[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

[bernbrue]

What did you take for R8? Solderbridge?
Bernd

 

[prescott]

Yes

 

[bernbrue]

Can you measure AC voltage coming from the transformer?
Cheers
Bernd

 

[prescott]

Hi!

I have a working unit now
I have ll1676 in 1:4 configuration and russian 6k4p-ev tubes.
Everything works fine, but I have too high output even at minimum output level on the pot.
When I increase the input I hardly get any compression, because the the output is too high for my interface.
I will change R32 as mantioned before, but I need some change on the output structure.
Any advice?

I have the following:

ll1676 1:4
6K4P-EV
12AT7 (jj gold)
6H2P-EV (EAA91 substitution)
6P1P-EV (EL-90) I had to use socket converter for these
Edcor output

I like russian millitary tubes because of their long life and (to my ears) better sound

The most important, the unit sounds awesome, just too much output

Thanks for this great project!

 

[prescott]

Another pic

 

[bernbrue]

Very nice! Interesting tube choice. The output pot is just a pad.  You can't cut down the output volume completely. And yes, the output is extremely hot. Have a look at the gates sa 38 schematic. There you'll find multiple output pads stacked. Just add a second output pad to the already included one. You can take the same values.
http://farm3.static.flickr.com/2680/4150163073_b7c2b5b94a_o.jpg
Cheers
Bernd

 

[prescott]

Thanks for the help!
Output level solved!

Is it possible to change the compression ratio, by changing a component value?

Thanks again!

 

[prescott]

I found a very interesting thing.

It felt like, if I incearse the input with the pot, it started to sound a bit blur.
I changed the pot for fix resistors and it sounds flawless.
I measured the pot in the middle position and on the first gang it was 50k on the second it was about 56k.
On a balanced signal this can be a problem, so I will use  stepped switches.
The difference is clearly audible.

Another:
1:4 for the input is a bit overkill knowing the unit's very hot level. I changed to 1:1 and now it's easier to adjust the controls.
I used ll1540. What do you think about it?

 

[bernbrue]

The unit needs a certain level to start compressing. There might be some impedance mismatch using the 1540, but it basically works as well.  Stereo pot tolerance is certainly an issue. Stepped pots with precision resistors will be better.
Can you give us some values measured at the test points on the pcb? What is your bias voltage at test point T? There is a nice explanation in the original manual of how to alter compression ratio. Definitely worth reading. Please report back.
Cheers
Bernd

 

[prescott]

Test points:
A:60,2
B:0
C:1.2
D:0.6
E:1.16
F:0
G:61.9
H:106
I:164
J:0
K:2.1
L:2.2
M:0
N:161.5
O:278
P:0
Q:277
R:279
S:200
T:16.8

R32 is 18k changed from 24 to lower the threshold point.

B+:283
F+:6.22

 

[bernbrue]

Can you measure at Q once again?
Which power transformer are you using? All in all your voltages are a bit on the low side but definitely nothing to worry about.
Bernd

 

[prescott]

Yes

Q is 0.03v

 

[prescott]

I have lower values...
Is this because of the lower B+?

I have custom made power toroid.

I have hum at around -75db. Can I lower it somehow?
The noise level is around -100-110db.
Its not in the box yet. Connecting the ground doesn't make any sense.

 

[bernbrue]

What are the specs of your costum made transformer? B+ coming from psu should be 300V. You should put it in a box before making reliable measurement. Anyway, your measured values are nothing to worry about.
Go ahead and show us pictures of your new compressor.
Cheers
Bernd

 

[prescott]

I had 320v B+ before, but I changed the resistor in the AC circuit. It's 1K now I think.
Now it's 283v. Tomorrow I will measure the voltages coming from the transformer.
The last thing is to lower the hum and the comp will be perfect
I will make R32 switchable to get different ratios. With 18k instead of 24, its smoother now. I want to use it for mastering as well so I need even lower ratios.
Soundwise its killer My favourite varimu now
Pictures to come after finishing the box!

What can be the final hum level in your opinion?

 

[bernbrue]

My unit has no hum at all. You need a proper grounding especially for the heater.  You'll find the measurement of my unit in the 3rd post. (Attached PDF)
Bernd

 

[prescott]

How can I ground the heater?
On a picture on the 3rd page I can see only I wire connected to the ground point of the psu pcb to the case.

 

[bernbrue]

You've got a GND connector on the psu. From there you can connect B- and F- with two wires to your star ground point.  Make the F- to star ground connection switchable.  Then connect audio ground  from main pcb (it's the third screw connector next to bias GND) to the same star ground point. Then connect the main ground from your iec inlet to the very same star ground point. Make shure that there is a direct connection to the metal of your enclose. Check continuity with your multimeter. Now you've got a proper grounding.

We need the switchable F- to ground connection for the balancing procedure. As soon as the F- connection to ground is interrupted, the unit starts to hum. Turn the balance pot (bias of the EL90 tubes) until hum disappears (or is hardly audible).  Then reconnect F- to ground. That's it.

Sorry, for me as a non native speaker it's a bit hard to explain such a technical procedure. I hope you got it.
Cheers
Bernd

 

[prescott]

It clearly understandable
Thanks!

 

[kosi]

Bernd, do you have a drill template for the boards ?