Connecting a meter to Telefunken U373a compressor

[Dr_J]

Thanks a lot Rob!

Now the game can begin... 

ROCK-ON!

 

[B-A-R]

I just salvaged this old GR meter from a busted Gates broadcast limiter! I don't know how accurate it is, but the needle seems to reflect the gain reduction going down..... There's a adjustment pot on the U373a pcb that seems to work in conjunction with the trim pot on the front for sensitivity..

What i'd really like to find is one of these:

 

[Dr_J]

Hi,

I just finished the racking of my U373  ;D
At the first glance everything looks fine... the needle of my hacked cheap meter is dancing...
Now I have to make a new scale, any hints for that around?

Has anybody experimented with bridging the two input and out put windings??
(PIN #3 / #4  and #23 / #22 => +6db Output level)

ROCK-ON!

 

[Rob Flinn]

you need a meter on the input & the output. Then turn a sine wave input signal up until you have compression.  Once you have compression you can mark the scale for how much the output signal is down compared to the input signal as you increase the level of the input.   

 

[Dr_J]

HI Rob,

thanks! That was although my rough plan.
Last night I did some testing, measuring and refreshing my knowledge about level math 

The manual says max 20dB GR, so this must be FSD (100uA), fine!
But when I measure the IN and OUT voltage and calculated the GR (20*log(U/U0)), with U0=1.24V for 0db the result on FDS is only approximately -6db 
(all roughly measured, just to get an idea...)

Below the threshold i see the +6db gain (Bridged PIN #23/#22), so that's fine.

What the hell i'm doing wrong her?? Grrrr.....
I have attached some rough test curves (1kHz sin) ... any hints...???

 

[Rob Flinn]

It might be the meter sensitivity ??? 

Or if you are using digital multi meters be careful because some won't measure ac above much more than 400Hz.  You are better off using a db meter.  Once the U373 starts limiting the output will not get any bigger, so you could use a dB meter to mesure this point, & then switch it to the input, because the output will be the same.

 

[Dr_J]

Hey Rob,

good point with the meter and the frequency... but when i can measure / calculate the +6db  without compression
the meter must be ok!? (But I'll check with a oscilloscope).
Meter sensitivity  is although a good point, but i don't think that i can adjust the meter (inst. eichen) to show -20db instead of -6db.. that was the rough testing all about....

I think it's not that easy as i thought... 

 

[Rob Flinn]

When I added GR meters to my U373 I didn't bother making a scale for them.  With use you just kind of learn what the meter is telling you.   

With the u373 it is quite fierce in that whatever you put through it over threshold will come out at 0dB (or whatever the o/p is set to...can't remember)

 

[Dr_J]

So,

here we go....

After some nights of beer and refreshing my brain i think i got it...

Here's a principal draft of  the signal measurement path to make my new meter scale. Maybe that will help someone out there ...

First I found out that I have to calibrated the U373 a bit (see first diagramm, U373 out No.1).
After a bit of tweaking I got the second curve (U373 out No.2, close enough so far).

Then i did a test row with measuring GR in ProTools and the corresponding meter values.
These values were fitted by a polynomial 5th degree (higher was not improving the quality).
The polynomial function was used for calculation the new meter scale values. Done.

ROCK-ON!

 

[Dr_J]

... here's the first diagram...

 

[Dr_J]

... polynomial fitting...

 

[Dr_J]

... result  ;D

 

[etheory]

I know this is an old thread, but, congratulations, this looks awesome ;-)
Your method, analysis and results are also fantastic. Great job.

 

[andre tchmil]

Does anyone has the schematic for the U373a , the two knob version as seen in the last picture ?
thanks

 

[Rob Flinn]

Does anyone has the schematic for the U373a , the two knob version as seen in the last picture ?
thanks

It's actually 1 knob really the black thing is something you need to put your thumb on to turn.  It's flush with the panel.  You will be able to get the circuit diagram here http://audio.kubarth.com/rundfunk/index.cgi

 

[andre tchmil]

well the one and a half version then.
Thanks , I will check there