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 Forum index » DIY Hardware and Software » Thomas Henry designs
VCF-1 drive & a few other questions
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Xorophone



Joined: Mar 01, 2018
Posts: 12
Location: Sweden

PostPosted: Fri Mar 02, 2018 4:54 am    Post subject: VCF-1 drive & a few other questions
Subject description: Could I add a drive control to the input of the VCF-1 filter?
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Hello! Is the "Attenuate" knob in combination with VCO-1 able to provide a sufficient amount of drive for the filter or is there an easy way to add a drive pot to get more energy out of the filter?

Also, how do you control the cutoff frequency? I can't find a cutoff knob on the schematics. Does it need to be controlled by an envelope generator/LFO? I'm planning on combining the filter with this envelope generator, but there's no volume out/cutoff pot on that schematic either. I want to be able to use the filter without any envelope too.

Thanks in advance!
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PHOBoS



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PostPosted: Sat Mar 03, 2018 5:36 am    Post subject: Reply with quote  Mark this post and the followings unread

I have no experience with this design so I don't know about drive but the cutoff frequeny
can be controlled with the two inputs (1V/oct, Envelope) and the Coarse and Fine controls.

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Xorophone



Joined: Mar 01, 2018
Posts: 12
Location: Sweden

PostPosted: Sat Mar 03, 2018 5:57 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
I have no experience with this design so I don't know about drive but the cutoff frequeny
can be controlled with the two inputs (1V/oct, Envelope) and the Coarse and Fine controls.


Oh, that explains things! So basically, I'd hook up my LFO to "1V/Oct", envelope to "Envelope" and then change the cutoff frequency using "Coarse" and "Fine"? But why is the fine there in the first place? Isn't that useless for normal usage?
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PHOBoS



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PostPosted: Sat Mar 03, 2018 6:20 am    Post subject: Reply with quote  Mark this post and the followings unread

I think the fine tune is usefull to tune it when it self-oscillates so you can use it as a 1V/Oct VCO.
So yeah, for general use you probably won't need it.

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Xorophone



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PostPosted: Sat Mar 03, 2018 6:25 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
I think the fine tune is usefull to tune it when it self-oscillates so you can use it as a 1V/Oct VCO.
So yeah, for general use you probably won't need it.


From http://www.birthofasynth.com/Thomas_Henry/Pages/VCF-1.html:

"The filter is controlled via a volts per octave signal input for keyboard tracking, a modulation input for envelope or LFO control, and, of course, it provides manual initial frequency control via a coarse tune control and a fine tune control."

Oh wow. I completely missed that when I read the page before. Embarassed Well now I know where to hook up my LFO too.

Ah, that explains things, PHOBoS! So I'll leave the 1V/Oct input and the fine control out for this build. Can I just get rid off both the resistors and the jacks/pots without affecting the behaviour of the other controls?

Edit: Or wait.. The 1V/Oct input might be good to keep for the keyboard tracking. I thought it was only there for if you want to use the filter as a sine oscillator. So since I'm not building a modular synthesizer, I might add a switch so that I'm able to turn keyboard tracking off. Might be a good idea to keep the fine tune pot as a trimpot too, just to be on the safe side.
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PHOBoS



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PostPosted: Sat Mar 03, 2018 6:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Xorophone wrote:
Can I just get rid off both the resistors and the jacks/pots without affecting the behaviour of the other controls?

yep, you could leave those out. Personally I would keep both as it is very useful to have 2 modulation inputs.

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Xorophone



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Location: Sweden

PostPosted: Sat Mar 03, 2018 7:05 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
Xorophone wrote:
Can I just get rid off both the resistors and the jacks/pots without affecting the behaviour of the other controls?

yep, you could leave those out. Personally I would keep both as it is very useful to have 2 modulation inputs.


Yes, but I'm doing all the wiring internally since it's not going to be modular, so I can just copy the envelope input to be able to hook it up to an LFO too.

Thank you for all the help PHOBoS! Smile Now we'll just have to see if anyone has an answer to the drive question too.

On the website it says this: "I've found that the filter can be overdriven to impart a very "nasty" sound, while attenuation of the input signal will also produce a smoother sound that is not lacking at all in signal level.", which indicates that it can be done without any modifications to the circuit, but I don't know if he used a VCO with a really hot output or if it can be done using any VCO, with the attenuation knob turned up.
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