[BUILD] SEM VCF

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roglok
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Post by roglok » Sat Oct 21, 2017 12:58 pm

i would also suggest checking your PSU. Maybe you are operating it near its limits or your power distribution and/or grounding is not appropriate...

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tito_tunes
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Post by tito_tunes » Sat Oct 21, 2017 9:05 pm

Ah figured it out. I knew that my Lp-N-Hp morph knob wasn't working but fixing that fixed my noise problem as well. Now this filter is amazing!!!
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oneoff
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Post by oneoff » Mon Nov 13, 2017 9:30 pm

while its not perfect, the build guide on our webstore (available free of course) is pretty well up to date.. hope this helps
thanks a lot!

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emmaker
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Post by emmaker » Thu Mar 22, 2018 4:07 pm

I'm finishing up my build of the filter for +/-15 volts (Frac) and have a few questions.

The CV2 frequency input has a pot on the front of it so you can scale your keyboard voltage. Without the pot is that input a (more or less) true 1 V/Oct input?

Looking at the frequency attenuverter (CV1, IC1D) I was trying to figure out what the ATV trimmer did and don't see any point in it. If you set R15 and R16 to the same value and remove the trimmer you get symmetry in the attenuverter and really don't need the trimmer. With the trimmer in there and not set for equal resistance (R16+trimmer = R15+trimmer) the input does not seem to have symmetry with the pot turned from one end to the other. I put the circuit into spice and it seems to work that way. Anyone know of anything different or have any other insight into just doing that?

Anyone using the VC resonance put a trimmer on the CA3080 to trim out the offset in the resonance circuit?

For the resonance attenuverter what are people finding the most useful the 27K or 100K setup?

Thanks in advance.
Jay S.

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pix
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Post by pix » Sat Mar 24, 2018 10:59 pm

just finished building one with the panel PCB. Sounds fantastic!

However I always thought that the inputs could be used as attenuators if nothing else was plugged into the B inputs. Any one tried to do this? Can I just ground the normalled pins of the B input jacks?

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emmaker
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Post by emmaker » Sun Mar 25, 2018 12:59 pm

pix wrote:just finished building one with the panel PCB. Sounds fantastic!

However I always thought that the inputs could be used as attenuators if nothing else was plugged into the B inputs. Any one tried to do this? Can I just ground the normalled pins of the B input jacks?
Yes if you put ground on the switch pin of the B jack the circuit will become an attenuator.

But there will be a couple of minor issues.

Normally for audio attenuation you would use an audio taper pot. Audio volume is on logarithmic scale (thus the audio taper) and the pot in the circuit is linear. At 100% the pot will give you full volume at 50% it will not give you half volume.

Also the pot will be reversed. For 100% volume the pot will be turned to the A side (left) and for 0% to the B side (right). Normally you think of more volume by turning to the pot's right.

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pix
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Post by pix » Sun Mar 25, 2018 3:37 pm

cool, thanks! Linear is fine. I think I'll ground all input's switch pins for more flexibility.

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oneoff
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Post by oneoff » Mon May 21, 2018 7:15 am

I’ve just finished building this incredible filter
And everything works as it should.
I just wonder if you’ve noticed this- the noise floor is quite high from
The outputs, especially the LP (fully open).
Is this normal?
I’ve tried to replace the 741 with 071 just to see
If there’s any difference, still the same...
Any ideas? :despair:

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Supervillain
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Post by Supervillain » Fri Sep 14, 2018 12:48 pm

I give a last try at fixing my build...
I created a topic for it viewtopic.php?t=206769&start=0
Anybody can help please?!

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tito_tunes
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Post by tito_tunes » Tue Jan 08, 2019 1:58 pm

I notice on mine after using it and loving it for a while that it takes more than 5 volts to get the filter cutoff to open all the way. My modulation sources only go up to 5v is there way to mod the FM inputs to lower the headroom? Thanks
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diode_destroyer
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Post by diode_destroyer » Sat Apr 13, 2019 11:50 am

Sounds great! I built the synthcube v8.0 kit.

My tuning proceedure (which is probably all wrong but seems good for me): use this pic to identify stuff files/semtrimmers01_311.jpg

No input signal, resonance knob down:

T9 Offset1: Adjust so that the BP output is zero VDC

T10 Offset2: Adjust so that the LP output is zero VDC

T12 V/oct: Turn resonance knob max, adjust so that with keyboard 1V/oct CV in non-inverted FreqCVin port (knob at max) playing octaves gives proper tracking (monitor LP output with tuner or ear) this can be difficult because the volume is low and a little noisy, but I did not have to adjust the trim pot (already tracking right)

Attenuverter: Turn resonance knob max, adjust so that with keyboard 1V/oct CV in attenuverted FreqCVin port (knob at center) playing octaves gives no change in freq (monitor LP output with tuner or ear)

T11 InitFreq: I turned my trim almost all the way counterclockwise to get the bass to disappear when the freq knob is down.
Last edited by diode_destroyer on Sun Apr 14, 2019 7:49 am, edited 1 time in total.

moogah
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Post by moogah » Sun Apr 14, 2019 12:52 am

Ahoy all :) After a multi year break from DIY, I'm actually back and involved again. Unfortunately I don't have enough of a setup to actually run one of my SEM filters to directly help out at the present time (moved into the city, need to convert to euro rack in order to actually fit a system in my apartment) but I've got my prototypes still assembled and will be getting them back on the bench to provide help in finishing these.

1000x thanks to everyone who has been providing measurements like above for builders to get a working module :)

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Post by tufraniuh@yahoo.com » Tue May 14, 2019 5:40 am

Nevermind that didn't work at all
Last edited by tufraniuh@yahoo.com on Fri Jun 07, 2019 2:28 am, edited 1 time in total.

moogah
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Post by moogah » Tue May 14, 2019 8:55 am

tufraniuh@yahoo.com wrote:I bought a module already made and it is very noisy. White noise.
Because I do not see that someone complains about the noise, my Moogah Sem is defective?
How to remove this HISS? What exchange? Transistors? Opamp?
Please, help.
Any chance you can post a sound demo? I've found most euro to have more hiss than I'd like, but if this is noticeably hissier then another filter then there could be a problem.

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Post by tufraniuh@yahoo.com » Tue May 14, 2019 3:23 pm

Nevermind that didn't work at all
Last edited by tufraniuh@yahoo.com on Fri Jun 07, 2019 2:27 am, edited 1 time in total.

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woodster
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Post by woodster » Tue May 14, 2019 4:35 pm

I've built 2 of the Moogah SEM Filters and I think they rock.
I certainly do not have any noise issues as described above, so I feel it is somewhat unfair to go straight into blaming the product as opposed to looking deeper into the build, or what kind of power solution is being used, or how close to the edge the PSU is being pushed etc.
Even though the noise might manifest at the SEM, the cause might not necessarily actually be the SEM...
A friend of mine has a Doepfer SEM VCF and prior to posting this I asked for his opinion on noise with it and he says his is fine, nothing out of the ordinary.
The Moogah SEM VCF does have additional functionality over the Doepfer, which for me makes it better, just my opinion, pinch of salt and all that.

HI Res pictures of both sides of the PCB, and also panel wiring would be helpful in enabling others to try to help you out with your unit.

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Post by moogah » Tue May 14, 2019 5:50 pm

I'm not surprised they sound so similar, since they should be identical (or nearly identical) circuits, mine is a full clone of the original circuit and I believe Doepfur's is as well. This is DIY product and that's where most of the value is meant to come from, personally I have a strong dislike of the jacks Doepfur used, and I really really like the look of Clarke68's panel designs.

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Post by Flareless » Wed May 15, 2019 4:15 am

woodster wrote:I've built 2 of the Moogah SEM Filters and I think they rock.
I certainly do not have any noise issues as described above, so I feel it is somewhat unfair to go straight into blaming the product ....
+1 on this point. We've sold a number of Moogah's SEM filter and they are awesome. A big favourite among my customers.

Quick question now that the thread revival has made me think of it;

Does anyone know actual color "white" used on the SEM? It's more of an off-white than actual white.

I ask because I'm designing a panel option in MU format that would reverse the traditional white on black look for a more "Oberheim" style. Here's what I have so far...

Image

I'd like to get both the color and font styled more like the actual SEM.
Rich

Image

What can this strange device be? When I touch it, it gives forth a sound - Neil Peart

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Post by keninverse » Wed May 15, 2019 8:28 am

Just wanted to quickly chime in...mine aren't noisy at all and at one point I had four. None really had any issues but I'm also running very large linear PSUs and distros with heavy copper pours.

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Post by tufraniuh@yahoo.com » Thu May 16, 2019 6:16 am

Nevermind that didn't work at all
Last edited by tufraniuh@yahoo.com on Fri Jun 07, 2019 2:27 am, edited 1 time in total.

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Post by keninverse » Thu May 16, 2019 8:24 am

tufraniuh@yahoo.com wrote: Four moogah sem ... do you make them for sale?
No. These were used in a 4voice set up. 2 were sold off later.
tufraniuh@yahoo.com wrote: Here is what one of the members of the forum wrote about CA3080 and it looks more like the truth...in his opinion, a circuit built on the 3080 is simply bound to be noisy.
Mine aren't noisy. Is there some noise? Sure. But I've used vintage and newer SEMs and the noise floor on my modules are lower than the the oberheims. As stated previously this is most likely due to my power scheme and grounding.
tufraniuh@yahoo.com wrote: We are talking about the noise of the filter itself in the open cutoff and without a signal at the input.
I am not so disturbed by such a large noise level as compared with the useful signal, like many others. Noise in the signal makes the sound more saturated and alive. But if I knew in advance - I would never have bought Moogah SEM. I prefer to be able to add noise to the signal and not to have it all the time.
I wouldn't have chimed in unless I know what I was talking about. You still haven't commented on woodsters' points so it sounds like you made up your mind and at this point you're just complaining and venting.

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Post by moogah » Thu May 16, 2019 9:03 am

tufraniuh@yahoo.com wrote: DotCom uses a standard 3080 OTA design. This typically requires a huge cut in signal level at it's input and then it is boosted by that amount at the output which also boosts the noise and any CV feed through. "
in his opinion, a circuit built on the 3080 is simply bound to be noisy.
Since I don't know how much noise you're hearing and how low you'd like the noise to be, I can't say if you're experiencing more than the normal amount. Having the doepfur perform the same way leads me to think it's working "normally" for your situation.

The note about the high gain involved with using a CA3080 like this is correct, and something that can't be adjusted without fundamentally changing the circuit. There are more modern SVF designs that tackle this, but part of the character of the SEM filter is found in this design constraint so changing it would mean it's no longer the SEM filter :)

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Post by tufraniuh@yahoo.com » Thu May 16, 2019 9:23 am

Nevermind that didn't work at all
Last edited by tufraniuh@yahoo.com on Fri Jun 07, 2019 2:26 am, edited 1 time in total.

moogah
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Post by moogah » Thu May 16, 2019 10:43 am

You may want to consider that a vintage circuit just isn't going to perform to your liking and save yourself the frustration of trying to figure out what exactly is going on.

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Post by Kampfzwerg » Thu May 16, 2019 11:03 am

Flareless wrote:
woodster wrote:I've built 2 of the Moogah SEM Filters and I think they rock.
I certainly do not have any noise issues as described above, so I feel it is somewhat unfair to go straight into blaming the product ....
+1 on this point. We've sold a number of Moogah's SEM filter and they are awesome. A big favourite among my customers.

Quick question now that the thread revival has made me think of it;

Does anyone know actual color "white" used on the SEM? It's more of an off-white than actual white.

I ask because I'm designing a panel option in MU format that would reverse the traditional white on black look for a more "Oberheim" style. Here's what I have so far...

Image

I'd like to get both the color and font styled more like the actual SEM.
flareless:

Since I am making a SEM clone (here) I have done some research on this.

Not shure about the color. I've figured it out and have written it down.
I'll look it up tomorrow in my office. I think it was something like RAL1015 "light ivory" or so.

The font is "Friz Quadrata"
Image

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