GSSL HELP THREAD!!!

[JAYJAYBEE]

WOW Sintech to the rescue yet again. Thanks a lot bro.

Ive been making this compressor for nearly 2 years now on and off, and im determined now to get it finished, so watch this space for more mindnumbing questions, haha.

THANKS again.

 

[JAYJAYBEE]

Also is this the right switch for the GSSL.

2 x 6 https://secure3.farnell.com/jsp/endecaSearch/partDetail.jsp?sku=1123696

4 x 3 https://secure3.farnell.com/jsp/endecaSearch/partDetail.jsp?sku=1123698

 

[gyraf]

You can't link to Farnell sessions - they time out after a while..

Jakob E.

 

[sintech]

JAYJAYBEE

Yeah they look good, both switches are Pcb pin type and have metric shafts.

 

[tommypiper]

Is there anyone here who understands this circuit?

I'm sorry to be here but it's been kicking my butt for a long time (a board someone else built). This thread has no information about tracking this problem. I can't be the only one! I don't have the experience to troubleshoot further without some hints. Please share your knowledge :grin:

Audio is fine. Compression is distorted. I've checked the whole board multiple times. The problem is in the compression / timing circuit. (see previous pages)

How do I track the timing signal? Where do I look (specifically, not generally). What am I looking for and how do I measure it?

Jakob said it's both DC and AC which confuses me.

 

[keefaz]

The AC carries the audio signal while the DC voltage is needed
for giving energy to the op amp's and VCA's

I had a distorsion problem on one channel, I swapped
the VCA's and I found that I had a bad one.
I replaced it and all went well

 

[bluezplaya]

Does anyone know the correct voltages for the GSSL. Mine is still not working and I guess I should check voltages across the IC's. Anyone have a .jpeg or .gif, or something to that nature?

Adam
:sam:

 

[tommypiper]

Adam, if you scroll back 10 or 20 pages you'll see the voltages as tested by myself and Fucanay at every pin on every IC socket. His voltages are correct.

Keefaz: Yep, I know that much about the AC and DC. :smile:
Jakob was saying measure the timed control circuit and look at it on a scope when it's coupled with AC, or something like that, as it is sent back to the channel VCAs. See my previous posts where I attempted to do that and have scope images which themselves are confusing. What I am looking for, he did not explain. I'm still waiting for some clarification!

I know how to measure voltages and get scope readings, and I'm up to speed on the VCA datasheets and Control Voltages in the circuit... I just didn't understand what he wanted me to do getting a mixed AC and DC reading, and what a correct reading would look like, etc etc...

And yep, I've replaced every chip multiple times, that ain't it, no how.

 

[bluezplaya]

Thanks. I compared voltages. Holy crap I'm off a ways! It seems the voltages around 12v/-12v and 15v/-15v are correct but some of the other voltages are way off. This is going to be a nice little journey tracing voltages starting from the transformer :?

Adam
:sam:

 

[gyraf]

[quote author="tommypiper"]Audio is fine. Compression is distorted. I've checked the whole board multiple times. The problem is in the compression circuit.[/quote]

If you have good audio in bypass, then the complete audio chain - including the two audio VCA's - is fine.

Now when you get distortion when switching the unit in, this must show that the changing-with-signal DC control voltage to your (known working) VCA's must be "dirty", that is, have too much AC components.

To find wether it's the sidechain in general or just the A/R rectifier, disable compression by turning the threshold all way up - and check that the "makeup-gain" does what it should. This control reaches the VCA's the same way as the compression CV, so if it works, we know that this path is fine.

(take a look at the schematic, and read the circuit description again)

If all this is well, we now know that it's in the sidechain rectifier and timing that there is a problem.

The rectifier part simply makes a single-polarity signal from the incoming audio, and this is fed through a diode to a (tantalum) timing capacitor. A resistor in series with the diode decides attack time, and a resistor in parallel with the capacitor (to ground) decides release time. This A/R circuit forms kind of a low-pass filter which should even out any dirt (i.e. high-frequency content) from the sidechain CV voltage.

So you need to check what sort of voltages are present at the A/R capacitor - and to verify that this voltage is also present at the output of the high-Z buffer opamp that immideately follows the A/R circuit.

In any case, check and recheck components, soldering, and inter-board connection - problem could very well be on the control board or in connection.

Jakob E.

 

[tommypiper]

[quote author="gyraf"]
Now when you get distortion when switching the unit in, this must show that the changing-with-signal DC control voltage to your (known working) VCA's must be "dirty", that is, have too much AC components.Jakob E.[/quote]
Yes. That was shown in my scope photos on the page 119. I had lots of AC test signal coming through the rectifier stage.

[quote author="gyraf"]To find wether it's the sidechain in general or just the A/R rectifier, disable compression by turning the threshold all way up - and check that the "makeup-gain" does what it should. This control reaches the VCA's the same way as the compression CV, so if it works, we know that this path is fine.Jakob E.[/quote]
Yes, make-up gain works perfectly. Problem is in the rectifier.

[quote author="gyraf"]If all this is well, we now know that it's in the sidechain rectifier and timing that there is a problem. Jakob E.[/quote]
Yes.

[quote author="gyraf"]So you need to check what sort of voltages are present at the A/R capacitor - and to verify that this voltage is also present at the output of the high-Z buffer opamp that immideately follows the A/R circuit.
Jakob E.[/quote]
This was weird. I fed a -20dB 1k test tone and measured voltages in various places. I believe I eliminated problems in the A/R circuit.

I checked at R47 and C32 (fast release parts) and the voltages were very hard to read, I thnk the cap was discharging into the meter. At one point I measured .85v AC, but it was not steady. I could not get a DC reading with a meter. I think the DC is swamped by the audio leaking in.

Next, I measured the TLO 74 at pin 8 (final output after A/R circuit).
With no compression:
.03v AC
.013v DC
With compression on:
.45v AC
-.6v DC

TLO 74 at pin 14 (post rectifier, pre A/R).
With no compression:
0v AC
.37v DC
With compression on:
.8v AC
-.32v DC

TLO 74 at pin 10 (post rectifier, post A/R).
With no compression:
.06v AC
.01v DC
With compression on:
.35v AC
-.57 DC

Is the DC control voltage supposed to be a negative value?

Clearly something is wrong with the rectifier stage, it seems. I have replaced the TLO 74, no help. I have checked the diodes, traces, etc.

What else can it be?

Many thanks.

 

[gyraf]

I checked at R47 and C32 (fast release parts) and the voltages were very hard to read, I thnk the cap was discharging into the meter.

Try with slowest attack and release - that will be easier to understand
The cap doesn't discharge into the meter, as there is a buffer opamp in between.

Is the DC control voltage supposed to be a negative value?

Yes - look at the diode and the polarisation of the A/R capacitors.

What else can it be?

Wireing, soldering, and shorts on the control board?

Jakob E.

 

[JAYJAYBEE]

Just recieved my parts from farnell this morning, yippee.

Just a quick question, how do you wire up the transformer, ive got this one.

http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?sku=9532102

I think the blue ond brown go to the iec socket, is this right? also where do i connect the orange black yellow and red.

Im gonna finish this if it kills me, and with my lack of electrical knowledge it probably will.

Thanks :thumb:

 

[sintech]

JAYJAYBEE,

Brown and Blue go to the IEC, connect the coloured leads like this:

http://i44.photobucket.com/albums/f23/sin-tech/power.jpg

If your using the corner marked "external transformer" tie Black and yellow to the middle pin, and Red and Orange either side.

 

[gyraf]

sintech,

:razz: one pic surely can say more than thousand words.. :thumb:

Jakob E.

 

[tommypiper]

[quote author="gyraf"]The cap doesn't discharge into the meter, as there is a buffer opamp in between.[/quote]
I was refering to the DMM.
[quote author="gyraf"]Wireing, soldering, and shorts on the control board?[/quote]
I've measured all around the TL074 and I see there is audio leaking through the CV when compression is on. But the diodes and everything on the boards are fine. I can't find where it is coming from.

The control board is fine.

The problem seems to be the rectifier. But it tests OK!

When compression is off, audio on the Left channel stays only on the Left channel. When compresion is on, distorted audio leaks directly from the Left channel into the Right channel - I guess this is going through the A/R circuit and entering the channel VCAs, and the channel VCAs pass it into the audio?

Is there another path that could make this happen?

It's diobolical! :evil:

 

[Luny Tune]

Have you checked the interconnection between the boards like Jakob suggests? It sounds like one of those weird confusing problems that that would create.

You say it's someone else who put the thing together (if I read you right) so maybe cables have been mounted incorrectly.

 

[tommypiper]

[quote author="Luny Tune"]Have you checked the interconnection between the boards like Jakob suggests? It sounds like one of those weird confusing problems that that would create.

You say it's someone else who put the thing together (if I read you right) so maybe cables have been mounted incorrectly.[/quote]

Yes, thanks. Someone else built it, but I've been over everything now 30 or 40 times, checking continuity and fixing as necessary. All the controls from the control board work as expected, but the audio is distorted during compression.

I'm sure it's somewhere in the rectifier section, or some audio is leaking into to the A/R circuit somewhere, or something similar. I don't know enough about chips and op-amps to go further than I have - which is eliminating all other problems down to this one area. But I don't find the source!

 

[enochjensen]

new poster here, and i could sure use some help!

just built a gssl for my studio and fired it up a few days ago and here is what i got:

audio only passes on one side. the side that DOES pass audio is compressing, BUT its behaving really strange. when the initial transient from a signal hits the compressor it reduces the gain so hard it almost goes silent. then it settles down and sounds like its working right, then at random will pump really hard again.
the ratio switch in other settings is also doing the same heavy gain reduction in a slow pumping way. almost as if i were modulating the attack settings by hand as i listen back.

any thoughts???????? yikes! :?

 

[gyraf]

Hi Enoch,

Welcome to GroupDIY!

There's no simple-explanation that I can come up with for your problem - could be a multiple-error.

Start making sure that you have audio passing in both channels, and take it from there. Read and understand the schematic. Trace signal through the pcb, comparing working with non-working channel to find the place of error.


If you read back through this long thread (which I think you should) - you will find that more than 90% of all errors are in three categories:

- Soldering. Mostly solder blobs shorting traces, but also sometimes bad or missing soldering.

- Component value and orientation. Including missing "wire links" on pcb.

- Interconnection wireing

Jakob E.