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4069 VCO buffer help
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hopefulsynthesis



Joined: Oct 12, 2015
Posts: 9
Location: Kent, UK

PostPosted: Thu Aug 18, 2016 12:06 pm    Post subject: 4069 VCO buffer help Reply with quote  Mark this post and the followings unread

Hi all

I've recently built Rene Schmitz's 4069 VCO on veroboard including the sawtooth buffer using my own layout. I'm running it from +-9V and used the stated component values, I'm not sure if it would have been wise to change the resistors in the buffer because that is shown for +-15V use.
From the pics you can see I am getting a square and saw wave out but the output from the buffer is clipped and seems to have become a downward ramp which is not what I expected. If anyone can shed some light on why this may have happened it would be great! I think I've got the layout correct but this is my first proper VCO so I could have missed something...
Any help will be greatly appreciated
Thanks


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JovianPyx



Joined: Nov 20, 2007
Posts: 1498
Location: West Red Spot, Jupiter
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PostPosted: Fri Aug 19, 2016 6:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

From the scope output, it looks like the buffer is clipping. You could increase the buffer's input resistor or decrease it's feedback resistor. And I'm assuming the top image is the buffer output.
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EL BO



Joined: Mar 15, 2015
Posts: 57
Location: hobart

PostPosted: Fri Aug 19, 2016 11:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

That is exactly the same output wave shapes I got on my breadboard this week!

Anyway, yes the buffer is clipping the top of the saw, so you need to tweak the resistor values somewhat around the transistor.

It is an inverting buffer which is why you are getting a saw/ramp. I believe they are more or less the same to your ears at audio frequency.
I think that to get a non inverting input it's a bit more work with the bias, but I don't actually understand transistor bias so don't take my work for it!


I wanted to use mine optionally as a vc lfo, so the output caps which are used to eliminate the dc offset meant that this didn't work properly so I actually just buffered both outputs with an opamp instead. This made it easier to get the saw in the right place, and I kept it inverting because I guessed a downward sloping ramp (aka a decay) is generally a more useful lfo shape for me.

If I want a saw or a triangle or a trapezoid or a sine from the lfo I'll just run it through my linear/log slew limiter (aka the yusynth one)

I have just made a single sided layout for it, but am yet to stuff and verify i, which hopefully will happen in the next week or so, and then I'll post the layout if no one minds..

Cheers

Lance
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hopefulsynthesis



Joined: Oct 12, 2015
Posts: 9
Location: Kent, UK

PostPosted: Mon Aug 22, 2016 4:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks guys, that really helps...and it now makes complete sense that an inverting buffer would flip the waveform!
I'm planning to breadboard the buffer when I get some free time. Another question: am I right in thinking that if I wanted a fine tune pot I would change the input summing resistor to a higher value and keep the pot the same?
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EL BO



Joined: Mar 15, 2015
Posts: 57
Location: hobart

PostPosted: Tue Aug 23, 2016 12:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes that's right about the Input sum r.

I actually went the other way to get the biggest sweep possible because I want to vc lfo it as well,, and if it's not good in actual use I'll change values later.
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EL BO



Joined: Mar 15, 2015
Posts: 57
Location: hobart

PostPosted: Thu Sep 01, 2016 6:55 pm    Post subject: Tuning out? Reply with quote  Mark this post and the followings unread

I built this up with my pcb design and it all seems to work fairly well and it makes all the right sounds, but I tried to get v/Oct scaling right last night anf it is just waaaay off.

I couldn't even get one octave, and the trimpot seems to just adjust the set point without much change in the scale.

I didn't have a 1.5 kresistor on hand so I just used a 1k rsistor and a2k. Trimpot but I can't see how that would matter, as I can still get the total 1-2.5k.range out of this.

I just set the main Freq. Knob at 30hz at 2v (for no particular reason) and then tried to get 60hz at 3v and 120hz at 4v, but adjusting the trim just seemed to shift the whole scale up and down. I was getting 58hz and 111hz at the offset which is obviously terrible!

What should I check, or is there a particular order of process I have to go through to tune it properly?

Do I just have to be more patient and try again? I'll admit it was late at night...

Cheers

Lance
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hopefulsynthesis



Joined: Oct 12, 2015
Posts: 9
Location: Kent, UK

PostPosted: Tue Sep 06, 2016 11:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Lance

Just a quick update here (I have seen your new topic about terrible tracking but have been away from any internet connection recently)...I don't know if it's just good luck on my part but I didn't match any of my transistors and I can get an octave out of mine. I did build a 1 oct tactile switch keyboard which used the current source from Ray Wilson's Single Buss controller and just took the output from the buss to my CV in. I would have thought you could also get at least an octave if I can with my (very) questionable set up!

Good luck
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EL BO



Joined: Mar 15, 2015
Posts: 57
Location: hobart

PostPosted: Wed Sep 07, 2016 2:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, I think there's something wrong with my build as I know it should track. Im pretty sure Rene Schmidt says you should be able to get 4-5 octaves out of it which would be great.
I don't even get a single tone, so there's clearly a hardware bug somewhere!
But it's gone down the priority list for me right now while other Jobs get done.
If I ever solve it I will update here, and post a working pcb if I'm happy with it.
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piedwagtail



Joined: Apr 15, 2006
Posts: 244
Location: shoreditch
Audio files: 3

PostPosted: Sun Oct 16, 2016 8:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

here's where I wish you Gentlemen had breadboarded the circuit out.

It's unclear who's using what buffer; the transistor or the inverting opamp.
Transistor's are like valves, namely great for doing some things but obselete in comparison to the advantages of an opamp amplifier.

In the other thread, there's an inverting opamp at the output of the saw integrator tap. Being configured as such it can efficiently do two things:

1. provide an opportunity to offset the waveform
2. provide gain or attenuation.

1. is achieved by the trimmer mixing CV into the negative input. This is maths, the output is the moment to moment analogue computation of the voltage set on the trimmer added to the moving sawtooth wave ac.

2. is achieved by the ratio between the series input resistors and the feedback resistor from the output back to the negative input or virtual earth. This is maths if the inputs are 100k and the feedback resistor is 200k, the gain is x2. If the feedback resistor is changed to 50k, the gain is halved.

So looking at the schematic in the other thread, the change in the pot is x-100; very sensitive; use a 20 turn or your waveform will fly off the 'scope. The change in the waveform is 100k/47k ie. x-2.
To reduce the voltage swing reduce the 100k to less than 100k with an empirical study on your breadboard.

Which is what Scott says above, but I think you may be using the single transistor instead.

If you want the inversion upward ramp just use the other side of a dual opamp to repeat the exact layout I indicate above but with the series and feedback producing another x-1. Work that out again on your breadboard!


R
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