Hi Z input on mic preamp

[swinginguitar]

Looking at some schematics for mic preamps, most of which include a line input

what would be involved in either a) adding a Hi Z input, or b) making the line input switchable to Hi Z?

IOW, what are some smape components/circuits that would bump the Hi Z down where it needs to be to play nice with the rest of the preamp, while maintaining good sonic specs?

[brett]

Hi
a good option for a high Z input is a JFET buffer (very simple design, but a *little* bit noisier than some other options).
There are many designs in the forum and around the www. The Stratoblaster is a buffer + booster that I like.
My only other suggestion is to avoid going too high with Z (ie into megohms), as the preamp will be very sensitive to incomming noise.
cheers

[swinginguitar]

So basically just a front end buffer like we're accustomed to seeing in stompboxes and amps presenting a Hi Z to the instrument and lo output Z to the preamp?

[swinginguitar]

Is the usual 1M Ohm about right, or you think lower than that?

[Mike Burgundy]

That's a good one to aim for. 1M is kind of the "standard". Way lower will definitely cost you highs, way higher might get you into noise pickup problems, although there are some pedals out there that have 5M+. I'd opt for an opamp stage (Jfet input, TL07x) for convenience. Also, keep in mind mic pre's expect a lower signal than a guitar or bass puts out - line level however is ahigher than a guitar - so you'll likely need to amplify (for mic in) or pad (for line in) the signal down somewhat, and/or maybe include an extra gain pot (trimmer?) in the opamp stage.

[Kesh]

I'm building a mic pre currently, INA217 based, and the input Z is set by a resistor that you can vary. The actual chip's input impedance is something like 60M