What input impedance may be too low and how is this an issue in a typical opamp circuit? I honestly cannot see it.
It was a guitar amp circuit that the OP posted. Your typical guitar is high impedance and there's rolloff into the 1st opamp in the signal chain for some circuit values and opamps. This is well documented. I agree it's not an issue for hifi. Also calls into question why OP wasn't forthright up front and wasted a lot of members' time.
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To find an overview consisting subjective opinions.
Given the current world population (>> 7 billion), too many dead ends is a reasonable yield.
Notice the OP's third & last contribution sofar is at #6.
Sorry, you came up with the suggestion that swapping opamps may affect the input impedance and cause audible differences. It would be fair if you explain how. "Well documented" is an unfair deflection. Circuit values are irrelevant in this particular discussion unless they are somehow tied to the opamp types.
Not sure I understand you are getting at. Do you agree different opamps may have different input impedances? Do you agree the circuit input network is in series and parallel with the input network inside the opamp?
@analog_sa has a point. All opamp ciruits employ feedback, and it is the circuit parts and feedback that set input impedances. I would like to see which opamps you propose to swap in which circuit to change input inpedance.
Jan
Well, you're making me do my homework Jan, but isn't closed loop input inpedance proportional to the differential input inpedance of the opamp? So a big change from BJT opamp to FET opamp would still result in a big change in input impedance, even if both were over 1M? It makes sense when we are talking about a passive guitar pickup.
Well, in the circuit below, using a 741 op amp in the first stage will result in a lower input impedance than with a TL071, if I am not mistaken. This could result in a different high frequency response when using an electric guitar. I think that is the point Leadbelly is trying to make.
Why would it? The input impedance is set by that 470k resistor
It makes no sense at all. Put numbers on it. Closed loop gain seems to be only around 10db, right? So, how much gain is left to increase the impedance through feedback? Would it make a difference if it is 100M or 10G?
And anyway, why does input impedance need to be particularly high anyway? Is there an electrical reason, or is it audiophoolery again? 😎
It's because guitar pickups have (very) high impedance and shouldnt be loaded with "low" impedance preamp input.
Surely the diodes in the feedback network render this circuit of interest only to guitar amplifier fans.
This thread belongs in Instruments and Amps instead.
This thread belongs in Instruments and Amps instead.
Ah. I thought the input impedance was this resistor in parallel with the input impedance of the op amp..
Yes, but the feedback makes the -input follow the +input so there is extremely low input signal difference, thus extremely low input current, much lower than the static input impedance would suggest*.
Jan
* the input current does not flow to gnd of course, but from one input to the other. Extremely low input voltage difference means extremely low input current. Say the opamp has 1V out and 80dB open loop gain at the signal frequency. That means the differential input voltage at the input pins is 100uV. Check the input impedance, it probably is many, many megohms. So the input current is extremely low and represents a gigantic input impedance.
Jan
* the input current does not flow to gnd of course, but from one input to the other. Extremely low input voltage difference means extremely low input current. Say the opamp has 1V out and 80dB open loop gain at the signal frequency. That means the differential input voltage at the input pins is 100uV. Check the input impedance, it probably is many, many megohms. So the input current is extremely low and represents a gigantic input impedance.
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Is it possible to put numbers on this? With no numbers any discussion is meaningless.
I have some doubts the pursuit for a high input impedance has much to do with unimpeded signal transfer. Could it be helping to preserve an undamped ultrasonic resonance? And if so, has anyone done any simulations to establish the highest preamp input impedance that is still worth pursuing?
Knowing zilch about guitar electronics i still find the subjective preferences interesting. And the quasi technical explanations that often go with them entertaining.
Guitar pickups typically have a DC resistance of several kilo-Ohms and an inductance of several Henrys with a self-resonant point at several kHz.
Volume potentiometers are 250k or 500k.
Thus, source impedance can easily exceed 100kOhms. You will want a FET input with low bias current to avoid any additional current noise. Voltage noise is already large enough.
Yes, OPA164x is truly excellent for low non-inverting gains from high impedance sources, notably buffers.
Btw, OPA828 is unity gain stable as well.