What you put at the input depends on the source it's connected to.
It's good practice to AC couple the input to the opamp and use a high (say 470k) resistor to define the DC conditions (ground reference). The AC coupling allows for "anything" that might end up getting connected to it that happens to have a significant offset. Something like a 5532 would give the best noise figure for this application, and allow a sensible value of coupling cap of around 0.47 to 1uf with 470k . The FET opamp is better noise wise at lower values of R, say 10k, but that means a much larger coupling cap in the 10 to 22uf range. The FET device would give a lower DC offset too perhaps not requiring a cap at it's output.
tbh, noise is the least of your worries with all this from a practical point of view... what you want is the correct matching and levels ultimately, so the OPA2134 is fine.
Greatness! I hacked together a buffer as above, replacing the 0,1uF caps the LM1972 wants with a pair of 0,47uF polyester caps followed by 470k resistors to ground. At the output of the opamp, I've a 2x1k voltage divider that's the input to the attenuator. Now the highs are crisp and clean again!
I tried setting the output opamp to a gain of 12dB but the output became too noisy. It's now at 7dB and it's back to its former levels. It's still a bit noisy (I can hear it but only with the ear about a foot away from the speaker) but I can live with that. It's about what I got from the chipamp before I put in the preamp.
There might still be slight problems with the pre, but I'll leave it for now until I get a hold of a scope and and design an updated PCB to accomodate the input buffer. I also have some slight DC issues, so I'll put back the 0,1uF caps before the LM1972.
Thanks all for your input! 🙂
I tried setting the output opamp to a gain of 12dB but the output became too noisy. It's now at 7dB and it's back to its former levels. It's still a bit noisy (I can hear it but only with the ear about a foot away from the speaker) but I can live with that. It's about what I got from the chipamp before I put in the preamp.
There might still be slight problems with the pre, but I'll leave it for now until I get a hold of a scope and and design an updated PCB to accomodate the input buffer. I also have some slight DC issues, so I'll put back the 0,1uF caps before the LM1972.
Thanks all for your input! 🙂
I guess the OP is done and happy.
But fiddling around with my own preamp I discovered this:http://www.rane.com/pdf/ranenotes/Unity_Gain_&_Impedance_Matching.pdf
And I wonder if it comes into play here...or for that matter at all in my preamp?
What became of this preamp issue?
But fiddling around with my own preamp I discovered this:http://www.rane.com/pdf/ranenotes/Unity_Gain_&_Impedance_Matching.pdf
And I wonder if it comes into play here...or for that matter at all in my preamp?
What became of this preamp issue?
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