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Mellotron-esque synth concept
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Potassium-40



Joined: Sep 16, 2013
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Location: US

PostPosted: Mon Sep 16, 2013 2:04 pm    Post subject: Mellotron-esque synth concept Reply with quote  Mark this post and the followings unread

I've been thinking of this idea for the past several days. It basically involves a keyboard controlling the speed of the motor of a cassette tape machine that has a tape recorded with 30 minutes of a continuous sound (i.e. a choir). I have two questions.

First, how can I build a low-note priority keyboard that outputs 0v when there is no key being pressed? I need to be able to tune the keys independently, and I need it to be able to be powered with a single supply.

Second, should I connect the output voltage from the keyboard directly to the motor, or should I control it with PWM through a 555 timer?
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JingleJoe



Joined: Nov 10, 2011
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PostPosted: Tue Sep 17, 2013 1:11 am    Post subject: Reply with quote  Mark this post and the followings unread

try a tape loop then you don't need to record so much and have effectively an infinate note time Smile
although, the motor and tape are likely to wear out after you've been pressing the key for about 10 years!

pwm is the best choice, you could use the standard comparator method, no 555 timer needed, unless you want to use one for the oscillator but you could use a an op amp schmitt trigger oscillator then you;d only need one dual op amp (like an lm358). don't drive the motor directly with that, drive it with a transistor, check the power it's rated for.

Just googleing "keyboard bus" should give you a lot of info into how this can be done. Make a common bus connected to your PWM comparator, connect the bus to a pull down resistor, say something in the region of 100k to 1Meg.
Your keys actuate switches connecting different points on a resistor chain, between whatever voltages you desire, to the bus.

This will give you voltagte control over pulse width Smile

It's going to be difficult to get your method in tune... I can think of a lot of ways this needs fine tuning but that's half the fun of experimentation Very Happy

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Potassium-40



Joined: Sep 16, 2013
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PostPosted: Tue Sep 17, 2013 10:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you for pointing me in the right direction. I will use a modified version of the Ken Stone's keyboard made for a nonlinear vacuum tube oscillator, where the notes are tuneable. The voltage from that will go into a PWM comparator, which is driven by an op amp Schmidt trigger oscillator, which drives the motor through an n channel MOSFET.
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Cynosure
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Joined: Dec 11, 2010
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PostPosted: Tue Sep 17, 2013 5:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

I had the same idea - I think that chords will sound especially cool when recorded and played back at different speeds.

My plan though, was to use an arduino for the PWM source. Then I could control it with MIDI. I could also use a dac or the other PWM outputs for CV control of a filter and vca.

Also - my plan was to record different things on the left and right outputs. For example, saw on the left and square on the right. Then you can pan to mix between the two.

Please post your project on the forums when you complete it. I am very interested in seeing and hearing it.

Good luck!
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Potassium-40



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PostPosted: Tue Sep 17, 2013 6:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

The only reason why I'm not going to use a uC is because the duty cycle/pitch ratio will most likely not be linear (or logarithmic), and I cannot easily tune it without changing a few numbers in the program.

I was also thinking of recording two different sounds on the channels. Maybe both a mixer and a switch? Then I can record two different chords (maj and min) and switch between them.
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jumunius



Joined: Apr 19, 2010
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PostPosted: Tue Sep 17, 2013 9:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

You might find this link interesting -- saw it on Muff's recently.

http://mysterycircuits.com/melloman/melloman.html

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amdagan



Joined: Jun 21, 2013
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Location: Iron mountain

PostPosted: Wed Sep 18, 2013 10:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Maybe use a stepper motor? That should simplify the tuning, anyway
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Potassium-40



Joined: Sep 16, 2013
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PostPosted: Thu Sep 19, 2013 7:24 am    Post subject: Reply with quote  Mark this post and the followings unread

That is a brilliant idea, but unfortunately a bit to complicated for my budget. I'd have to use either a microcontroller or CMOS logic chips, neither of which do I have access to.
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