Hello.
I’m testing AOUT module, and I don’t have the same voltages on all 8 channels.
I’m using for 10k Rx resitor, precision ones with 0.1% tolerance (and I picked by hand the same values from a lot).
But even with the same resistors, one chanell gives me 1.0v-2.1v-4.2v-8.4v, one 1.1v-2.3v-4.6v-8.8v, one 1.0v-1.98v-3.8v-7.8v etc.
Any suggestions ?
I should test many TL074’s until I find the right ones ?
Thank you.
I’m using the AOUT for Midibox CV V1
I should test many TL074’s until I find the right ones ?
No, you need potentiometers (in the schematic called R_p) for the calibration to compensate the usual device tolerances.
Best Regards, Thorsten.
Sorry. I explained the wrong way.
I have the pots installed. With the pots I correct the intervals from the voltages.
But when I adjust the intervals, the voltage also raise or lowers from 1V (test voltage).
So, I will have good actave tracking for all outputs, but they are not centered at a base voltage. It will boe at 1v test voltage : 0.9, 1, 1, 0.9, 1.1 etc.
If put all the voltages at 1v from the pots, I will have bad intervals.
There is a way to adjust both (starting from the same voltage and intervals) ?
(of course, I can use note base from the VCO’s. If the intervals are correct, I can fine tune the voltage coming from CV)
Some of the pots are for the interval, you want 1.00V/Oct, some pots should take care of the offset. Normally this is done so that you try to dial in one or two octaves on the interval, adjust offset, move to higher octaves etc. Just like in Maths you’re doing a linear curve fit here. It’s an iterative process. If everything has been soldered correctly it should be relatively easy to get at least 5 octaves that are very exact and 7 octaves that are exact to okay.
That said, only the bipolar mode of the AoutNG let’s you adjust both gain and offset with trimmers. It says on the AoutNG page how this calibration is done. One thing to try is to swap TL074 to TL084 since the latter is optimized for lower offset voltage in the opamps. However, these days with modern semiconductor manufacturing the difference should be very small.
Some products and projects will use manual calibration to get as close to a perfect curve as possible, then add 1-3 points per octave in order to build a linearization table to get around small flaws and deviations from the ideal curve.
That calibration process can’t be done with the old Aout, because it only has pots for gain. The voltages do look, like there’s some offset though.
Strat-1, did you check the if the voltage coming from the reference source (e.g. at pin6 of max525) is exactly 2,048V?
Ahh, saw that after a little looking. I’d attack the shunt in case it’s not doing its thing. Chances are that a non-pro multimeter is worse than the shunt though. Looking at the module design it went from the *censored* LM324, an op-amp that has nothing going for it unless you want a rotten vintage band-limited sound to a TL-074. A 084 might be better, or you could try another low-offset opamp using that common pinning.
Thank you for your support.
Voltage reference is perfect 2.048v.
I tried tl084, but with worse results.
I changed with other TL074’s and obtained better results. Like this : (on the first 4 outputs)
1v-2v-4v-8.01v
1v-2.01v-4.02v-8.05v
1v-2v-4.01v-8.03v
1v-2v-4.02v-8.04v
It should be enough for good note tracking. (I wanted to buy precision op’amps, but I think, that I will not obtain perfect 1-2-4-8v tracking)