3340 stripboard - pulse width detuning

[Myleswise]

Hi Gang,
hope everyone is doing alright in these crazy times I’ve built Sam’s 3340 VCO strip board 2 separate times, and both times I have problems with the PWM detuning the oscillator. I’ve even breadboard it and I have the same issue. All of the features work fine, I can calibrate it to track 1v/OCT, but if I touch the PWM pot or the PW pot it will slightly detune the frequency of the oscillator. I would consider myself an intermediate to semi-advanced DIY’er, I’ve quadruple checked my work and it all checks out. Anyone else have this issue ? I’m using the AS3340 chip FYI, and I have even swapped out multiple IC’s to make sure I didn’t have a dud chip but the results were all the same. Thanks and much love !!!

[fredrik]

It’s a known issue with the 3340s, see e.g. this subthread:

1222 Tuner VCO Module

Oh, as a note, I built my #1112 Oscillator’s with ferrite beads as listed on the diagram, and they seem have enough resistance to cause PWM pitch drift when you change the PWM knob directly or apply a CV to the PWM input. This is a pretty common problem with the CEM3340’s as they are super sensitive to changes in current (on pin 3 if I remember right), but when I built my eurorack VCO Maximus boards, which are also 3340 designs, I used the same ferrites without issue so figured I would be ok. Un…

[binarymob]

I just recently found this "To improve stability of VCO during PWM excursion, simple solution can be used: " - Is this related to OPs issue above?

[Caustic]

ah this basically uses a diode to reduce voltage swing.

[Trumac22]

Interesting! Might give it a go - I had this issue pretty bad on mine. I did a few other tricks to tame it a bit which I posted here:

AS3340 VCO /w hard/soft sync, FM modulation

So, I kept playing with it and managed to make some headway - I’ve now limited the pitch drift to about the the last 10% of the duty cycle. Not perfect but certainly much better! I stumbled across this thread: https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=231130 and saw lots people repeating the problem (especially with the AS3340, like I’m using). I managed to treat the issue by including: 10uf electrolytic caps on the power rails *100nf ceramic cap from pin 4 to ground *changed t…

[analogoutput]

See also this

TH-VCO-MAXIMUS.pdf

which is the build guide for the Thomas Henry VCO Maximus. It’s a different circuit than Sam’s but might provide some ideas:

As usual Thomas Henry did design this project for 15V, alas, time
has shown that this circuit could not be used with 12V w/o a lot of
changes. Especially the PWM did have an impact on the overall
pitch. I am very thankfull that ‘chrissugar’ from Romania did a lot of
valuable work and tests to tackle the PWM/pitch-issue. I amended
this documentation accordingly.

Evidently one of the modifications referred to is to power the 3340 with +11V from a TL431.

Also here:
https://modularsynthesis.com/mrmrshoes/maximus/maximus.htm

The PWM control did affect the frequency as has been noted in various forums. I changed the 10R power series resistors to ferrite beads and added a -5V regulator which stabilized the frequency when changing the pulse width.

(Not clear where that regulator fits in.)

[fredrik]

You can use a -5V regulator for the negative supply, instead of relying on the built-in zener.

(not getting the feeling that these mods are very scientific, though )