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 Forum index » DIY Hardware and Software » Thomas Henry designs
The VCO Maximus
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fonik



Joined: Jun 07, 2006
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PostPosted: Sun May 13, 2018 12:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Last week I had an email exchange with a tech of one of the current 3340 manufacturers.

In general the 3340 is very picky regarding the PSU and even he has not been sure about the actual minimum voltage supply needed.

Thomas Henry designed this circuit for 15V, when oyou build it according to his documentation you will have no issues at all.

My conclusion for 12V supplies so far:
1. Use a nice and reliable PSU with enough juice.
2. Don't resistors in line at the power entry of the module.
3. Don't use the TL431 to further down-regulate the power.

I will update the documentation accordingly ASAP.

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chrissugar



Joined: Sep 23, 2008
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PostPosted: Sun May 13, 2018 4:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Matthias

I did some investigations about this problem. Please read my posts at Muff so at least you will know what I tried to test.

As a short summary I will mention that I tried every possible idea I had without much success.
The PWM to pitch influence is present regardless of:
- power supply (eurorack PSU or laboratory bench PSU)
- +/- 12V or +/- 15V
- TL431 enabled or disabled
- with or without 10 ohm resistors at power input
- different value for R4 as suggested by Alfa
- no mater what chip, CEM3340 or AS3340
- corrected load resistor value for AS3340 at pulse out (51k)
- etc

Looking forward to your thoughts

Best regards

chris

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fonik



Joined: Jun 07, 2006
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PostPosted: Sun May 13, 2018 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread

I am astonished about this result. Last week I tried a NOS CEM3340 on 12V w/o recognizable change in pitch with PWM, and I did not even touch the load resistor on pin 4.
But I will check again.

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fonik



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PostPosted: Sun May 13, 2018 6:34 am    Post subject: Reply with quote  Mark this post and the followings unread

And with 15V? This is new to me.
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philpeery



Joined: Nov 08, 2006
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PostPosted: Sun May 13, 2018 9:33 am    Post subject: Reply with quote  Mark this post and the followings unread

I am very surprised by this. I've built 6 copies of this design so far with Fonik's pcb, using both revG and 1982 date code CEM3340's. I do all of my stuff with +/-15V, use PowerOne power supplies, and my format is 5U (eurorack is too small for my eyes and fingers!). I have high confidance in my PSU's, so I did not use the TL431 regulation.

These modules are STABLE. I have not had any tuning issues, and I make heavy use of both the PWM control and the PWM cv input, often varying the PWM cv past its range. No tuning problems here.
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fonik



Joined: Jun 07, 2006
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PostPosted: Sun May 13, 2018 10:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you. This is what I discovered so far. It is dependend on the supply voltage. The lower, the worse. I guess it has something to do with the internal Zener?
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elmegil



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PostPosted: Sun May 13, 2018 10:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Has the current limiting resistor for the zener been adjusted for the lower voltage?

I was poking around the data sheet and some ideas last week, and it seemed to me that it ought to be dropped to 560R from 820R.
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chrissugar



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PostPosted: Mon May 14, 2018 2:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi all

I have to mention that my built worked perfectly from the beginning. All waveforms are correct, the sub output and switches do what they are supposed to do, it is frequency stable and 1V/oct on several octaves, FM and SYNC inputs also work great when I modulate them from another VCO.

I was sure everything was working right but a few days later when I started to manually rotate the PW knob I discovered that it affects the pitch. The VCO is frequency stable at any static PW position.
After doing some measurements I discovered the error was about 0.5% regardless of frequency (at 1000Hz a 5Hz detune when pulse width is varied from 1% to 99%)

Today I had some more ideas and tested them. Posted them in the VCO MAXIMUS thread at Muff.
Should I post my finding here or is it OK for you to read there?

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elmegil



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PostPosted: Mon May 14, 2018 5:32 am    Post subject: Reply with quote  Mark this post and the followings unread

A link to the Muff's thread would be helpful, but I don't see any reason you'd need to post the same things both places.
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chrissugar



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PostPosted: Mon May 14, 2018 8:56 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, here is the thread:
https://www.muffwiggler.com/forum/topic-193367.html

I have to mention that there are a few other projects based on 3340 and many members mention this PW to tune error so it is not just a VCO Maximus specific problem. I'm more and more convinced it is an implementation problem.

After reading all the complains I thought to check PCB photos and service manuals of various 3340 implementations and what I find surprising is that roland Jupiter6 and MKS80 have some very interesting solutions.
They developed some custom resistor networks and installed them very close to the VCO chips, also decoupling caps close to the pins and a sophisticated ground scheme.
Also interesting is that in Jupiter6 and MKS80 the 3340 chips are powered at +10V and -5V which makes all the discussions about +/-12V or +/-15V pointless.

Looking forward to your thoughts.

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elmegil



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PostPosted: Mon May 14, 2018 9:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Is this where you were seeing those other implementaitons?

https://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/

If not, there's some useful info there.
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elmegil



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PostPosted: Mon May 14, 2018 9:36 am    Post subject: Reply with quote  Mark this post and the followings unread

the +12/-5 is due to the supply limits of the chip. It basically bypasses use of the zener, which limits the - supply to 6.5V internal to the chip.
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chrissugar



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PostPosted: Mon May 14, 2018 8:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Is this where you were seeing those other implementaitons?

https://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/

If not, there's some useful info there.


I checked the Electricdruid page at an early stage of building my VCO Maximus. No, I was talking about checking how manufacturers implemented circuits with 3340 chip at a phisical level, on real PCB not only schematic.
What I mentioned about the Jupiter6 and MKS80 was by checking how the real PCB looks. For example here:
http://www.subidiom.com/webfiles/image/mks-80/mks80_interior_zoom2.jpg
As I mentioned they used a pair of resistor networks custom made for each 3340 chip. All connection very short. Check also the service manual to see the PCB traces.

On Muff there are several eurorack module projects based on 3340 chip like Kassutronics VCO3340, Timo Rozendal CEM3340, Nonlinearcircuits NLC 3340 VCO, OPENCEM3340, etc. Reading the specific threads gives you the idea that the problem I mentioned is not something specific only to VCO Maximus. I think this is an implementation problem.

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elmegil



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PostPosted: Mon May 14, 2018 9:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

It looked like you'd done the OG CEM3340 and one other; I'm curious if a detailed comparison across CEM/Rev G/AS/CoolAudio has been done.
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fonik



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PostPosted: Mon May 14, 2018 11:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yesterday one of the team powered his MAXIMUS from 11V using a bench supply w/o any trift (beads ISO 10R on the modules power entry point). Now it is getting weird.
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chrissugar



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PostPosted: Tue May 15, 2018 12:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi all

It looks like I found the solution.

After checking every possible thing and having no idea what to do to move the things in the right direction I started to find something common in most service manuals.
One thing started to look similar was the fact that contrary to what is suggested in the data sheet Memorymoog, Jupiter6, MKS80, Prophet5, SH101 all have -5V at pin3.
In the Jupiter6 service manual if you look at pin3 of 3340 it has a mysterious resistor going to a WIDTH bus. At some point it is mentioned in the manual:

"WIDTH - computune data for each vco ideally approximately 5V. It may vary with the characteristics of the VCO IC. If the value greatly differs from the ideal value, the corresponding VCO is judged to be defective, unless the computune operation is improper "

This made me think what if I would go in the opposite direction suggested by Alfa and others to reduce R4 for +/-12V operation.

I started to increase the value and also measured the voltage at pin3 and the PWM to tune error.
First I tried 910 ohm, voltage went lower and error was a bit lower. Then step by step I tested 1.2k, 1.5K and finally 1.6K.
With R4=1.6K the voltage at pin3 was 5.05V and the PWMto tune problem was gone at 1000Hz.
Increased the frequency to 5000Hz and the error was now detectable but very low (around 3Hz, which is at this frequency is excellent)


So as a conclusion the goal is to have on pin3 as close as possible to -5V and the PWM to tune error will not manifest itself.
You should check what R4 value is optimum for your build depending it is +/-12V or 15V and your specific 3340 chip. You should not go lower than -5V.


P.S. Later as a curiosity I checked the Juergen Haible HJ2VCO dual CEM3340 VCO. Juergen divided the negative voltage with a resistor pair to -5V and buffered it with a non inverting opamp to pin3 this way also creating a local low impedance voltage source. I think this is the right way to do it.

http://jhaible.com/legacy/tonline_stuff/hj2vco.gif

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chrissugar



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PostPosted: Tue May 15, 2018 12:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Also please do not forget to change R19 to 51K if you use AS3340. Original CEM3340 works with 10K.

I also increase R35 to lower the load of PULSE/SUB circuit on pin4.

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fonik



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PostPosted: Tue May 15, 2018 1:17 am    Post subject: Reply with quote  Mark this post and the followings unread

chrissugar wrote:
So as a conclusion the goal is to have on pin3 as close as possible to -5V and the PWM to tune error will not manifest itself.

Ah, now this makes sense! I will check this with my build later this week.

Thank you very much for your efforts and for sharing! Cool

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chrissugar



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PostPosted: Tue May 15, 2018 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
Thank you very much for your efforts and for sharing! Cool


It was a real pain but I'm glad I finally managed to solve this puzzle.
The main PCB started to look like a test board Smile

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chrissugar



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PostPosted: Tue May 15, 2018 12:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here are my final conclusions about the measures needed for a correctly working VCO MAXIMUS (in Eurorack format +/-12V)

- need to power the module without 10 ohm resistors. Current variations induced by the logic chips produce voltage variations and it reflects on tuning. Install jumpers or inductors instead of 10 ohm resistors.

- change R4 to a value that will give you -5V at pin3 of 3340. Do not go below -5V.
In my case it was 1.6 Kohm but you should test the right value for your construction/3340 chip

- decouple pin5 of 3340 with a 1nF capacitor directly between pin5 and pin12 (GND) below the PCB.

- decouple pin3 of 3340 with a 10uF elco directly between pin3 and pin12 (GND) below the PCB (careful about polarity)

- you need to enable TL431 for a stable voltage at pin16 of 3340

- in case you install AS3340, use R19=51k as a load for pin4 (pulse out). Original CEM3340 works with R19=10k.

- increase R35 to 47Kohm to reduce load on pin4 (pulse out)

- I suggest to narrow the voltage range at pin5 to limit the pulse width from approximately 2% to 98%.
It has three advantages:
- you reduce the risk to expose pin5 to voltages above or below the suggested limits (0-5V).
- your pulse wave will not stop working (like it happens when you are out of the correct range)
- reducing the voltage range, you also reduce the level of PW to tune error
For this you need to remove R62 (10Mohm) because it generates a negative voltage (around -150mV) at PW input
Also change R53 to 91Kohm, this will limit the upper range or the pulse width.
Rotating now the PW knob from CCW to full CW will give you a nice pulse wave with varying pulse width from approximately 2% to 98%.


Enjoy your MAXIMUS Smile

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chrissugar



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PostPosted: Tue May 15, 2018 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Matthias

I forgot to mention I discovered a small error in the schematic from the build document.
PW INIT potentiometer (R50) in the schematic is connected to GND and +12V.
It should be connected to GND and -12V.

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fonik



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PostPosted: Tue May 15, 2018 12:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

chrissugar wrote:
- change R4 to a value that will give you -5V at pin3 of 3340. Do not go below -5V.
In my case it was 1.6 Kohm but you should test the right value for your construction/3340 chip

Did you use the ALFA A3340? I installed a trimmer and with the A3340 I ended up with something more 1k5, whereas for a NOS CEM3340 I trimmed about 1k2 (using my nice and clean linear 12V rail for the 3340).

Anyways, thank you so much for sharing. With your consent I will incorporate your findings into the documentation later this week.

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chrissugar



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PostPosted: Wed May 16, 2018 12:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:

Did you use the ALFA A3340? I installed a trimmer and with the A3340 I ended up with something more 1k5, whereas for a NOS CEM3340 I trimmed about 1k2 (using my nice and clean linear 12V rail for the 3340).


Yes, I used the AS3340.
I'm glad you confirm you used a similar resistor value to what I used.

fonik wrote:
Anyways, thank you so much for sharing. With your consent I will incorporate your findings into the documentation later this week.


You're welcome. Yes, please add my findings to the document.

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