1222 Tuner VCO Module

Per Mouser, the differences are the L7805ABV provides higher output current, higher PSRR/ripple rejection, and lower line regulation, and can operate down to 40°C. In other words, it can only do a better job.

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(not sure the amp difference is correct (datasheet says “up to 1.5A” for both, and they have the same °C/W and max specs), but the A version does indeed have better voltage tolerance and regulation, and is available for wider temperature ranges. Not that any of these specs matter here, though :slight_smile: – you’re regulating regulated 12 V, and if the Arduino needs more than 1 A or you find yourself jamming in −40 °C :cold_face: you may have bigger problems.)

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ok, so when i removed the molex connector to wire directly, seems i damaged a track on the pcb. see pic below.
if i read the schematic correctly, the middle pin of the 12-way-switch basically goes directly to pin3 of the TL074, is that correct? do you think i could try wiring it directly?
cheers

Yes. But Discourse will not allow me to simply say so without dumping a load of other garbage into my reply.

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yessss!!! finally it works!!! thanks for all of your help! going to calibrate the thing now! cheers!

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I finally got around to powering up my #1222 VCOs, I was waiting for the mica caps to arrive from AliExpress but they’re still at least a couple weeks away, so I gave up.

While poking around trying to figure out why I can’t seem to be able to calibrate the frequencies (more on that later), I noticed a resistor getting quite hot.

It is R42, the resistors in series with the 4.1 voltage reference.


I did a bit of math :
The current through R42 is I=V/R= (12-4.1)/220 = 35.9mA which seems a bit high.
The power dissipated by R42 is P= VI = (12-4.1)x35.9mA = 0.28 watt which is more than the quarter watt that my resistor is spec’d for.
Again, that seemed high so I looked at the spec sheet of the LM4040AIZ to see if we could safely reduce the current.
image
Not only can we, but we should reduce the current as the maximum recommended current is 15mA.

So @lookmumnocomputer, I recommend replacing R42 by a 1K (or more) resistor to reduce the current to a safer 7.9mA.
The power dissipated by a 1K R42 would then be reduced to about 1/16 of a watt.

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Is there any reason it could not be much higher, say 10k or even 47k? The minimum for the diode evidently is 68 µA, more than 100 times lower than that 7.9 mA. Am I missing something?

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The effective load is the four 10K “precision” resistors R16, R17, R18 and R21 forming the voltage divider, so a 40K load.
I’d like to be about an order of magnitude lower than that so maybe up to 4.7K for R42, but I would not recommend more than that.
You could increase the precision resistors too, as their load is the very high input impedance of an opamp, which in turn would allow to further increase R42

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Thanks for sharing! I’m going to try swapping out mine.

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Is yours running hot too?

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I didn’t notice, but I guess I should check that first :crazy_face:

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hey Antoine! ok 1k ill pop that as a note on the site! thanks. have you changed it out and has it made it less sizzle?

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Hello Sam,
no I haven’t swapped a 1k for R42 yet, the reason being that I was having trouble tuning/calibrating the VCO and I wanted to figure out what was going on before I changed anything.

With no external CV plugged in, I used the REF trimer to adjust the the octave switch’s reference voltage to exactly 4V and checked that the position of the switch gave very close to exactly 0V, 1V, 2V, 3V and 4V.
I then used the octave switch and the TRK trimmer to calibrate the oscillator over the 5 octaves.
I then plugged the output of my BeatStep Pro into the 1V/oct input and found that even though the BSP had a 1V/oct output, the oscillator did not track very well at all.

After some analysis, this is what I found out. (Sorry here comes another long post).

  • The voltage provided by the octave switch is turned into a current by the R19 100k resistor.
  • The external CV on the 1V/oct input is turned into a current by the R22 100k resistor.
  • The voltage from the fine tune knob is turned into a current by the R24 100k resistor.
  • The CV from the link interface (if used) is turned into a current by the R25 100k resistor.
  • The voltage on the external CV input is attenuated and turned into a current by the R20 470k resistor.
    The sum of all these currents is what determines the frequency of the oscillator.

The attenuated external CV does not need to be in tune so let’s ignore it.
The other voltages are all converted into currents by 100k resistors, but unfortunately they are not all exactly 100k.
Like most people probably do, I used 1% precision resistors.

Let’s look at a worst case example involving only R19 and R22, the summing resistors used for the octave switch and 1V/oct external input respectively, as nobody uses the link yet and the fine tuning knob is less critical.

At 1% precision, the octave switch summing resistor could be as high as 101k and the 1V/Oct input’s summing resistor could be as low as 99k.
When one calibrates the oscillator using the five voltages provided by the octave switch, the oscillator is adjusted to 1V/101k= 9.9µA per octave, but the external voltage provides 1V/99k= 10.1µA per octave, a difference of 2%. That doesn’t sound to bad, but unfortunately it adds up for each octave.

For middle C the BSP outputs 5V which pushes 5V/99k=50.5µA into the oscillator which is expecting 5x9.9µA = 49.5µA, the difference of 1µA corresponds to more than one tenth of an octave which is more than a semitone and it just gets worse for higher notes.

I see two solutions to this problem.
a) Use closely matched 100k summing resistors for the octave switch, the 1V/oct input, the link CV and the fine tune knob.
Or, if you don’t care about the Link CV and the fine tune exact range.
b) Calibrate the oscillator using the external 1V/oct input and TRK trimmer, and then use the REF trimmer to adjust the ref voltage (to not quite 4V) in such a way that the octave switch gives exact octave jumps (not exact 1V jumps).
That’s what I did, and combined with the HF trimmer, got good tracking from 10Hz to 10240Hz (eleven ten octaves).

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11 octaves! blimey. thats pretty good going!

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Sorry I counted wrong, 10 to 10240, is really “only” 10 octaves.

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You could write a book, “Synthesizers For Dogs”

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wow excellent work!
1512662773982

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Nice work!

Of course the problem returns if you have more than one oscillator and want to use the link.

I see Mouser has 0.1% 100k through hole resistors at about $0.50 each. For about $7.70 you can even get 0.01% — though they have only 9 in stock!

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measure each resistance with the multimeter before choosing them

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In most cases, you probably won’t use the link CV and the external 1V/oct input at the same time, so you could turn the link CV into the “normal” connection of the 1V/oct input by removing R25 and connecting the CV link to the normally closed tip connection of the 1V/oct jack. That way, the external 1V/oct input overrides the link CV whenever something is plugged into the jack.

With this mod you probably don’t need any high precision resistors or manually matched resistors.

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