Reality Check: DC Offset from Passive Pickup is Impossible?

[peskywinnets]

The method I spoke of (biasing the opamp +ve input pin via the pickup coil itself), would indeed need a low impedance 1/2 VCC source ...but bearing mind the OP is sorted, (& has gone the cap route) leaves us all arguing the toss over something that no one gives a toss about!

[Mike Sulzer]

The method I spoke of (biasing the opamp +ve input pin via the pickup coil itself), would indeed need a low impedance 1/2 VCC source ...but bearing mind the OP is sorted, (& has gone the cap route) leaves us all arguing the toss over something that no one gives a toss about!

It does not need a low impedance 1/2 Vcc dc source. You can use large resistors and bypass the 1/2 Vcc source to ground with a cap (which you should do in any case). The advantage of doing it in this way is that you can use a single pair of resistors and one capacitor total for the two op amps in the circuit. The other circuit needs separate caps and Rs for each op amp.

[Joe Gwinn]

I think you mean 1/100 V+; so there will be about 100 mv on the input and the amp will not function since it needs to keep the common mode voltage about 3 volts above the negative rail (it can equal the positive rail.)

Oops. Right.

I suspect that there was more to this than just shorting out the capacitor.

Well, we have enough already to explain the reported effects.

[peskywinnets]

What is it about this forum?

My posts were in response to this....

I' I was looking at building blocks for a starting point and thought maybe I could omit an input cap, because it seems there is going to be no DC offset from a simple coil, right?

Now look where we are...

It does not need a low impedance 1/2 Vcc dc source. You can use large resistors and bypass the 1/2 Vcc source to ground with a cap

The OP asked about NOT using a cap...I therefore fielded a capacitor-less a method where you can bias the opamp via the pickup...but you do need a low impedance 1/2 VCC source if going that way. (I wasn't saying it was the best, optimum blah blah...I was just saying it could be done)

There a several way to skin a cat, but your methods are now involving ....erhm capacitors. (remember? those little things we were trying to avoid using here?)

[Mike Sulzer]

What is it about this forum?

My posts were in response to this....



Now look where we are...



The OP asked about NOT using a cap...I therefore fielded a capacitor-less a method where you can bias the opamp via the pickup...but you do need a low impedance 1/2 VCC source if going that way. (I wasn't saying it was the best, optimum blah blah...I was just saying it could be done)

There a several way to skin a cat, but your methods are now involving ....erhm capacitors. (remember? those little things we were trying to avoid using here?)

Yes, it "works". But most engineers will tell you that a power supply, that is, something provides a dc voltage above ground in this case, should look as much like ground as possible at ac That means bypassing it with one or more capacitors.

I do not know why you are attempting to cut off all discussion that you consider not directly related to the original question. Let's leave that to you and your therapist. By the way, the original question says "input" cap, not all caps.

[peskywinnets]

I do not know why you are attempting to cut off all discussion that you consider not directly related to the original question. Let's leave that to you and your therapist. By the way, the original question says "input" cap, not all caps.

I'm not talk away...but the OP is sorted & I'm only pointing out that the bits you're disecting are being taking out of context - again, the OP wanted a input cap-less solution, that's what I posted, I never commented whether it was the best way or not.

Anyway, gotta run, I'm late for my "how to avoid being sucked into threads that are straying off" therapy class.