x0x wrong envelope and filter resonance
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x0x wrong envelope and filter resonance

by mex on Mon Jul 11, 2016 4:45 pm

Hi community :)

I did build up my x0x some yeras ago, like 4 to 5 I guess, and it did never work properly ( well it did work good enough to make music) but it always botheres me, to take a loot inside again.. so now here I am, open x0x with some questionmarks remaining.

This thread is for experienced and nerdish people only :) as I am an electric engineer, I am not looking for obvious error, although they happen a lot to me too, but lets pretend I did solder perfectly and every resistor and cap is in the right spot, sinc I do get acid sound out of my device.

There are 2 main points, firts the filter section, witht he "moog" diode filter, I think there is some amount of wrond resonance in mine, I will show you in a second.
The second thing is It might be that my output section is just too quiet, I have to pull the volumn always to full amount and have it boosted in order to fit into my mixes, the x0x itself is too quiet, It might have something to do with the out mixer, maye i put and preamp inside, or you guys can help me, maybe.

Both screens have the same setting: 5ms per box ( sorry no native speaker :) ) and 100mV the y axis

res max, cutoff max, env max
IMAG1555.jpg (212.29 KiB) Viewed 738 times

cutoff min res min
IMAG1556.jpg (193.21 KiB) Viewed 738 times

So As you might notice, the Vpeak peak voltages are way too high, in the adafruti manual its maybe a facotr 2 maximum, and I have a factor of maybe 10

I would say it is a resonance peaking of the filter, but I got no clue where this is coming from, I would assume my whole x0x is working fine, but something, little thing inside the ladder maybe got wrong so there is the overschhot.

Also when the peak peak feedback voltage gets higher than around 2V the volumn increased suddendly, as my best guess would be one of the transistors, that are there for the emitter long pair current of the last filter transistor pair, are saturated then, and are fully conducting, whicch causes this.

Any help maybe someone more expirienced than I am know which resistor and or cap and or transisstor is to be replaced :) As if it were that simpel :)

Well thanks anyway for reading, goodnight.

Posts: 13
Joined: Mon Oct 01, 2012 8:52 am

Re: x0x wrong envelope and filter resonance

by antto on Tue Jul 12, 2016 12:16 am

in the build manual, there are "oscilloscope" screenshots from a PC app xmexoscope (or whatever it was called)
that is basically feeding signals as audio directly into the soundcard thru a capacitor/resistor
my point is, the app "looks" like a scope, but the actual vertical (the voltage) is not accurate at all, because it's hard to calibrate
if that's what you're using to compare against your scope, and if you also know that your scope is fairly accurate - don't worry

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Joined: Thu Apr 15, 2010 3:21 pm

Re: x0x wrong envelope and filter resonance

by mex on Tue Jul 12, 2016 1:50 am

ok, hadnt thought of that, so thanks. BUT :)

there is aweful clipping , also the higher voltages lead to really harsh volumn differences when I pull the cutoff potentiometer form one side to the other, so this is definetly not right.

I also found an old thread of mine, stating, that I changed some circuitry around the VCA, but my best guess is as that stage is after the VCF stage, it should not have anything to do with my "wrong" VCF behaviour.

Ah also as I think of it:

I have the oscilloscope screens, with which I am comparing mine, in the service manual of the x0x, so those are definetly some oscilloscope shots, not some LCD computer screenshots. So I am pretty sure, I hit some strange resonance.

Anyone of you got some decent SPICE *asc file for me to be lazy and just play around, what cap or resistor could be bad in my design, I am still thinking that I have a resonance peak waaaay too high, o that is why my x0x is behaving like this. But that as always is a wild guess until prooven right :)

Posts: 13
Joined: Mon Oct 01, 2012 8:52 am

Re: x0x wrong envelope and filter resonance

by mex on Wed Sep 21, 2016 4:03 am

For those who might still follow :D

I did get a new BA6110 chip and desolderd resoldered that part, I could only try the waveforms and Audio for a little bit but it seems that its alright now.

I also checked my other post about concerning the VCA and resistor values for its current source, I did get back to the original values with the new , now plugable, ;) BA6110 and it seems a lot louder and without the lot of disstortion and cut off that I had before.

I will endtest my x0x and maybe post the results.

Posts: 13
Joined: Mon Oct 01, 2012 8:52 am

Please be positive and constructive with your questions and comments.