Threshold values in a vari-mu comp

[ruairioflaherty]

The tradition with feedback limiters is to provide an input gain control and an output level control. The threshold is fixed inside of the device, but by using these two controls, you can place the gain reduction wherever you want relative to your signal levels. Many classic ones use switched attenuators, so you can set and reset easily, or match two channels.

You could fold all of that up into one switch, but it becomes very complex and counter-intuitive for those who have used classic feedback limiters.

If you want to change the actual threshold level of the limiter, you can change the DC value sent to the rectifiers that sample the output plate voltage. However, all you're doing is changing the gain structure inside of the limiter, which will usually just make the limiter noisier. Most vari-mu stages are prone to hum from imbalance and power supply ripple, since one can't put feedback across them and still get variable gain. So, using anything lower than the 'standard' threshold voltage will probably just add hum.

I say there's no problem to fix - you just need to get used to non-VCA type processors. The whole idea of 'threshold' is from feedforward processors that have a dB-linear VCA and a real level detector, something that didn't exist until the 70s.

Good points all but as I outlined above there are some good reasons to want to have some control over a notional "Threshold" point.  I make use of that advantage daily.

There may be some noise penalty to having a threshold control in a Vari-Mu (Vari-GM) design but I'm sure like every other engineering problem ever it's a compromise.  Both the MDE that I use and the EAR 660 (2nd best I've heard) have a variable threshold and neither have issues with noise.

I'm a huge fan of the minimalist design of yore but I don't make records in 1960!  It's a very different musical landscape now and some extra control of color is very very useful, if not essential in my work.

 

[ruffrecords]

The whole idea of 'threshold' is from feedforward processors that have a dB-linear VCA and a real level detector, something that didn't exist until the 70s.

This statement is incorrect. There were compressors around in the 70s with threshold controls e.g Neve 2254 and he LA2A that did not use feed forward.

In a feedback compressor you always need two controls. The two common ways of arranging these are:

1. The loop consists of a fixed gain amplifier with a threshold control setting the amount of this amp's output that is fed to the sidechain. The amp is followed by a gain make up amp with a level control (this amp is outside the loop so it does not affect the threshold). Both the Neve 2254 and the LA2A work this way

2. The loop consists of a user variable gain amplifier (usually a fixed gain amplifier with an input level control) the output of which is fed directly to the sidechain.  The threshold is therefore controlled by the input level control. The amp is again  followed by a gain make up amp with a level control (this amp is outside the loop so it does not affect the threshold). This control is usually marked output. The Allison Gain Brain and the 1176 are typical of this arrangement.

Because FETs are easily overloaded it is important to keep the signal fed to them quite low. For this reason, many FET compressors use an input level control followed by a 20 to 30dB amplifier to achieve this. In normal operation the input control is set for 20 to 30dB of loss which is made up by the following amplifier. For this reason, most FET compressors use method 2 of setting threshold.

Cheers

Ian

 

[abbey road d enfer]

The tradition with feedback limiters is to provide an input gain control and an output level control. The threshold is fixed inside of the device... The whole idea of 'threshold' is from feedforward processors that have a dB-linear VCA and a real level detector, something that didn't exist until the 70s.

Yet the iconic Fairchild 660/670 have a Threshold control that adjusts the amount of signal hitting the side-chain amp.
The output attenuator is used as a "Trim".

 

[scott2000]

Yet the iconic Fairchild 660/670 have a Threshold control that adjusts the amount of signal hitting the side-chain amp.
The output attenuator is used as a "Trim".

The Federal AM-864 ? How would it's threshold be described?

 

[abbey road d enfer]

The Federal AM-864 ? How would it's threshold be described?

The AM864 has two means of controlling the threshold: the input level control AND a dedicated Threshold pot that controls the level driving the side-chain.
Increasing the overall output level involves increasing  both the input level and the Threshold (i.e. attenuating the signal to the side-chain).
It is less convenient than a dedicated output control; however, in the context of very strict normalized operating conditions, it is adequate. Much less in a studio environment.

 

[Monte McGuire]

Thanks for the info! I did not know the details of these devices, so take my statements within my 'cone of ignorance'.  It's always a goal of mine to learn something from these discussions!

To the OP, I think that if you want to do this complex gain scaling, you might be able to adapt some of the pre-made rotary ladder attenuator switch controls, re-wiring them, using your own resistor values. Building such a switch with raw parts would normally be prohibitively expensive, but I bet that one of the pre-made, discrete resistor ladder volume controls could be re-worked a bit to get what you want.

 

[johnheath]

Thanks for the info! I did not know the details of these devices, so take my statements within my 'cone of ignorance'.  It's always a goal of mine to learn something from these discussions!

To the OP, I think that if you want to do this complex gain scaling, you might be able to adapt some of the pre-made rotary ladder attenuator switch controls, re-wiring them, using your own resistor values. Building such a switch with raw parts would normally be prohibitively expensive, but I bet that one of the pre-made, discrete resistor ladder volume controls could be re-worked a bit to get what you want.

Thank you sir

As Ian mentioned the signal is fed from the output before the output transformer and as it is now that signal is controlled by a 2-pol/6-pos switch that is wired like a stepped potentiometer... It works very good and since it is resistors I can easily make the switch  with matched resistors.

I was just curious about the values of the resistors and if it could be determined what values to use to "get" specific threshold values. If I understand it correctly by the information given here, those threshold values are changing by the given gain set by the input control.

I guess I can live with the fact that I will not have specific threshold values and do a little like ruairioflaherty mentioned and dial in wanted gain reduction by ear

Best regards

/John

 

[scott2000]

@ ruairioflaherty....

The Magic Death Eye was mentioned somewhere in this thread and I was wondering if you had any opinions you're allowed to share on the plug in.... I  randomly came across it today.....

 

[abbey road d enfer]

I guess I can live with the fact that I will not have specific threshold values

That is how the 1176 operates and it hasn't prevented it being successful.