Formant VCF Ladder/3046? Huh?

JRock

Hi.
I'm back with another newb question about this Formant VCF...

http://dropmix.xs4all.nl/rick/Emusic/Moog/moogvcf_schematic.gif

At the top of the transistor ladder IC 2a and b (Pins 1-5 of a CA3046?) are shown with their emitters connecting to different sides of a capacitor and separate sections of a dual opamp.

But according to the datasheet for the CA3046

http://www.datasheetcatalog.org/datasheet/intersil/fn341.pdf

The emitters for the first 2 transistors on the chip (the matched pair?) connect already at pin 3.

So if I connect C7, A7, A8, Q6 and Q7 Won't they all just converge on Pin3 of the CA3046?

Is that right? It seems too clustered to me and not exactly the way it's been drawn...

Am I missing a secret trick (or something glaringly obvious Razz

) about the CA3046?

Thanks. I feel like I ought to check with someone wiser than myself on this cause It's the sort of thing I will blunder through and wonder why it doesn't work Laughing

Trying to take my time and do this right.

Thanks!!!

magman · Joined: Feb 04, 2009 Posts: 363 Location: Liverpool, UK

JRock,

The trick here is to not re-invent the wheel.

You can always refer to the PCB layout as another source of information to cross check, even if you aren't going to make one yourself.

For this design, and referencing the transistor numbers on the Data Sheet, I would see the following mapping for the transistors on IC2.

CA3046 Ref -> Design Ref
Q1 -> Q14
Q2 -> Q15
Q3 -> Q6
Q4 -> Q7
Q5 -> Q16

You actually don't have to follow this convention in your own design though, as you may find it easier to route connections on matrix board to different transistors - as long as you can connect to the seperate emitters as you've already discovered (For example, you may want to use Q5 on the 3046 ref as Q6 on this design).

Good luck on your design, and don't forget to share it when you get a bit further along.

Regards

Magman

JRock · Posted: Fri Dec 03, 2010 3:25 pm Post subject:

Once again MagMan to the rescue!!

Ha! Of course Laughing

I can use these transistors in any configuration that's convenient for my layout Cool

I guess that only makes things... a million times easier Wink

I knew I'd better wait to solder this section together till I had all the pieces in place. Runnin' out of room on my little 2" x 3.5" piece of board...

As for postin' this, It's not gonna be real pretty... I'm just tryin' to build lots of modules in hope that Some of them work and Maybe I can start to understand how this stuff fits together and why.

I'll be happy to post it though... How do you think the best way to draw it out would be? I'll post a photo but there's a bunch of wires blocking the view of some of the components...

Thanks for your time and advice! You really got me out of a jam Very Happy

magman · Joined: Feb 04, 2009 Posts: 363 Location: Liverpool, UK

JRock · Posted: Fri Dec 03, 2010 4:54 pm Post subject:

That's Awesome!!! Thanks a bunch magman!!! thumb up

JRock · Posted: Sat Feb 26, 2011 12:17 pm Post subject:

I just wanted to mention that it didn't work Crying or Very sad

I'll try again some time. Maybe lay it out first Very Happy

forbin · Posted: Mon Feb 28, 2011 5:11 am Post subject:

Danger Will Robinson!

There is a catch with the 3046 and it has bitten me! see: http://forbinthesynthesizer.blogspot.com/2010/07/thats-spoilt-its-looks.html

If you look at the data sheet (see: http://www.intersil.com/data/fn/fn341.pdf) the pin called "substrate" MUST be the lowest potential of all of the voltages running around the chip... There are protection diodes off the other pins to the substrate and it all goes a bit pear shaped if you have chosen poorly...

Pantheon · Joined: Feb 17, 2011 Posts: 36 Location: Edinburgh

wow, thanks for that 3046 tip. I must have missed that when reading the data sheet. I'll have to re-examine the layout of my project. I would have totaly missed that.

Ta.