Installing a Turbo board in a GSSL with deleted SC resistors

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Emperor-TK

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Joined
Jul 14, 2004
Messages
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Location
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This is a warning to everyone who wants to install a turbo board into a GSSL with deleted resistors in the side chain. About 2.5 years ago when I built my GSSLs, there were discussions on weather or not to delete the 47R between pin 4 and ground, and the 10K between pins 3 and 5 of the SC VCA. Based on the THAT data sheets, it seemed like these resistors might not belong when using the newer 2180/2181 VCA's. At least a few of us proceeded without stuffing these resistors, and my GSSLs seemed to work fine....until I went to add a turbo board today.

My builds use both 2180LB and dbx C1252H2 VCAs without the SC resistors mentioned above. For the sake of symmetry, I also deleted these resistors on the turbo board. I fed the GSSL a 1K sine wave through a mixer and panned L to R. Relative to the center, the left channel rose steadily a few dB when panned. The right channel however, dipped down a dB or two. If I wiggled the pot randomly from center to right, I could get the gain reduction to drop another dB or two from hard right.

I stuffed the missing SC resistors into the Turbo board and tried again. This time, the right channel was good, and the left channel was off. Although this time the gain reduction increased slightly, it still had that same random behavior when I wiggled it (from center to left this time). It also never rose as high as the right channel.

So, like the story of the three bears, the third time is the charm. I stuffed the missing SC resistors onto the main board along with the Turbo board. Panning both left and right now lead to smooth monotonic increases in gain reduction with no random behavior.

The moral of the story: Put the missing side chain resistors back on the main board if you want to go turbo.

-Chris
 
So you're not talking about the reistors in the left/right (10K/1M/68R and 3K9 which was changed to 5K1) vca but 'sidechain' on main board?
 
Yes, the sidechain VCA resistors, not the resistors on the two signal path VCAs. I'm not sure how many people actually wound up deleting these, as it may have just been a passing "fad".

-Chris
 
No problem. I jump at any chance where I can be useful. Maybe someone can comment on what function those resistors are actually serving? I guess that the 10K resistor is the one that is actually making the big difference. It seems like it is putting a DC bias on the CV input, right? How would loosing that DC offset from both SC VCAs lead to an unbalance in the L-R response?

Just curious.

-Chris
 
[quote author="Emperor-TK"]Yes, the sidechain VCA resistors, not the resistors on the two signal path VCAs. I'm not sure how many people actually wound up deleting these, as it may have just been a passing "fad".

-Chris[/quote]

Thanks, I didn't delete those in the 1st place :thumb:
 
Looking back over the threads, it looks like many folks deleted the 10K resistors between pins 3 and 5 on the audio path VCA's (including myself). Why would it be desirable to delete them on these VCA's, but not on the SC VCA? Does anyone know what function they serve? Should I be putting them back on my audio VCA's too?

-Chris
 
interesting, I need to check this with my turbo GSSL's and update the infos on my webpage. Unfortunately I'm out of town for some time but I'll see that I get to it. I mainly followed the THAT application notes...

thanks, Marten
 
Marten, I noticed that you too recommended deleting the 10K resistor from the SC VCA. Surely there must be more than a few of us that have done this (however, I noticed that the latest revision of Jakob's schematic/layout still does not call for the deletion of these). Has anyone else who deleted this resistor had similar weirdness with a turbo board? If you have this problem, you might not notice it unless you actually do the pan test or listen critically. I first tested it with a 2 buss on a mix I wasn't familiar with. While in hindsight it seemed the vocals and snare were leaning a little left, the result sounded pretty decent over all.

I'm still trying to figure out weather I need to put the 10K's back into the audio path VCA's.
 
Is there a diagram showing the connections for the turbo board? Also, if the THAT 2180 LB vca's are used your saying to put the omitted 47R resistor back into the sidechain? Im finishing up my third comp and I have a turbo kit coming from Ptownkid. Is the turbo kit also omitting the 47R resistor?
Thanks
 

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