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 PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW. 
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Post Setting Scale trims on the 9720 VCO
I'm copying the section of the P9700S overview below with some added comments which might be useful to you in getting your Scale trims set.

Also, imagine the Scale trim as being an adjustment for the slope of the v/oct scaling -- at one extreme the frequencies produced for octave-spaced key presses will be more than one octave and at the other extreme, they will be less than an octave (doubling or halving of the frequency).

Think of the panel controls in the VCOA and VCOB sections as ones for setting the initial Pitch or Frequency that will be varied by the connected Pitch Control Voltage.

The procedure has three objectives:

1) the setting of the MIDI2CV8 DAC Trim for 1 volt changes for one octave key presses.

2) the setting of VCO A scale trim in response to changes of 1 volt (and increments thereof) for octave spaced pitch changes.

3) the setting of VCO B scale trim as was done for VCO A.

MIDI2CV8 Pitch CV and VCO Scaling Adjustment (procedural supplement)

The DAC Tune trim can be set by connecting the pitch cv output to a multitester set to measure dc volts in a 0-10v range and then pressing octave spaced keys on the connected MIDI controller while adjusting the trim for 1v change.

Later the VCO will be adjusted with this adjusted MIDI2CV8 output. Note too, it is easier to adjust the VCO pitches when they are in the lower range obtained when the Low-key transpose option is selected as outlined at the top of the Operating Modes pull-out (power-up the MIDI2CV8 while a the lowest key on the connected MIDI controller is pressed--then, the first message received is a note-off (when the pressed key is released) which selects the low-key transpose option).

While a tuner can be useful, it can throw you off if the input is overloaded, clipping the wave and introducing harmonics.

Because it isn't always consistent as to what MIDI Note Number is used by a MIDI controller for the low note of the keyboard, the MIDI2CV8 can be set so it treats this lowest key as the one which produces zero volts of pitch control voltage. Generally, the lowest key of a MIDI equipped keyboard is not going to be MIDI Note Number 1 ($0) so it is good to develop the habit of asserting the low-note transpose function by powering the MIDI2CV8 while holding down the lowest note on the keyboard -- then, as MIDI2CV8 powers up and begins executing the programming in its firmware (watching for any MIDI Input), the first note-message it "sees" will be a Note Off (sent as the low-key that had been held down is released) and it understands this as a command to understand this as the lowest MIDI note.

Here's a way to get them by ear... First set the MIDI2CV8 to be outputting 0V for the low key pressed on the controller, by powering the 9700K while holding the low key on the controller so that releasing the key will cause the first message input to the MIDI2CV8 to be a note-off (it interprets this as a signal to output zero volts for this note number). A dvm can now be used to check that octave spaced key presses are outputting a 1v pitch cv change (down to a hundredth of a volt or so) as set by the trim on the MIDI2CV8. The most accurate setting will be obtained if you press octaves that start a few notes above the lowest key, ie if your low key is a C, press D or higher for the lowest octave reading.

The MIDI2CV8 trim is now adjusted so octave spaced key presses produce volt spaced Pitch CV Output changes, right?

The usual tuned setting for the VCO scale trims is about a 1:00 setting for the pointer of there disk which covers a cw range from about 7:00 (ccw) to 5:00 (cw). Start with the trim at 1 o'clock.

So, when you've completed the following procedure, if the trim isn't at about 1:00, this might be an indication something is wrong. Maybe the scale was adjusted to produce a third or a fifth instead of an octave, or, it's time to go back over your work -- are board and panel solders true, are parts in their respective designation, etc...

After having been powered for a minute or two patch the low-key transposed pitch CV over to the VCOA P2 input. Set the two pitch controls to they're both in-tune at unison at about a mid-rotation setting for the low key pressed on the controller.

It isn't mentioned here, but to hear both VCO A and VCO B, a waveform output from each will have to be patched to a mixer (the L and R inputs of the VCA) or a passive mixer constructed using two 10k resistors twisted at one end and with a 1k resistor to ground at the stem (the two branches are the two inputs and the stem is the output with the 1k to ground working to attenuate the high-level signal to match the input of a line amp or mixer). Or, don't worry with listening to the other VCO, but some other tone or sound generator -- you just want to be able to have a reference as the key presses, panel control and board mounted trim changes affect the VCO in question.

Play an octave higher and the adjust scaleA for an octave relationship with B. Go back to the low note and adjust the A panel pitch control for unison and then, again, press an octave higher and adjust the scaleA for an octave relationship with B. After going back and forth like this another time or two, you should find the scale is about as close as it can get. Then, do this all again pressing some note a few keys higher than the lowest key (with VCOB adjusted to match at this new low pitch) and confirm/tweak the scale to match for higher pressed octave relationships with B (using the A panel pitch to realign A with B after adjusting the scale). When the scaleA adjustment is complete, move the pitch control over to VCOB P2 and make the panel Pitch and scaleB trim adjustments to get unison and octaves with the VCOA pitch (adjusted to match B for the low key pressed and pitch CV sent to it).

So, in these steps we've used the panel Pitch controls to get each VCO at an initial or starting pitch in response to a key having been pressed down towards the lower end of the keyboard, and, then adjusted the Scale trim so the pitch produced for a key pressed an octave or more higher is an equal amount of octaves higher, ie one-octave-spaced key presses result in one-octave-spaced relationships with the reference, two-octave-spaced key presses result in two-octave-spaced pitch relationships with the reference,....

After the scaleB is set, move the Pitch CV over to the VCOA P1 input and listen to the two vcos tracking across the keyboard range (again with the low-key transpose asserted) after aligning their pitch while a higher note is pressed, say the third or fourth octave. Confirm they track for a higher octave pressed and hold as you press keys going down towards the low key. You should be able to turn up the sound of the controller or some other MIDI Sound Module and hear that the tones made from the VCOs can be aligned using the panel pitch controls and follow as you play different ranges on the keyboard (make the alignment between the two when a key pressed in the middle C, A 440 range is pressed--its easier to hear the beating slow as they are tuned).

Your panel Pitch settings for getting the two VCOs to track each other, and another instrument will be closest when the adjustment is made while they are producing pitches in the upper range, ie press a key towards the top of the keyboard to set the pitches of the VCO to each other or another instrument.

It helps to keep the VCO Modulation control at minimum so there isn't any possible pw modulation which can make the VCOB pitch change slightly (often, a change of R59 from 91k to 27k prevents this interaction).

Unless R59 is a 27k, the VCO Modulation control being advanced (and affecting VCO B PulseWidth), influences the pitch of VCO B slightly which is not good when using VCO B as a reference, or setting this VCO's scale trim.

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Scott Lee
scott@paia.com


Mon Sep 24, 2012 1:26 pm
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Joined: Mon Sep 10, 2012 8:02 am
Posts: 15
Post Re: PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW.
Thank you Scott, this is a lot of help!!

I'm still having some difficulties making it go 100% perfect the whole range, but I think I can try tweaking it again.

I couldn't manage to find out the other problem though (VCOB/VCOA sync not working properly), so I think I still need some help there.
Quote:
I get different sounds for each key. Sometimes both follow the pitch perfectly, but on some keys, there's no significant pitch change when playing them.


Anyways, at the moment I'm renovating my whole apartment, meaning wallpaper torn all over, no walls, no floor, no paint...
My girlfriend somehow means that the apartment has a higher priority than my PAiA 9700, so I guess it has to wait a couple of months.
Women! :roll:

Could be nice anyways to take a short break, and get a new approach. Christmas time is always nice for these projects.

Again, thanks Scott! Your comments helps me a lot, my understanding of the internal electronics on these synths is growing, but I'm still at a lower level :oops:


Tue Oct 09, 2012 3:44 pm
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Joined: Mon Sep 10, 2012 8:02 am
Posts: 15
Post Re: PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW.
I'm finally getting finished with most of the refurbishing.. *puh*.

I'm hoping to test the PAiA again this weekend. Will start from scratch with adjusting the MIDI voltage etc., and go over to testing VCO etc.

Will post my experiences, and will most likely need some further help. I was hoping to be able to be absolutely finished with the VCO now, so I could move on to the VCF next weekend, which is finished, but not yet tested. Then I'm just missing the VCA, which still is in its bag.


Thu Apr 25, 2013 6:14 am
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Joined: Mon Sep 10, 2012 8:02 am
Posts: 15
Post Re: PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW.
Who am I kidding, I couldn't wait to this weekend. I dug up my PAiA project from the moving boxes, and started again. Instantly I found something strange;

I just did the simple VCO tests first, putting both VCOs to max, and alternating between them to hear if there were any differences.

I had both Scales set to 01:00 o clock, and they were definitely not in unison. When lowering the VCO A pitch, it would go much lower than the VCO B. I therefore adjusted the VCO B's Scale, so that both VCOs had the same pitch at max and min.

In other words, VCO B is higher pitched, by maybe a half octave. I actually had to set ScaleB to 11:00.

So 11:00 and 01:00... That's strange?

Anyways, I kept on going:

I went to the step where you're supposed to check the pulse output, and I did definitely find something weird, this was there before also, but I didn't think about it.
It starts at a louder volume when I plug in the mini jack, and immediately goes down. Sounds like a ground issue or something.

Please watch this video to see what I mean:
PAiA 9700s - OSC B Pulse not working properly

Where could the error be Scott?

Thanks again for helping out. It's so nice to start on this project again!

Btw:

Quote:
It helps to keep the VCO Modulation control at minimum so there isn't any possible pw modulation which can make the VCOB pitch change slightly (often, a change of R59 from 91k to 27k prevents this interaction).

Unless R59 is a 27k, the VCO Modulation control being advanced (and affecting VCO B PulseWidth), influences the pitch of VCO B slightly which is not good when using VCO B as a reference, or setting this VCO's scale trim.


I'm not sure if I have a 27k or 91k, will check it this weekend. A local retailer has a set with a bunch of carbon film resistors, hopefully one is 27k.
I only have metal film resistors.


Thu Apr 25, 2013 1:04 pm
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Joined: Mon Sep 10, 2012 8:02 am
Posts: 15
Post Re: PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW.
I can't get them in unison, and set to track perfectly with both scales at 1 o'clock... What could be wrong? Both are beneath 12, one has to be set to about 11 for the VCOs to be at unison. If I set both to 1 o'clock, VCOB is several notes higher.


Fri Apr 04, 2014 10:35 am
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Joined: Wed Jan 01, 2014 1:36 pm
Posts: 61
Location: Toronto
Post Re: PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW.
perhaps the pots are not the same?

- A10k vs B10k ?
- labeled the same but not ?
- well out of tolerance ?
- a fixed resistor in series with the pot ?

( I have not looked at a 9720 so I may be off base )

stupid suggestions
- pots not mounted the same ?
- knobs not installed the same ?

btw, you could just place the knob differently for now

Bill


Sat Apr 05, 2014 5:49 pm
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Joined: Mon Sep 10, 2012 8:02 am
Posts: 15
Post Re: PAiA 9720 VCO Pulse Width - Goes quiet when fully CCW.
Turned out I was following the wrong manual, so I set the scale instead of pitch.
Near 100% correct between the OSCs now, and tracking a digital synth correctly.


Mon Apr 07, 2014 9:50 am
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