[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

[DonnieDarko]

I completed the build and started the unit up.

All voltages check out good, they are in a 5% range to that listed by Bernd in the second post if not even closer.

Audio passing fine, meters can be calibrated nicely, no hum oder noise present.

Input Iron is UTC M-8369 wired to 600k/50k from Douglas, Output are the Edcors from Don-Audio.

I did the BIAS calibration method mentioned with ground F- to case, minimizing the hum, but  something still seems wrong with the BIAS.

Some small issues:

• The input pots don´t seem to work correctly, even though I double checked the wiring
• Same goes for the output pot, it doesn´t do much
• The "Single Mode" seems to get me some compression, "Double" and "Triple" are heavly thumping and not usable
• The first four positions of the time constant switch work somehow, the last two get the unit to oscillate and heavy distortion

Any help would be appreciated!

Thank you in advance guys!

Here is the completed unit sitting in good company:

IMG_9190 by Donnie Darko, on Flickr

IMG_9189 by Donnie Darko, on Flickr

 

[bernbrue]

Hi,
I assume, you basically know how this compressor behaves.

Output pot: it's only a variable 6dB pad, so even turned fully counterclockwise the unit passes audio.

Input pot: did you wire the dual pot according to the wiring guide in the first posts? In my unit compression already starts with input pot at 9 o'clock. When turning up the pot, the unit doesn't really get louder, it just compresses more heavily.

modes: there must be some wiring error with the lorlin. In your pictures I can see, that you didn't solder the lorlin directly to the control board, so I would start there.  Check, if the two control boards are connected (3  connections).

So check second post 2), 7), 11)
I'm shure we are going to fix it.
Bernd

 

[DonnieDarko]

Hey Bernd,

Yeah, I did double check the Lorlin and the connection of the two small boards, they appear to be correct.

I used stepped switches for Input and Output, but also wired them accordingly to your drawing that you added recently in the second post.

I guess it´s the bias that is somehow wrong or maybe I need to address the input pad.

 

[bernbrue]

Hi,
I'm quite shure that you error is in the side chain, i.e. on the control boards.
You could check a few things:

Pull the 6AL5 tube, so that the side chain is deactivated and only the amp can be check. Verify that the input and output pot work as they should. The input pot should behave like a volume pot.

Check the orientation of the 3 pin molex connector from main board to control board.

Check the orientation of the mode and recovery boards ( component side/solder side). Maybe you turned them around

Check max. Voltage rating of  c10s/c11s

Report your finding, please.
Bernd

 

[DonnieDarko]

Thanks Bernd! I will do so!

 

[bernbrue]

Just to clarify, the trim pot between the two output tubes is just for balancing the bias of these two tubes and has nothing to do with the bias voltage derived from B+.  The 25V for the 6AL5 tube, derived from B+ is setting the bias of this tube and influences the threshold/compression curve of the unit.
Bernd

 

[DonnieDarko]

Yeah, sure.

 

[bernbrue]

Hi Donnie,
any news?
I had a closer look at your pictures and I saw that you used metal standoffs for the two controlboards. One of these standoffs is very close to one of the caps next to it, so it might be better to use plastic standoffs here. I´m not shure if this helps but at least you could avoid that there is any contact , except the three bridging wires, between the two boards.

Another things I realized is that for the mode switch basicly only a few wires are needed. In you picture I can see a whole bunch of wires. So I would really double check the wiring of the control boards.
Bernd

 

[DonnieDarko]

Hey Bernd,

yeah, I got some news.

First off, Recovery and Mode Switches are working good and wired all correctly (even though some of the wiring might be obsolete). Caps are allright, everything else checks out good, too.

I had a tech look at the unit with some fresh eyes and he found that I installed a 50K lin output pot, which was noted in the version of the BOM that I went with.

Upon checking, this was changed now to 1k lin. I will correct this.

Also in some recovery settings, the compressor seems to jump and click when turning the input pot. Is this normal?  I can´t check with my other STA since it doesn´t have the Recovery Mod.

Now for the  calibration procedure:

I disconnect F- and temporarily connect this to case ground, which is about 160 mV different fromt the F- Ground. Then I will look to minimize the occuring Hum and then put the ground in F- again for use.

Right?

 

[bernbrue]

Hi,
Clicking and jumping when turning the input pot is not normal. There is still something wrong. 

Balancing procedure:  the heater supply minus needs a reference to ground, otherwise you'll get hum. Make this reference to ground switchable. As soon as the reference to ground is interupted and the unit starts to hum, you can trim the 100r trim pot for minimal hum. 
As far as I can see, you have one wire for B- and H- to ground in your psu. Please rearrange it, so that B- has a fixed reference to ground and the H- reference to ground is switchable. Don't disconnect H- from the tubes.
Bernd

 

[DonnieDarko]

Hey Bernd, you mean F- instead of H- right?

 

[bernbrue]

Yes, filament minus. (F-)
Bernd

 

[bernbrue]

Donnie,
I attached a file in the second post, explaining the grounding of F+. Sometimes a picture tells more than 1000 words.
Bernd

 

[DonnieDarko]

Ah, allright, thanks!

I did exchange the output pot to a new stepped switch reading 1K, double checked all the wiring, resoldered the input put.

The unit works - on  one channel.

I made you guys a video:

http://we.tl/TVpoB5Rti5

You only hear one channel at once.

First, you will hear the left channel. This is the bad one. On 0:10 for example you can see and hear, how the compression bangs in with a THUMP, then only very slowly returns.

After that, i switched to the right side. This side sounds better and reacts very fast, off course depending on the setting.

I did switch back and forth the Input Pot, Recovery + Mode switch,  Input tranny and most of the tubes from the good and the bad channel, to rule them out as causes for the problem.

 

[bernbrue]

Ok, saw the video. A few observations.
It´s strange, that you have to really crank up the input pot before the unit starts to compress.  Your input transformer has quite a high ratio (1:9?) and should deliver such a strong signal that the unit should start to compress very early.
For me it seems that the bad channel has got some kind of ground issue. Since your psu is external I would check ground connection from psu to main unit. Measure if the control boards have connection to ground as well. I would like to see the results of your measurement. Maybe a bit stupid but you should check the XLR wiring as well. My unit behaved almost the same as yours until I found out that my XLR were not wired correctly. 
Bernd

 

[DonnieDarko]

Hey Bernd, did check the gorunds, they seem to be fine.

XLRs also double checked.

Yeah, i found the late compression strange too. However, we are driving this rack with our rednet units, maybe the signal from them is not lound enough or was turned down a few db before. Will look into that!

 

[bernbrue]

Hi Donnie,
Just for comparison a video with some vocal compression. Input poti is only around 9 o'clock. Output pot is turned fully ccw.

http://we.tl/Z5hTvnesB8

Bernd

 

[mrcase]

wow donnie, your gear looks absolutely fantastic!

 

[bernbrue]

Just a guess, but maybe you hooked up the input transformer the wrong way, i.e. step down instead of step up?
Bernd

 

[DonnieDarko]

I will change the input with the output of the transformer, easy thing to do.

However, I think that the input of the interface might be turned down a notch.