Budget active subwoofer lowpass, buy, eval, mod

[Max Headroom]

Not what I meant.
I mean resistive mixing/adding is not the same as virtual earth mixing/adding.

Dan.

 

[keilau]

Not what I meant.
I mean resistive mixing/adding is not the same as virtual earth mixing/adding.

Dan.

Learn something new everyday. What is "virtual earth mixing/adding"? Can you show a conceptual schematic to demonstrate the idea?

 

[Max Headroom]

Audio Signal Mixing
This should fill you in.


Dan.

 

[DonVK]

Thanks, its a good article. I know what you're referring to now.

Preferred summing at the inverting input of the op amp would reference all inputs to the op amp's virtual gnd (input) and minimize any potential cross talk between those channels because the mixing point can't float. It's importance increase with the number of inputs mixed and I agree with that.

I have only 2 inputs, and I'm only stripping off the LF for a mono subwoofer. This should be OK, even according to the article's author, and I have not seem a problem with.

Thanks for the heads up. If I start having issues when I measure the entire system I'll have at least one place to start looking.

 

[keilau]

Audio Signal Mixing
This should fill you in.

Dan.

Thank you for the article and it is a very good read. But I am confused by the author's conclusion. While Rod Elliott agrees with you that "resistive mixing/adding is not the same as virtual earth mixing/adding." But they are virtually the same for simple stereo to mono summing.

...….both methods are compromises. As is probably obvious by now, the benefits of the virtual earth mixer generally outweigh any disadvantages. This is shown by the fact that it is almost universal for any mixer with more than two channels.
For very simple mixers, simple resistor mixing stages are common and are well suited to the task. Common uses for such simple circuits are to convert a stereo signal into mono - either full range or only the bass frequencies.

BTW, I am a long time visitor of the ESP website and have a lot of respect for Rod Elliott's writing.

 

[DSP_Geek]

I ran a few freq sweeps using a REW and a soundcard loopback. The filter behave as a bandpass. Those extra caps are doing something.

[...]

Issue #3 : The first input cap is a polarized electrolytic. The distortion is too high and I think this cap is causing a lot of it.

Reviving a necrothread...

<Genie>
Rule number three, I can't bring people back from the dead. It's not a pretty picture. I don't like doing it!
</Genie>

Oddly, the distortion seems to happen at subharmonics of 60 Hz, or what one might expect if induced line frequency were somehow leaking into the measurement. The hint is 30 Hz has excess 2nd harmonic distortion (60 Hz), 20 Hz too much 3rd harmonic (60 Hz), and so on. Maybe a ground loop?

The other thing to notice is some of the distortion decrease noted in a later post could be linked to bandwidth shrinkage: if you're measuring sixth harmonic distortion at 30 Hz, don't be too surprised if it goes down substantially should a modification attenuate frequency response at 180 Hz.

Given all that, the distortion still seems lower than before, even including possible measurement artifacts. You just gotta be careful of what you measure because, to extend Dick Feynman's dictum, nature may not be fooled but she can take every opportunity to fool you!