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2 vco's phase issue
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isak



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PostPosted: Fri Sep 06, 2013 4:11 am    Post subject:  2 vco's phase issue Reply with quote  Mark this post and the followings unread

Hi guys.

I build a bass synth with 2 vco's, I have a phase issue that I wanna try fix.
They are going through a mixer with diffrent channel for each one, from there to the vcf and so on..
If i play C2 in vco 1 and play C3 in vco 2 does they suppose to phase?
I understand that if both on C2 they will phase but if I change one of the vco's to another octave (higher or lower) should they still phase?
if yes How should I fix this?

I thank you in advance,
Isak.

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elmegil



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PostPosted: Fri Sep 06, 2013 11:50 am    Post subject: Reply with quote  Mark this post and the followings unread

What's the waveform?

If it's a pulse/square, could it be that there's interference that's more like pulse width modulation, that would sound similar to phasing?
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isak



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PostPosted: Fri Sep 06, 2013 12:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for replying.

It's the ody vco so only saw or square, does not matter if vco1 is saw and vco2 is square, it's phasing.
The PWM is off.
I think the mixer should help this matter? well, of it suppose to help, it doesn't.

cheers.

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JovianPyx



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PostPosted: Sat Sep 07, 2013 8:43 am    Post subject: Reply with quote  Mark this post and the followings unread

A phasing sound is generated when two waveforms are not exactly synchronized. I.e., when their frequencies differ. This is also true of waveforms that are supposed to be exact integer multiples of each other such as when one is an octave higher than the other. If the ratio of their frequencies is not precisely 2:1, then you will hear phasing. As elmegil noted, other artifacts can cause a phasing like sound as well, but my guess is that the tuning tracking needs to be adjusted on one or both oscillators. In an expo pitch CV system, this can be a PITA if the tempco circuit isn't quite precise - as such, the circuit can behave differently at different temperatures (such as phasing at low temperatures, but it magically disappears when it's warmer).

Not to hijack the thread, but you can get a note plus a note an octave down using a toggle flipflop driven by one VCO. In this case, the flipflop forces synchronization and there can never be a phasing sound from the two sources (VCO and flipflop outputs). Granted, the flipflop will output a square wave which may be a good thing, but it can also be filtered. This is actually a very common technique and my guess is one reason it is popular is that it avoids the phasing issue altogether.

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isak



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PostPosted: Sat Sep 07, 2013 11:18 am    Post subject: Reply with quote  Mark this post and the followings unread

hi JovianPyx and thank you for replying.

well, i got an answer about this matter from a friend with the Oddy VCO.
as i understand if you put 2 oddy vco's together they will phase no matter what.
it seems that the real ARP odyssey acted the same, after looking at you tube i saw and heard this.
you can see and hear it in the next link..(move to 43 sec, you can clearlly hear the phase among the 2 vco's)
http://www.youtube.com/watch?v=9OwSNz7lLLo

to solve this i need to put 10uF in the power rails in addition to the 10uF i already got there.
dont know if it will work but this is the answer i got.

i did the flipflop (4013) thing using the sh101 subosc scheme, sounds amazing.

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JingleJoe



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PostPosted: Sun Sep 08, 2013 11:09 am    Post subject: Reply with quote  Mark this post and the followings unread

you've only got 10uF on your power rails!? well there's your problem Smile

Bump it up to something more like 100uF.

As the old saying goes: your device is only as good as it's power supply.

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isak



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PostPosted: Sun Sep 08, 2013 11:33 am    Post subject: Reply with quote  Mark this post and the followings unread

A 100uF?
We're talking about the 10uF's that on the vco it self, yes?
do i need to rais them to 100uF?

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JingleJoe



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PostPosted: Tue Sep 10, 2013 1:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

isak wrote:
We're talking about the 10uF's that on the vco it self, yes?

the power supply filter capacitor, which is what I assume you were talking about, not the timing capacitor... if you have a 10uF timing capacitor that is probably electrolytic and will cause a lot of problems because they are a bit crappy, great for high capacitance and psu filters but bad at most things.

I'd use 2200uF minimum as a power supply filter cap for any synth even a little one (like 2 vcos and one of most other things) LOTS more for a bigger one because of the increased power draw.

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isak



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PostPosted: Tue Sep 10, 2013 1:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi JingleJoe and thank you for your reply.

i'm talking about the caps that on the VCO, not the time caps.
PDF attached you can see them on the power rails.
its on bottom right of the PCB.


arp_odyssey_vco_layout_287.pdf
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JingleJoe



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PostPosted: Wed Sep 11, 2013 6:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes those are the power supply filter caps. I'm a little surprised they're only 10uF but I'm thinking maybe that circuit is designed to be connected to another power supply which already has big caps?
Maybe the designer of that circuit has calculated that 10uF is all they need to addequate PSU noise surpression...
Whatever the case, if you're modifying it, I'd use something bigger: like 1000uF on each rail.

Personally, while I like a good clean power supply, anything over 100nF is going to do some good for one's simpler and ramshackle circuits Wink

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isak



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PostPosted: Wed Sep 11, 2013 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

its connected to this power supply...
http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php

so i think 3300uF is more then enough, dont you?
or i totally didnt understand you Confused

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Thomas_Henry



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PostPosted: Wed Sep 11, 2013 10:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Gang,

The 10uF caps are neither timing nor power supply filter caps. They are simply decoupling caps to ensure this module doesn't interact with other modules (in either direction). 10uF is a standard value for that application, and what I've always used in my designs.

Without them, it is possible for two VCOs to lock to each other when close to each other in frequency, and no phasing effect would be heard.

I remember fixing a guy's Organtua (design by Craig Anderton) that had this problem. He complained that when the oscillator banks were tuned close to each other, there was no phasing; the clocks locked to each other. Popping in some 10uF caps took care of it and a very pleasing phase effect was created.

Thomas Henry
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isak



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PostPosted: Wed Sep 11, 2013 1:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Thomas and thank you for join in Smile

so as i understand if i take the 10uF's off there will be no phase at all?
can i do that?
will be safe to remove them?

cheers.

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varice



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PostPosted: Wed Sep 11, 2013 2:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

No, I would recommend that you keep those decoupling caps in place.

It sounds as though you want to sync one VCO to the other. Have you tried that yet?

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JovianPyx



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PostPosted: Wed Sep 11, 2013 3:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

I agree with varice.

If you take those caps out you will likely get "soft synch".

I have two Fatman synths, they both had soft synch. It was fixable, so I did the Lee Diode mod on one and replaced the 555 timers with 7555 timers in the other. Soft synch gone. Soft synch is bad when you want to tune the VCOs really close - but you want the phasing.

Then I designed a synch circuit which allowed exactly what isak is talking about. After adding the synch circuit, I'd tune the oscillators as close as I could (like an octave or a 5th apart) and then flip the synch switch on. Phasing disappears.

Purposeful hard synch also allows other new timbres anyway.

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isak



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PostPosted: Wed Sep 11, 2013 10:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

I love the synch switch, I use it all the time to get stable vco's.

Ok, I understand, don't takeoff the decoupling caps.
So can I get this phase off my neck?
How moog did it with the minitaur synth.
When watching tube clips I could hear the phase among the 2 vco's but when the octave was higher ...no phase, how do you think they did it?

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JovianPyx



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PostPosted: Thu Sep 12, 2013 6:33 am    Post subject: Reply with quote  Mark this post and the followings unread

isak wrote:
I love the synch switch, I use it all the time to get stable vco's.

Ok, I understand, don't takeoff the decoupling caps.
So can I get this phase off my neck?


With the synch switch... Other ways would include adding the flipflop suboctave. The phasing is coming from the two VCOs not tracking perfectly. Synching forces the tracking. You can also try adjusting the tracking to get it perhaps close enough?

isak wrote:
How moog did it with the minitaur synth.
When watching tube clips I could hear the phase among the 2 vco's but when the octave was higher ...no phase, how do you think they did it?


That sounds like the tracking of the VCOs was less perfect at the low end than the high end. This is something that you can do by careful adjustment, but it can be some work to get it exactly the way you want it.

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JingleJoe



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PostPosted: Thu Sep 12, 2013 8:51 am    Post subject: Reply with quote  Mark this post and the followings unread

isak wrote:
its connected to this power supply...
http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php

so i think 3300uF is more then enough, dont you?
or i totally didnt understand you Confused

that's precisely what i was saying Smile you already have enough!


Did someone mention sub-octave frequency division already? that could be the solution to anyone who wants two octaves precisely phase aligned Smile but only for square waves unless you make some wave shaping circuitry.

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steffensen



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PostPosted: Thu Sep 12, 2013 8:56 am    Post subject: Reply with quote  Mark this post and the followings unread

I have a feeling there is a missunderstanding here.
I believe isak is talking about the "phasing out", "locking", "crossmod" between the 2 ARP VCO's.

Edit: Just to make myself even more clear. This happens when the 2 VCO's are tuned somewhat close in pitch. Be it 1 oct apart, same oct, etc.
If it were 2 FM operators, i could obviously see the reason causing the phasing out, but as these are analog, it shouldnt happen - theoretically.

This happens both on Odyssey and the ARP2600.
I cant say to what degree they do this on the above, but it would be sure nice to find the culprit on the matter so we could fix it.

I have personally tried grounding all the input sockets in a chain (instead of grounding them thru the panel) - didnt help.
The Sync switch is grounded too when not in use.
The only thing i can imagine is the decoupling caps you already are talking about.
Might also be that the 4027-1's are behaving too similar as a result of the circuit design, and its not possible to change that.
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varice



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PostPosted: Thu Sep 12, 2013 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

steffensen wrote:
I have a feeling there is a missunderstanding here...

"What we’ve got here is failure to communicate." - Prison Warden in the film Cool Hand Luke Laughing

Isak, I’ve looked at the Ody vid you linked above and I don’t hear anything wrong with the sound of the two VCOs. They are slowly going in and out of phase as is normal for slightly detuned oscillators, without any undesirable phase locking. There is a slow volume change, but that seems to be caused by the filter cutoff being modulated by an LFO, which of course, has nothing to do with the function of the VCOs.

At this point, I’m not sure what the “phasing” problem is that you want fixed. Would you please post a short sound clip here and/or try again to explain exactly what the problem is.

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isak



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PostPosted: Thu Sep 12, 2013 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Please watch the tube clip I did for the synth I built (sorry its in Hebrew)
Move to 5:30, you will here vco1 plays saw, few sec after, I add the second vco and playing square.
Here you can hear the phase.
Then I turn the freq pot up to one octave above and I can still hear phasing.
http://www.youtube.com/watch?v=AaYcMA1p0Xw&feature=youtube_gdata_player
This is what I'm talking about.
Even when I fine tune it to be as close as possible to each other they phase.

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PostPosted: Thu Sep 12, 2013 6:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

I hear what you’re talking about now. That sure does not sound right at all. But, I can’t tell by watching the video what might be causing this problem. Sorry that I can’t offer more help.
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isak



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PostPosted: Fri Sep 13, 2013 2:31 am    Post subject: Reply with quote  Mark this post and the followings unread

I thank you for trying Smile
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AndyR1960



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PostPosted: Fri Sep 13, 2013 11:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Isak,

Thanks for uploading the vid... I have to admit that I've never come across this problem (I've got 9 of my ARP VCOs in my modular, and none of them exhibit this behaviour).

Can I ask how you're mixing the output signals, and is there anything you've added between the out of the VCOs and the input of the mixer i.e. level shifters or similar (to remove the DC offset of the VCO outs).

Something strange is definitely going on there.
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isak



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PostPosted: Sun Sep 15, 2013 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

AndyR1960 wrote:
Hi Isak,

Thanks for uploading the vid... I have to admit that I've never come across this problem (I've got 9 of my ARP VCOs in my modular, and none of them exhibit this behaviour).

Can I ask how you're mixing the output signals, and is there anything you've added between the out of the VCOs and the input of the mixer i.e. level shifters or similar (to remove the DC offset of the VCO outs).

Something strange is definitely going on there.


thank you for jumping in Andy Smile

from VCO 2 saw output is going to audio buffer and split to subosc and to vco1 FM
this is what i did...
http://www.eecs.tufts.edu/~dsculley/tutorial/opamps/opamps2.html
this is my layout..
Posted Image, might have been reduced in size. Click Image to view fullscreen.

the mixer i used is the Yusynth
http://yusynth.net/Modular/EN/MIXER/Mixer-1U.html

beside that... our friend steffensen have the same problem, i think he didnt used any buffering for his outputs to other modules.

anyway...did i did something wrong?

i thank you for your help, i know your busy man Smile

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