# CV input-protection
**URL:** https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383
**Category:** DIY STUFF
**Created:** [July 23, 2024, 8:39am UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383 "2024-07-23T08:39:00Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@NEOZEO](https://lookmumnocomputer.discourse.group/u/NEOZEO)
#### Post date: [July 23, 2024, 8:39am UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/1 "2024-07-23T08:39:00Z")
Hey people!
I want to build VCO based on a 33340. I tried out the Hard- & Soft-Sync-capabilities and put a ~ ±4V rectangle signal on both inputs with the 1nF cap in between. It works, yeah!
In the datasheet of a CEM3340/AS3340, there are no maximum input voltage ranges, just this info:
I am in fear that I fry the chip for a ±10V gate/cv signal. What would you do? Saturate the input to 0V-5V with zener diodes? Or do nothing and hope?
Thanks in advance!
---
### Author:  [@clarionut](https://lookmumnocomputer.discourse.group/u/clarionut)
#### Post date: [July 23, 2024, 11:20am UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/2 "2024-07-23T11:20:55Z")
I built my AS3340 VCO based on the [Digisound 80 schematic](http://www.digisound80.co.uk/digisound/modules/80-2_files/80-2.pdf). I used the sync input circuitry exactly as shown - it works fine for me and (tempting fate here!) nothing has blown up (yet).
---
### Author:  [@NEOZEO](https://lookmumnocomputer.discourse.group/u/NEOZEO)
#### Post date: [July 24, 2024, 8:52am UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/3 "2024-07-24T08:52:09Z")
ok thx! And you can put a +10V gate on it?
---
### Author:  [@clarionut](https://lookmumnocomputer.discourse.group/u/clarionut)
#### Post date: [July 24, 2024, 2:25pm UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/4 "2024-07-24T14:25:49Z")
Mine is happy with 10V signals on the sync inputs.
Quick update - I’ve just checked mine again and it needs pulses of at least 9.5V on the -ve hard sync input to work at all using the component values from the digisound schematic. Might be worth investigating other CM/AS3340 VCO schematics as well.
---
### Author:  [@Jos](https://lookmumnocomputer.discourse.group/u/Jos)
#### Post date: [July 24, 2024, 5:45pm UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/5 "2024-07-24T17:45:23Z")
> [@NEOZEO](#):
>
> Or do nothing and hope?
Na, that won’t work. Not with your kinda luck.
Most ICs are not up to voltages higher than +V and lower than -V.
There are standard ways to protect inputs, often using diodes and series resistor against +V and -V. If you want to clamp against lower voltages, then a zener comes to mind, but anyway, I’d suggest you do a search for ‘voltage clamping’ or ‘input protection’ and you will find lots of examples.
---
### Author:  [@NEOZEO](https://lookmumnocomputer.discourse.group/u/NEOZEO)
#### Post date: [July 24, 2024, 10:04pm UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/6 "2024-07-24T22:04:20Z")
Thanks! Makes sense. Yeah, I still need to learn a lot 😅. There more you tinker with diy modular synthesis (and ask questions about schematics from the internet), the more side-quests open up
---
### Author:  [@clarionut](https://lookmumnocomputer.discourse.group/u/clarionut)
#### Post date: [July 25, 2024, 7:11am UTC](https://lookmumnocomputer.discourse.group/t/cv-input-protection/7383/7 "2024-07-25T07:11:12Z")
According to the datasheet the hard sync input on the 3340 should be connected via a 1n capacitor (or a slightly more complex but still AC coupled pair of inputs for separate + & - hard sync). Internally the pin is connected to -2.5V via a 6k3 resistor. As long as you’re only applying sync pulses within the normal ±10V range, and using one of the input schemes shown in the datasheet, there should be no problems. I certainly wouldn’t try a direct DC coupled connection though.
Voltage clamping can - in general - be very important on sensitive inputs, but I don’t think the sync input on the 3340 is one of those cases.