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VCF problems on M.Koster x0xi0
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VCF problems on M.Koster x0xi0

by Neyah on Mon Jan 16, 2017 5:57 pm

Hi everyone,

I have a trouble issue on my Marteen Koster x0xi0. The sound on the output is very very low (-24dB with the cutoff ans resonance full), and she isn't soung good : I have pretty much no signal if the cutoff isn't far away the 3/4 of his course.
So I test the VCO output, it sound good and loud, then i test the VCF output and the problem become : I have no sound if the resonance isn't in max, but the sound is very very low.
So I think the problem come from the VCF, and more from the resonance, perhaps the hi-resonance mods ?

Someone have this trouble on any x0xb0x ? any idea ? or maybe someone have the contact of Marteen Koster will be good.

Thanks to community.

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by rv0 on Tue Jan 17, 2017 7:15 am

His mail (slightly obfuscated) is m dot j dot koster86 at gmail dot com
however i'm not sure if he still supports it, also he didn't want to share his schematic in the past.
you can however use the BC x0xI0 schematics (posted on this forum, somewhere on the 20th page of this topic: viewtopic.php?f=13&t=11539), they should more or less be similar.

issues i've seen with MK x0xI0's in the past were due to soldering issues (shorts).

good luck.

rv0
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Tue Jan 17, 2017 8:42 am

Ok great, thanks for the answers, I send him a mail I wait for his answer.

Shorts in SMD it was very difficult to find no ?

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Tue Jan 17, 2017 1:51 pm

Ok I have test different wiring with an externe source.
So, ext. source > VCF in - Main out = no sound
ext. source > VCF in - VCF out = exactely the same thing when I use internal VCO
ext. source > VCA in - Main out = no sound
ext. source > VCA in - VCF out = sound is equally lound but cleaner. The weird thing is th reaction with the resonance : when the pot are 0% = no sound, sound appears at the second mark. when the pot are 50% the sound is at the best lound and clear he can (so ever not very lound...), and he dicrease when he goes to full resonance (that it's normal i think...)
But I'm very obsessed by this results : first, how I can ear sound in VCF out if the source enter in VCA in ? it's strange no, the VCA Input should be bypassed the VCF no ? And two, why this resonance pot act in the signal like this ??

In first place I have think about the amplifier stage of VCF, maybe a capacitor was burn or something, but the strange behavior of resonance pot make me a little confused...

I would like to add I haven't huge knowledge in electronic, I try to deduct by logic.

So, someone can help me to understand what's happends here ?

thanks a lot to community

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by antto on Tue Jan 17, 2017 6:12 pm

Neyah wrote:I have pretty much no signal if the cutoff isn't far away the 3/4 of his course.

this probably means that the cutoff frequency is very low (which is not normal UNLESS there's a "extended cutoff range" mod, and there probably is)

I have no sound if the resonance isn't in max, but the sound is very very low.

this smells like the main output from the filter isn't reaching the VCA
some explanation:
the filter output (from C14) goes to two places:
1. to the VCA (via C21) .. this is the main path
2. to the resonance pot, from where splits and goes to yet another two places:
2.1. back into the filter - this is the feedback path, and creates the actual "resonance" effect
2.2 to the VCA (via C20) .. this is used as gain compensation..

since the ladder filter loses a whole lot of gain when the resonance is increased (since it's achieved with negative feedback), the circuit has a compensation:
the more you increase the "resonance" - the more of the filter output (coming from the resonance pot) is also going to the VCA to bring the gain up

in your case, if you only hear the filter when the resonance is increased - it smells like the filter's main output path (the one going to the VCA via C21) is missing (and you're only hearing the "compensation" part)

i hope this helps

antto
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Tue Jan 17, 2017 8:07 pm

ok thanks for the explanation.

If I understood well, I loose the signal before C21, but as I hear a signal (signal from the gain compensation circuit that goes to c20) everything before C14 (the output of filter) is working. So the problem will be between C21 and C14, which mean I have a short on the path or an issue with the resonance pot, since there is nothing between C21 and c14 except VR4 resonance pot. Am I right or it's a complete nonsense logic ?

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by antto on Wed Jan 18, 2017 12:55 am

if you can hear the filter actually "resonating", then the resonance pot should be okay
the filter itself should also be okay (minus the cutoff issue, but that might be because of the other mod)

now, the two outputs from the filter which go towards the VCA pass thru the capacitors C20 and C21, then thru resistors which act like passive mixing of the two audio signals, R122 and R121, then they connect together and go to the input of the VCA.. also, at this point the passively mixed signal is "pulled up" to 5.33V

the "compensation" output goes thru resonance pot, C20, R122
the main output - C21, R121

possible things i can think of:
- one side of C21 might be shorted to ground or to a power rail
- R121 might be wrong value (much bigger than it should be) which would result in a much lower gain when the two signals get passively mixed
- broken C21 or broken R121

antto
 
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Re: VCF problems on M.Koster x0xi0

by rv0 on Wed Jan 18, 2017 4:44 am

Neyah wrote:Shorts in SMD it was very difficult to find no ?

The shorts I've seen were just the housing of pots touching other things.
no SMD involved.
Just have a good look at the pcb on both sides to look for anything suspect.

rv0
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Wed Jan 18, 2017 1:45 pm

I've test R122 and it's good 220k as expected. I've test c21 with a voltmeter and i've no result, so I try to test c20 and I have no result too. So I exchange the capacitors c20 to c21 and c21 to c20 to see if I have more signal with no resonance but it would seem I have exactly the same response than before... So the problem isn't here no ?

I forgot to say there is a noise seems like pass trough a bitcrusher or something like that on background, that respond to gate or to bpm when the sequencer don't running.

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Thu Jan 19, 2017 8:30 pm

So now, how i test if c21 is shorted ?

I made some photos of that seem unconventional in the circuit.

First, ugly soldering in R110 and r111, and they are in the VCF block. How can I test if this ugly soldering make something wrong ?
Image

I can record the sound of the different wiring if you want, just ask me.

Thanks again to you.

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Fri Jan 20, 2017 3:54 pm

OK some news :

- I've make a continuity test all the way from resonance pot where C14 was connect, to pin 1 ba6110. All the wiring between component seems good.
- I've test with ohmmeter R121, I have 220k, so I think is good.
- I've make continuity test from the two side of C21 to : -12v, +12v, +5v,, +5.33v, +6v and GND. And I have no result...

oh, and i've test the ugly R110 and r111, they've got the good resistance value, the wiring between them and Q21 are ok, and the wiring between them and +12v don't bleep but say me 100 ohm. It's ok ?

So I love treasure hunt but I don't know where to go now... any suggestion ?

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by antto on Sat Jan 21, 2017 6:49 am

i don't think i can help any further, this is a heavily modified x0xb0x, not a normal one

antto
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Sat Jan 21, 2017 3:52 pm

Arff I understand... But I think you're right about the spot where the problem come from, it's definitly from the VCF, because when I wire the VCO OUT to VCA IN it sound good. And the weird response of the signal with the resonance pot which I describe above makes well think at an issue on the main path... And at this part, no mods. The closer mods is the VCF out/VCA in, which is on the jonction of R121 and R122.
I have some question if you don't mind :

1- Is it possibly C14 is broken rather than C21 ? and what about Q19 ?

2-Is it a good idea to track the main path with an audio probe to ear if has sound or not and maybe compare to compensation path ?

3-and a last one, I know someone who have the same gear and he can do some test of value for see if i have the same result, and find the faulty component maybe... but I don't know what to test ! I thought to go up the circuit from the end of VCF section and further and further... But with mine poor electronic knowledge I don't know what to search really... Can you tell me what's the right way for debugging empirically ?

Ok I finish for today I think, thank you for your time by the way.

edit : 4 - I've notice that the resonance pot doesn't act like resonance, but more like a volume pot in fact. So the problem will be perhaps not in the main or compensation path but more on the feedback path no ? I don't know my brain is burning... I want my synth baaaaaaack !!! sorry i'll smoke a cigaret and calm down héhé..

Neyah
 
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Re: VCF problems on M.Koster x0xi0

by antto on Sun Jan 22, 2017 11:47 am

hypothetically, if you prevent the "main" filter output from reaching the VCA, then what's left will be the "gain compensation" path reaching the VCA
then the resonance pot will indeed work like a volume pot.. on the low side - you won't hear much, and on the high side would hear the filter (lacking some of its bass) resonating

in your case, are you saying you hear the filter but it's not resonating? then something must be broken in the feedback path
it could help if you record a short audio clip of that

probing the circuit with an audio probe can help, just watch out for a few things:
in some places the audio signals are loud (a bunch of volts) while in other places they can be very quiet

also, in some places the audio signals are "weak" - i mean, if you load them you affect the normal operation of the circuit
that's why measuring instruments try to have very high input impedance (ver tiny load), as high as possible

with all that i mean:
if you probe with a computer soundcard or headphones - it's a good idea to put a series resistor (100K for example) which would limit potentially high currents from killing your soundcard/headphones
if at certain place you don't hear anything - this doesn't automatically mean there's no signal, it could be your probe loading the circuit too much, or the signal could be very quiet and you're just not hearing it
if at certain place you hear something and it sounds wrong - this again could be due to your probe affecting the circuit
so have these things in mind when measuring with an audio probe, if you want to be on the safe side - use cheap headphones which you don't care about

actual measuring instruments (like multimeters and oscilloscopes) have a very high input impedance (higher than 100K) and yet even then you have to watch out in certain places of the circuit.. for example: measuring around the pitch circuit causes the circuit to be affected and the VCO sounds out of tune

what could actually help is to use a buffer/amplifier:
it will provide a very high input impedance, and it can have adjustable gain, so you can turn it up when you're probing those quiet places in the circuit
it should also have limiting on its output to prevent the signal getting louder than what's safe for a typical audio card

this basically isn't too far from the description of a distortion effect, minus the tendancy of distortion pedals having extra stuff to make the sound more interesting

anyway.. check if you actually have resonance

antto
 
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Re: VCF problems on M.Koster x0xi0

by Neyah on Sun Jan 22, 2017 2:16 pm

OK I have done a record : Volume pot = full, Env Mod = 0, Decay = full, Accent = 0.
The first is like I record it, and the second is the same with normalize.

https://soundcloud.com/neyacid/0001-3-audio

https://soundcloud.com/neyacid/sans-titre

I make mark with Cutoff & resonance pot on the second.

Neyah
 
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Please be positive and constructive with your questions and comments.