Calculating Quad 405 Input Impedance

[DarrenHW]

Can anyone help me calculate the input impedance of my Quad 405? I'm thinking of buying an Oppo 105 to use as an AV PreAmp with 3 Quad 405's for Front, Centre and Rear channels. The recommended amplifier input impedance for the Oppo is 47k, the Quad stock is 20k. I have changed the input sensitivity of the Quad from 0.5v to 0.775v by changing R6 to 220K, R4 to 15K and C4 to 68nF (and could increase it further to 1.5v), am I right in thinking this increases the input impedance?

13376

What calculation with which component values do I need to establish the input impedance?

[tubehunter]

Hi Darren

Basically I understand that R3 sets the input impedance of the inverting op amp used here

[DarrenHW]

Hi Duncan,

Thanks for the response, I didn't realise it was that simple :doh:. I've done some more reading and decided I'm going to implement Keith Snook's non inverted mod. This makes changing the impedance simpler, C1 just needs to be changed so that C1 x R3 = R6(22k) x C4(680uF). So I'll replace R3 with 47k and C1 with 300uF, not quite equal but I guess as there is tolerance within capacitors anyway this should be within spec? Any thoughts as to how / whether the amp will change by increasing the input impedance?