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555 VCO calibration
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author 555 VCO calibration
xnor
I am failing to find documentation about how to calibrate Random Source's Thomas Henry 555 VCO. I've seen forum posts about Thomas providing documentation about calibrating but can't seem to find the actual docs. Can someone point me to those?

there are 5 trimmers.
V/oct
Ramp connect
Sine Round
Sine Symmetry
HF Trim

The v/oct seems self explanatory but I'm not exactly sure in what mode to look at and what to look for with the other trimmers.
search64
Look for the Thomas Henry 555 build docs, which this is based on. You'll need a scope for some of this, although you can get close by ear.

Sine round and sine symmetry are about the shape of the sine. HF trim is the v/oct trimmer for the higher range. Ramp connect I'm not aware of, not sure if the original circuit had that. I suppose it's to line up the positive and negative parts of the ramp's slope?

Mutable Instruments' Module Tester is great for the v/oct trimming by the way.
Rumblin_Cynth_Rampo
The way I did it was;
Do the ramp connect first (helps if you have a scope but as mentioned you can do it by ear)

Next do the Sine round then the symmetry. I found this to be quite an iterative process as you adjust the symmetry the round goes off to you have to tweak each trimmer back and forth. The first time I did this it took ages. I found setting the pots in the middle of their ranges to help. I found I had to use a scope for this both times I did it.

Next do the V/Oct. When I did this first I used Ray Wilson's MFOS V/Oct calibrator, which worked out ok and a scope. The second time I did it was when I changed my rack PSU and used Yarns Midi to CV and the a tuner plugin on my DAW which I found to to be much easier use. Also you can play scales on the keyboard and check that you are getting the right frequencies.

Then finally do the HF. One of my 555 VCO I found I had to go back and forth between the V/Oct trimmer and the HF.

As a previous poster mention if you have access to a MI Module tester it is even better for doing the V/Oct
xnor
Looks like the calibration doc is here:

http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html#Calibration _and_Adjustments

I'm not exactly sure what the 'ramp connect' is?

I've built the MFOS v/oct calibrator which is working pretty well for me.. one question on that though, if I calibrate using 1 power supply [in my shop] but then put the module into my case which uses another power supply, both setup for +/-15, will the slight differences in actual voltages provided by the 2 different supplies effect the v/oct tuning? Hopefully the differences are so subtle that it wouldn't be noticeable, but I'm curious anyways.

Thanks for all your input!
xnor
HMM, after looking closer it looks as though the VCO-1 is not the same as the 555 VCO.. though the calibration info there seems good enough for this. I thought he published this info publicly? I wonder why I can't find the build docs for the 555 based VCO..
Rumblin_Cynth_Rampo
All I remember having was the schematic, the BOM and I think (but am not sure) there was a bit description of the operation of circuit over on electro-music. I remember there being lots to read through

The ramp connect is used to align the slope of the ramp. See diagram

Picture file
flagada
Thomas presented his VCO555 in this topic: http://electro-music.com/forum/topic-54623.html
xnor
Rumblin_Cynth_Rampo wrote:
All I remember having was the schematic, the BOM and I think (but am not sure) there was a bit description of the operation of circuit over on electro-music. I remember there being lots to read through

The ramp connect is used to align the slope of the ramp. See diagram

Picture file


oh huh, I guess I'll look at the ramp on the scope, maybe in LFO mode and see about that. Thanks
jschussler
I think I'm reasonably calibrated (the scope shows "normal" looking waveforms) but for one thing: the sine out is a) not very loud an b) +5v above the others.

Any idea why that might be?
flagada
Sine waveforms just are not loud anyhow. There are no harmonics, so they are less audible than the other waveforms. In addition, the amplitude of the sine of the VCO555 is smaller (at least in mine) than the other waveforms.

Is the bottom of your sine at 0 volt and the top at +10 volt?
jschussler
No, it's running from 4-7 volts.
humax5600
Maybe the same problem here :
https://www.muffwiggler.com/forum/viewtopic.php?p=1619648#1619648
jschussler
Yup, that did it. I missed the change of R25 to 300K. Sine sounds perfect now. Thanks for the pointer.
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