I would use higher values around U1B, like 10x. Don't forget the 3.9k resistor in series with the meter.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 )
Ok thanks, I'll scale up. The meter actually has another buffer so I should be ok.I would use higher values around U1B, like 10x. Don't forget the 3.9k resistor in series with the meter.
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 ..
+4dBu=0vu=1.228VCan I ASSume you are switching between +4dBu (re 0dB = 0.7745V) and -10dBV (re 0dB= 1V)?
JR
Added 100Kohm in parallel and adjusted values which should alleviate the momentary open loop condition.Definitely put a fixed feedback resistor directly from non-inverting input to output (pins 6 and 7) of that second stage.
Bri
The issue with the lower value is that the NFB circuit dominates the load (the meter + series resistor itself should appear like about 7k).Ok thanks, I'll scale up. The meter actually has another buffer so I should be ok.
gotcha that makes sense. I don’t think it will be an issue in this case but I will adjust for proper freq. responseThat feedback cap will form a LPF with the feedback R.
47pF will be OK as long as feedback Rs are 100k or less.
JR
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.
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...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.....
I really don’t understand why you wouldn’t use the schematic I suggested. This 100kohm as Rf is unnecessary and superfluous..... 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...
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 really don’t understand why you wouldn’t use the schematic I suggested. This 100kohm as Rf is unnecessary and superfluous.
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.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...
Am I wrong in thinking that the trim pots will interact when they are in parallel? Maybe it doesn't matter.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.
You could use only one. That's what the JLM VU buffer does. Usually, when doing this kind of clever optimization, it makes things a little more convoluted and difficult to troubleshoot.I was attempting to use less opamps.
And it can't be done any other way. The gain should be set to -10dBu first, and then using a second trimmer for + 4dBuHowever, when adjusting the trimmers, if you cal the -10 pot first and then the +4 it shouldn't be a problem.
I like this one :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
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