*technical elec question* DC offset/input impedance relation

[richie00boy]

concerning offset input voltage.

this holds true when the pot is set to top position, but in
normal use when it sets at a lower position the parallel
resistance is lower, hance the voltge drop is lower.

That is right, and is the reason why I specified at this postition -- it will be the worst case.

Is the shunt resistor from +input to gnd needed if I have an input capacitor between it and a pot/attenuator? How would the DC offset be calculated if I remove the shunt?

The shunt resistor is absolutely vital if you put a cap there as without it the non-inverting input would not be able to get a DC bias from anywhere.

 

[Sheldon]

It helps me when I look at these things to remember that the amplifier is trying to make the voltage at the non-inverting input exactly the same as the voltage at the inverting input (i.e. no differential). It doesn't matter where the voltages come from, just add em all up (in fact, you can add a small DC voltage of the appropriate sign at either input to reach a desired output DC). If you get a little lost, do a reality check based on that.


Sheldon

 

[homer09]

Here are graphs showing the curves of the different R(shunt) ratio values. I think the best value would be somewhere in between what janneman recommended and the 1/20 i have in the schematic. What do you guys think? (if anyone wants to see the spreadsheet, just ask)

 

[jan.didden]

[snip]The shunt resistor is absolutely vital if you put a cap there as without it the non-inverting input would not be able to get a DC bias from anywhere.

Why not use a cap at the top of the pot where the signal comes in? Then you can get rid of the shunt. That shunt is bad, because either it is low for low offset, and then it messes up the pot law, or it is high not to mess up the pot law and then it doesn't anything to help with the offset.
Just use the pot, and for offset balanced values use 1/3 of the pot value. In practise, it means that the offset varies with the pot setting, but it should be low enough not to matter.

Jan Didden

 

[jan.didden]

The idea here is that the pot is of the linear type. The load resistor is used to change the law of the pot (aprox a log pot) Taking this into consideration, should i boost the value of the 680 ohms? What other effects does changing this value have?

Any insight on what would be the best values for pot, R1 and R2? My requirements are to keep the value of the pot 20x the value of R1 because i think this is a good ratio for the fake law. Also, i would like to keep the feedback at the same value. basically as little change as possible would be nice.

OK I see. Yes. Well, for the balance condition for min offset you would want each amp input to see the same DC resistance to gnd. So I would put a cap in the pot top, then calculate the equivalent value of the pot with the shunt say at 1/3 rotation, and then select a fb network with the correct ratio and a parallel value close to the pot/shunt etc calculated earlier.

Since your gain will be around 20, the parallel value of the fb network will be close (good enough for government work ) to the resistor that goes to ground, so select that to be the same as the pot etc.
To keep noise down, you would want all values to be as low as possible that can be driven from the sources you are going to use.

Jan Didden

 

[homer09]

OK I see. Yes. Well, for the balance condition for min offset you would want each amp input to see the same DC resistance to gnd. So I would put a cap in the pot top, then calculate the equivalent value of the pot with the shunt say at 1/3 rotation, and then select a fb network with the correct ratio and a parallel value close to the pot/shunt etc calculated earlier.

Since your gain will be around 20, the parallel value of the fb network will be close (good enough for government work ) to the resistor that goes to ground, so select that to be the same as the pot etc.
To keep noise down, you would want all values to be as low as possible that can be driven from the sources you are going to use.

Jan Didden

Yes, this sounds like a good design sequence. I dont understand one thing. If both imputs should see similar impedance to ground, then why is it on Brian and Peter's minimalist gainclone schematic that there is a large imbalance?

Brian's schematic

The way i calculate for this schematic:

NI to ground: 22K + 220 = 22.22K (i guess this is in || with Source, how do you calculate then?)
I to ground: 680

Why does this design get away with it?

 

[jan.didden]

Well, of course you can get away with it. The offset will be unnecessarily high, but probably not catastrophically so. If they accept that, hey, it's their amp. They will probably claim that it sounds better that way

On the other hand, if they couple to the source without a cap then it may actually come out quite nice, if they have a couple 100 ohms output resistance in the source. Then again, that source, if DC coupled, will have some offset of it's own, so that is a dangerous path. Anyway, some builders will use DC coupling, some AC coupling, some will not even know what type of coupling their source has, so the results are quite unpredictable. But that's DIY for you, you pays your money and you takes your chances.

Jan Didden

 

[homer09]

They will probably claim that it sounds better that way

hehe, of course they would. not to say their amp doesnt sound great, but there will always be diverging opinions and compromises.

Which leads me to my opinion, that no cap is the best cap. So Janneman, is there a way to follow your last recommendation (i think it makes lots of sense) but without adding a cap? im willing to sacrifice lower dc offset as well as more stable input impedance.

 

[jan.didden]

hehe, of course they would. not to say their amp doesnt sound great, but there will always be diverging opinions and compromises.

Which leads me to my opinion, that no cap is the best cap. So Janneman, is there a way to follow your last recommendation (i think it makes lots of sense) but without adding a cap? im willing to sacrifice lower dc offset as well as more stable input impedance.

Sure, if you are sure that your source doesn't give out too much DC, there's no reason not to couple directly. Then you can keep the low 680 ohm in place. The low end will also be a bit more tight with DC coupling, although depending on your speakers it may be a bit too much to your liking.

Is that what you had in mind? Maybe I misunderstand your intention?

Jan Didden

 

[homer09]

My sources should have low DC output since they have caps at their outputs. So yes, i want to couple directly.

So if you could just look over the first schematic i posted (first thread post), would i be ok with those values? or would a lower/higher pot value (while matching resistor within fake law ratio) be better?

I have the choice of the following linear pots: 10K, 15K, 25K, 35K, 50K, 100K, 250K, 500K, 1meg.

And any standard resistor value for shunt.

Thanks for all the help Jan

 

[carlosfm]

OK I see. Yes. Well, for the balance condition for min offset you would want each amp input to see the same DC resistance to gnd. So I would put a cap in the pot top, then calculate the equivalent value of the pot with the shunt say at 1/3 rotation, and then select a fb network with the correct ratio and a parallel value close to the pot/shunt etc calculated earlier.

Since your gain will be around 20, the parallel value of the fb network will be close (good enough for government work ) to the resistor that goes to ground, so select that to be the same as the pot etc.
To keep noise down, you would want all values to be as low as possible that can be driven from the sources you are going to use.

Jan Didden

Good advice.
But there are other ways also.
Like a good fet-input op-amp as an input buffer (between the pot and the power amp chip).
The input stage will give stable impedance to the (bipolar) power amp chip.
The coupling cap can be used between the two stages, or in alternative before the pot, as it protects the pot too from DC-offset from some distracted diy experiences.
For those who sometimes connect portable devices to their systems a coupling cap is mandatory. Most of those devices have a shared headphone/line out plug, and they don't have DC with headphones (low impedance) but as line-out they have sometimes 5~10mv or more DC-offset, and that will be amplified by the amp's total gain.

 

[homer09]

Richie, here is the fake-law simulation spreadsheet you wanted.

My only request in exchange is if anyone finds a mistake or makes a useful improvement to share their work here.

The sheet is pretty rough, but its functional. Just change pot/shunt values that are highlighted in yellow, everything else should update on its own. I didnt know what function to use for ideal log pot, so if anyone has a better aprox to an audio log taper, do share. The sheet also calculates input impedance, useful for choosing pot value.

download at yer own risk, no guarantee of accurate calculations, always double check!