduce the amount of nonlinear capacitance seen at the input. Furthermore, R34 presents the opportunity for a 2nd-order input LP, far more effective than first order, at RF filtering. This means an additional cap across the zeners.

But ait there's more! If you make R34 82R and put a 220p capacitor across the zeners, you ill be giving your input cables a termination netork hich prevents them from resonating and causing RF ingress or oscillation.

Just so you are aare of the possibilities.
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Old 2nd October 2012, 02:08 PM   #1110
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Quote:
Originally Posted by danielwritesbac View Post
Okay, howabout this?
One possibility to equalize the offset voltage is to insert a 12k paralleled with 4µ7 in Q14's base.
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