The
FFOSC Project
A project brought to you by Tommytones, Kubi
and Fred F.
and other good members at the LAB.
Updated 30 December
2004
Fred Forssell
Opto Side Chain
rev 1.b
Before we begin, this project is available for anyone to download and build for non-commercial purposes only. This project is based on an opto side-chain designed and donated by Fred Forssell. Most of the files will deal with the PCB's that I made and so many of you purchased, but there are some .pdf files of the boards for the Home Brew crowd to download. It is a 2 sided layout so it might be out of reach to the un-experienced DIY-er but if you're a brave soul you should have a go.
This is a link to the original development thread from where this all began.
Here you will find a lot of information on the history of this side-chain and
it's consequent growth into the PCB's and project it is now.
Official
Forssell Limiter Build thread at the LAB
Fred Forssell
http://forsselltech.com/
Many of the images here were originally source from Fred's web site. Do give
it a vist.
For those who might have missed it, this is the link to the PCB thread where
folks purchased their boards.
PCB ..... I don't know where this thread is ??? someone let me know.
This next link is to a member's site (Kubi). Here you'll find some alternate
pcb layouts and some schematics from the early days of the project. Certainly
worth a look! There's some good info here. Also, take some time
to visit Kubi's site as he has a lot of good stuff!
Kubi
http://audio.kubarth.com/
http://audio.kubarth.com/fredopto/index.html
Here's a quick link to a front panel design made by Frank (NRGRecording)
for use with the Front Panel Design software. I'm not sure if this will
fit my PCB's or not but it's a very good starting point for sure. Frank
has a lot of great info at his site so definitely take some time to poke around.
He's got lots of front panel designs and plenty of build info for various DIY
projects.
NRG Recording ... http://www.nrgrecording.de/html/index.html
Here are the main files for this project. Schematics for each individual section of the design and a basic block diagram to help understand the setup of the PCB.
• NE5534
+ Discrete Input buffer
• PSU
note - see the download section for Jason Hall's parts list
Here's a head's up on the parts list....C4 is not listed for the parts list,
but is required for the discrete output servo amp. It's a .68uFd/16V.
http://recforums.prosoundweb.com/
Some comments from Fred
A question or two in a popular DIY forum :
Kris (DrFrankencopter) asks ...
" Fred, if you have a chance would it be possible
to get a little walkthrough of the sidechain circuit. "
Fred replied :
Yeah I can do that.
Q : (Kris) - The gain reduction
element is effectively a variable resistor, whose resistance is inversely proportional
to the current driven into the LED. This resistance, then acts to pad down the
input to the output stage.
A: (Fred) - Yes, the LDR acts as a variable
shunt element as in a L-pad, working against the series resistance created by
R43 and R44.
Q : (Kris) - The threshold control
appears to be a level adjustment on the sidechain input buffer amp. After this,
the signal is paralleled and one side inverted then the two signals are rectified
by the small signal diodes. Out of curiosity, why not use an absolute value,
or precision rectifier type circuit here? Maybe the forward diode drop isn't
significant given the response time of the compressor/Vactrol...or maybe you're
using the forward drop to set the threshold of compression (that's what I'm
currently thinking, and would explain how you can have a threshold control without
a comparator to see if the level is over or under threshold).
A: (Fred) - Yes, the threshold control is a simple
level control that adjusts the signal level feeding the side chain. U5A is configured
for some gain (appox +24 dB) to allow the compressor to function at low signal
levels. U5B is a unity gain inverter. The outputs of U5A and U5B are rectified
through D9 and D10.
The reason I used this scheme to create a full-wave rectifier instead of the
more commonly encountered precision full-wave rectifier is simple; I wanted
to use D9 and D10 to prevent C24 from discharging into the very low source impedance
of U5A/B. While a precision rectifier could be built with one opamp, its output
connection would cause C24 to discharge very rapidly preventing a long release
time setting. Making C24 larger to try to improve the release time would be
problematic. The only drawback to my approach is that signal levels lower than
one diode drop (about 0.6v) will not trigger the compessor. Adding gain to U5A
took care of that problem.
The main thing to think about from this point on in the side-chain is that everything
after D9/D10 is DC.
So what happens is that the output of D9/D10 is
a series of positive going AC waveforms. C24 charges to the peak value of the
outputs of D9/D10 and discharges through the release time pot and R26 to ground.
The input impedance of U6A is very large compared to the pot/R26 path to ground,
so it doesn't play much of a role in the release time characteristics. R26 determines
the quickest release time. The 500k pot determines the longest release time.
U6A buffers the release time R/C network from the
Attack time R/C network and provides enough drive current to charge the attack
time cap quickly. It also help reduce interaction between the attack and release
time controls.
C25 and the output charactistics of U6A determine
the fastest attack time (if we ignore the LED/LDR time constants) when the 500k
attack time control is not creating a resistance in series with C25 (fully CCW).
In this case, C25 will not add to the release time characteristics because it
discharges rapidly into the output of U6A. As we increase the attack time by
moving the pot CW, C25 does take longer to discharge, so it is important to
use a fairly small value C here. R27 is there to provide a ground ref to the
input U6B and needs to be large so as not to attenuate the drive to C25.
U6B is a unity gain buffer that provides a good
high current drive to the LED/LDR cell(s). The input of U6B is also a good point
to passively sum drive signals for a multi-channel compressor.
Yes, too hard to
read,
see above and download and print the drawings to refer to in detail
Q : (Kris) - The ratio control
looks like an variable current limiting resistance on the output of the sidechain,
which makes sense to me as it will change how hard the LEDs are driven.
I notice too that the circuit is set up as a feedforward
compressor....I wonder how hard it would be to tweak to be feedback....I guess
I'd want to pad down the threshold an amount equal to the amout of make-up gain
I'm applying in the output stage (may need a two deck switch).
A: (Fred) - Yeah, that's how the ratio works. Another
way to do that is to place a variable R in series with the LDR (to ground),
and for a long time I did it that way, but it makes is difficult to design a
GR metering circuit that tracks the amount of gain reduction as you vary the
ratio. If one were to check the tracking on other opto compressor design (as
a function of ratio control setting), you'd find that almost none of them (none
of them?) track properly. It does on my design, if you use my metering circuit
(once you get it calibrated), at least well enough to be useful.
With regard to feedback/feedforward-- my desire
was to design a reasonably simple side-chain. There is a bit of feedback and
a bit of feedforward on this design, at least with regard to how the side-chain
functions. The point at which the signal to the side chain is picked off determines
this. If you were to reduce R43 to zero, you'd have an all feed-forward design
(and you'd have to increase the value of R44 by 4.99k). If you were to reduce
R44 to zero, you'd have a feedback design. I chose values for the R43/44 ratio
by trail and error. You might have fun playing with these values to see what
effect it has on the design.
It is possible to use this design as a gate instead
of an compressor. The LDR would be connected in series with the signal (like
R43/44 are now) and you'd need a resistor to ground where the LDR is now. You
would want the side-chain pick off point to be prior to the LDR, therefore it
would be a feed-forward design. Max attenuation would be determined by the ratio
of off resistance to the shunt R resistance (and may vary as a function frequency,
so be aware). Min attenuation (gate on) is determinded by the ratio of the LDR
on resistance and the shunt R.
Anyway, that's kind of the idea. I hope my explaination
makes sense and that it addresses your questions.
I wish a great new year for all of you.
_________________
Fred Forssell
many thanks to Fred
Downloads
Parts List
from Jason Hall
http://recforums.prosoundweb.com/
Parts List - rev 1 - Excel spreadsheet
A little incomplete for now and Jason would like us to have a look over it generally and in particular, the capacitor section and help to fill in some blanks.
" ... please use the "best" components
if you add things in. "
So if you do have some suggestions , please send an email to myself or Jason
and we will update the sheet.
Even as it is , this sheet should be of use to most people looking to take on
this project.
an update to spread sheet
Parts List - rev 2 - Excel spreadsheet
PDFs of the PCB
Kubi has some PCB layouts as well - see link above
Just to wet the
appetite,
Kubi has a graph to look at.
we have a zip file some where of the gerbers ( should we post that ? )
ERRATA
So
far so good. Check back for any updates in this area.
Just the parts list changed to rev 2.
Pics
pics to come ... stay tuned !!
Enjoy!
Our Tommy sitting at the controls of an SSL
this page is still under development
and more will be added
brought to you by Tommy and the DIY Team at :
www.diyfactory.com