Author Topic: MN3207 and Zombie conclusions (kinda long)  (Read 1357 times)

bioroids

MN3207 and Zombie conclusions (kinda long)
« on: November 14, 2003, 07:46:22 AM »
Hi everyone!

well I finally got time to keep testing this thing and I can make now a few conclusions:

1) It doesnt really matter wether you connect Vdd and Vgg together or not, so I simply ended up hooking both to the 9v supply (no matter how I biased the input, no change in sound when pin 4 was at 14/15 of pin 5)

2) Is required to isolate the bbd from the preceding opamp using a capacitor and biasing it separately.

3) The exact bias value for max headroom (someone remember him?) with the particular components I used and a 9.01v supply is 5.52v. The best way is to use a trimpot to adjust it exactly, a good aproximation is 30K and 47K for the voltage divider (gives like 5.49v).

4) Anyway the max headroom available is not enough for a hot guitar signal (like my 335 imitation with power chords). I tested it biasing the output of the bbd to different points and disconecting the following opamp stage (filter) and it has nothing to to with it.
This seems to be a problem inside of the bbd itself. According to the datasheet, you get 0.4% THD with a 0.25v signal at 1Khz. But what about lower frequencies? I thinks that's pretty much the wall I hited. I tested first with a 440hz signal, then a 220hz signal. The distortion got higher as I lowered the frequency. So what I did was to cut some lows with a 0.01uf coupling cap. I dont think anyway that big bass is needed on the modulated signal of the chorus, as long as there's bass on the clean part of it. This reduced clipping a lot. I tried using a series resistance and higher cap but for my guitar this worked best.
Maybe another solution is not to get that little bit of gain the buffer opamp is set to give.

Now I notice there's a good amount of hiss on the final chorus sound, maybe I should check the filtering stage (as I also got to check the lfo part). If anyone could explain me how this "3 pole filter" is supposed to work or point me to some web I'll thank him/her very much.

Well that's all I can say about this, I hope its usefull for someone. When I'm done with this I'll post a schematic with all the mods, a pcb if I get get time and some sound samples maybe.

Good luck and thanks!

Miguel
Eramos tan pobres!