TFE Broadcast Console Limiter

[David_H]

Greetings everyone!
I have yet another fun unit of German Broadcast land to contribute to the already stellar info that can be found on GroupDIY - and I am also seeking some advice. I have acquired an old Broadcast mixer from a Company that's called "TFE studio Gmbh". They built mixers for the first private radio station in Germany and also got some of their designs licensed by BFE, which some of you may recognize as a houshold name in outfitting German broadcast facilities.
My mixer is out of the pro series 10 (I will post docs in the documentation subforum), and has a Limiter built in like many of the consoles form that era had (late 70s, early 80s). A schematic is attached. Now to me this looks like a FET Limiter of some sorts (I'm pretty bad at understanding schematics though...). Now from this video I learned that the "ratio" buttons in the 1176 are actually somewhat of a "threshhold" control.

Now to the question: In the schematic there is a threshhold control (German "Schwelle") internally - does this function the same as the ratio in an 1176 (Because then I could just wire a pot in there)? Reason being, I like the sound of the compressor but the "limiting" is really that - it makes bricks out of sine waves, and I'd like to have a little more control over the type of compression.

Also, I'd be very interested if anybody else has the same unit or if somebody could shed light on how this circuit compares to the Studer/EMT/ANT Telefunken etc. offerings of the time.

As always, thanks for all the free knowledge.

 

[E7b9b13]

From what i gathered by looking at the schematic, the „Schwelle“-pot actually does work like one half of the ratio buttons in the 1176. But there‘s also the „Rücklauf“ pot, which looks more like a Ratio control to me.

 

[David_H]

Nice, hanks for the input! Do you think I could just wire a regular pot and use it as a ratio control without messing up the bias of the circuit? I'm not completely sure but I think "Rücklauf" should be the release control which is on the outside - but of course with these older designs one could very well influence the other.
Attached is a picture of the limiter in question, although in my console it's part of the aux master module (bad) photo attached as well.

 

[E7b9b13]

Yes, I got it totally wrong with the Rücklauf, now the word makes much more sense. My next guess on how to install a Ratio control would be to replace R23 with a pot to vary the OpAmps gain, but I’m not an expert, especially not when it comes to OpAmps.

 

[E7b9b13]

But I would maybe wire a resistor in series with that pot.

 

[ruffrecords]

It is a limiter so there will be no ratio pot. The Schwelle sets the threshold. It has no effect on ratio. Ruchlauf is the release time, again nothing to do with ratio.

Yes, it is a FET limiter. Quite a neat design - full wave rectified versions of both channels go via a kind of biggest wins citcuit and then to the detector circuit which produces a common control volrage fed to both FETs.

Cheers

Ian

 

[ruffrecords]

. Now from this video I learned that the "ratio" buttons in the 1176 are actually somewhat of a "threshhold" control.

This video is rather misleading and in places very inaccurate. For starters he seems to think 20dB is a voltage ratio of 100 times when it is 10 times. His statement that you cannot change the ratio is also incorrect. Altering the dc bias on the FET alters the part of the curve it operates on which changes the ratio. This is what the right hand bank of switches does. Also, it makes sense to raise the threshold has the ratio is increased and this is what the first bank of switches does.

Cheers

Ian

 

[David_H]

Ian,

Thanks for the schooling! Then I'll leave erverything "as is" - I really didn't want to "mod" it as much as explore the possibilities that are alread there. As I said, I really like the unit for what it is, but for tracking it's a bit much. On the other hand, even though the waveforms look severly altered you have to work really hard to make it sound "bad" or "overdone".

 

[ruffrecords]

The distortion cancelling circuits in FET compressors and limiters are always the weak point but at the same time that is what lends them their characteristic sound. For broadcast use, the important thing is not to over modulate because this could lead to out of band signals being broadcast which can lose you your licence. Better to suffer a little audio distortion. Most listeners will not notice.

Cheers

Ian