By the way, if power efficiency is an issue (due to the < 0 dB gain, and lower impedance load presented to the amplifier), an alternate solution could be to connect parallel RLC circuits in series with the ER4S.
Originally Posted by luisdent /img/forum/go_quote.gif
I don't understand how it differs whether it is before or after my amp? If my player is a sansa fuze for instance, the output impedance is 1ohm roughly. The c5 amp is 2.2 ohms, but that itself would have no effect on the er4s itself. So why would it matter it the passive filter were before or after the amp?
It can matter because the impedance of both the load (headphones/IEMs, or amplifier line input) and the source (amplifier or line output) affects the frequency response. Also, although it might not be an issue with the Sansa Fuze, a line output may not like driving a low impedance and reactive passive filter circuit.
Originally Posted by luisdent /img/forum/go_quote.gif
I just figured I'd use it because there would be a gain loss. But if it isn't too much of a loss I may not even need the amp. I'm not sure what to expect. I use a -6db cut on the fuze EQ and I'm fine with that.
In the above example, the filter has about -2.4 dB gain. It also reduces the load impedance to the amplifier to about 40 ohms at 8.2 kHz, but that is likely still acceptable.
Originally Posted by luisdent /img/forum/go_quote.gif
And second, how the amp/source effect the circuit. I was told that i need to use the "alternate" values on the page with my low impedance amp output. I guess I don't get why that's the case. I had electronics courses back in high school quite a long time ago. I understand the basics and then some, but my brain just isn't following all of this. I get the concepts, but not the literal stuff I need for implementation.
The output impedance of the source (amplifier or Sansa Fuze) is added to the first, serial resistor (R3 in my circuit). Increasing it makes the notches deeper than originally intended, so R3 needs to be reduced by the same amount for the correct frequency response.
If you want to modify the filter:
- a lower R4 makes the 2.5 kHz notch deeper, while reducing R5 does the same for the 8.2 kHz notch
- the center frequency of the first notch is 1 / (2 * PI * sqrt(L4 * C2)) Hz; the second can be calculated similarly with L5 and C3
- increasing L4 and reducing C2 by the same ratio to keep L4*C2 unchanged makes the first notch narrower (again, the second one is controlled by L5 and C3)
For the simulated frequency response, I used LTspiceIV.