Potato Cakes

Re: GSSL add-on help thread
« Reply #100 on: October 01, 2019, 02:20:54 PM »
Ok so l got to the bottom of my problem, and i had bugger all to do with the operation of the Turbo PCB....It was me...or to be more specific "The stupid balanced signal I have been feeding my GSSL from my mobile phone all this time" FACE PALM!!

Since my soldering/projects desk is some distance from my computer...and Cubase, I was playing audio from my mobile via a small Stereo jack going to two large jacks...just for convenience. so I decide to check my meter calibration whilst I was fault finding the turbo issue, so I hooked everything to Cubase, suddenly my feeble meter readings became hefty meter readings! another FACE PALM! hooked up the Turbo again and the needle is wiggling away nicely.

Now just to button up this sexy beast and put it to work.

I was going to remove this rather long winded post but it does end with a rather poignant "What not to do" so I will leave it for other GSSL builders.

And thanks again to Harpo for his help.

You had a question on GSSL page about the LCR compression characteristics of the the Turbo board, so not to pollute the stock build only thread with more mod questions, I'm replying here. If you viewed the video on Expat's website regarding this circuit, you get a full explanation of what it does and why. In the original SSL buss compressor circuit, the panning of the material source in the stereo field does not affect gain reduction. If I recall your post on the other thread, you mentioned you saw someone had posted a video of the turbo and the gain reduction changed with panning the material, and if that is the case this is not correct. If your GSSL with the turbo mod has the same gain reduction when applying tone and panning it between the left and right inputs, then you have a working mod. Anymore I when I build anymore of these compressors I permanently wire the Turbo circuit in as this gives the GSSL the compression characteristic why I would use an original buss compressor in the first place.

Thanks!

Paul


Re: GSSL add-on help thread
« Reply #101 on: October 01, 2019, 03:10:47 PM »
Hi Paul.

Yes that made more sense to me as the turbo was designed to give an equal side compression as to match the centre, it was a YouTuber known as "really nice audio" perhaps go take a look? My query related more to the fact that with the external sidechain engaged I got a imbalance when panning, I have since decided not to worry about it as the difference is small...it's just annoying.

Thanks for replying.

Potato Cakes

Re: GSSL add-on help thread
« Reply #102 on: October 01, 2019, 03:41:19 PM »
Historically, doing Turbo and SSC boards (which requires two of them as you know) has been a little tricky to get a perfect balance. If there is an unbalance with just the external in on the SSCs, then either the signal getting to both boards is not the same or the signal getting to the ICs on each board has some sort of variation if indeed there is balanced GR on all of other settings.

On a side note, I tend to agree with the Expat guys that an HPF for the sidechain in the GSSL is not necessary with the Turbo function. A parallel mix function gets much more use (at least for myself), which seems most any DAW incorporates in their external insert functions. Possibly I could see using an SSC for the Thrust characteristics, but I have a MixBuzz that I use for FOH mixing and I have yet to want to use that function, so I'm not sure if I'll ever actually want to have it for any future GSSL builds. An external input for the sidechain could be very helpful in the studio, but I would want to explore a simpler method to do this rather than the extra work (and expense) of wiring up two SSC boards.

Glad you've sorted everything out and enjoy your now working GSSL.

Thanks!

Paul

Re: GSSL add-on help thread
« Reply #103 on: October 01, 2019, 05:24:19 PM »
Historically, doing Turbo and SSC boards (which requires two of them as you know) has been a little tricky to get a perfect balance. If there is an unbalance with just the external in on the SSCs, then either the signal getting to both boards is not the same or the signal getting to the ICs on each board has some sort of variation if indeed there is balanced GR on all of other settings.

On a side note, I tend to agree with the Expat guys that an HPF for the sidechain in the GSSL is not necessary with the Turbo function. A parallel mix function gets much more use (at least for myself), which seems most any DAW incorporates in their external insert functions. Possibly I could see using an SSC for the Thrust characteristics, but I have a MixBuzz that I use for FOH mixing and I have yet to want to use that function, so I'm not sure if I'll ever actually want to have it for any future GSSL builds. An external input for the sidechain could be very helpful in the studio, but I would want to explore a simpler method to do this rather than the extra work (and expense) of wiring up two SSC boards.

Glad you've sorted everything out and enjoy your now working GSSL.

Thanks!

Paul

Yeah I did actually build the two SSC's in readiness but have decided its probably not worth the hassle, or potentially giving myself more problems to sort out.

I had even moded a new front plate to accommodate the SSC frequency rotary but that rotary is now going to become a glorified Turbo on/off switch....yes I know why not have the turbo fixed? well ive got to fill the hole with something :-)

Oh on the subject of external sidechain,  can you tell me how to wire a ts jack to this filter?, wouldn't mind having the option to feed a signal into it.

Cheers.

Script

Re: GSSL add-on help thread
« Reply #104 on: December 08, 2019, 05:52:52 AM »
-- Problem moved here to keep Forum clean --

Turbo board problem:
different control voltages between L and R sidechains
Turbo board alone yields compression, but only when cranking up the input (1/8 turn up on mixer output knob. I know that not a very precise statement.)

Removed all bells and whistles from the circuit for now. Thus, a basic and 'reasonably' working gssl plus Turbo board flywired at 47K SC IN junction for now.
- Only modification still in place right now is the '27K unity gain' trimmers feeding the audio path VCAs instead of the 15K (as specified the rev #7 board by pcbgrinder).
- As well as a '100K resistor disconnect switch' (labelled 'Feedforward' on the front panel) before pin 2 of the 1/2 TL072. This resistor is in place for all measurements.
- And two hardbypass relays (powered from 15V rail) wired so that the unit always compresses but with the INs switched directly to OUT in default/OFF.

SC VCA (2181B) test:
3K9 resistors installed to set the SC VCA voltage (as the SC 2181Bs are both powered from 12V). Pin 4 disconnected, 10K between pins 3 and 5 disconnected now.


RATIO MEASUREMENT
(across R6 [1meg] on Turbo board and equivalent on main board). Switching ratio on front panel yields this:
      main / turbo
1:10  582 / 618
1:4   270 / 296
1:2   191 / 155
This tells me that the RATIO switch is connected correctly. However, ratios are off. But could that account for such a difference in compression as I hear and see on the meter?

SC MEASUREMENTS (all DC in mV)
SC 2181B (all ICs installed, steady mono IN signal from a synth on L & R)
(gssl board / turbo board)
pin 1: -3.7 / -2.3       IN
pin 3: -9.8 /-9.8                   (CV to SC connection)     
pin 5: -2.9V / -2.9V  Ec-  (both 3K9s definitely installed, voltage almost ideal as per datasheet at -2.85)
pin 6: -2.7 / -2.7      Gnd
pin 7: 12.10V / 12.10V
pin 8: -2.5 / -3.3      OUT
Looking kind of good. Although the INs and OUTs are a bit off.

TL074 (all ICs installed, steady mono IN signal on L & R)
gssl board / turbo board
pin 1: -267.1 /  14.3 !!!
pin 2: -3.7   /  -1.8
pin 6: -3.5   /  -3.3
pin 7: -3.8   /  -6.2
pin 8: -311   /  +59   !!! @ 2:1
                -37   /  -76   !!! @ 4:1
                 -1.9 /  -90   !!! @10:1
pin14: -256.1 / +159   !!! @ 2:1
Now that is seriously off.

Will have to go through the entire troubleshooting as explained by Keith, including AC measurements.

Will do,  but am I totally off in suspecting the rectifier diodes around pins 1 and 2 of the TL074?
« Last Edit: December 08, 2019, 06:19:18 AM by Script »

Script

Re: GSSL add-on help thread
« Reply #105 on: December 09, 2019, 09:27:50 AM »
I put in new diodes around the TL074, but it didn't fix it. I have checked all solder joints  and connections , the placement and orientation of components, as well as for broken traces. No problems there as far as I can see.

I did troubleshooting using Keith's run-through
https://groupdiy.com/index.php?topic=28113.msg339348#msg339348

With no input signal, AC at pins 1 and 7 are fine.
However, these look suspicious.
Pin 7 DC -1.9mV / -2.3mV
Pin 1 DC -99mV /  -139mV
Is this the problem already?

Pin 7 with mono input signal and varying the threshold control gives this
AC 0.021V ~ 0.074V / 0.048V ~ 0.074V

And at pin 1 with input signal it all goes bonkers (threshold CCW ~ CW) )
DC -243mV ~ -347mV / +2mV ~ +61mV
AC  here reads 0.054V ~ 0.07V  / 0.2V ~ 0.4V


Before I headlessly replace more components, maybe someone can point me in the right direction where to look and what to look for, please.