# Line-in level from OpAmp

This is a follow on question from here: Simple Audio Amp for FPGA audio to PC in which I'm trying to get audio from an FPGA -> delta-sigma DAC -> op-amp -> Line Level.

Following the advice given in the above mentioned question, I've come up with this:

By explanation:

• Left side of circuit connects to 3.3V output pin on the FPGA
• The 3.3K and 4.7nf form a simple RC filter as part of the delta-sigma DAC
• The opamp is half of an opa2314

I've connected the RHS to microphone input on my PC and it records quite well, however...

I'd prefer to connect this to line-in. I've tried many different values for the two other resistors, including putting a 10K pot in for each, but can't seem to get enough gain for a line in signal.

Am I missing something, or am I simply hitting the limit of what this op-amp can do?

• Is your pc setup for recording line inputs? – Andy aka Aug 5 '13 at 7:33
• What are you actually expecting? It looks like you are expecting a 10kHz LPF followed by a gain of 80 ... in which case it won't work. – MikeJ-UK Aug 5 '13 at 7:48
• Check this out: windows.microsoft.com/en-us/windows-vista/… – Vasiliy Aug 5 '13 at 7:57
• What level of peak-to-peak voltage are you seeing at the input, the - terminal of the opamp, and the output? – pjc50 Aug 5 '13 at 9:20
• @Andyaka Yes, there's a gui for specifying the jack as line-in or microphone. – Brad Robinson Aug 5 '13 at 11:42

Signal that is blocked by the capacitor (i.e. that is in your filter's low pass band) sees a gain of $-8/3.4$.
So basically your amplifier tops out at a gain of $-8/3.4$ gain.