Lower volume by switching from FET to BJT input op-amp?

[wwenze]

This post here got me thinking:

One very noticeable difference between the two is the volume level. If I have the knob at 9:00 for the AP, I need to turn it to 11:00 for BP to reach the same volume.

Is it possible that the OPA627BPs are fake op-amps that are using BJT inputs, and the resulting lower input resistance results in lower volume compared to when used with a real OPA627AP with FET inputs?

[AndrewT]

show your circuit.

[Mooly]

Nah....

and you can't read anything meaningful between pins on an opamp on a dvm... all you might do is increase the noise figure permanently if you reverse bias the input devices.

[pacificblue]

How low does the input impedance of an op amp need to become to influence the volume of your device?

[Mooly]

Originally Posted by pacificblue

How low does the input impedance of an op amp need to become to influence the volume of your device?

Depends on the circuit. No competent design should be device critical in that sense. Opamps are assumed to have infinite input Z. In practice they have not (bjt types) but it would be spectacularly bad design if gain were "opamp input impedance dependant". The ratio of applied input voltage to input current gives the figure... and it's tiny (I) making the resistance huge, even for a device like a 741. If you built a circuit using 20meg resistors then (apart from other issues) you might notice the design really was device dependant, and had a different gain if you swapped a 741 for TL071 for example, but for audio it doesn't come into it with any normal design. DC servos are one example where low input current (and consequently high Z) is of utmost importance.

[wwenze]

Thanks for the answer.