Compressor questions

[Kipper4]

Hey Guys

I know I could google it but you guys have a wealth of tacit knowledge that I might be able to understand more you explanations.

What's the difference between feedback and feed forward compressors?

Which ones are the best?
Just kidding...


Examples for comparaison might help.

Cheers
Rich

[EBK]

[anotherjim]

Feedback closes the stable door after the horse has bolted.
Feedforward closes the door before the horse is stabled.

[Kipper4]

I need more horse power.

[Mark Hammer]

Feedback is more typical of compressors and feedforward of limiters.

Think of feedback like two good friends, one of them better off and the other penniless.  The wealthier one gives the penniless one a chunk of money to help out, because the friend asked.  But now the wealthier one has less than the previously penniless one and asks for some help, which his buddy responds to.  Bit by bit, they both endup with exactly the same amount of money.

Feedback is used when the goal is to have a constant level.  The gain reduction is a function of the peaks in the signal, but since the gain reduction created by the feedback affects those peaks, there are fewer/lesser peaks to direct that gain reduction and a steady level is achieved.

Feedforward is analogous to the rich friend saying "Listen, when you run into difficulties, gimme a shout".  The rich friend occasionally bails out the poorer one when times get tough, but in between those times, keeps the remainder for himself, only stepping in when needed.

Feedforward affects identifiable peaks but has no bearing on the state of the overall incoming signal like feedback does.

Of course, as the literature for the SSM2166 conveys, if one sets the point whereby gain reduction is applied low enough (i.e., anything above this teensy tiny level), the audible consequences of feedforward and feedback are negligible.

Make sense now?

[bluebunny]

Make sense now?

Hang on...  Do we need to know whether Rich is rich?                                         

[Kipper4]

I'll reread after work.
Thanks.

Btw Rich by name. Poor by birth right.

[Kipper4]

Regarding the Dyna comp.
Schematic for reference.



This is how I call it.
Q2 is a concetina splitter feeding 2x out of phase envelope detectors.
But Q3 & Q4 resembles a current mirror with additional Timing cap C8.
RV1 being a CLR.
R13 the designers minimum CLR feeding Iabc.

Assuming I'm right.
What If one was to futz with R13.
maybe 10x bigger
  "       10x smaller.
I guess it would affect the release time of the cap right? how low or high could one go and feasably have a functioning effect.

Has anyone tried swapping the value of C8?

Thanks for helping educating me.

Rich

[Mark Hammer]

R13 is what many manufacturers that include an "Attack" control replace with a 10k fixed resistor in series with a 150k pot.

[reddesert]

Careful, there are two R13's in that schematic. The 27K is the minimum sensitivity, and I think Mark was referring to the 150K on the collector of Q3.

C8 is the integrator to get the envelope from the signal.

[Rob Strand]

Feedback:
- The natural behaviour is 2:1.   You only get more than 2:1 if non-linearity is introduced into the rectifier.
  An example of strong non-linearity is the Dynacomp.
- The Attack time naturally different to the Release time.   Generally you can only (easily) make the Release to Attack
   ratio larger than the nature value.  Attack is made faster by feedback. 
   Understanding the dynamics is actually quite complicated but tends to be closer to what you want without doing much.
- The gain of the amplifier inside the feedback loop has a strong effect on threshold and the dynamics.

Feedforward:
- The natural behavior is infinite compression.  You get less than infinite by making the control voltage a weighting between the input magnitude + a constant.   When it's a constant you get 1:1.   There's other ways as well including non-linearity.
- The Attack and Release time are naturally the same.  To get asymmetrical time constants it must be done
  by force with different attack and release resistors (and some sort of diode.)
- It is easy to define the threshold.

In both cases the threshold must be done with some sort of dead-zone in the control voltage.

More variations are log detectors and all that.

See here,
http://www.experimentalistsanonymous.com/diy/Datasheets/SA571%20AN.pdf
http://www.rane.com/pdf/ranenotes/Dynamics_Processors.pdf

[rankot]

Regarding Dynacomp - would it be better to connect output (C9) to Q2 base, not emitter?

[anotherjim]

would it be better to connect output (C9) to Q2 base, not emitter?

OTA output is fairly weak current, not the low source impedance drive of an op-amp. It needs buffering to produce voltage drive. Note the included buffer parts in other OTA like LM13600.

Envelope is rather clever. Q3 & Q4 are doubling as charge pump (C6 & 7) feed diodes and mixers for an aggregate envelope  via R13/C8. Q5 is a follower buffering the envelope cap C8. RV1 & R13 (R17?!) turn the voltage into control current into Iabc.

[Mark Hammer]

Careful, there are two R13's in that schematic. The 27K is the minimum sensitivity, and I think Mark was referring to the 150K on the collector of Q3.

C8 is the integrator to get the envelope from the signal.

You're right.  My attention was drawn to that part of the circuit diagram where I knew it would be located.  Never occurred to me to check the rest of the drawing, just in case there were other resistors using the same part number. 

[duck_arse]

I need more horse power.

you'd then need a stable genius to handle the extra.

[ElectricDruid]

Just to repeat the warning about R13/27K. Don't mess with that one - it's the current limiting resistor for the Iabc input of the OTA, and if you reduce it and then turn the sensitivity up, you'll cook the poor old CA3080. And there aren't enough of those left in the world that we should be treating them so roughly!

Mess with the R13/150K by Q3 as much as you like.

Tom

[Mark Hammer]

Just to repeat the warning about R13/27K. Don't mess with that one - it's the current limiting resistor for the Iabc input of the OTA, and if you reduce it and then turn the sensitivity up, you'll cook the poor old CA3080. And there aren't enough of those left in the world that we should be treating them so roughly!

Mess with the R13/150K by Q3 as much as you like.

Tom

Damn straight!

[rankot]

OTA output is fairly weak current, not the low source impedance drive of an op-amp. It needs buffering to produce voltage drive. Note the included buffer parts in other OTA like LM13600.
Envelope is rather clever. Q3 & Q4 are doubling as charge pump (C6 & 7) feed diodes and mixers for an aggregate envelope  via R13/C8. Q5 is a follower buffering the envelope cap C8. RV1 & R13 (R17?!) turn the voltage into control current into Iabc.

I asked that because I've read about people experiencing distortion coming from bottom of the envelope detector. Wouldn't be better to add one more transistor parallel with Q2 to feed the output and make it unaffected with envelope detector?

[anotherjim]

Wouldn't be better to add one more transistor parallel with Q2 to feed the output and make it unaffected with envelope detector?

Probably. And it is is in principle a bad thing to load the phase splitter unequally - anything could be loading the final output. Somebody is going to use it plugged into a 10k line input sooner or later, not necessarily for guitar. Q2 emitter ought to be clean under most conditions...

...but, TBH, I find it hard to see what the clipping limits would be of a raw OTA "current" output and where that leaves the bias, and clip limit, of the following transistor. So it may be difficult to set up another transistor that's any better off than Q2.

[Kipper4]

Wow.
Thanks guys.

Some great stuff.
Thanks for the warning too. I know better than to mess with the 27k to Iabc.
Sorry about the poor choice of schematic. 2x R13.
Meantime if found the page it came from and it turns out Q3 Q4 are being used as switches to allow current to flow. End of bullet point 5.

Link
https://www.electrosmash.com/mxr-dyna-comp-analysis


I'm going to go reread some answers.

I'm sure I have a couple of ne570. Might have to hunt those down.

Choices choices.