Changing resistor values for DBX 202C

[rob_gould]

I am attempting to build the Mixbuzz 500 compressor from Igor, but using DBX 202C gold can VCAs instead of the VCA boards that were supplied with the kit.

The compressor passes signal with compression engaged, but seems loose about 7dB of gain.

I’ve been getting some assistance in the project-specific thread, but it’s dead in there now so I was hoping that perhaps I could get some advice from here about which resistors to exchange to set the gain values correctly.

The recommendation was for these values.  You can see the component numbers on the attached schematic.

DBX 202C GOLD CANS
RXX APPLY TO BOTH L AND R (R100L, R100R)
R6 40K2
R2 1MEG...1.5 MEG
R100 710R
R4 JUMPER/FERR BEAD
R3 40K2
C1 5PF

And looking here :

http://www.gyraf.dk/schematics/Dbx202.pdf

And here :

http://www.thatcorp.com/datashts/dn127.pdf

Some of the values advised on the datasheets are different to what I copied and pasted above.  So I have a few questions…

Why is R100 710Ω instead of 650Ω in the  on the datasheet?

Why is R3 specified as 40k2 instead of 50k on the datasheet?

Should either of these ^^^affect the gain through the VCA?

Should the values of R6 and R3 always be the same?

In short, can I swap resistor values (I’m guessing R6?) in order to try to correct the drop in level that I’m getting through the Gold Can VCA?

Thanks!

 

[RuudNL]

As far as I know, with R3 = R6 and a control voltage of 0 V. there should be unity gain. (0 dB)
Anyway between CNB2 and CNB3.

 

[rob_gould]

OK, useful to know thank you.

Harpo recommended that I remove the DBX 202C from one channel and put a wire between I in and I out.  when I do this, I do get unity gain on that channel, which he indicated would mean that the surrounding circuitry is OK.

 

[mitsos]

Find the SB4000 schematic. It has a table showing the component changes needed for each VCA. I've used that chart with dbx black cans, gold cans, as well as new THAT vcas, and can confirm they work.

The jumper test shows that your input and output stages are working so you need to check your sidechain and vca. 

Check the control voltage with the threshold fully clockwise. Check with Harpo but I think it should be zero. 

 

[rob_gould]

Hi Mitsos,

SB4000 schematic attached...

Thanks - yes I'd examined the SB4000 schematic before and noticed that the values looked different to the Mixbuzz project.  Also, some of the schematic is a little different which put me off comparing them as like for like.

I made an analysis of both, and have worked out what I believe are the equivalent components between the two schematics :

Mixbuzz w/ DBX 202C	SB4000 w/ DBX 202C
R6 40K2	VR50K or 27K fixed
R2 1MEG...1.5 MEG	R76  1M
R100 710R	R82 / R77  1K
R4 JUMPER/FERR BEAD	Not present on SB4000
R3 40K2	R49 15K
C1 5PF	C69 100pF

Would you agree with this analysis? 

You mentioned that you've used these component values with success before - I was not expecting that a fixed resistor of 27K could be put in place of VR2, because R49 is only 15k.  I would have expected these two values to be the same.  Did 27K and 15K work together for you?

I'll check the control voltage with threshold fully clockwise tonight...

Thanks,

Rob

 

[rob_gould]

Ok, so measuring at EC on the DBX 202 referenced to GND on the DBX 202,  I get the following

+15dB    -0.003V

0dB    0.381V

-15dB    0.971V

This is measured with Ratio at 4:1

So the compressor is (near as) unity gain (EC = 0.003V) when the threshold is at +15dB.  Is this correct?  My assumption was that unity gain should be at 0dB threshold...

Thanks,

Rob

 

[RuudNL]

Strange... There should be a linear relation between the control voltage change and the gain (dB) change.
So the (control voltage) difference between +15 dB and 0 dB, and 0 dB and -15 dB should be the same IMHO.

 

[rob_gould]

Hmm, another strange thing then.

Well, I am quite certain now that there is no way I can get my unit to be unity gain at threshold 0, and this seems to tie up with some calibration instructions I found in the Mixbuzz 1 thread which should also apply to the 500 model.  There the instruction is to check unity gain with threshold at +15.

I can get unity gain just before I hit +15 on the threshold, so perhaps I need to try to carry on with the calibration and see whether or not the compressor seems to be doing the right things in each test.

 

[mitsos]

I wasn't sure what you meant by 0dB at first, I forgot the scale usually goes from -15 to +15.  Did you have the threshold set midway?  For calibrating unity gain, the compressor can not be compressing,  you do this by setting the threshold at the highest setting, the other controls should have no effect.

So, to be sure, the threshold knob should be set fully clockwise for unity gain testing. That seems to be the case in your last post, so it's working fine.  Go on to the next calibration step and see how it goes. 
good luck

 

[voitto]

Hey Rob, did you reach any conclusion on this? I'm almost finished building a Mixbuzz one with 202c's and Igors suggested values, did you end up changing them in the end?

Cheers!
Robin

 

[rob_gould]

Hi Robin,

The first part of calibration seemed to go OK with Igor's suggested resistor values. I haven't had a chance to complete the calibration (checking ratios etc) yet, but I'll report back when I do.

But yeah, the reason for my strange results initially were that I had the threshold control centred, not fully CW.  That was just down to my misinterpretation of Igor's instructions...