# [sdiy] Single supply op-amp filter biasing question

Tom Wiltshire tom at electricdruid.net
Fri May 5 17:02:54 CEST 2017

```Thank you very much, Roman.

Where does 3.5 times greater come from? I could see 4.5 times greater, since 9/2=4.5, but how does 3.5 work?

Thanks,
Tom

On 5 May 2017, at 15:25, Roman Sowa <modular at go2.pl> wrote:

> yes, you could, but I would just add the "X" resistor straight to 9V and skip 100k trimmer and 22u.
> 56K in parallel with X is your new value of the first resistor for filter calculations so keep that in mind.
>
> To rise bias by about 2V, you need X to be 3.5 times greater than R1 (let's call that what you have there as 56K). And R1 in parallel with X should be 56k.
>
> Roman
>
> W dniu 2017-05-05 o 16:03, Tom Wiltshire pisze:
>
>> capacitor. But since the signal that I'm trying to retrieve with this
>> filter is a DC level, using a capacitor (AKA highpass filter) to
>> connect the input doesn't seem feasible. So could I do something like
>> this, and how would I calculate the value of X?
>>