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A couple of questions about Limiting Amplifiers (FET Compressors)

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A couple of questions about Limiting Amplifiers (FET Compressors)

PostPosted: Fri May 15, 2020 5:03 pm
by Dolmetscher007
After fumbling about with the native stock Logic Pro X plugins for quite some time, I have finally just decided to throw in the towel and go back to basics. I obviously cannot afford to buy $200,000 worth of outboard analog gear just to learn how it all works, but I can read websites, subscribe to SOS magazine again, (*just re-subscribed after 15 or years), and ask questions. I am watching a YouTube video about how Steve Albini records drums. He goes through every setting of every mic, and talks about what he's done and why. He talks about the Universal Audio 1176LN Limiting Amplifier. I have a few questions about what this thing really is.

1. The Universal Audio website refers to the 1176LN as both a FET compressor and a Limiting Amplifier. I know that compressors work by reducing the volume of the louder parts of a signal, so the quieter aspect "sound" louder. Usually a compressor has a gain knob to re-boost the signal's amplitude to recover from the inherent volume reduction. So does this just mean that a Limiting Amplifier is simply literally a synonym for compressor?

2. Since the 1176LN only has one input and one output, does that mean if you want to use it on multiple microphones when tracking drums, you literally have to buy 2, 3, 4... or however many 1176's?

3. On the front panel of the 1176LN there are buttons "Meter" that are labeled, GR, +8, +4, and OFF. Also, on the back of the unit there is a bank of what appears to be screw lugs, labeled IN, +/- C, GR, +/-C Meter... What is all this about?

Re: A couple of questions about Limiting Amplifiers (FET Compressors)

PostPosted: Fri May 15, 2020 5:24 pm
by MOF
Compression is gain reduction at various ratios up to about 20:1 when it becomes Limiting.
Yes you will need more than one device to handle multiple mics and/or a stereo group.
I think you should persevere more with Logic’s software versions of this very device, read the books/articles and watch videos.

I notice that you never responded to my answer here
https://www.soundonsound.com/forum/view ... 08#p665408
when I suggested that you do more research. Might I suggest that if you’ve got a budget for the 1176N then it might be better spent getting some individual tuition.

Re: A couple of questions about Limiting Amplifiers (FET Compressors)

PostPosted: Fri May 15, 2020 5:27 pm
by Kwackman
MOF wrote:
Yes you will need more than one device to handle multiple mics
Unless you use a mixer, route the mics to a (mono) group and insert the compressor in the group.

Re: A couple of questions about Limiting Amplifiers (FET Compressors)

PostPosted: Fri May 15, 2020 5:30 pm
by Hugh Robjohns
Dolmetscher007 wrote:The Universal Audio website refers to the 1176LN as both a FET compressor and a Limiting Amplifier.

The FET bit describes the active element (field effect transistor) which is used to provide the variable attenuation that creates the gain reduction. The FET serves effectively as a remote-controlled variable resistor in a potential divider to attenuate the input signal. At the time of the 1176's launch, FETs were relatively new and cheap, but also very fast-acting in great contrast to the common but slow optical Compressors, and the equally common but expensive variable mu valve compressors!

The main difference between a compressor and a limiter is the ratio. Ratios of around 10* and higher are considered to be limiting, and as the 1176 offered 8:1, 12:1 and 20:1 as three of its ratios Urei were probably right in calling it a 'limiting amplifier' (with a stiff compression mode thrown in for fun!).

*What the 10:1 ratio is saying is that for the output signal to rise by 1dB more, the input signal has to increase above the threshold by 10dB.... which is quite a lot. And for me, if the input has to rise by 8dB before I see 1 more decibel at the output, I call that limiting in every practical sense too!

I know that compressors work by reducing the volume of the louder parts of a signal, so the quieter aspect "sound" louder.

Kind of....

Compressors and limiters work by pulling down the loud bits (those above the threshold), and in so doing they reduce the signal's overall dynamic range.

The way you make the quiet bits louder is by taking that squashed signal with its smaller dynamic range, and boosting its level. That's what raises the quiet bits to sound louder. The loud bits that were turned down, are now restored to something close to their original level, of course.

Usually a compressor has a gain knob to re-boost the signal's amplitude to recover from the inherent volume reduction.

The 1176 works slightly differently to most traditional limiters/compressors in that it doesn't have a threshold control. Instead, it has a fixed internal threshold, and you determine how mush of the signal to squash by adjusting the input level. The higher the input level, the more signal will exceed the internal threshold, and the more compression (and gain reduction) will be applied. You then adjust the output control to set the peak level of the resulting signal where you need it to be. The output control is the same as a Make-Up gain control on traditional compressors.

So does this just mean that a Limiting Amplifier is simply literally a synonym for compressor?

It's what it says it is: a limiter first and foremost with its 12:1 and 20:1 ratios.... but with the addition of stiff 4:1 and even stiffer 8:1 compression modes too. (To be honest, I'd call 8:1 a limiting response too in all normal practical terms!)

Since the 1176LN only has one input and one output, does that mean if you want to use it on multiple microphones when tracking drums, you literally have to buy 2, 3, 4... or however many 1176's?

If you want to process individual mic channels, then yes, you'd need to buy individual 1176s.

Of course, most people will buss the drum mics together and put the 1176 on the bus... if mono is acceptable... However, it is possible to link two 1176s for stereo working (such that they both apply the same amount of gain reduction, regardless of which channel calls for the most, so that stereo imaging remains stable), and there are other tricks that can be employed to use two unlinked 1176s...

On the front panel of the 1176LN there are buttons "Meter" that are labeled, GR, +8, +4, and OFF

GR makes the meter display the amount of gain reduction -- the normal working mode to show how hard it's working and how much it's attenuating the signal.

The other buttons configure the meter to show the output level. and the +4 and +8 refer to the signal level equating to 0VU on the meter: +4dBu (studio standard) or +8dBu (if you're running very hot signals).

Also, on the back of the unit there is a bank of what appears to be screw lugs, labeled IN, +/- C, GR, +/-C Meter... What is all this about?

The original Ureis were designed to wired up with spade connections on screw terminals -- it predated the XLR standard. So IN and Out refer to the terminals associated with the input and output. Each has a +/- connection for the hot side of the balanced input signal, and a C connection for the cold side of the balanced signal. It had transformers in and out so could accept unbalanced signals just as conveniently. The GR (or GND) terminal provided a connection to the earthed chassis for the input and output cable screens.

The two Meter terminals were effectively wired directly across the unit's own VU meter and were intended to allow a second remote meter to be installed in the mixing console etc to make it easier to view while mixing (assuming the 1176 was bolted in a rack out of sight somewhere).

HTH

Re: A couple of questions about Limiting Amplifiers (FET Compressors)

PostPosted: Sat May 16, 2020 12:25 am
by DanDan
Afaik the 1176 was intended primarily as a safety Limiter for Broadcast.
You would not want to over modulate those Megawatt Transmitters in the USA desert....... The 8dB Metering reflects the higher normal level used in Broadcast. In order to keep things safe Limiters, particularly this one, have extremely fast attack capability of 20 microseconds. There were a few Urei's all a bit different. The LA2A was a Levelling Amplifier, does what it says on the tin, slowly. It had a Limiter switch but I think nowhere near as fast or brick wall as the 1176.

Re: A couple of questions about Limiting Amplifiers (FET Compressors)

PostPosted: Sat May 16, 2020 12:55 am
by CS70
Dolmetscher007 wrote:1. The Universal Audio website refers to the 1176LN as both a FET compressor and a Limiting Amplifier. I know that compressors work by reducing the volume of the louder parts of a signal, so the quieter aspect "sound" louder. Usually a compressor has a gain knob to re-boost the signal's amplitude to recover from the inherent volume reduction. So does this just mean that a Limiting Amplifier is simply literally a synonym for compressor?

It just means that it is fast - i.e. has very fast attack.. so can push down a steep peek better than its predecessors.

2. Since the 1176LN only has one input and one output, does that mean if you want to use it on multiple microphones when tracking drums, you literally have to buy 2, 3, 4... or however many 1176's?

Yes. Sure you can record each microphone and bounce the results after passing it thru a single box, but you'll soon lose the will to live (and can't hear the overall result in real time).

Studios used to have many 1176 in racks one on top of the other exactly for that reason.

3. On the front panel of the 1176LN there are buttons "Meter" that are labeled, GR, +8, +4, and OFF. Also, on the back of the unit there is a bank of what appears to be screw lugs, labeled IN, +/- C, GR, +/-C Meter... What is all this about?

In old times, before XLRs, that's how you wired the box: IN is line input (plus, minus and common of a balanced wire), G is chassis ground, OUT is line output (plus, minus and common), and METER are for plus and minus of a metering device.

My MC77 has similar sockets for audio and chassis ground. Modern renditions of the LN will have XLRs I guess, but maybe for some they've kept also the old connection strip (don't know).