4xTHAT2181 VCA board for GSSL etc builds, DBX 202X replacement

[sircletus]

Funny, Igor, I had just finished making my own!  PCB layout is ready, but I suppose I'll hold off on ordering my own while I check out yours. Cheaper that way!  Mine is literally a DBX 2002 from the THAT site with eight 2181s. I bought a bunch of 2114s (instead of 5534) before they got hard to find.  For those wondering about the 5534: as Igor said, it is used to buffer a DC control voltage, so the most important factor is LOW NOISE.  Also, for those unfamiliar: if you don't have a distortion meter, just use THAT2181As, as they're pre-trimmed with adequate distortion performance.  This allows you to skip distortion trimmer pot.

 

[ptron]

i thought pretrimmed were only 2180X`s

 

[ptron]

Also i wouldve thought on the gssl the distortion trimmer is already there on the main pcb or im mistaken?  I just want to understand what i must do before i start dismantling my unit

Thanks

 

[sircletus]

Sorry, pton, I mistyped.  You are correct: The 2180s are pre-trimmed.

 

[ptron]

hey guys sorry to keep asking same questions over again but im really itching to install these boards.... im not an electric engineer and not know in reality anything about the physic that makes this compressor work D: let alone the vca. (even if i would guess that i know what Rmagic pot does i couldnt really knwo for sure untill some of u post about it here ) ..
hehehe sorry

anyways im still stumped by the lack of this R-magic in the boards and is the one and i think only question i have, if i can work out somehow what parts from the circuit should be removed in the original GSSL emulation circuit then i can start making my frankenstein ssl beautiful from the insides .

thanks a lot!!!

 

[ptron]

eheheh another bump sorry

 

[ptron]

please help out guys im still clueless on this subject any help will be very apreciated... i think from all this maybe u just needed 1 trimmer if u using 2180 but then again this trimmer (gssl board) goes to the leg 4 of the 2181 so would be the inactive thd trimmer ... or symmetry and what i need is Rmagic no? (still not sure what that does)
please any help would really help

thanks !

 

[Harpo]

please help out guys im still clueless on this subject any help will be very apreciated... i think from all this maybe u just needed 1 trimmer if u using 2180 but then again this trimmer (gssl board) goes to the leg 4 of the 2181 so would be the inactive thd trimmer ... or symmetry and what i need is Rmagic no? (still not sure what that does)
please any help would really help

With pretrimmed THAT2180s you don't need a trimmer, though you could have the trimmer installed to override the chip-internal pretrim value for maybe intentional displaced symetry. This is the external trimmer for symetry adjust on FI the GSSL board. You will need a THD meter for adjusting to min.displacement/THD with audio signal present and VCA forced into bypass condition (Ec=0.000V). For the GSSL this bypass condition can be set with the bypass switch. Other compressors, coming with a hardwire bypass, might need a second look to get the VCA into this state.
RMagic is a rheostat on a THAT2002T VCA to adjust for least TDH for the paralelled VCAs with signal present and Ec set for max.control voltage (the makeup gain might come handy for increasing Ec). As this is interacting with the symetry trim, you'll have to repeat this trimming procedure. Igors pcb, coming without this rheostat, is not a THAT2002T clone.

 

[ptron]

Man as usual many many thanks for your explanation... Now i understand a bit better. So without this rheostat its not really what i need this board? .
Im sorry just what does "Ec" means?

Thanks a lot harpo

 

[Harpo]

Im sorry just what does "Ec" means?

Ec=Control voltage (better read as milli voltage), in this case driving the VCAs Ec- port with Ec+ held at 0V reference voltage.
If for whatever reason you want the RMagic trimmer installed and you have the required measuring equipment to get useful results, just wake up your DIY brain cells. A pcb has two sides ...

 

[ptron]

hehe thanks harpo!   i better leave my compressor with single vca ?

im not sure if i get it 100% ... both trimmers work for thd? do i need both ?? ..

if i wanted to put the trimmer for R magic...  i just go from leg 3 to 4 of any vca with 10k in series with the trimm ya?  


thanks a lot!!

 

[Harpo]

hehe thanks harpo!   i better leave my compressor with single vca ?

try it and you'll know if you like it better. Be aware, the 27K Rin for this type of VCA with internal 4x20K in parallel=5K Rin in series needs change as well as the feedback resistor and compensation cap in the VCA following I2V stage to make use of improved noise figures from paralleled VCAs...).

im not sure if i get it 100% ... both trimmers work for thd? do i need both ?? ..

with paralleled THAT2181s at least the symmetry trim, with pretrimmed THAT2180s maybe none at all.

if i wanted to put the trimmer for R magic...  i just go from leg 3 to 4 of any vca with 10k in series with the trimm ya?

ya. I doubt you will notice much difference in thd+noise without some measuring equipment, but as always YMMV.

 

[ptron]

thanks!! that makes it much clearer! i think im going to start populating the boards today see where it takes me hehe

i just got a couple of questions as usual maybe more than a couple  ;D

1- Rin 27k at the moment. u say i got to change for some other value. if Rin is 27k i change to 22k? (im not sure i know what i got to do tbh)

2- the compensation cap im not sure which one that is.. is it22u/35v just before sidechain?

3- also i wouldnt know what value to put instead of compensation cap would that be a standard range of uF for this vca?... 

4. im guessin i got to completely remove all parts from inside the original GSSL vca emulation circuits before putting the new ones in?

as usual thanks a lot harpo

 

[Harpo]

1- Rin 27k at the moment. u say i got to change for some other value. if Rin is 27k i change to 22k? (im not sure i know what i got to do tbh)

In Igor's plan there are already 4* 20K Rin in parallel, each limiting current from the debalancing receiver to below 1mA to not exceed VCAs limits. (In the GSSL with NE5534 as line receiver, max.peak input voltage in front of these resistors is about 13V, as this type of opamp doesn't swing rail2rail. 13Vpk/20K=0.65mA. Other type of opamps or higher rail voltages might exceed parts limits). I'd substitute this 27K with a nada ohm wire link.

2- the compensation cap im not sure which one that is.. is it22u/35v just before sidechain?
3- also i wouldnt know what value to put instead of compensation cap would that be a standard range of uF for this vca?... 

as already posted '.. the feedback resistor and compensation cap in the VCA following I2V stage'
In VCAs bypass condition (Ec=0V for current-in=current-out) you might want unity gain behaviour. With Rin 5K (4* 20K in parallel=5K) the formaly 15K feedback resistor from GSSL schematic would decrease to same value 5K in an unbalanced-out config or half this value =2K5 (=2* 5K in parallel) in a balanced out config as shown in GSSL schematic to account for the +6dB from the last inverter. A THAT218x has a typical output capacitance of 15pF. 4x 15pF in parallel = use at least 60pF for compensation, or the I2V stage might get unstable. This cap across the 5K or 2K5 feedback resistor (use next higher standard value, maybe 100pF, but have an eye on opamps GBW) will form a -6dB slope LPF at -3dB cutoff frequency in Hz of 1/((2*PI()*R*C) with R in ohms and C in farad, giving rolloff 318kHz with 5K/100pF or 289kHz with 2K5/220pF.

4. im guessin i got to completely remove all parts from inside the original GSSL vca emulation circuits before putting the new ones in?

Sure. Will also be a much easier fit without any parts below these modules.

 

[ptron]

wow thanks a lot!! i will read this a few more times and sleep on it and tomorow ill order everything from mouser i think get this started!!
thanks man!

 

[Harpo]

i will read this a few more times and sleep on it

sorry, I know my 'english' sucks .....

 

[ptron]

man your english is great i understand perfect what u mean (i think haha) ... but my technical knowledge and "know how" is the problem here... hard to know i *really know* even tho u explain so well.

thanks tho for the help and lot of patience to my extreme ignorance !!!

 

[ptron]

At last i got a couple of weeks to get this one nailed! So ok.. Now that i read.. I think i understand (i think)...  If my outputs are balanced i put instead of 15kohm / 100pF(after vca in gssl schematic) , 2.5k ohm / 220pF is this right?
Many thanks!

 

[Igor]

AH!!! Yep. Lul'by little.  Can't understand yr french. Draw the schemo, will try to analyze)))
Got my D-class amps on subs working, 20 mins ago. Dats cool, now, have at my home system 800W RMSx2 sub!!!
Listening Das Ich (Erde Ruft) and Alan Parson, Eye In The Sky. F@@@ck IT!!! Class D sounds cool on  subs. At least on my home setup)

 

[ptron]

heheh that system must sound pretty nice indeed. i love how your DB studio looks. amazing rack hehe (is something i would say to a girl but u deserve it my friend)

ok so this is what i mean with my french heheh ... this is what i understand from what harpo explained so patiently hehe .. im so thick i want to know im not missinterpreting anything

in the replacement vca i would have 4x2181A and 1xopa604

2- in the calibration for the GSSL using only 1x2181A i had to go from leg 4 of VCA to 680K in series with 50k trimpot (symmetry)
do i have to change this 680k for 160k (like the "VCA notes Rogy" says)

thanks a lot!