[sdiy] The annual I/O impedance/protection thread! (bumped)

[Harry Bissell]

----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 12 Jul 2011 15:58:14 -0400 (EDT)
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (bumped)

-----Original Message-----
From: Harry Bissell [harrybissell at wowway.com]

>>Q:...and if Yes to the above - were would the AC coupling cap go? Between Input and 100K (formerly 100R) or between Zeners and inverting input?
>>

>ugly any way you slice it. The cap should go between the inout and the zeners imho, with ANOTHER resistor to ground right at the input, so both sides of the capacitor have a bleed-off path to ground (no pops when you jack in) AND the non-inverting input can get bias current (or the opamp runs to the rail and stops working.
>

Harry,

For an AC Input - can you clarify the order for the components you've suggested above?

Input Socket, Resistor to Ground, Cap, Zener protection Resistor in Series, Zeners?

What value would you give the resistor to Ground - or what would dictate the value?

Much appreciated,

J


OK this is for a non-inverting input. DO what you wrote above, but add one more component, a resistor from the non-inverting input to ground.
The first resistor (by the input socket) can be 1Mohm, the one near the non-inverting input should be at least 100x the zener series resistor...
with a TL07x that could be in the 100K to 1M range without any troubles...

The higher the resistance at this point, the lower the frequency response will extend for a given cap size. It would make a hipass filter, you can choose the cutoff from the values you use. Making the resistors bigger allows you to make the cap smaller (so you might use a film cap instead of
an electrolytic and not worry about input voltage polarity...)

H^) harry


-- 
Harry Bissell & Nora Abdullah 4eva