MN3207 and Zombie chorus conclusions (kinda long)

[bioroids]

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 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. I thinks that's pretty much the wall I hit. 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

[gez]

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

If you wired the buffer for unity gain AND attenuated the signal going into the BBD, then recovered the loss afterwards using a active mixer at the output, you'd solve the problem of clipping.

It might be an idea to filter the signal before the BBD as well as after to help with the hiss (similar filter would be appropriate).  This might even solve the clipping problem as it would cause some attenuation of the signal into the BBD.

[bioroids]

I think the input buffer should be used as unity and then attenuate a little in the lows. That shlud keep us away from clipping.

I also think introducing a little low pass filtering before the bbd should help to attenuate a little more and control the hiss. I'll try first with a simple capacitor to ground before muving to the active filtering zone (wich I dont understad very well :oops: )

Luck

Miguel

PS maybe this will benefit with a little companding but i dont want to spend more money on expensive chips for this project  :wink:

[Mark Hammer]

You realize, of course, that all of this is moving the Zombie closer and closer to a CE-2, minus the FET switching.  That is not a criticism, either of the Zombie itself, or what you are doing.  Rather, minimalism has its costs as well as its benefits, and as different people start addressing each of the different costs, the circuit gets more and more complex, until it is finally identical to the basic Boss design.

On the other hand, it has been an interesting exercise in essentially re-justifying all the aspects of their commercial design, by incrementally "solving" all the problems they solved.  A bit like an 8-month apprenticeship in their R & D unit.

It's funny, you know.  I think of chorus and fuzz as being a bit like trumpet vs flute.  Flute is one of those instruments that, even when you do things all wrong, it still sounds interesting and even pleasing.  You can have a perfect embrochure, or a sloppy, spitty, breathy one, it still sounds nice.  Trumpet, on the other hand, unless you blow a certain way, sounds like crap, and holds little room for experimentation in note or sound design the way flute does.  

Similarly, so many folks here work in the fuzz domain, where there is considerable latitude in design because almost anything that aims for distortion can still sound interesting in some fashion.  BBD-based effects like chorus, however, place very strict constraints/boundaries on what you have to do - the essential housekeeping, like filtering, headroom management, etc - in a manner that tends to push for uniformity of design, and work against experimentation.  From their experience with fuzz design, many people have high and probably unrealistic expectations about what can happen with respect to BBD-based designs.  In the end, almost all BBD-based designs get funneled in the same direction.  That's not a complaint, merely an observed reality, and the rules of the game.

[Aharon]

I have a chorus that uses those chips and it sounds cristal clear and chimey (a little too much if you ask me) but maybe it's time to reverse engineer this thing and see how they did it.
.It also has no hiss whatsoever.
Let me know if you guys would be interested.
Aharon

[bioroids]

Well said Mark.

I always thought, looking at the schematics of comercial effects, "are all this components really necesary?".  Maybe they are, as their effects are designed to work in multiple and different rig settings. When you're focused on your guitar-amp only you can ommit a few things (or take the fast road)

I'd like to check out the CE-2 schematic, does anyone knows where may be posted?

Regards

Miguel

[bioroids]

I'd like to see that circuit Aharon. I have a DOD stereo chorus I like very much, but uses the MN3007 wich has much more headroom.

I also own a danelectro flanger that uses the mn3207, but i dont like how it sounds (is a flanger anyway)

Luck

Miguel

[Aharon]

Ok,I'll try to scan the PCB and go from there.
Aharon

[bioroids]

That would be nice.

I'm having trouble now when switching to the clone mode, no sound gets out the BBD, could this be again a input biasing problem? maybe the different clocking speed needs to bias it differently? that would really suck :x

I get different voltage reading on the bbd output and filter stage when i switch to clone mode. Thats weird because it only switches a cap out of the 4046  loop.

Regards

Miguel

[Ansil]

Well said Mark.

I always thought, looking at the schematics of comercial effects, "are all this components really necesary?".  Maybe they are, as their effects are designed to work in multiple and different rig settings. When you're focused on your guitar-amp only you can ommit a few things (or take the fast road)

I'd like to check out the CE-2 schematic, does anyone knows where may be posted?

Regards

Miguel

http://home.hetnet.nl/~chrisdus/download/ce2b.gif

[bioroids]

Thanks Ansil, I'll check this out.

Luck

Miguel

[bioroids]

I keep strugling with this. Apparently the biasing point is not the same at different clocking speeds  

I'll be checking this again later.

Luck

Miguel

[bioroids]

Well, I just have to give up. I think the bbd may be broken or something.
I can't get any sound of it if the clocking speed is less than some value. I cant get the 50ms delay time is supposed to get.
I checked the clock generator, its a very nice square wave, like 15khz (the bbd is supposed to work between 10khz and 200kz), or more, 20khz, but no matter how I bias the input (including pin 4 at various ratios of pin 5), no sound gets out of it.

I'm stuck with a maximum delay time of like 12ms, i dont think that qualifies as a chorus. Does anyone knows what's the delay time I should get in the non-clone mode?

Thanks and luck

Miguel

PS: I checked again and yes, a little more headroom is available when pin 4 is a little less than 14/15 of pin 5

[bioroids]

Well, I just have to give up. I think the bbd may be broken or something.
I can't get any sound of it if the clocking speed is less than some value. I cant get the 50ms delay time is supposed to get.
I checked the clock generator, its a very nice square wave, like 15khz (the bbd is supposed to work between 10khz and 200kz), or more, 20khz, but no matter how I bias the input (including pin 4 at various ratios of pin 5), no sound gets out of it.

I'm stuck with a maximum delay time of like 12ms, i dont think that qualifies as a chorus. Does anyone knows what's the delay time I should get in the non-clone mode?

Thanks and luck

Miguel

PS: I checked again and yes, a little more headroom is available when pin 4 is a little less than 14/15 of pin 5

[puretube]

the BBD`s output-DC varies at different clockrates.
maybe you have to capacitively (again) decouple the BBD-output
from the output (filter-) opamp,
in order to not un-bias that one. (and provide that one
with an extra bias, of course.....

[bioroids]

Well, i will try that one. I dont know why the output ties pin 7 and 8 together when in every other circuit are separated by 5k6 resistors.

I should have breadboarded this one, the pcb looks like a survivor of a nuclear war :lol:

Thanks and luck

Miguel

[uncle boko]

Maybe the ADA flanger is a better bet.