[IN STOCK] Eurorack: Discrete State-Variable VCF v1.4

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medbot
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Post by medbot » Mon Aug 25, 2014 12:16 am

Also, while I have the floor I'll take a minute to say how just bad ass this filter is. I honestly wasn't super sold based on the lone demo on the first page but figured I'd build it anyway since MA has yet to disappoint me, and I'm really glad I did. Combining the filter with the transistor mixer was an inspired idea, and I already wish I had another one or two in my rack. This thing would kill in a polysynth.

:bacon:

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LektroiD
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Post by LektroiD » Mon Aug 25, 2014 4:27 am

medbot wrote:I don't know that I've ever used a high pass filter that closed all the way, but I've only got a handful to test right now (ssvcf, sem, minibrute).
I've made a quick video to illustrate how to get rid of the bleed at the top end of the cutoff on a hipass filter.

I just need to know which resistor adjusts the level of the inverted signal of this filter so I can fine tune it. Any schematics available for this?

[video][/video]

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mcop
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Post by mcop » Mon Aug 25, 2014 12:10 pm

Not sure if this helps, but here's a link to Scott Stites original thread where he presented his schematics.

http://electro-music.com/forum/viewtopi ... torder=asc

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paulstone
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Post by paulstone » Mon Aug 25, 2014 12:21 pm

pcb's arrived today :tu:

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LektroiD
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Post by LektroiD » Tue Aug 26, 2014 1:57 pm

negativspace wrote:For calibration (and I'll add this to the build page) you definitely want to set the -6V rail up properly first, so do the little fix and then set the trimmer. There's a test pad onboard as well as a spare ground pad for the black probe so calibration should be pretty simple.

I couldn't find the ground test pad, only +/- pads, so I hooked up my probe to the chassis.
Once that's done you can set the null. This was a really useful trimmer in the standalone CP3 with the bipolar output as it could be used to zero out any offset at zero-output. (12:00) I learned in making this version, though, that the trimmer only effects the +OUT. So you have 2 options:
What do you mean by zero output? Nothing plugged into the filter?

I have nothing plugged in.
1) Trim as you would in a single-output situation and make the +OUT 0V
This makes even less sense "single output situation???" It's a filter, nothing plugged in, do I plug something into one of the outputs?
2) Trim as you would the standalone version and match the absolute value of the offset present on the -OUT. (Memory fails but I think it's roughly -70mV on the -OUT, so you'd want +70mV on the +OUT)
Now I'm really flummoxed... "standalone version"? You made other versions of this filter? How do I know which one I have?

Furthermore, trimming will not get these results, the two trimmers interract with each other, so turning one makes the other completely wrong output. I simply can not get 70mv out of both at the same time, no matter what I do.

I get 70mv out of the +, then the - is messed up, and vice versa. This thing is impossible. Am I supposed to be rotating the mixer trim too? That seems to do nothing.

Should I use a scope, or a multimeter?

I have absolutely no idea what those instructions mean. If anyone has worked it out, could you please translate to plain English.

& I've still not worked out how to reduce that hipass bleed, it's unusable as a hipass like that.

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woodster
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Post by woodster » Tue Aug 26, 2014 4:47 pm

Hey LektroiD,

I think Jason is referring to the CP-3 Mixer when talking about a standalone version.
As he says, his 4hp Euro CP-3 offering only has a single output which is a sweepable bi-polar output, as opposed to the dual separate + and - implementation as used as part of this Filter build.

"What do you mean by zero output?"
Again, this refers to the CP-3 with the single bi-polar output, Zero output is the output knob positioned at 12 o'clock (Like an Attenuverter).

I'll have to dig out my SVVCF PCB's to see if I can locate the test point for you...

I had a problem trimming the -6v on one of the three CP-3's I built.
It was related to a chip in the solder mask right next to a pad, and the inevitable solder finding it...
Do you have any decent PCB pic's ?

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Post by gbiz » Tue Aug 26, 2014 5:06 pm

LektroiD, the calibration method for the -6V & NULL pots is exactly the same as you would for a standalone CP3 mixer. Once you have the -6V calibrated, with nothng present at the inputs, adjust the NULL pot so OUT+ of the mixer section is at 0V.

The ground test pad is the one marked "G" adjacent to Q14.

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LektroiD
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Post by LektroiD » Tue Aug 26, 2014 6:03 pm

I don't understand why Jason would be referring to a different module which requires different callibration techniques, as there is nothing that zeroes at 12:00 on this, just makes it all the more confusing.
gbiz wrote:LektroiD, the calibration method for the -6V & NULL pots is exactly the same as you would for a standalone CP3 mixer. Once you have the -6V calibrated, with nothng present at the inputs, adjust the NULL pot so OUT+ of the mixer section is at 0V.

The ground test pad is the one marked "G" adjacent to Q14.
Now I'm even more confused... So what does Jason mean when he says "-70mV on the -OUT, so you'd want +70mV on the +OUT"?

I can't even get mine to go as far as -6V.

I'm even more confused now...

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Post by windspirit » Tue Aug 26, 2014 8:01 pm

The point is that the cp3 mixer circuit is the same as the summing section on the filter, i believe for the CV inputs. Therefore it uses the same calibration procedure.

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LektroiD
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Post by LektroiD » Tue Aug 26, 2014 11:12 pm

windspirit wrote:The point is that the cp3 mixer circuit is the same as the summing section on the filter, i believe for the CV inputs. Therefore it uses the same calibration procedure.
We're just going round in circles here... It IS different... There is no zero point at 12:00 on ANY of the pots, no Standalone, no "single output situation". It makes NO SENSE!.

Where are the notes about the mixer trim, which appears to do absolutely nothing?

This clearly requires a different calibration method to the CP3, which I am not even familiar with.

Still not resolved that awful HP bleed either

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medbot
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Post by medbot » Tue Aug 26, 2014 11:23 pm

:deadbanana:

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Post by ym2612 » Tue Aug 26, 2014 11:31 pm

From the CP3 thread:
Adjust the 2k trimpot [labeled -6v] until the output of the LM337 is -6.0V.
Adjust the 100R trimpot [labeled null] until the output has no DC offset.
That's all there is to it.

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LektroiD
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Post by LektroiD » Wed Aug 27, 2014 12:27 am

Adjust the 100R trimpot [labeled null] until the output has no DC offset.
Which output? Hipass, Bandpass, or Lopass?

And what is Jason talking about when he mentions -70/+70mV on the +/-?

If I trim for -6V, these settings go all over the place.

And... What is the MIXER trim for?

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Post by nikmis » Wed Aug 27, 2014 3:45 am

1. -6 volt trimpot

First you should turn the -6v trimpot until the output pin of the lm337 is -6volts. After that, you are finished with that trimpot, don't touch it


2. DC offset trimpot, null on the pcb,
Which output? Hipass, Bandpass, or Lopass?
You would measure using the mixer output, mix+ on the panel, I believe. So before it ever gets to the filter section. Try to get it to 0 volts in reference to ground.

3. 1v/o trimpot
And... What is the MIXER trim for?
It's not a mixer trim, its a volt per octave trimmer. The word mixer is just close to it. Set it to self-oscillating and try to get as close as possible to 1 volt per octave. Use the low pass output


I wish I could help you with the highpass bleed, I haven't built mine yet

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Post by gbiz » Wed Aug 27, 2014 4:12 am

Adjust the 100R trimpot [labeled null] until the output has no DC offset.
Which output? Hipass, Bandpass, or Lopass?
None. The (pre-filter) OUT+ output from the mixer section.
And what is Jason talking about when he mentions -70/+70mV on the +/-?
You have two options for calibrating the pre-filter mixer stage. With no input signal, either (1) simply adjust the NULL pot until you see 0V on the OUT+, or (2) adjust the NULL pot so the offset at the OUT+ matches whatever the offset is at OUT-, which will likely be in the order of +/- 70mV. There's no adjustment for OUT-. FWIW, i used (1) on mine, & with 0V at OUT+ i see 125mV at OUT- on one & 135mV on the other.

As Jason mentioned way back in this thread, the Mix+ & Mix- test point labels on the PCB are reversed, so be aware of that. The outputs on the panel are correctly labelled.
If I trim for -6V, these settings go all over the place.
I'm not going to suggest this is the cause, but have you tried reversing the 337, as Nigel picked up a few pages back ?. Might be worth a try. -6V doesn't vary here on both of mine.

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Post by negativspace » Wed Aug 27, 2014 4:23 am

Holy crap, this isn't hard. :doh:

No cables in the module. No inputs, no outputs. Use the testpoints on the board.

1) As mentioned, the ground pad is over by the lower screw hole on the left-hand side of the PCB.

2) Adjust -6V trimpot until the test point "-6V" reads -6.00V. This test point is down by the LM337. Once you set this, for fuck's sake leave that trimpot alone!

3) Forget everything else I've said about the CP3 circuit in this thread.

4) Seriously, just take a deep breath and forget all of it. I'll give you a moment.

5) ...

6) Measure the test point labelled "-". This incorrectly corresponds to the jack labelled 'mix +' on the front panel. It should read somewhere between 55 and 110mV, give or take. The null trimpot won't effect this measurement, so just note the voltage. It may bounce around a bit. Just... ballpark it.

7) Measure the test point labelled "+". The null trimpot will effect this voltage so you now have two choices:
7a) Recall the voltage from Step 6 and adjust the null trim until the voltage at TP "+" reads the inverse.
7b) Forget Step 6 and adjust the null trim until the voltage at TP "+" is ~0.00V.

8) You're done with the mixer.

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Post by negativspace » Thu Aug 28, 2014 4:39 am

Added the pinout diagram to the build page for you non-control-board builders.

It is double-checked but not verified!

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paulstone
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Post by paulstone » Thu Aug 28, 2014 6:07 am

negativspace wrote:Added the pinout diagram to the build page for you non-control-board builders.

It is double-checked but not verified!
thanks for the picture , i will chek soon :tu:

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Post by Jop » Fri Aug 29, 2014 3:15 am

Just finished wiring it the old fashioned way. I can confirm the pinouts are correct :sb:

Also trimming was easy due to the testpoints and clear instructions!

Great filter, now time to explore all the possibilities.

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LektroiD
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Post by LektroiD » Fri Aug 29, 2014 8:27 pm

Thanks for the detailed calibration guide. :tu:

Anyone else have a quiet spot in the middle of the notch pot? I wonder if a lower value pot might sort it? I have 100K's in as per the BOM. Moving the mixer trim doesn't do anything to help (I still have no idea what that trim is for).

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Post by Stinktier86 » Sat Aug 30, 2014 5:31 am

I happen to have a clip of LM337 of the TO-220 variety. While these are flat-pinned and wont fit through the holes without adaption, the dimensions are basically the same. Is the input/putput/variable pinout the same as the TO-92 also?

Or maybe i should just order a bunch of modern LM337s?

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Post by nigel » Sat Aug 30, 2014 7:42 am

Stinktier86 wrote:I happen to have a clip of LM337 of the TO-220 variety. While these are flat-pinned and wont fit through the holes without adaption, the dimensions are basically the same. Is the input/putput/variable pinout the same as the TO-92 also?
NO, the pinout on the LM337L TO-92 and the LM337 TO-220 packages are different. The TO-92 is Adj / Out / In, the TO-220 is Adj / In / Out.

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Post by qfactor » Sat Aug 30, 2014 7:58 am

Also remember the silkscreen on the PCB of the Lm337 is reversed too! :tu:

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Post by designator » Sat Aug 30, 2014 2:04 pm

Fetched boards and panels from the customs yesterday. Great stuff! :banana:
Thank you very much!

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Post by bsmith » Sat Aug 30, 2014 2:47 pm

qfactor wrote:Also remember the silkscreen on the PCB of the Lm337 is reversed too! :tu:
Mine are working fine with the lm337 oriented as shown on the screen.

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