What's the real reason that laptop batteries are made not-accessible?

[David Hess]

Does that solve ground loop issues though? Other than using optical coupling of some kind, I can't think of an alternative to a transformer for that.

With sufficiently well matched resistors, the CMRR of a differential amplifier is more than good enough.

I will definitely try that (thanks @David Hess as well). 

The audio transformers sold for this purpose on Amazon did not measure well...   to put it mildly!     It shouldn't be hard to make an op amp work better than that...

If you need better rejection, then 0.1% resistors are inexpensive, but the AC common mode rejection becomes more important and will need to be adjusted using a trimmer capacitor.  I have only seen instrumentation go to this much trouble.  Audio applications do not often need this level of performance.

Below is an example of the common mode rejection of a Tektronix 7A22 differential amplifier which has performance limitations from the same mechanisms as an instrumentation or difference amplifier.  The fall in CMR at low frequencies is caused by using input AC coupling capacitors.  If AC input coupling is required, then there is another way to do it.  The lower curves (10^3 is 60 dB) are similar to what would be seen with an audio instrumentation amplifier with 0.1% resistor matching.  The fall in CMR starting at 1kHz is caused by mismatch in the AC CMR and shows why adjustment with a trimming capacitor would be required.  1% resistor matching would put the CMR below the bottom of the graph at about 10^1.7 or 50 where no trimming of the AC CMR would be required.

Good audio transformers have their own limitations which makes using an instrumentation or difference amplifier a very competitive solution if galvanic isolation is not required.

[SilverSolder]

Does that solve ground loop issues though? Other than using optical coupling of some kind, I can't think of an alternative to a transformer for that.

With sufficiently well matched resistors, the CMRR of a differential amplifier is more than good enough.

I will definitely try that (thanks @David Hess as well). 

The audio transformers sold for this purpose on Amazon did not measure well...   to put it mildly!     It shouldn't be hard to make an op amp work better than that...

If you need better rejection, then 0.1% resistors are inexpensive, but the AC common mode rejection becomes more important and will need to be adjusted using a trimmer capacitor.  I have only seen instrumentation go to this much trouble.  Audio applications do not often need this level of performance.

Below is an example of the common mode rejection of a Tektronix 7A22 differential amplifier which has performance limitations from the same mechanisms as an instrumentation or difference amplifier.  The fall in CMR at low frequencies is caused by using input AC coupling capacitors.  If AC input coupling is required, then there is another way to do it.  The lower curves (10^3 is 60 dB) are similar to what would be seen with an audio instrumentation amplifier with 0.1% resistor matching.  The fall in CMR starting at 1kHz is caused by mismatch in the AC CMR and shows why adjustment with a trimming capacitor would be required.  1% resistor matching would put the CMR below the bottom of the graph at about 10^1.7 or 50 where no trimming of the AC CMR would be required.

Good audio transformers have their own limitations which makes using an instrumentation or difference amplifier a very competitive solution if galvanic isolation is not required.

That makes sense - if the capacitances in the balanced "legs" of the difference amp are not matched, things will go more and more pear shaped as the frequency goes up...   maybe even causing instability?

[ogden]

That makes sense - if the capacitances in the balanced "legs" of the difference amp are not matched, things will go more and more pear shaped as the frequency goes up...   maybe even causing instability?

Reactance of capacitor decreases at higher frequencies. Meaning the higher frequency - the less important is "legs" capacitor [capacitance] matching.

[David Hess]

That makes sense - if the capacitances in the balanced "legs" of the difference amp are not matched, things will go more and more pear shaped as the frequency goes up...   maybe even causing instability?

That is right although instability is no more likely than any other feedback circuit.  The 7A22 example I showed has much more complicated input networks than would typically be found in an instrumentation or difference amplifier which is why the residual AC CMR is not trimmed out, so could be regarded as a worst case situation.  An audio instrumentation or difference amplifier should be better with AC trimming.

That makes sense - if the capacitances in the balanced "legs" of the difference amp are not matched, things will go more and more pear shaped as the frequency goes up...   maybe even causing instability?

Reactance of capacitor decreases at higher frequencies. Meaning the higher frequency - the less important is "legs" capacitor [capacitance] matching.

I think you mean the opposite?  As the reactance drops due to increased frequency, the capacitance dominates the gain and common mode rejection.

[SilverSolder]

Update on the replaceable battery thing...

I got hold of a V20, and - since you can actually replace the battery in this phone - I put a bigger one in.

If it makes it to the estimated end of the charge, it will have run for 22 days on a single charge -  Not bad! 





I tried the "quad DAC" headphone port.  It sounds awesome on a set of Grado SR80e headphones.  Not as loud as an external amp, but the quality is there.

[PlainName]

Aren't phones supposed to be smaller than a desktop screen?

[SilverSolder]

Aren't phones supposed to be smaller than a desktop screen?

Yeah, fixed it...

[PlainName]

Thanks

[coppice]

[Can you give an example of even one computer or smartphone where replacing the battery is "absolutely impossible"? I'm not aware of one. Even devices that have the battery glued in it can still be replaced, even if you have to take it to a technician that is properly equipped. Some batteries are held in by adhesive, but they are not potted in epoxy, they can still be replaced. Most people who buy these sort of devices don't care if they can replace the battery themselves, there is no business reason to invest the engineering and materials in making a robust mechanism to make the battery easily accessible.

There are plenty of phones glued together to the point where the quality of the phone after a repair is a bit hit or miss - depending mostly on how good the technician was who did the job. However, I have yet to see a laptop that couldn't reliably be opened by anyone with simple tools (although I've heard nasty things about some Apple products, but I never use them). A few screws usually gets you access to replace or expand disks, memory and the battery. The battery may be stabilised in its place with some glue, but this is reasonably easy to separate and replace. It only has to be functional, rather that super neat. The glue holding an entire phone together is a PITA.

[Bassman59]

Does that solve ground loop issues though? Other than using optical coupling of some kind, I can't think of an alternative to a transformer for that.

With sufficiently well matched resistors, the CMRR of a differential amplifier is more than good enough.

I will definitely try that (thanks @David Hess as well). 

The audio transformers sold for this purpose on Amazon did not measure well...   to put it mildly!     It shouldn't be hard to make an op amp work better than that...

If you need better rejection, then 0.1% resistors are inexpensive, but the AC common mode rejection becomes more important and will need to be adjusted using a trimmer capacitor.  I have only seen instrumentation go to this much trouble.  Audio applications do not often need this level of performance.

You need high levels of performance when your input stage is connected to a mixer which drives a big PA.

Anyway, the easy solution to the problem is to use one of the THAT differential amplifiers. Matched/trimmed resistors built in. Yes, you can degrade the performance with whatever you connect to it, but at least the amplifier is solid.

[eti]

Guess... you need it explaining?