Compressor Knee?

[Ian MacGregor]

Hi all,
A friend who is farily experienced engineer in LA has been trying out my SSL clone for the past few days. One of the comments he had was that it seemed more aggressive and had a sharper knee than the original.

I'm curious as to what (electrically/functionally) gives a compressor its knee??

I'm wondering if not using the original DBX VCA could be the cause. Any ideas??

Ian

 

[JustinS]

Hi Ian,

Have a search through the SSL pages... I think it was SSLTech who alerted everyone to the fact that the newer THAT 2180 / 2181's may require an additional resistor in series with the ratio switch to get the VCA to spit out the correct ratios... On some units 10:1 hits infity:1 2:1 closer to 4:1, so what your friend may be hearing are much more aggressive ratios rather than the knee being substantually different...

Justin

 

[soundguy]

Im with ian's friend, definitely steeper knee, I hear it at 2:1. I was going to post about this but was saving it until I had time to actually work on it. If there's a way to adjust the knee (which Im betting there is) Id love to know about it for the future.

dave

 

[Steve Jones]

Wow, I noticed this also, as did a friend who borrowed mine last week, that compared to the original Al Smart box that we had the GSSL sitting on top of, the GSSL in his words "got up and boogied" a bit faster than the Smart box. Maybe this resistor placing is why? Apart from that, we could find no sonic differences between the two.

This would be a different issue though to a difference betweem DBX and "That's" VCA's. Was it Keef or Greg that had the post about calibrating the ratio's?

 

[Ian MacGregor]

[quote author="buttachunk"]
he has a 10k resistor bridging the ratio lead 'A' with the 1m resistor below. on the original schematic, that is a direct connection,, and the 10k goes between that junction and the 20k/diode junction.

this biases the ratio 'A' lead quite differently to both paths.[/quote]

Wow... yes. Thanks for pointing this out. I don't have an open unit in front of me to tweak tho. It's damn hard to get these things back if you let anyone borrow them, which says a lot about Jakob's design!

Is Jakob around? Does he have any comments??

Ian

 

[hitchhiker]

I just lifted one unit out of my console and on the PCB ver7, the ratio switch "a" terminal trace runs straight to pin 13 of the tl074. That would be correct to the original schematic ...right?

 

[Ian MacGregor]

[quote author="hitchhiker"]I just lifted one unit out of my console and on the PCB ver7, the ratio switch "a" terminal trace runs straight to pin 13 of the tl074. That would be correct to the original schematic ...right?[/quote]

It seems so. If you look at the Gyraf SSL schematic, the pole of the ratio switch goes to TL074 pin 13 via a 10k resistor. On the original that buttaCHUNK posted, it goes directly to the inverting input of a 741.

I'm really wondering what'll happen if we switch it around on our compressors. Is it going to make the compressor knee a little more like the real deal? Or is it gonna change all sorts of things?

Ian

 

[Steve Jones]

I'd be inclined to leave it Jakob's way, it sounds great, but I will experiment anyway and see what happens. Gotta love a good DIY mystery...

 

[ChrisA]

there appears to be a lot of differences between the two schematics right around there...more than just that 10k resistor...am I wrong?