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I have built a prototype of cmoy headphone amplifier as a guitar monitor from this scheme (click to open). It works perfectly but anyway now I want to do some modifications. I was searching everywhere on internet to answer my following questions last two days but without success. So here are my questions, hope somebody can answer and if yes, thank you very much in advance.


1) Since I want my final version much smaller than it is now, I have bought 9 mm mono pots with integrated switch (instead of my original 16 mm pots without switch). The problem is, that the small ones are 10k and the original (pictured on my scheme) are 100k. I have read many discussions about swapping pots in cmoys but all of them were about changing the value of R2 (according to the discussions it should be approx. 10x higher value than the pot value) in non-inverting scheme. But there is one big issue in which I am not sure...since my scheme is inverting one (positive signal from input goes to op amp's pin 2), I am not sure if there is "such a R2" in my scheme. The only resistors in my scheme (R6, resp. R7) following after the pots are not connected to GND. I am not so good in theory but aren't those R6 and R7 in series config with the pots? If yes, it would mean that swapping of the pots should be followed by swapping of the R6, R7 = 100k, right? If not, can someone please tell me what to change in the circuit if I want to use 10k pots?


2) I am considering usage of a single Li-pol battery (with some cut-off protection) as a power supply. I was measuring the lowest operating voltage of my amp and it is 8,8 V. As a compromise between dimensions and voltage I would prefer some 4-cells Li-pol unit which gives 14,8V. But it means to use one of the Tangent's "Simple Buffered Virtual Ground Circuits"...would be "Buffer-based virtual ground driver" good choice? Current of my amp is around 17 mA (if I was measuring it right).


3) If I prefer only one 9 V battery, or smaller Li-pol with 3 cells (11,1 V), how should I change the values of parts in my circuit? Of course I would change also the power supply part of circuit to one with virtual ground...but how to change the amp itself? I have seen some scheme, where were changed only C3 and C4 (both on my scheme) from 1 uF to 3,3 uF. Can I do it like that? The mail goal of this should be to decrease the lowest operating voltage (which now is 8,8 V as mentioned above) because the lowest possible voltage of 3 cells Li-pol is right under 9 V.



Note 1:

Actually the best option for me would be to use this battery (single) but according to this measuring the real voltage output is 8,9 V - 6,3 V. So it would mean that I need to decrease the minimum overall operating voltage of my amp under this level. Is it possible?


Note 2:

Well, during writing this text I am more and more convinced about using two of those li-ion batteries and there wouldnt be necessary any circuit modification at all :D But application of single one would maybe allow me to put everything in classic Altoids case (two batts are too large for it since I need to use three 6,3 mm jacks), which would be perfect.



4) Is there possible to do any modification (like changing resistor or cap values) to decrease the total consumption (current) of the whole circuit?




Thanks, Ondra