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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
"Single Buss 1V/Oct Keyboard"
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Liquids



Joined: Jun 06, 2011
Posts: 23
Location: CT, USA

PostPosted: Tue May 01, 2012 10:58 am    Post subject: "Single Buss 1V/Oct Keyboard"
Subject description: Synth newbie questions
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I'm attempting to enter the world of keyboard-controlled synth DIY after a few years of guitar pedal electronics and tube amplifiers.... Shocked

Never thought I'd be here, but if I can get this (or another keyboard) to work as a controller, I'll be 'in!'

Anyhow, I've got Ray's PCB for this project in hand, just started populating it. However, since I don't entirely understand how everything works (if at all), I want to make sure I get this correct.

On the page, Ray first mentions a 37 key keyboard, and then at the end mentions calibration:

"Calibration is easy. This procedure assumes that you have the circuit connected to the keyboard resistor chain, one end going to circuit board point KBTP (keyboard top) and the other end going to ground. We want to have .8333 mA flowing through the keyboard resistor chain which means that .8333 mA will be flowing through R5 (475 ohm 1% metal film resistor). When .8333 mA flows through a 475 ohm resistor is causes a (.0008333 Amp x 475 ohms) 0.3958175 volt drop across R5. So with the highest impedance most accurate DVM you have adjust R3 until you have 0.3958 volts across R5 and voila you will have .8333 mA flowing through the resistor chain which in turn will give you 1 volt per octave from your keyboard controller."

Currently if all goes well with my attempt at refurbishing an old organ's keyboard, I'll be using a 5 octave controller. On one hand, I assume that if the amount of octaves mattered or would require more than the 1k trimpot's range of adjustment, it'd be designed this way.

However, I want to make absolutely sure... Very Happy

Q: outside of the +/-15v or +/-12v variation and appropriate changes noted on the page and schematic given that variable, will all values and calibration function and procedure be the same as specified, utilizing a 5 octave (61 note) keyboard controller?

http://www.musicfromouterspace.com/analogsynth_new/SINGLEBUSSKEYBOARD2007/SINGLEBUSSKEYBOARD2007.php
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Liquids



Joined: Jun 06, 2011
Posts: 23
Location: CT, USA

PostPosted: Sat May 05, 2012 10:48 am    Post subject: Reply with quote  Mark this post and the followings unread

I am a synth newbie. So please bare with me.

I've populated the PCB, but I don't have it hooked up to an oscillator nor a real keyboard (just a string of resistors on a breadboard); I'm merely making connections and checking the bus, CV, trigger, and gate voltages, etc.

When I 'disconnect' the buss from the resistor string, the gate voltage does drop. So that is a good thing. The gate is working.

However, MY confusion is, when I disconnect the buss from the resistor string (i.e. no notes are played), the CV outputs remain at the previous voltage for a while....unless I disconnect power.

Is this the way it should work? I understand there is a sample and hold involved here. but I was assuming that the sample and hold would stop 'holding'when the gate signal went low by default, and hence, when there was no note, the buss voltage AND the CV output would go low.

Is the idea that the VCO will continue oscillating given the sustained CV and the gate/trigger is there for controling even the simplest of VCAs, a VCA of some kind always being needed - so that if nothing else, when the gate goes low (no note is played), the VCA passes no signal, but opens back up upon trigger or gate going high again?

I suppose that if CV dropped at the end of every gate, that would be an 'effect' or an easy thing to do circuit wise with the gate...and if CV went to 0v (or -.xx volts?) everytime no note was being played, you'd just get a drone from any oscillator, huh? Laughing

What is the typical solution here? I intend to add ASDR-based VCF and VCA, however, for those 'straight synth lead' sounds I hear, without resonant filtering, I'm still hearing a gate/trigger VCA effectively doing 'on/off' switching post-oscillator set like a switch which is 'closed in' upon attack to let the VCO tone through, and switch 'disconnected' when no notes are played, I take it?
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richardc64



Joined: Jun 01, 2006
Posts: 614
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PostPosted: Mon May 07, 2012 9:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Liquids wrote:
However, MY confusion is, when I disconnect the buss from the resistor string (i.e. no notes are played), the CV outputs remain at the previous voltage for a while....unless I disconnect power.

Is this the way it should work?

Yes, that's how it should work, for the reasons you've given in the rest of your post. The VCO wouldn't stop oscillating, but instead drop to a lower frequency with zero CV. The S&H prevents that.

Quote:
On the page, Ray first mentions a 37 key keyboard, and then at the end mentions calibration:

"Calibration is easy. This procedure..."

Easier than trying to measure 0.3958175 volts would be to measure the output CV while alternately pressing keys one octave apart until you get 1volt difference between them.

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Liquids



Joined: Jun 06, 2011
Posts: 23
Location: CT, USA

PostPosted: Thu May 10, 2012 9:23 am    Post subject: Reply with quote  Mark this post and the followings unread

All this was with the resistors and VCO on the breadboard connected to the PCB for the keyboard...so keep that in mind.

However, I am using an LF444 chip and a 10n polystyrene cap as prescribed. However, I seem to still get some droop in CV/pitch over time.

I seem to get notably less droop when I parallel a big 1uF film cap. ....

I'm not sure if I'll get less droop in general once my resistors are actually on the keyboard....

however, even if so, is there logic in using a larger value film cap here instead, paralleling a film cap in there with the 10n polystyrene, adding/swapping/paralleling/putting in series the biggest silver mica cap I can find...

Other options are...putting a film cap in series with the 10n polystyrene, paralleling another 1 or 2 10n polystyrene caps rather than a film cap or putting the 2 other 10n polystyrene caps I have on hand in series, paralleling a film cap and the polystyrene cap(s), etc?

Likewise, overall, is there an total amount of capacitance that should not be passed on either end (biggest, smallest), or, is "as big as possible" work just fine so long as the TYPE of cap(s) is or are well chosen (for their leakage characteristics, I assume)?
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