Headphone amplifier "Smart Kit 1215" - possible improvements?

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Wrt r9/59: the opa1641 is a jfet input but that doesn't change anything about r9/59. Either you use a 100k linear pot and keep them or you switch to a 10k log pot and remove them.

Wrt c8/58: If you install opa1641 instead of ne5534 and remove c8/c58, leave their positions open. Actually, pins 5 and 8 aren't connected on the opa1641 so you can leave c8/58 installed, they won't do anything. Even easier ;)

Excellent.
Thanks a lot.

Regarding the potentiometers, I only have linear ones.
A 10kΩ one and an 100kΩ as well.
Is there a way I can use any of those linear ones and still be able to remove R9, R59?

Can i ask you one more thing?
Regarding the values of resistors R7,R57 and R14,R54, I replaced the original ones (100Ω) with values of 270Ω and 330Ω respectively.
According to felllow member sgrossklass, this would improve things with input bias current.

So, am I supposed to do the same with R6, R56 and R13, R63?
Aka replace the original 100Ω ones for the same values as above?
 
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Regarding the potentiometers, I only have linear ones.
A 10kΩ one and an 100kΩ as well.
Is there a way I can use any of those linear ones and still be able to remove R9, R59?
Hardly.
Can i ask you one more thing?
Regarding the values of resistors R7,R57 and R14,R54, I replaced the original ones (100Ω) with values of 270Ω and 330Ω respectively.
According to felllow member sgrossklass, this would improve things with input bias current.

So, am I supposed to do the same with R6, R56 and R13, R63?
Aka replace the original 100Ω ones for the same values as above?
No. Look up how a non-1:1 current mirror works. The ratio of resistor values influences the ratio of currents in both legs, up the the point where it is almost directly proportional when currents are high enough. Hence swapping out the resistors on both sides would defeat the objective of increasing output current.
 
Thank you sgrossklass!
Current throught the R6,R56 resistors is measured at around 3mA.

Something I was thinking about, is adjusting the gain on both stages to something a bit less.
Now it is around 2, if i understand correctly.

I might change the value of the feedback resistors to something around half current value.

I believe I don't really need any more than gain: 3 for my current use.
 

PRR

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> adjusting the gain
 

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Thank you both for the ideas!
Yes, actually I'd like to keep the rec out active.

I was thinking of doing something like the following:

Do what PRR suggested, with only one difference.
Keep the input signal connected to stage 1.
So, both stages will be fed by the input, while remain decoupled otherwise.

First stage will function only as rec out.
Second stage will function as the headphone amplifier.

Will it be fine for the input to simultaneously feed both stages?
Will it affect negatively on their input impedance?


On a separate question:

If I omit C7, C57 (i've already omitted C1, C51), what'll be the purpose of R10, R60?
Do i still need those?

Thanks in advance!
 
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Will it be fine for the input to simultaneously feed both stages?
Will it affect negatively on their input impedance?
Should work just fine.

On a separate question:

If I omit C7, C57 (i've already omitted C1, C51), what'll be the purpose of R10, R60?
Do i still need those?
I'm not sure why you think that these parts have anything to do with each other. Having R10 in is pretty much standard, it can prevent oscillation @ min/max volume and gives input protection a better chance @ max volume.
 
I'm not sure why you think that these parts have anything to do with each other.
Basically, I got confused by the function description given in the manual (pdf).
There, it describes R10 as "a protection of the non-inverting input against the capacity of C7".
And continues to say that R16 "protects the output of the op-amp in the same manner".

Go figure...

Having R10 in is pretty much standard, it can prevent oscillation @ min/max volume and gives input protection a better chance @ max volume.

I'll follow your guidance and leave R10 as it is then.
It makes sense that it helps limit oscillation.

Should I leave R16 as well (what's its function really)?

By the way, I removed C7 entirely.
I also lowered the gain from 2 to approximately 1.45.
Hope it doesn't affect stability too negatively.

It seems that my headphones, would also work fine with a unity gain (use the op amp as a voltage follower).

But from what I read, the lower you go with gain, the more it can affect stability.
Of course, I have 22pF comp. capacitor attached but that doesn't guarantee stability, does it?
 
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