VU meter Input

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RSRecords

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So trying to switch between a +4VU and -10dBV signal and have both be independently calibrated to 0vu. Won't be heard just level referenced. VU buffer is it's own thing. This gonna work?( edit r12 is 3.83K )1631639728948.png
 

abbey road d enfer

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So trying to switch between a +4VU and -10dBV signal and have both be independently calibrated to 0vu. Won't be heard just level referenced. VU buffer is it's own thing. This gonna work?( edit r12 is 3.83K )
I would use higher values around U1B, like 10x. Don't forget the 3.9k resistor in series with the meter.
 

JohnRoberts

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tl072 is only rated to drive 2k load total, so you need to use larger resistor values.

Can I ASSume you are switching between +4dBu (re 0dB = 0.7745V) and -10dBV (re 0dB= 1V)?

JR
 

moamps

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In addition to what has already been said I would modify the switching a bit more to avoid a small moment when there is no Rf resistor present between the output and the inverting opamp's input. To be honest, it's not so critical here ..
 

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JohnRoberts

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A small capacitor from output to - input would prevent squirrel barks during switch transitions.

JR
 

RSRecords

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Thanks folks!

I would use higher values around U1B, like 10x. Don't forget the 3.9k resistor in series with the meter.
Ok thanks, I'll scale up. The meter actually has another buffer so I should be ok.

In addition to what has already been said I would modify the switching a bit more to avoid a small moment when there is no Rf resistor present between the output and the inverting opamp's input. To be honest, it's not so critical here ..

wouldn't this put them in parallel in the NC switch position? I'm sure it could be worked out but I think I'll add a fixed resistor so I'll only have to work out the individual parallel resistors. That way the trims won't (shouldn't?) interact.
Can I ASSume you are switching between +4dBu (re 0dB = 0.7745V) and -10dBV (re 0dB= 1V)?

JR
+4dBu=0vu=1.228V
-10dBV=.316v

Definitely put a fixed feedback resistor directly from non-inverting input to output (pins 6 and 7) of that second stage.

Bri
Added 100Kohm in parallel and adjusted values which should alleviate the momentary open loop condition.
I'll add small cap in parallel as well per JR's suggestion. I usually just roll with a 47pF in that position. Any reason that wouldn't work here?
 

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JohnRoberts

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That feedback cap will form a LPF with the feedback R.

47pF will be OK as long as feedback Rs are 100k or less.

JR
 

abbey road d enfer

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Ok thanks, I'll scale up. The meter actually has another buffer so I should be ok.
The issue with the lower value is that the NFB circuit dominates the load (the meter + series resistor itself should appear like about 7k).
The lowest value for the NFB circuit should be no less than 3k.
 

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Paul Wolff
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"A small capacitor from output to - input would prevent squirrel barks during switch transitions.

JR"

What John said, and put it on both amps, 47pf. The 200K feedback R is essential as well, so the meter doesn't ping. Never let an I-amp have a moment of no feedback, it goes to full gain, which is where the term "open-loop gain" comes from.....
 

RSRecords

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That feedback cap will form a LPF with the feedback R.

47pF will be OK as long as feedback Rs are 100k or less.

JR
gotcha that makes sense. I don’t think it will be an issue in this case but I will adjust for proper freq. response
The issue with the lower value is that the NFB circuit dominates the load (the meter + series resistor itself should appear like about 7k).
The lowest value for the NFB circuit should be no less than 3k.

"A small capacitor from output to - input would prevent squirrel barks during switch transitions.
JR"

What John said, and put it on both amps, 47pf. The 200K feedback R is essential as well, so the meter doesn't ping. Never let an I-amp have a moment of no feedback, it goes to full gain, which is where the term "open-loop gain" comes from.....
Makes sense. The 100k will stay in fb loop and the parallel resistors and trims will switch. This means momentarily a gain of 10. Which might bounce the meter but shouldn’t blow anything up...
 

moamps

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.... The 100k will stay in fb loop and the parallel resistors and trims will switch. This means momentarily a gain of 10. Which might bounce the meter but shouldn’t blow anything up...
I really don’t understand why you wouldn’t use the schematic I suggested. This 100kohm as Rf is unnecessary and superfluous.
 

abbey road d enfer

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I really don’t understand why you wouldn’t use the schematic I suggested. This 100kohm as Rf is unnecessary and superfluous.
As you know, there are many ways to skin this cat. The switching could be at the input and done in at least two different ways...
I'm not sure the OP is alert at the implications of all the possibilities, either. Sometimes it's dfficult to see the forest for the trees...
 

RSRecords

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As you know, there are many ways to skin this cat. The switching could be at the input and done in at least two different ways...
I'm not sure the OP is alert at the implications of all the possibilities, either. Sometimes it's dfficult to see the forest for the trees...
True. many ways to skin cats. I was attempting to use less opamps. Not a huge deal to add another I know but this is only 1 channel of 4.. I know of at least 3 ways I can build this. Unfortunately I actually need to pick one and build it.
I really don’t understand why you wouldn’t use the schematic I suggested. This 100kohm as Rf is unnecessary and superfluous.
Am I wrong in thinking that the trim pots will interact when they are in parallel? Maybe it doesn't matter.
Though I think I'll try and add a jumper incase.

Also, this will never be heard. it is only for metering and while it doesn't have to be perfectly accurate it does need to be fairly precise (eg trim pots)
 

Brian Roth

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There will be interaction but without a closer analysis I can't say how much. However, when adjusting the trimmers, if you cal the -10 pot first and then the +4 it shouldn't be a problem. I still vote for totally independent trims and a large value fixed R across the feedback loop to prevent the opamp going open loop.

Bri

Am I wrong in thinking that the trim pots will interact when they are in parallel? Maybe it doesn't matter.
Though I think I'll try and add a jumper incase.
 

gswan

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Meter movements like to be driven with a current source. Adding a series resistor with an op amp voltage source sort of approximates this. Alternatively you can use a transconductance amplifier to create a current source output from a voltage source input.
 

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