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Using a simple (non-Spice) calculation: Of course, tolerances will affect this but it's interesting that the "120Hz" filter should be that far out when they go to the trouble to specify a 92 nF cap for the "800 Hz" (calc 746 Hz). It would be interesting to see the curves.

NickS wrote:Using a simple (non-Spice) calculation:

Code: Select all

Nominal	Calculated
freq   	freq
50	     49
120	    153
400	    394
500	    533
800	    746
4.5k	   4823
10k	    10526

Of course, tolerances will affect this but it's interesting that the "120Hz" filter should be that far out when they go to the trouble to specify a 92 nF cap for the "800 Hz" (calc 746 Hz). It would be interesting to see the curves.

What was your calculation method, out of interest?

Here are some curves for mine, taken with an Android app called Spectrum Analyze, via a Behringer UCA202 USB audio interface fed with a white noise sample. The green line was live; the red line is remembered peaks.

As you can see, at full cut on all the sliders, it's all over the place. The best thing you can say is that with all sliders in the centre, it's pretty level.

It's hard to see much detail at low frequencies, with this method, so I wouldn't read too much into the apparent bass response. That's not really the issue.

The file names explain the conditions they were taken under.

The first two are with the EQ sliders all the way down, with Level full up in the first one, then centred in the second.
The next one has all EQ sliders full up except the last/top one, which is full down:
Then all sliders full (if I've labelled that right - maybe not) - not sure where the level was: Then all centred: Doing one at a time might be more instructive, but you get the idea. The app and the audio device are both very affordable.

(No edit button?)

I used the white noise sample from here:

https://soundcloud.com/onlinetonegenerator

NickS wrote:Using a simple (non-Spice) calculation:
Of course, tolerances will affect this but it's interesting that the "120Hz" filter should be that far out when they go to the trouble to specify a 92 nF cap for the "800 Hz" (calc 746 Hz). It would be interesting to see the curves.

OK, I note that it's not 92 nF, it's 82 nF which brings the calculated frequency to 790 Hz. Not sure where I got that from, nor the values that gave me 153 Hz; the values on the Rev B schematic there work out to 125.
Today I used the gyrator calculator at http://www.muzique.com/lab/gyrator.htm. I thought I used that last time. They could have got closer using a 2400 pF 5% SMD cap for the 4.5k gyrator.