Taking @eric 's advice I did one heck of a kludge, using a spare TL071 to buffer the VREF as it comes out of the trim pot and before it goes to the rest of the circuit. I’m still getting 4.07 on the LM4040AIZ but the VREF (and thus the tuning) has stayed consistent for three power cycles, which is a first. Too soon to call this a win but I’m letting my self dare to hope this issue might be solved. Sadly it totally invalidates the stripboard layout I had planned to share. I guess potentially serving as a reference for 12 DAC channels at once was too much for the little guy. In any case, thanks a million to you both, Analogoutput and Eric, for the solid advice and talking me through a brief bout of hobby electronics hysteria.
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Have you measured the pot resistance from top to wiper? If the DACs were presenting a VERY low impedance then to get your 2.07 V you might have to sit close to the top of the pot giving a load of much lower than the ~50k you’d assume. Then that would increase the current the LM4040 has to supply. Still, to bring it up above the 15 mV it’s specced to handle you’d need R = 4.1/.015 = 270R or 0.3% of the pot, which is hard to believe. Also hard to believe the DACs would put that kind of load on their voltage reference. It still makes no sense.
Added: Input impedance for unbuffered V_Ref is supposed to be 165k.
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Well, the TL071 VREF buffer seems unneeded from a theory perspective, but it seems to have solved the problem, so I’m going to ignore it for now since that part of the circuit will be eliminated in the final design anyway.
This thing has been rock solid and super fun to play with for the last 5 days, and now I’m looking into one of the last hardware feature’s I’d like to add - analog slew rate limiters. I might start a new thread on the topic, but does anyone know of a good slew rate limiter circuit that meets the following criteria?
- Relatively simple
- No need for “true bypass” (pot rolled all the way to zero will cause almost no slew rate limiting)
- Has appropriate response curve for oscillator portamento (exponential curve? IDK)
I plan to make two four channel busses with behind-the-panel micro switches that will allow the user to dictate which channels are used for driving oscillators (or keyboard tracking) and should be subjected to slew rate limiting. The tricky thing here is that all four channels will be affected by a single pot, rather than individually. I’m not sure if this can be done with one pot, or if a 4 gang pot will need to be used. All channels don’t need to be perfectly matched, just in the ball park. Here is a block diagram of what I’m talking about:
Since there will be two instances of these 4-slew-rate-limiter-circuits, this means I will be cramming 8 slew rate limiters in - obviously simpler is better in this context, but I would like to go one step beyond a simple resistor and cap, if it will have any noticeable impact on the quality of the circuit.
If anyone knows of any good portamento circuits that I can model my efforts after, feel free to post some links!
Cheers
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Slew, Glide, Portamento ? i don’t know really the difference between each
but i made this Glide
i added this on mine
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I’m using slew, glide, and portamento interchangeably here, but I suppose glide and portamento are synonyms describing when slew is applied to oscillator pitch. Thanks for showing me this circuit, I will check it out!
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Ok. I’ll have a bash;
A Slew is note with an initial offset ± to the one note/voltage it resolves to.
A Glide is a slide (usually linear) between two notes.
Portamento is to ‘carry’ one note/sound over to the start of the next. (you can wibble, slide,glide,slew, bend or simply step the next note). Though commonly a synonym for a glide the portamento notation is useful for breathing, bowing, hitting or plucking and in synths? Well mostly used as a linear ‘changer’,
In a synth I think portamento works really well with odd waveforms, for me that’s mostly FM sounds and odd “ringy/bongy*” sounds. On a simpler square wave I think of any portamento as a cute pitch bend but otherwise I avoid it.
Ok, I’ve thrown too many brackets, N3Ts*, and probably meds down this logical rabbit hole. Enjoy!
- N3T = a Not True Technical Term
a.k.a. MUF - Made Up Fact
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