MUFF WIGGLER Forum Index
 FAQ & Terms of UseFAQ & Terms Of Use   Wiggler RadioMW Radio   Muff Wiggler TwitterTwitter   Support the site @ PatreonPatreon 
 SearchSearch   RegisterSign up   Log inLog in 
WIGGLING 'LITE' IN GUEST MODE

MFOS schem questions (VCCS)
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author MFOS schem questions (VCCS)
Orgia Mode
I am trying to tune a 13700 in the floating point resistor setup and came across MFOS's Sub Commander schematics which I believe shows an exponential Voltage Controlled current source.:


original


I have my own linear VCCS schematic that seems to work very nicely in my VCA, but I want to know more about the MFOS VCCS.

I can't get it to simulate in LTSpice. EDIT: I mistakenly disconnected the power on one op amp. It simulates now, but the window of current sourced is quite small.
I still have the questions:
1. Does this setup need a tempco (R74) and matched NPN's (Q3 and Q4)?
2. What purpose are R77 and R78? The LM13700 has a maximum rating of 2mA on the Iabc pin and coming from the collector pin of Q5, the source is already a current, right?
jorg
This looks like a DIY question.
Orgia Mode
Yep, the curse of having too many tabs open at once. I chose the wrong one.
Orgia Mode
Playing around a bit more I was able to widen the resistance range and better calculate each section of my own board using the floating resistor model.

Should be fun to make a fast VC LPF. I just have to determine the different values I need for each section. 5-poles should be a good start.
guest
that is a bizarre circuit, and i wouldnt reccomend using it. switch the transistors to PNP (2N3906), and just feed the OTAs directly, rather than going thorugh Q5. R75 will need to go to -12V as well. R77,78 help balance the current into the 2 OTAs, and also limit max current. not strictly neccessary, but a decent thing to do. you might want some temperature compensation, but for a VCA its not really required.
Orgia Mode
guest wrote:
that is a bizarre circuit, and i wouldnt reccomend using it. switch the transistors to PNP (2N3906), and just feed the OTAs directly, rather than going thorugh Q5. R75 will need to go to -12V as well. R77,78 help balance the current into the 2 OTAs, and also limit max current. not strictly neccessary, but a decent thing to do. you might want some temperature compensation, but for a VCA its not really required.


Thanks guest! I'll update later when I simulate both of the versions, but for now how would you suggest using a current mirror instead of a bunch of parallel resistor on one 3906? For my purposes, I have 6 LM13700's.
guest
the issue with 6 13700s, is that you need to decide how much current you drive the OTAs with, and how much headroom you want to have. the 13700 likes 500uA as a max, with 250uA being a more reasonable top limit. but, multiplying these by 6 gives 1.5mA to 3mA, which is a bit much for an exponential converter to run at and still be exponential. it will tend to go flat at the top, which might be ok for a VCA, as it will act sort of like a compressor for high input values. using more transistors in parallel will give more current, but it might make the currents not match exactly to each one.
Orgia Mode
guest wrote:
the issue with 6 13700s, is that you need to decide how much current you drive the OTAs with, and how much headroom you want to have. the 13700 likes 500uA as a max, with 250uA being a more reasonable top limit. but, multiplying these by 6 gives 1.5mA to 3mA, which is a bit much for an exponential converter to run at and still be exponential. it will tend to go flat at the top, which might be ok for a VCA, as it will act sort of like a compressor for high input values. using more transistors in parallel will give more current, but it might make the currents not match exactly to each one.


Oh no, not that way. I don't want to multiply the current; I want to mirror the one current converter across 6 separate OTAs. Kinda like this:


I am using 6x LM13700's as 6x floating point resistors in a VC LPF. If I mirror the current to each section rather than just splitting the current with 12 resistors in parallel, the floating point resistor calculations will be more accurate.
guest
there are a number of different ways to mirror the current. the quickest path from where you are at, is to take the bases of your PNP transistors, and connect them to the NPN emitters. then collect the collector of the NPN to ground.
MUFF WIGGLER Forum Index -> Music Tech DIY  
Page 1 of 1
Powered by phpBB © phpBB Group