Some questions about optical bass compressor

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

L´Andratté

Well-known member
Joined
Jan 17, 2013
Messages
793
Location
Hamburg Germany
Hi everbody!
It is not just courtesy of me to tell you, I like this forum and it´s, hmm, inhabitants? ;D
You know, I have become one too...

I have just posted in the documents section the schematic of the Roland SIP 301, an middle aged bass preamp, that I own and love. That is especially because of the brilliant optical compressor.
http://www.groupdiy.com/index.php?topic=45158.msg677240#msg677240
(upper right)

Now I would like to build me a standalone box of that, feat. also variable time constants (lacking in the original, but very useful e.g. for guitar-sustain).

But there are some things, that I don´t understand really:
there are three paths, one the gain cell, then the (feedback) sidechain path, but then also some kind of dry mix, regulating the ratio - called ´Dynamic Range´. This is mysterious to me as it is also contributing to the sidechain,
upping it´s gain when the threshold is reduced.
I just can´t wrap my head around it, but I´d really like to, so please someone likes to explain to me?

I know that you can ;)

Another thing: per Vactrol application notes the two optos should give different responses, one being a voltage divider, the other in the fb-loop of noninv. op-amp. Of course these are not vactrols, so this is maybe dead end-thinking (or >>THE SECRET OF THE SIP<<)!

This is the page I got the service manual of, it has also some nice demos:
http://www.joness.com/gr300/SIP-301.htm

Cheers, L´Andratté :eek:
 
> the two optos should give different responses, one being a voltage divider, the other in the fb-loop of noninv. op-amp

They both do the same thing.

Analyze the gain equation.

(PH2/(PH2+R111)) * (((PH1||R113)+R112)/R112) -- (I think!)

Assume the LDRs idle over 1Meg and fall to a few K.

At 1Meg, PH1 starts to cut gain.

At 100K, both PH1 and PH2 cut gain.

At 2K2, PH1's gain-cut doesn't get much deeper; however PH2 is still cutting gain.

At 2K2, PH1 sets opamp gain to 2.0, PH2 is causing a loss of 0.02 against R111. Total gain is 0.043. Assuming threshold at THRESH pot wiper is 0.15V, input can be 3.48V, very large by instrument standards. If THRESH is turned-down so threshold is 1.65V, maximum input is 38V, which is impossibly large (actually we can't get more than 8V through the Q101 input buffer).

> also some kind of dry mix, regulating the ratio - called 'Dynamic Range'.

You have the unit. What does that knob do? It appears to bypass the limiting action. In strict-protection limiting, this is "bad". The manual says it "compensates for loss of attack".

To slow the limiter attack, raise R127. To slow/speed the release, raise/lower R128. You can't have a very small or very large attack/release ratio with this type filter.

There is another trick. If signal _stops_, Q102 Q103 short-out the time capacitor for a quick release. (This is backward to some program limiters: if signal stops, they "hold" the gain level so silence is not brought-up.)

Someone thought-about, and experimented, this plan a LOT. I would be very hesitant to try to change it.
 
Thank you, that was quite helpful.

PRR said:
Analyze the gain equation.

(PH2/(PH2+R111)) * (((PH1||R113)+R112)/R112) -- (I think!)

Assume the LDRs idle over 1Meg and fall to a few K.

At 1Meg, PH1 starts to cut gain.

At 100K, both PH1 and PH2 cut gain.

At 2K2, PH1's gain-cut doesn't get much deeper; however PH2 is still cutting gain.

At 2K2, PH1 sets opamp gain to 2.0, PH2 is causing a loss of 0.02 against R111. Total gain is 0.043. Assuming threshold at THRESH pot wiper is 0.15V, input can be 3.48V, very large by instrument standards. If THRESH is turned-down so threshold is 1.65V, maximum input is 38V, which is impossibly large (actually we can't get more than 8V through the Q101 input buffer).

Now after you explain, it all seems so clear... ::)
Though what I meant was, they give different response ´slopes´ in both implementations...but that is probably
dilettant-thinking!

PRR said:
You have the unit. What does that knob do? It appears to bypass the limiting action. In strict-protection limiting, this is "bad". The manual says it "compensates for loss of attack".

This thing is not about protection limiting, i think, just about sound. Fully clockwise it sounds like a very subtle ratio setting, if at all, but with turning ccw gets very tight very quick, I never used it past 12 o´clock on bass and that only with high threshold.

What I like about it is that it *always* sounds very defined, never mushy (?sorry, as everybody probably noticed already, I´m no native english speaker) like e.g. the RNLA that I have (not that it´s bad).

I think this relates to the attack-compensation and signal shorting, you mentioned.
PRR said:
Someone thought-about, and experimented, this plan a LOT. I would be very hesitant to try to change it.
I´m really interested now, though not in copying the unit, but in understanding the principles behind it,
thinking and experimenting a lot I can do also, though the thinking will be the harder part!  :p
But not now, because https://www.youtube.com/watch?v=T0Oac-IErMI

Thanks a lot and please everybody continue to share your thoughts!

 
> they give different response 'slopes'

Yes, but it is a feedback (servo) system. The sidechain gain reduces any nonlinearity in the gain control element's slope. In other words, it only reduces gain as much as it needs to, whether that means a small or large change of control signal (light).

> This thing is not about protection limiting, i think, just about sound.

Certainly not. I come from radio broadcast and recording limiters. If a 50KW transmitter is pushed past 100% modulation, it "splatts" signal into adjacent channels, and the stations on those channels complain. If a disk-cutter cuts one groove into the next groove, the record is unplayable. Tape is more forgiving; still for "pure clean" sound we prefer to turn-down rather than push the tape headroom; in live performance overloads can happen unexpectedly.

"Limiting" a bass is a different problem. It's an awkward instrument. Decay is quick; it probably should be scaled to 7'/2m string length). It is hard to play at steady 80% of amplifier power. A 20% note or decay is weak, a 110% attack note is distorted and rude. With ample amp/speaker, the limiter can tend to hold 70% of maximum to extend decay, yet bop-up a bit on attacks so they are not mushed.
 
PRR said:
> they give different response 'slopes'

Yes, but it is a feedback (servo) system. The sidechain gain reduces any nonlinearity in the gain control element's slope. In other words, it only reduces gain as much as it needs to, whether that means a small or large change of control signal (light).

Ah, well
L´Andratté said:
Now after you explain, it all seems so clear... ::)

What you say about the response of bass(guitar) is very interesting. Though it´s hard to believe it took me some time to figure out, you were actually joking and not measuring decay in some arcane unit per string meter! ;D
7 feet of decay are a LOOONG decay ;D

In the meantime I am trying to recreate the circuit block on my breadboard using the same component values but my standard opamps and tansistors- just as a starting point- we´ll see...(hear?)

On a sidenote, only while doing this I noticed that this funny little circuit is also feedback and feedforward simultaneously!

EDIT: From a Tape-Op tutorial by Mike Caffrey
(http://tapeop.com/tutorials/61/two-stage-compression/

`Suppose we have a volume scale of 1-10 with 10 being the loudest. In the verses the drums are quiet, around 2. In the chorus they're loud, around 8. Our average volume is 5 [ (2+8)=10/2=5 ]. Now, add heavily compressed parallel signal, with a lot of make up gain. It's level is 8 and never fluctuates, so 8 is also the average. Figure out the new average [ (8+5=13) and divide by 2, because it's two sources and you get an average level of 6.5. Blending the compressed signal in parallel brings up both the average and the minimum levels.´
 
The SIP301 limiter is pure feed-back.

Its noise-gate is feed-forward. Noise gates usually are.
__________

> From a Tape-Op tutorial

When is TapeOp gonna fix their IE-broken website??

It isn't working smooth in FireFox either.

That 2-stage idea seems like a hack to do what good broadcast limiters did 40 and 67 years ago. Barry Blesser wrote some good papers in the late 1960s.
 
Back
Top