GSSL HELP THREAD!!!

[beatnik]

with no signal on pin 4 and 11 of the TL074 I have respectively +12 and -12 Volts and 0.36V on pin 14

 

[MeToo2]

on the return of the compressor I have 5dB difference between compression in or bypass.

and the meter shows gain reduction, so I think compression is happening and it's not just volume decrease

i tried to disconnect the super sidechain board and put the two 47K resiistors back on the main board, but what I got is there is even more gain reduction, about 10dB

How do you know it's GR and threshold related, and not just a general gain/loss problem, or no compression, or your SSC filters?

Have you tried sending tones of -88 -78 -68 -58 -48 -38 -28 & -18 -8 and drawing a graph of output level versus input level ?
Have you tried at a few different frequencies?

Compression will curve the top of the graph. You should be able to clearly see the "knee" where the threshold is kicking in and compression starts.

You should also then be able to measure the slope of the GR (far to the right of the knee) to check the ratios.

If it's pure a gain problem, the graph will be shifted up or down vertically to what you expect.
If there's no compression happening, the line will be straight.
If it's purely threshold level related, the knee should show up as being shifted too far off to the left on the graph.
If it's ratio related, the slope will be incorrect.
If it's frequency/filter related, the slopes will be different per frequency.

 

[beatnik]

I'm sure because if I use the gssl on some audio tracks, I hear compression.

and the ratio, attack and release controls and sidechain filter work as they should.

Anyway, I do right now the procedure you mention and see if there is a problem

 

[beatnik]

with 1kHz sine wave until -32dBFS i have no difference between input and output level, then compression kicks in

-38 --> -38

-28 --> -33

-18 --> -32

-8 -->  -25

with 100Hz

-38 --> -38

-28 --> -33

-18 --> -25

-8 --> -18 

and with 10KHz

-38 --> -38

-28 --> -33

-18 --> -25

-8 --> -23

Seems the compressor is working but the threshold spot is very low. What you suggest me to check? Thanks guys

 

[beatnik]

ok i did the check with all the three ratios and the values are so wrong! there is obviously a problem there.

keep in mind that I've taken out the sidechain filter circuit for now

 

[MeToo2]

ok i did the check with all the three ratios and the values are so wrong! there is obviously a problem there.

keep in mind that I've taken out the sidechain filter circuit for now

Your results do seem to suggest a very low threshold. When I had a problem like that it was simply due to a faulty threshold pot going intermittent.
Replacing it solved the problem.

Check the value of the 220K resistor pin 2 TL072.
Check the value of your threshold pot is 50K LIN.
Check that the voltage here at the 220K = "point F" varies linearly between -12V to +12V as you turn the pot from full cw to full ccw.
[The meaning of a pot marked "A" varies in meaning depending on manufacturer, region, and age, and can sometimes mean log ]

Please also bear in mind it is normal for the knee to move when you select different compression ratios.
Measure the ratios by looking at the slope of the input/output graph far to the right of the knee.

 

[beatnik]

voltage on F point varies from -12 to +4 Volts so there's a problem here. I try to replace the pot and see.

 

[beatnik]

replacing the pot had no results.

if i wire just the pot i have +/- 12V varying on point F of the board

if i put 47k in series with one leg goes from +12V to -0.69V

then i noticed a strange thing. different behavior between different types of TL074

with the TL074IN I had in there, we know whats happening.

but I tried to replace it with a TL074CN and... output goes down about 40 dB, like threshold goes even lower

what's that?!?

 

[Harpo]

voltage on F point varies from -12 to +4 Volts so there's a problem here. I try to replace the pot and see.

There seems to be too much load connected to your +12V rail, causing the 78L12 going in self protection mode. Did you connect any additional parts (status LED's, incand.bulbs, relais, iluminated switches, meter ilumination, ..., cigarret lighters) to this rail ?

then i noticed a strange thing. different behavior between different types of TL074
with the TL074IN I had in there, we know whats happening.
but I tried to replace it with a TL074CN and... no output!!

Only parts difference from datasheet is temperature range and input offset voltage. Both parameters don't matter here (except you want to operate it at arctic venues). More likeley your TL074CN was broken to begin with or maybe IC pins bent inwards, so didn't make connection to IC socket.

... output goes down about 40 dB, like threshold goes even lower

check for broken trace and solder connections around/at TL074 socket, parts and ribbon cable connector. One opamp stage might be running open loop, causing exessive control voltage for your audio VCAs.

 

[beatnik]

Harpo you were right!
I redid all the soldering joints on the main board around the TL074 and now the problem is gone. Maybe I had a bad soldering or  small bridge somewhere! Now the threshold is in the correct spot!

As you pointed I have a relay and an illuminated switch to do the bypass, and a combo of vu meter and bargraph led to do the metering. They have their separate 12V regulated taken directly from the power transformer secondary.

However, there is a latch problem on the +12 regulator, I've ordered a replacement from a different brand, hope that will solve the problem...

One last question. Right now I have the vu meter to show output level of the L channel.
How should I do to display L+R?

 

[Harpo]

.. and a combo of vu meter and bargraph led to do the metering. They have their separate 12V regulated taken directly from the power transformer secondary.

Transformer is AC, the usual suspects want DC, but my cristal ball is a little blurry what your circuit behind might be.

However, there is a latch problem on the +12 regulator, I've ordered a replacement from a different brand, hope that will solve the problem...

hopefully. Already tried my latchup fix for the +/-12V ?

One last question. Right now I have the vu meter to show output level of the L channel.
How should I do to display L+R?

I need a better cristal ball ...

 

[beatnik]

There is a led meter driver board which has a 7812 regulator on it, then I bring 12V from there to vu meter, relay, illuminated switch.

78L12 on the main GSSL board is only for the sidechain circuit.

I'd really like to know your option and try that before changing regulators. Do you have a schematic maybe?

 

[rainton]

Hey everyone,

I have a problem with my GSSL. It also has a turbo board but I don't know if this matters in this case.
I haven't built it myself (I bought it from a friend before I started to DIY myself)
So here's the thing: As soon as I patch the GSSL in the chain the sound drops a little (level wise) and gets duller and less defined - kind of a high end roll-off
This also happens when the rotary switch on the front panel is switched to "bypass". (the bypass is actually working)

Actually the whole comp is working - it's compressing and all - it just doesn't sound good (anymore)

I just moved the entire studio and I had not experienced this problem before.

It would be great to get some initial help as I'm just working my way into the topic

I also took a pic of the main pcb
Thank you!!

 

[Harpo]

There is a led meter driver board which has a 7812 regulator on it, then I bring 12V from there to vu meter, relay, illuminated switch.

there need to be more on this whatever it is board or your 7812 won't surwive the every next halfcycle reverse polarity from transformers AC voltage.

I'd really like to know your option and try that before changing regulators. Do you have a schematic maybe?

http://www.groupdiy.com/index.php?topic=47.msg626822#msg626822
for the +/-12V fix you only need the two righthand side diodes.

 

[beatnik]

So here's the thing: As soon as I patch the GSSL in the chain the sound drops a little (level wise) and gets duller and less defined - kind of a high end roll-off
This also happens when the rotary switch on the front panel is switched to "bypass". (the bypass is actually working)

Actually the whole comp is working - it's compressing and all - it just doesn't sound good (anymore)

Maybe are you sending unbalanced signal to the GSSL?

 

[beatnik]

In the attachements I've posted the power supply portion of that board. I'd really appreciate your opinion.
The 7812 regulator takes the +/- V from the main GSSL board, post rectification. It's kinda the same as the main GSSL regulators are connected, so I though it was ok.

Thanks for your anti latchup fix, I'll try that today.

I also have another strange thing, every time I power on/off the compressor there is a spike in the audio path.
Maybe it's related to the regulators latching?

 

[Harpo]

In the attachements I've posted the power supply portion of that board. I'd really appreciate your opinion.
The 7812 regulator takes the +/- V from the main GSSL board, post rectification. It's kinda the same as the main GSSL regulators are connected, so I though it was ok.

No schematic attached, but should work. Your wording was 'taken directly from the power transformer secondary' and that would likley let the smoke out.

Thanks for your anti latchup fix, I'll try that today.

my pleasure. Did the trick for some members, but might not work for all brands of Vregs.

I also have another strange thing, every time I power on/off the compressor there is a spike in the audio path.
Maybe it's related to the regulators latching?

No. Just don't power on/off while listening or mute/turn down your monitors. ITB plugins don't do this, because when activating there isn't any real world cap to charge or rail voltages to be stabilized and when deactivating there isn't any cap to drain from not equal current demand of the different +/- rail voltages. Active hardware unfortunately suffers from these shortcommings. A workaround for this would be a hard bypass circuit by means of some relais that could only be activated for normal/usual operation until a timer circuit after some seconds enables the relays control voltage. Switching the unit 'off' would immediatly cut this control voltage, setting the relais back to their hard bypass mode again.

 

[beatnik]

Thank you again Harpo

 

[rainton]

Maybe are you sending unbalanced signal to the GSSL?

Well, my GSSL sits indeed on the unbalanced inserts of my console's master bus. I didn't think that would be a problem - but I just realized all the other gear I occasionally use on the master bus is transformer balanced...

...so what are my options here. I googled it and found another GroupDIY  thread about this topic and the conclusion was to either solder special-cables where only (+) and (0) is connected or to disconnect (-) from the XLR-pins in the GSSL (?)
But then I would still experience the -6db drop, right? Is there any way to avoid this?
Help would be highly appreciated!

A second question:
I also went through some of the 280pages of this thread last night because I also experience the "latch-up" problem when switching on the GSSL. And since the LED of the meter stays off when the comp doesn't boot correctly the problem has to be with the +/-12V regulators. I checked Harpo's anti-latch-up solution:
So I get where the diodes need to go - but harpo wrote:

The 100nF caps in front of the 7815/7915 are already shown in the schematic, but are in front of the 78L12/79L12 on pcb instead.

Where exactly do I need to put the 100nF caps in this case?

Again thank you so much for your help!

 

[gyraf]

The position of the 100nf's won't change the latch-up.

latch-up occurs in this kind of differential supplies when one side's voltage rises much quicker than the other at power-up, which (through the secondary stabilizing capacitors) pulls the other side's regulator too far to the wrong side of the supply, which in turn sets it into protection mode.

Positive regulators goes into protection if they see more than ca. -0.6V at their output
Negative regulators goes into protection if they see more than ca. +0.6V at their output

But sensibility to latch-up is quite different from brand to brand (and possibly from batch to batch) - that's why we recommend trying a different one.

If larger-than-recommended supply decoupling capacitors are used in the GSSL, latch-up will be even more likely.

Another solution could be to replace the 10 Ohm voltage-dropping resistors before the 78L12/79L12 with some higher value, which will slow things down by limiting power-on inrush current.

Jakob E.