MFOS VCOs

[Captain Biscuits]

Hi
I've recently finished three MFOS VCOs. They are all producing sound which is a good start. My problem is that the sounds they are creating all sound very similar. I'm getting no distinct variation between the different wave shape outputs except for the square wave where the Pulse Width Modulation does change the sound as it should.
I don't have an oscilloscope but have found a programme called AudioXplorer which seems to be letting me see a waveform which is clearly not the shape I want it to be. I've attached a couple of pics of the output in case that helps.
When I use one VCO at its lowest setting as an LFO to modulate another VCO I do get modulation which is broadly in line with what each wave shape might produce although the sine, in particular, is a pretty ropey and not as smooth as it should be. All of them seem to have jumps and flat areas as the graphs seem to illustrate.
All three VCOs are behaving similarly suggesting it's not a dry joint or shorting but, as I built all three together I guess any incorrect component value would probably have been replicated over all three as I grabbed three components and bunged one in each.
Does this ring any bells with anyone as to what might be wrong and where I might start looking for faults.
As it is I'm pleased that I have 3 oscillators to play with but am looking forward to getting them right so I can get the full range of tonal variation that I would expect from oscillators with 4 wave forms.
Many thanks for any help or suggestions anyone can offer
Cheers
Ian

---

Ramp.jpg
Description:
Filesize:	70.15 KB
Viewed:	198 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

---

Triangle.jpg
Description:
Filesize:	69.64 KB
Viewed:	193 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

[Captain Biscuits]

Apologies for my foolishness.

I was bunging my oscillator signals straight into the laptop and the AudioXplorer software. Far too much voltage for audio meant that I was getting a mess from the oscilloscope. A bit of attenuation was all that was needed so now I have recognisable waves which I can adjust to my heart's content with the trim pots and I'm getting reasonable wave shapes and outputs with sharp ramp sounds and smooth sine sounds.

I'm learning gradually but it must sometimes look a bit slow and like a lost cause to the old hands around.

Thanks for reading and sorry to have been asking such a stupid question.

Cheers

Ian

[Don Erskine]

Congratulations!

The lesson to be learned is that test equipment, when connected to a circuit, becomes part of that circuit and can affect it's operation. Also, the test equipment must be suitable for measuring what you want to measure, and not be overloaded, too slow, etc.

For example, nowadays Digital MultiMeters have high input impedance, and tend not to load circuits unduly, but even stray ('parasitic') capacitances from test leads can cause (or stop!) phenomena like high frequency oscillation.

Let us know how you get on with these VCOs, as I am thinking of buying quite a few. I like the look of design of the temperature compensation and HF trimming sidechain on the Voltage Control and it seems to have a good selection of output waveforms.

Are you using the MFOS 2009 VCO or the earlier version MFOS PCB available from Bridechamber?

My VCO choice is a toss-up between the latest MFOM VCO, and the Oakley VCO which seems to have similar functionality (but I won't get the schematic of the Oakley until I buy a board).

[Don Erskine]

whoops, duplicate post

[Captain Biscuits]

Thanks Don - it was your reply to my other question (about AC/DC) which made me suddenly realise why my laptop, with its audio oscilloscope, was struggling to make sense of a 10 volt signal. Doh!
The three VCOs I've got came direct from Ray at MFOS and are the 2009 version.
I've built them without the temperature compensation for now. I'll see how they go. I suspect I will end up using them for sounds more than for music so a bit of temperature induced drift may well not bother me. I may add a couple more with the tempcos later.
Now I can see the signals and have had chance to have a play it didn't take long to get the trim pots adjusted to give shapes which I was happy with. The Sine wave gave me some trouble but it turned out that I could use the Ramp trim pot to centre the Sine wave and all was well. I'll post pics later when I have a bit more time.
I'll let you know how they go but should warn you that my lack of experience does mean that I may not see the pros and cons that someone whose been doing this for a while might
Thanks again for all the help.
Cheers
Ian

[Captain Biscuits]

This is what I'm getting now I have some idea what I'm doing.
Looks fairly good to me.
I'd be interested to know if I'm still doing anything wrong.
Cheers
Ian

---

Sine.jpg
Description:
Filesize:	60.08 KB
Viewed:	208 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

---

Ramp.jpg
Description:
Filesize:	60.62 KB
Viewed:	193 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

---

Square.jpg
Description:
Filesize:	57.71 KB
Viewed:	202 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

---

Triangle.jpg
Description:
Filesize:	56.7 KB
Viewed:	193 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

[Don Erskine]

They look pretty good to me.

I recommend you add the Tempco resistor, there will come a day when temperature drift will be unwanted in some application.