[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

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barthman.de

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May 26, 2007
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Erfurt / Germany
Hi I hope you are well.

I’m building two STA-Level-Compressors  (P2P wiring, original schematic incl. 5Y3 and 6386. I use Sowter 9062e on input and edcor on output. The first one is ready to test. So at first I had to add an extra 14dB H-Pad on output to get input level and ouptut level matched. This works fine, all sounds and looks good.

I use the input pad like shown on the original schematic. R32 is 24k and I get 26V between R32 and R39. If I go in the STA with an -12dBFS sinus signal and the input pot is full CW I get not more than -10dB GR. This is good for the most requirements but the box is known for higher GR.

I’m interested in your practical knowledge. Is it enough to lower R32 to 18k to get higher GR or have I change the input setting too (remove input pad and wiring sowter 9062 in other way - pink/white cable?). How much gain reduction did you get with this modifications? How change these the Ratio?

Thanks.
 

Dr_J

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Jul 5, 2011
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Germany
Hi Sebastian,

great to hear from you.  :)

What's the voltage (RMS) you're feeding in? ([email protected]?)
(dBFS doesn't say much; FS = Full Scale Deflection  ;) )

I'm currently as well working on, testing the compression behavior of my unit.
Maybe i'll post some curves later...

How did you measure, calculate your -10db Gain Reduction?

ROCK-ON!

 

barthman.de

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May 26, 2007
Messages
83
Location
Erfurt / Germany
Dr_J said:
Hi Sebastian,

What's the voltage (RMS) you're feeding in? ([email protected]?)
(dBFS doesn't say much; FS = Full Scale Deflection  ;) )

I'm currently as well working on, testing the compression behavior of my unit.
Maybe i'll post some curves later...

How did you measure, calculate your -10db Gain Reduction?

Hey Dr_J,

-12dbFS @ 1kHz is 1,212V AC between hot and cold.

My output pad I 've calculated here:

http://www.nu9n.com/tpad-calculator.html

600 Ohm in and out; had 0,1% 200R, 0,5 W resistors in stock, so I calculated a 14dB Pad and put it directly behind the pad shown in the original schematic (4x200R and 1x 249R as bridge resistor)

BTW: 1k pot is the right one on output :)

Have Fun

Sebastian

 

Dr_J

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Hi Sebastian,

-12dbFS @ 1kHz is 1,212V AC between hot and cold.
Why not using the standard +4dBu with 1.228V?  ;)
This would/could be as well the "Bezugswert" (U0) to calculate dB values.
Have a look at good old sengpiel  ;)

I'm currently try to figure out how to measure the transfer function (dbout over dbin) to check the compression behavior of my unit.
I'm not quit in the ballpark yet...  ???

Where did you get your (only) -10db GR value from?


ROCK-ON!
 

barthman.de

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Joined
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Messages
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Erfurt / Germany
Dr_J said:
Hi Sebastian,
Why not using the standard +4dBu with 1.228V?  ;)
This would/could be as well the "Bezugswert" (U0) to calculate dB values.
Have a look at good old sengpiel  ;)

Where did you get your (only) -10db GR value from?

+3,89dBu and +4dBu is nearly the same  ;D

the -10dB gain reduction shows the Simpson Meter when I turn the input full CW

I'll change R32 to 18k in the second unit at first and we will see what happens....
 

schrobbelbop

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Mar 7, 2010
Messages
133
Location
Eindhoven, Netherlands
My Dual sta from drip is still thumping in the low end when compression kicks in
especially with fast release times.

I've tried the 6ba6 t-bar's with no luck, other pairs of 6v6, other brands... no diffrence.

myguess is the psu, it measures rock steady on all voltages points, the only
thing i've chosen for is not to use a choke but an extra resistor in that b+ path.
i've ordered the 5mh choke and see if it will help.

could it be something with the power feed? how to meaure that? there is an diode rectifier
into seperate voltage regulators for both channels.

i've read something about old 6AL5 tubes that could be causing this too?

I must admit it's one of the best sounding compressors on guitars, vocals and bass lines.
super rich harmonics, adds nice detail of highend character and rock steady bottomend.
try it in parallel :)
 

barthman.de

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Joined
May 26, 2007
Messages
83
Location
Erfurt / Germany
barthman.de said:
I’m interested in your practical knowledge. Is it enough to lower R32 to 18k to get higher GR or have I change the input setting too (remove input pad and wiring sowter 9062 in other way - pink/white cable?). How much gain reduction did you get with this modifications? How change these the Ratio?

I'll change R32 to 18k in the second unit at first and we will see what happens....

A few more notes:

- the answer to get more compression is to remove the input pad completely; now more than 40db of compression  :) - I did not change the sowter 9062 wiring
- seems to be a good idea, to make the input pad switchable
- to adding a second H-Pad (around 15dB) after the original output pad is a good idea (I leave it like that)
- changing R32 seems to change the comression ratio; lowering R32 to 18k lowers the voltage on 6AL5 to something about 20V
- after some short tests between my to units (one with R32 18k; one with 24k) some signals seems a little  distorted at the fastest settings on the 18k unit; so I tend to go with original 24k @ R32; I will test this again longer next weeks

Here some pics of the finished units:

sta_front1.jpg

sta_back.jpg

sta_inside.jpg
 

schrobbelbop

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Mar 7, 2010
Messages
133
Location
Eindhoven, Netherlands
schrobbelbop said:
My Dual sta from drip is still thumping in the low end when compression kicks in
especially with fast release times.

I've tried the 6ba6 t-bar's with no luck, other pairs of 6v6, other brands... no diffrence.

myguess is the psu, it measures rock steady on all voltages points, the only
thing i've chosen for is not to use a choke but an extra resistor in that b+ path.
i've ordered the 5mh choke and see if it will help.

could it be something with the power feed? how to meaure that? there is an diode rectifier
into seperate voltage regulators for both channels.

i've read something about old 6AL5 tubes that could be causing this too?

I must admit it's one of the best sounding compressors on guitars, vocals and bass lines.
super rich harmonics, adds nice detail of highend character and rock steady bottomend.
try it in parallel :)

I 've put in the choke, no diffrence. still thumping alot. i'm now up to changing the 6AL5's
there's cheap new siemens brand from conrad.nl. now ordered some nos telefunkens, let's see..
 

Dr_J

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Jul 5, 2011
Messages
270
Location
Germany
Hi Sebastian,

what a nice pair!!
What cases did you use? They're looking great.

... but I'm missing Bernd's PCBs...  ;D


ROCK-ON!
 

Dr_J

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So...

I did some more testing and measuring.
Here's my compression curve. (Sowter 6062 & 9185, 2x 6BA6)
As a reference I took the curve from the original manual.

I applied a 1000Hz Sinus.
Setting the compressor to SINGLE & FAST.
Taking Uout after Output-TX.

My first try was to measure Uin after the input pot at test points #F and #B (#3, #7).
But somehow the result curve looked like as if it was compressed  8) along X-axis, after the knee point.
I don't know why this doesn't work... :mad:
If anyone can explain that to me, that would be GREAT!!

Next try...
I cranked up the input pot fully cw and controlled Uin via ProTools, measuring at XLRin.
U0 = 1.228V

I shifted my curve to fit the knee point of the original diagram.
I think the curve looks pretty good even if it's at the upper tolerance limit according to the manual.
Maximum gain reduction is ~ [email protected]
(Leaving the input pad out would of course give more... ;D ... if it's linear... don't know)

Update:
I did another run - changed the mode to SINGLE and SLOW.
Changing to SLOW Recovery increases compression.
Now my compression curves fits the original even better  ;D


Next comes a new meter scale... next problem...
(the compression behavior is never linear, should the meter scale be linear..??)


ROCK-ON!
 

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Dr_J

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Next: Meter Scale.

I have an old Simpson Meter in my build.
It's a 200uA FSD.

Here's the meter scale curves i took from my tests (see above).
It's in no way linear...  ???

I think to represent the real compression behavior of the unit, i'll have to make a non-linear scale.
(looks like some kind of Sigmoid function...
... a polynomial 5th grade fits quite well...)

Any comments on this?


ROCK-ON!
 

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barthman.de

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Messages
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Erfurt / Germany
Dr_J said:
Hi Sebastian,

what a nice pair!!
What cases did you use? They're looking great.

... but I'm missing Bernd's PCBs...  ;D


ROCK-ON!

Yes no PCBs I'm sorry  ;)

I had 3 housings bent at Laserteileonline.de. One is still reserved for a La2a that I will build next year. I have drilled all the holes. I also painted with spray cans and gained my first experience in this regard. It took me a couple of tries to get the paint right. Frank from frontpanels.de printed the front panels. Overall, this was my most complex project so far. I also built a stereo version of the La3a on the side, which was much faster (Standard housing, PCB and front panel). I find that the STA Levels sound extremely good even with high gain reduction. In this respect, the effort was worth it for me.
 

halftime

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Nov 7, 2018
Messages
2
I'm looking for a replacement input transformer for my vintage Sta-Level.  Does anyone know if the Sowter 9062 is still available?  On their website, the status shows "Catalog suspended"... not sure what that means. 

Or what are the current recommended replacements from other manufacturers?

Happy new years to all!
 

Dr_J

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Messages
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Location
Germany
... here we go, my new matched meter scale  :)

There're 3 conclusions for me:

- a 200uA meter seems not be a perfect choice
  (the meter range is only one-fifth of the original value range)

- Applying the original meter scale (to 200uA FSD) is not a good deal
  (my scale read -25db when the original reads -40db approximately)

- For a good meter scale measuring the real compression behavior is essential
EmRR said:
Any time you hack variable ratio into a vari-mu like this, the meter becomes a 'general indication of action', and not a thing resembling meaningful precision. 


ROCK-ON!

 

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Dr_J

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... here we go, that's what I ended up now.  :D

It was a long run of 5 years.. ::) ... ok with a serious break in the middle...
... who cares... it's running now!

Frontpanel and Case from Frank, Knobs, Power-TX from Don-Audio.

That was a really cool project, THANKS YOU EVER SO MUCH Bernd & Zayance!!


ROCK-ON!
 

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Dr_J

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... and here's the inside.

- Sowter 6062 & 9185
- 2x 6BA6
- 2 fine PCBs  ;)
- No relays board
  (I ordered the relays (by mistake 2 times...  ::) ) but didn't put them in...
- 15db H-Pad on Output 
- Nothing special else
- ok, wiring could be better, nicer...  8)
- ...


ROCK-ON!
 

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TLRT

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Jan 9, 2014
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Hungary
Anyone solved the "thumping" issue? I have one unit which works great,another one puts popping noise to processed material. I selected the tubes with curve tracer,doesnt really helped. Sowter input /Edcor output transformer, voltages are normal.
 

Dr_J

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Messages
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Germany
Hey TLRT,

if one unit is running, it must be a build error  ;)

If the tubes are fine, i personal would check the mode pcb!
Bowie gave me that hint, which solved all my problems.
Have a look: https://groupdiy.com/index.php?topic=59017.msg967904#msg967904

How're the test points voltages?


Good Luck & ROCK-ON!
 

HerbertR

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Mar 3, 2020
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Location
Austria Voecklabruck
Hi, I have also nearly finished the the build. But with Problems:

-> B+ Resistors R3 fried after a few Minutes but B+ Voltage looks good with about 314 VDC

->also R12S on the Motherboard gets very hot after a few minutes

-The EL 90 Tubes from RSD also gets really hot

The Voltages are nearly like the Voltages on the first page. Only Testpoint A at the GE 6BA6 Tubes has about 65V instead of the 82V...
Any Ideas???
Thanks for Help...
 

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