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

 

[JohnRoberts]

I expect Ian is more correct than not... Brickwall or "Hard" limiters are called that as compared to "soft" limiters that are not as strict. Limiters have "thresholds" not ratios.

Of course there are (very) many ways to skin this cat.

JR

 

[Det van Rijn]

I don't think it makes much sense for me to get involved in a discussion where blind people talk about colors.
That's why I'm going to delete my account.
And always remember: let the big boys have the big toys!!

 

[JohnRoberts]

@JR:
I think you're a good reader!
Did I write anyhow and anywhere that limiters have a ratio? But I was mainly responding to his claim about the overmodulation of radio transmitters, which is technically incorrect. When a limiter is "overrun", it only ever emits a maximum set NF voltage, no transmitter carrier is overmodulated.
I am a graduate engineer in image and sound technology with a focus on audio engineering, have planned turnkey outdoor studios and was most recently the head of the entire transmitter and frequency management of the first nationwide broadcaster "Deutschlandradio", the former DLF (Deutschlandfunk)

I don't speak German but I can drink in German ( ein bier bitte).

The OP is inquiring about adding a ratio control to a broadcast limiter. Ian correctly (I think) advised him that the "Schwelle" establishes the threshold, limiters don't have ratios.

Ian's explanation "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 license." Sounds accurate to me in the context of US FM broadcasts.

I didn't (and don't plan to) watch the video so I have no comments about that, or Ian's response to the video.

If you really want your account deleted I/we can help you.

JR

 

[ruffrecords]

I am confused. Is there a missing post or two?

Cheers

Ian

 

[JohnRoberts]

he deleted some of his own posts....
===
You are not confused but there may be a language problem....

JR

 

[abbey road d enfer]

Actually, for many years, a compressor with a high ratio (>10) was considered a limiter. That's what this particular design is. Ratio is essentially variable, constantly increasing as level increases. There is a small range where it acts with a moderate ratio. By tweaking the side-chain gain, it is possible to shift this range towards upper levels. Then by another tweak, it's possible to shift down that range. That's exactly what's done in the archetypal 1176. E.g. modifying R12 & R48.
I believe E79b13's (is he a pedal-steel player?) solution deserves to be pursued, since it doesn't cost much and is without any risks.

 

[MidnightArrakis]

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.

>> Just to clarify.....your attached "V101 Limiter Schematic" is of this unit?.....

This looks like a "500-Series" format module.



/

 

[Matt Syson]

The BBC in the UK designed a limiter/compressor to be used on their 'live' desks (live to air) which were pretty sharp knee compressors and to prevent possible under limiting the main audio path went through a delay line consisting of a bunch of inductor capacitor stages so that the sidechain would always be faster than the audio. Of course transmitters had zener diodes (extremely hard limiting) as overdriving can be very messy and expensive! the schematic shown earlier is quite nice and at least they understood that the sidechains must be combined after the rectifier to preserve left/right balance.