[SOLVED] Bass goes away as I turn up resonance

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fredrikstolpe
 
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Joined: Thu May 14, 2009 10:55 am

[SOLVED] Bass goes away as I turn up resonance

Post by fredrikstolpe »

Hello people

I have built a Willzyx x0xb0x. It works fine and sounds good in general, but the filter doesn't seem to work like it should. As I turn up the resonance the base disappears.

I'm an experienced solderer and I've been quite careful when building. Willzyx agrees that it doesn't sound right and has been helping med out, but so far without success, so now I'm reaching out here for suggestions and help.

Here's a demo: http://www.youtube.com/watch?v=Gp4_SU18iF0

As you hear, there is bass when resonance is low, but it goes away as I turn it up. Sure, that's normal for resonance in low pass filters, but not that much, right?

So, I've been adjusting the trimpot for the filter but found no setting that's any better. I've been double-checking for solder bridges and component placement and it all looks good. The tests all work out well, although it's hard to tell if the filter looks correct in the scope.

Hope there's anyone out there that might be able to suggest things for me.

Regards
Fredrik
Last edited by fredrikstolpe on Thu Jan 16, 2014 4:08 am, edited 1 time in total.

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antto
 
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Re: Bass goes away as I turn up resonance

Post by antto »

hi

your x0x filter is 100% healthy

this is how the 303 filter works, love it or hate it
it's a diode ladder with negative feedback

one example is the moog ladder filter:
it's four poles, negative feedback
when there's no resonance - the gain is 1.0, the signal goes thru the four poles and that's it
when you introduce resonance, you get the output signal subtracted from the input
that's where you need to amplify the feedback by 4.0 to reach self-oscillation
and as you do that, the output gain drops to 0.2 (-14dB)

in contrast to the moog ladder, the 303 filter has a few differences
the poles are somewhat pimped up (read, less ideal in a way) and the filter requires even more than 4.0 amplification of the feedback
it requires 17.0 amplification of the feedback to get it to self-oscillation, which means that the output gain drops even more than the moog ladder - -25dB
however, that's only at the output of the ladder
there is built-in gain-compensation circuit in the 303:

the output of the ladder goes to two places:
1) VCA
2) resonance pot

from the resonance pot, it goes into yet another two places:
1) filter input (this is the negative feedback right here)
2) VCA - there, gain compensation!

this compensation adds up but isn't amplified additionally, so it makes things better, but the output gain even with this "hack" is still not 1.0
what i heard in your video is perfectly normal for 303/x0x, trust me

and one more thing, the filter cutoff frequency when there's resonance shifts up with 41% ... unlike the moog ladder where it remains in it's place
in other words, when you turn the resonance down - the effective cutoff actually goes lower

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fredrikstolpe
 
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Re: Bass goes away as I turn up resonance

Post by fredrikstolpe »

Hey

Ok, thanks a lot for the reply.

Still I just think, after watching other videos of the x0x that there is bass present event with high resonance, for example:

http://www.youtube.com/watch?v=NX7xgwMAEoM

http://www.youtube.com/watch?v=i7BU9xmits8

If you compare this to mine, don't you agree?

Later
Fredrik

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antto
 
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Re: Bass goes away as I turn up resonance

Post by antto »

nope

i recorded the audio from both those two videos as well as your video and looked at the waveform

video 1 - signal looks normal
video 2 - signal is EQ'ed (bass freqs are boosted)
your video - signal looks normal

for video2, the description says that the x0x is ran thru a mixer (which gives the overdrive) and reverb
the volume at first is very low, yet the x0x is loud, then he simply turns up the volume which results in overdrive from the mixer
you can ask the author whether he used EQ from the mixer or afterwards, but i tell you, it's EQ'ed

EDIT: here's one of mine, with just some delay FX:
https://soundcloud.com/antto/weird-pattern
the volume knob was not touched during the whole recording ... the 303/x0x is very dynamic

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fredrikstolpe
 
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Re: Bass goes away as I turn up resonance

Post by fredrikstolpe »

Ok, well thanks again! :-)

/Fredrik

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