A means of closing a JFET switch with a postive voltage?

[(*steve*)]

droop, drift, yeah its all part of the problem. There's no particular reason why it will always fall, so I said drift.

Another solution would be to interface these keys to a microcontroller and use it to create the analog voltage.

 

[CDRIVE]

Another solution would be to interface these keys to a microcontroller and use it to create the analog voltage.

I guess it's possible but highly improbable that this option wasn't proposed somewhere lost in this epic odyssey.

Chris

 

[AFex54]

droop, drift, yeah its all part of the problem. There's no particular reason why it will always fall, so I said drift.

Another solution would be to interface these keys to a microcontroller and use it to create the analog voltage.

right from the start I was deluded by the MFOS 1V per octave keyboard when I proposed using the sample and hold,
something obviously wrong is that the noise toaster is only 18mV per octave!
there no way in hell holding a stable CV with a basic analog sampling circuit, a few μV of drift will send a note out of tune.
I wanted this project to be purely analog but this is so foolish

I do like the idea of using a microcontroller, I will treat this a seperate project and leave this synth on hold for few weeks.
could you give me some of basic information?
for example: the only microcontroller im aware of is the arduino, was this what you had in mind (is this the go-to for basic digital needs)?
or would I have to find one thats most suitable?
I not sure how you intended for me to implement it but I hope that It could function as some sort of digital S&H so I can use the 34 existing analog pots.
but a microcontroller is very flexible right? it would open up many different paths forward?
and finally can 1 of them multitask? like can I sample 2 seperate CVs and get 2 seperate outputs simultaneously ?

My brother uses arduinos for some his projects so I will talk to him about this, and I will do a lot of reading too.

 

[(*steve*)]

The way I envisage it is that you would have only a single trimpot and use it for all the keys. The micro controller would only sample the voltage generated by it when you are "tuning" the keyboard with the digital value stored in eeprom.

I'd probably skip trimpots entirely and opt for rotary encoder or similar, however there's no reason you'd have to.

The issue with using a micro controller is that you will be limited to discrete voltage steps.

 

[CDRIVE]

right from the start I was deluded by the MFOS 1V per octave keyboard when I proposed using the sample and hold,
something obviously wrong is that the noise toaster is only 18mV per octave!
<<SNIP>>I not sure how you intended for me to implement it but I hope that It could function as some sort of digital S&H so I can use the 34 existing analog pots.

How can that be? 9V divided by 34 = ~ 265mV.

Chris

 

[AFex54]

The way I envisage it is that you would have only a single trimpot and use it for all the keys. The micro controller would only sample the voltage generated by it when you are "tuning" the keyboard with the digital value stored in eeprom.

I'd probably skip trimpots entirely and opt for rotary encoder or similar, however there's no reason you'd have to.

The issue with using a micro controller is that you will be limited to discrete voltage steps.

EDIT: this post is null and void, you can ignore
I just read this here:
''The Arduino board contains a 6 channel (8 channels on the Mini and Nano, 16 on the Mega), 10-bit analog to digital converter. This means that it will map input voltages between 0 and 5 volts into integer values between 0 and 1023. This yields a resolution between readings of: 5 volts / 1024 units or, .0049 volts (4.9 mV) per unit. The input range and resolution can be changed using analogReference().''

does this mean the discrete voltage step you mentioned would be minimum 9mV If I were to sample the voltage from the pots?

 

[AFex54]

How can that be? 9V divided by 34 = ~ 265mV.

Chris

If i were to tune it to a typical musical scale the key voltages would only be a few mV apart for each note.
btw there are only 24 keys, 10 of them have 2 pots for polyphony

P.S Im glad you questioned me though because it made me query the NT being ''18mV per octave''.
there is no way this is correct,
that would mean there is 500 octaves from 20Hz to 20000KHz when in reality there is only 10!
Im sure I misinterpreted it but if you read the description of VCO here you will see the '18mV per octave' thing.
anyway when you consider with 1V per octave the whole audible spectrum would be contained within 10V the NT is pretty much 1V per octave.

so the S&H along with its drift problems is still a perfectly viable option since the MFOS 1V/octave incorporates the same simple circuit.

 

[AFex54]

Ok so I decided what I will do is build the sample and hold with a TL074 ,
I already explained how this will work here:

If this wouldnt work I now think the TL074 could maybe work!
my reasoning: when I was messing around with noise toaster before this project i distinctly remember that the frequency pot (top left of the schematic,this is what im duplicating for every key) must be turned up at least 1/6 before anything became audible,
a few minutes ago I tested what voltage a potentiometer voltage divider would put out when turned 1/6 the way up, it was around 1.75V.
the TL074 input offset voltage is 1.5V and its output tops off at around 8.75V , and it sample and holds perfectly, so that 1.5-8.75V range would be fine

(you can actually see visual evidence of the low voltage cut off in the NT demo video at this point)

If the S&H works well in the finished synth then I will purchase the correct rail-to-rail and its just a matter of swapping the TL out from the socket.

Also Steve pointed out the necessary delay, since the voltage input will cease right as the sample switch is opening .
i think to eliminate this problem I will use a very short monostable pulse to sample and hold as soon a key is pressed rather than when you let it go, this is how the MFOS keyboard does the sampling I should point out.

but first i need to buy the schottky diodes and the electronic switch.

 

[CDRIVE]

Ok so I decided what I will do is build the sample and hold with a TL074 ,
I already explained how this will work here:

The TL074 will produce an output offset voltage ~1.5V when used with a single ended 9V power source.

I told you waaaaay back (in which topic title I don't remember) that the first basic feature that your application needs is an OpAmp that sports Single Ended Supply operation. This information is always found at the top of the Datasheet under the heading .. "Features". I also remember giving you an OpAmp PN that looked promising.

Chris

 

[AFex54]

The TL074 will produce an output offset voltage ~1.5V when used with a single ended 9V power source.

I know I explained this in that post where I quoted myself, did you see?

 

[CDRIVE]

I know I explained this in that post where I quoted myself, did you see?

I probably missed or forgot it. It's been a long journey. I'm guessing that 1.5V offset is now acceptable to you. I know it wasn't at the beginning.

Chris

 

[AFex54]

I probably missed or forgot it. It's been a long journey. I'm guessing that 1.5V offset is now acceptable to you. I know it wasn't at the beginning.

Chris

''I know I explained this in that post where I quoted myself, did you see?'' I sound like im talking about a post that was made a week ago when I just meant post #68... I think Ill go to bed now haha

 

[CDRIVE]

Hey I'm an old fart! You'll get there someday (maybe?) and you'll find an hour ago, a day, 6days, maybe a month, all seem to run together with absolutely no discernible difference between them. Hell, I told my wife today that the Corn Beef Hash & Eggs she made for breakfast was great! She looked at me with a expression and said.. "Thanks but that was last Thursday"!

Chris

 

[AFex54]

So I will use a CD4066 for the sampling switch,

For the schottky diodes, I think im looking for one with maximum values as close to my supply as possible for maximum efficiency and minimum voltage drop.
I looked through a load of them and this SD101C has the lowest, 40V and 30mA.

Also a possible choice for rail to rail op amp is AD822, the input bias current seems low enough not charge the sampling cap but I dont trust my judgement.
Input bias current is apparently a max of 25pA with a 5V supply, the TL074 is max of 200pA with +/-15V supply.
if theres any wrong with it ill just stick with the TL074 for now.

Spot any problems with these components?

 

[(*steve*)]

why not try it with the components you already have (aside firm the 4066) and see how it works.

Of course, if you have to buy parts and pay freight, it may make more sense to buy everything now.

I'm most concerned with the schottky diodes as they have significant leakage and you may end up seeing them operate more like a resistor at very low currents.

depending where you're using these diodes, low leakage may be important. I'd have to go back to check...

 

[(*steve*)]

Ok, for the diodes in the last circuit I drew, you want normal silicon small signal diodes like 1N4148.

 

[AFex54]

why not try it with the components you already have (aside firm the 4066) and see how it works.

Of course, if you have to buy parts and pay freight, it may make more sense to buy everything now.

I'm most concerned with the schottky diodes as they have significant leakage and you may end up seeing them operate more like a resistor at very low currents.

depending where you're using these diodes, low leakage may be important. I'd have to go back to check...

I have tested with the TL074 and polystyrene cap by manually sampling with 2 wires and it works good.

https://www.electronicspoint.com/attachments/updkb2-png.21470/
the diodes for the CV in green would be the schottkys.

 

[(*steve*)]

yeah, you may find that the thoroughly high leakage causes problems. But give it a go and see.

 

[AFex54]

yeah, you may find that the thoroughly high leakage causes problems. But give it a go and see.

I dont have the schottkys now but Im after testing the voltage drop of a standard 1N914 from 0-9V and I doesnt go over 0.4V.
that should be ok but i wonder If I did use schottkys would the drop be almost non existent (0.1V)?

anyway I will just use the 1N914 since I own lots of them already and there are other possible issues with schottkys.

At €7 each the only thing Im worried about is buying this AD822.
The datasheet seems to imply that is has very low input bias current but I dont know if its low enough i.e as low as the TL074

 

[AFex54]

Hey I'm an old fart! You'll get there someday (maybe?) and you'll find an hour ago, a day, 6days, maybe a month, all seem to run together with absolutely no discernible difference between them. Hell, I told my wife today that the Corn Beef Hash & Eggs she made for breakfast was great! She looked at me with a expression and said.. "Thanks but that was last Thursday"!

Chris

so my S&H is finally working and I can continue my project!
my synth also has another S&H that will be used in the traditional way to make some cool synth effects.
Im wondering about those unsuitable rail-to-rail Op Amps that a I bought a few weeks- the TLC2264.
the datasheet says the maximum supply voltage is 8V,
before I knew this I tested it in the keyboard S&H circuit and it worked fine until the input went over 8V, any input below 8V, even with the 9V supply, worked perfectly.
The inputs to this traditional S&H will be small ,4V max, so I could potentially make use of these Op Amps.
the 1V overvolt doesnt seem to be breaking it so I think I will use them.
Is this is a always a bad idea or can some components work fine with a bit of overload?