Forssell Opto Compressor

[TomWaterman]

Hi,

a 20 segment meter only need take up about 50cm width of panel space if people use kingbright 10packs ala massenburg. Should require two 16-pin ICs on the board....it could be made with VU or PPM ballistics with a full wave rectification circuit. Maybe even switchable modes.....how about having it switchable so that it can monitor GR or output level??

I have been doing PCBs for a similar meter in Eagle...but its my first attempt at board layout....I will try to help the best I can but I'm very green.

It all sounds soo good so far - I think it will be an awesome tracking comp but I am curious as to how it will sound on the mix buss.

I like the idea of power supply included but if everyone wants two units - its double the parts. An external powersupply takes up rack space too.

I say go for separate PSU with regulation on the compressor boards.

Great stuff!
Cheers Tom

 

[tommytones]

I was thinking of having both options for led and vu metering. Switching in, out & GR is a good idea as well. I'd like to have those abilities.

 

[soundguy]

Its probably because Im the village idiot, but LED meters are totally meaningless to me, I could never get a feel for mixing on them. With 40 or 50 VU's pumping away during a mix, the brain has to totally switch modes to see the LED meters. I mixed on a soundcraft ghost for about three years exclusively and I swear I never ever got used to the LED meters... Having the option to put a VU or PPM on this project would be MEGA good.

Regulation on the compressor cards with a third satellite card for the main power supply would be good. Pads for input transformers would also be excellent. I think for anyone using transformers, having the satellite board is good, if the power supply needs to move around in the box or out of the box completely, its an option. If this is as transparent as everyone says, this might be a good one for lundahls.

In/out/GR is great on the meter, whatever the meter winds up being. Every limiter should have that.

I dont know what kind of room there will be on the PCB's but if black gate standard series could fit, that would be awesome.

dave

 

[tommytones]

Well, I've started the layout and I must say that as we keep adding things here, the board is growing. I agree with Dave that the led thing is more for rough reference than anything else and that a vu meter would be preferable. I'd need for you to point me to a suitable schem for the vu thing.

What about input buffer...are we using one or just straight into the make up stage and out again? Transformers are cool but should be left off the board I would think. Too many options for those.

 

[tommytones]

Here

This is the 1st attempt. I haven't conected the dots yet cause I'm sure there'll be changes. Rough dim are 4" x 7.5".

 

[hitchhiker]

hi Tommy,

The original circuit that Fred had on his site had a VU circuit without resistor values. It's no longer there but maybe Fred can give values
and I'll test it in my proto with the true API VU that I have.

It's a simple looking circuit.

Lance

 

[soundguy]

butta-

Id usually prefer caps to the very very few servo circuits Ive heard, would be interesting to hear your comparison of the two if you build them.

Id be giddy to get this built to use it to mix the record Im tracking right now.

dave

 

[Svart]

looking good tommytones! what cad software are you using? I have an older version of circuitmaker/traxmaker but never get enough time to use it. from the dimensions in adobe, it shows the card to be 4.49in*7.75in. that should allow the cards to sit vertically in a 3u rack box(edit: i just thought about that.. a lot of boxes are much smaller than their face plates..)! awesome, i can fit a bunch in one rack!

and one more request, on the cap through holes, can we make several holes so that we can use different size caps if need be?

cheers!

:guinness:

 

[tommytones]

Yup, I'm gonna add a few extra holes for some of the caps to allow varying sizes. The pdf pic is a little larger than the actual board would be at this point. I didn't put an outline yet. So the dim are a little closer to what I indicated. Certainly will fit vertically in 3RU.

So what about input buffer? Needed or not? The vu circuit would be very cool to get a hold of. Any other things to consider be fore I begin drawing traces?

 

[tommytones]

Oh ya, I use CirCAD'98 from Holophase. Simple, not too expensive and works quite well. I like it better than Circuit Maker and others I've tried. There's a lot of key commands with CirCAD so I don't have to click the mouse all the time which results in me working a lot faster.

 

[Svart]

simple input buffer might be nice, something like the gssl? simple 5532/4?I also have orcad and eagle but never use them.

 

[DrFrankencopter]

I have a nice input buffer design that I can lend to this project....it's the same one that I am using on my SSL variant. Uses a Forssell DLR-1 as an input receiver and an OPA-177 as a servo amp. Or, you could just couple with a 10uFd poly cap and be done with it.

Cheers,

Kris

 

[FredForssell]

Hey everyone,

Looks like ya'all are progressing along with this project at a good pace.

Based on what I've read here and seen with tommytones pcb lay-out, I have a couple of quick thoughts/comments which you can take with a grain of salt.

On the subject of metering... I personally do not like to use VU meters to indicate gain reduction because they do not accurately indicate GR on short above-threshold transients. Their ballistics prevent this, so they indicate nothing even though there actually is gain reduction going on. Typically I use very little compression in my recordings (acoustic music) and I want to know exactly what is being compressed and by how much. LED meters work way better for me. YMMV.

As for increasing the number of segements-- While it should work fine, I would caution you not to expect the kind of accuracy you may hope for when using a 20 segment meter. You may be able to mess around with the meter design enough to get accurate 1 dB per segment indication, but having it track the Ratio control accurately throught its range may be problematic. If someone comes up with an accurate 20 segement meter circuit, I'd like to see it and perhaps use it myself in some up coming projects. It is a subject that I've not spent any time on myself.

If you want to use a movement type VU meter, then you need to rethink the GR meter control circuit. When I have used VU meters in the past (a long time ago) I used a 500k lin pot connected across the meter with its wiper connected to +24V (this is the GR zero adj). One side of the meter also goes through a resistor to ground (something like 10-30k ohms) and the other side of the meter also goes through the LDR cell to ground (not +24V as I show it in the schematic for the LED meter). You'll have to mess around with the values, but I think these are reasonable accurate.

I encourage you to use a differential to single-ended input stage on this project as it will allow a completed compressor to be used in a number of patched signal paths far fewer grounding problems. The output can be single-ended without creating grounding problems most of the time. I almost always design gear with differential inputs and single-ended output, though I do use the impedance "balanced" method (not true balanaced, IMO) on the outputs.

With regards to tommytones layout (which looks good to me), I would recommend a couple of things...

1- I'd change places with the ratio and threshold pots (unless this board is going to be mounted upside down), but that's a personal preferrance.

2- I'd use a pin footprint for the pots that uses 0.100" spacing (more pots available using that footprint).

3- I'd move the LED/LDR location to be very close to series connect/signal dropping Rs and the input/output section of the board. This will keep the audio signal path as short as possible and avoids routing it near the power supply and side-chain circuitry.

4- I like that tommytones discrete opamp footprint includes the offset trim pin. I been thinking about using that pin on my next opamp design.

I hope this project turns out to be a valuable tool to all of you.

 

[TomWaterman]

Hi Fred!

Thanks for your input once again. The LM3916 datasheets have ideas for extended L.E.D meters, however I think they will always be 3db per segment.

The sheets have a nice full wave rectifier which seemed to have PRRs seal of approval. It could be made to be PPM specification (faster than VU) and uses one dual LF353 or a TL072 chip.

I think you just feed it to the LM39xx series, which can be cascaded. I'll have to check again but its something I want to play around with. I'm not too sure if its possible to set the segment divisions to 1dB?

What would be the main cause for the meter not tracking the ratio control properly?

Kris, cool you're offering your servo circuit. Is your buffer performing well. I remember reading your SSLServo document and you were slightly worried about possible noise probelms with a high value resistor??? I'm eagerly awaiting your modded SSL.......

Its all sounds great so far
Cheers Tom

 

[DrFrankencopter]

Hi Tom,

I've since lowered the resistances (this helps give the integrator a little more authority). I haven't built it yet, but it's working well in simulation. I have written a few more pages in my document. I'll publish it this weekend, plus a little blurb on Jung regulators.

Cheers,

Kris

 

[tommytones]

So, back to business here. What are we using for dif input circuit? Is there any fav?

 

[soundguy]

might be nice to incorporate a circuit using the same opamp as the output, if possible. Will make the build more expensive, but would be cool to keep it all the same idea in there...

dave

 

[tommytones]

Sure, we can do that. Makes laying it out a bit more interesting for me but can be done. Do we have a circuit to use?

 

[hitchhiker]

My bread board model had an opa604 in the makeup because it's a fet input for high input impedence as Fred recomended.I don't like the change in sound that 604's make so I tried op275 and it gives a loss of mids and fullness,and a slight boost of highs. So I pulled the supply V to the chip down to +-19.5v with 750 ohm resisistors and put in a 5534. I hear much less of a tone change and I would like to see these work here. The gain control is too hot so it's at it's lowest setting and that's about right.
Is there a way to buffer the side chain enough so that the impedence interaction is minimal?
If so we could use 5534 or 5532 input as well.

cheers,
Lance

 

[hitchhiker]

BTW I also pulled the sup V down for the op275.