Buffered Mult, is this correct?

Hi there I designed a buffered mult but I don’t have a lot of experience I wanted to know if you guys think it would work and if not what are the necessary parts. I don’t mind the signal being inverted if it stays within 0 - 1V. Thanks.
AN

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There’s generally a 499r resistor on the output & a 100K resistor to buffer the inputs of the op-amps of video synth modules.
Calling a few of the engineers around here: @aladan, @Fox, @rempesm @reverselandfill, @syntonie
Any obvious component requirements or circuit improvements to suggest to @Jestern?

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There’s a thread somewhere on here that covers the basics of video synth circuits.
(update: found it I think)

Is RV2 a potentiometer that acts as a VCA?

I have two coming as well. One is a 1:3 Distribution Amplifier and the other is a YRGB Distro Amp.

I’ll share the schematics via DM for now; don’t want prototype version floating around you know.

As for the schematic you drew, you should have a 100K pulldown on the TIP connector of your input and a 499R resistor at the output. The location of your pot will act as an attenuator only.

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At first I thought I was on the wrong thread as I incorrectly thought I had seen your RGB Crossfader panel :sweat_smile:
Cheers @Fox for confirming the resistor values. I’ve got to read and learn what “pull down resistor” means.

For anyone like me who hasn’t a clue about electrical circuits.

Maybe it should be called termination resistor in this case. Pull-ups and Pull-downs are terms typically used to digital electronics, but each is kind of ambiguous.

The 100k protects a disconnected input from oscillating but also creates a voltage divider with the signal’s series impedance. They also prevent a wire’s resistance from increasing t infinity, but that really isn’t relevant in this case since most user’s wires are a few feet at best.

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It will only act as an attenuator in this case. For it to create gain, it would have to be in the feedback path of the op amp.

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That pot on the output is bad news. If you turn the output all the way down, the output will be ground. If you use stacking cables for example, that would force all signals on the stack to short to ground. Put the attenuator on the input.
100k to ground on the input jack, through 499ohm to the pot. Wiper of pot to the input of opamp. 499 between output of opamp and jack.

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Thanks everyone :slight_smile: that pot was a mistake. Is it ok like this?

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If you add 100nF bypass caps between V+ and GND as well as V- and GND on the LM6172, that looks right to me.

Here’s one I posted in another thread:

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This then

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This is starting to look a lot like a proper buffered mult :slight_smile:

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if you have an unused opamp stage (like in your schematic above), you need to do this:
image

but I recommend to just add another buffer on your pcb.
in the LZX schematics the output jacks switches are not connected to GND.

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On the input section the 100K is there as a pull-down resistor but what is the 499r doing after? Adjusting the Voltage to a bandwidth of 1V or stopping or limiting a hot signal into the op-amp?

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ok what about now?

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Best not to wire the socket switch pins to ground - that will mean you short the tip of the plug to ground briefly as you insert it. On the output side shorting the op-amp to group (via 499R) when no plug is inserted loads down the op-amp unnecessarily. In general leave the switch pin unconnected on outputs and use a small series resistor on inputs.

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A nice trick to add to your circuit is to normalise the second input to the first input (might by incorrectly phrased), that way you’ve six cloned outputs if nothing is plugged into the second input.
I’ve seen this done on other threads by some of the various circuit designers here. A resistor was required but where exactly the first & second inputs connect, I can’t say sadly. I’ll scout for some of the threads & post a link if I get lucky.

@jestern, here’s a couple of screenshots of other circuits found on here that show what Adrian was talking about.

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Perfect thanks guys!!!

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