Gain reduction metering and needle deflection

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soundguy

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Im trying to work out a metering solution for the fred forssell opto limiter project and its brought up some greater questions about gain reduction in general.

Directly, how does the left deflection of the VU meter in an 1176 in GR mode work? Ive looked at the metering circuit a bit and really cant translate the schematic into a working understanding of whats happening there. Im guessing that the circuit presents a voltage to the meter when at rest, when the limiter kicks in, the fet just drops the voltage to the meter?

Assuming this is how it works, could I use the optocoupler present in the forssell circuit to work in place of what the fet is doing in the 1176 circuit?

Im most used to gain reduction metering like that and would really like to incorporate it on a VU meter on the forssell project. I wired a VU as shown in fred's original schematic and get right deflection, so thats not too much use. Im guessing I could graft the 1176 meter circuit onto the forssell project, but would like to try to understand how it works before kludging something together in a shotgun method.

If anyone can suggest perhaps a simpler circuit or a better solution, that would be beyond cool.

dave
 
Mount the meter upside down.

Actually, when the gain-cell is inexpensive and works on DC, just duplicate it and feed 1V DC into it, meter the output.
 
PRR's suggestion is cheap & cheerful, here's how I did it in all my own compressor/limiter designs that have wanted a VU meter:

From the same drive stage that drives the GR opto, drive a second opto as well.

Build a simple DC-coupled op-amp stage and feed some DC in through a trimpot, feeding into the same resistor as before the GR opto in the audio chain, then shunted to ground by the second opto. Take the output to a VU meter through the usual 3.6k resistor, and you got yourself a GR indication.

The optos have to match halfway decently, but this is true in the T4 way of doing things, and same as in the 1176 where the FET stages are the same.

The DC trimpot sets the 0VU position with no GR.

A simple and short description is that you build a 'mimic' stage of the attenuation part of the compressor, feed a little DC in to move a meter, and meter the output of it. Whatever it's doing to the audio, it will do to the meter, given approximate matching. That way, if the meter is in any way accurate for steady-state audio, it will automatically be accurate for steady GR indications. Since the ballistics are also the same, the display performance will be familiar. If you like the meter on audio, you should like it just as much for GR.

Here's one way that I did it, -look at the audio path and then at the GR DC path from the trimpot:
http://www.celestial.com.au/~rosswood/data/CompressorSchematic.pdf

Keith
 
Those four diodes (in the schematic) are just there to protect the meter against voltages higher than +/-1.2V

If you don't run the risk of overloading the meter, just omit those. I always use a diode shunt like that on valuable meters or in circuits that are not "finished" yet..

Jakob E.
 
If the voltage across the meter needed to deflect the meter to it's full scale is less than 1.2V, then the scheme shown will work fine. If it's less than 0.6V across the meter, just use two anti-parallel diodes.

And make that 1N4001 or the like, 1A diodes.. ..just in case..

Jakob E.
 
Old topic but...

I'm working on my whatcomp and metering on it,
I made this simple proto (see pic) with 2 trimmers and one ldr.
And it works nice when I tested it with some 1xx-uA meter,
but..
with my 10mA-meter (that is small enough for my 1U case and I planned on using)
I get only tiny amount of movement.
I can adjust the maximun point with trimmers, but with that smaller current meter I've got whole meter range of movement when i lit the ldr.

Is there a simple solution to scale this for 10mA meter, or do i need some opamp driver or paraller ldr's or new meter:)
 

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> ...or paraller ldr's....

If it works with 100uA, but you have 10mA meter, "all you need" is one hundred LDRs and identical light on all of them.

Opamp buffer makes more sense. Buffer the LDR voltage. Put enough resistance on the output to hit 10mA. For example: 1V on dark LDR, you want 1V/10mA or 100 ohms. The meter has some resistance too. Probably best to let the meter loop resistance be 100+ ohms, and trim the LDR source DC voltage a hair over 1V to get full swing.
 
..and check the guts of your 10mA meter to see if there is an internal shunt resistor across terminals, that could be cut out to increase sensitivity (many modern meters are basic 1mA's with different internal shunts)

Also remember: with a 10mA (or any linear DC-reading) meter, your resulting GR reading will NOT be VU-scaled - you'll need to scale ityourself.

Jakob E.
 
ok, thanks for tips! I guess I do it with opamp then:)
the meter has a scale of 250mA and after removing inner shunt-resistors it had 2.5Ohm resistance.
I measured that roughly meter and external resistor in series were 150ohm and with 1,6v battery meter hit maximun
and with 150 resistor it was about halfway..so 1.6V/150=10,6mA what must be 10mA.
 
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