Hi-Z signals driving LED bar graph meter

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TijuanaKez

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Jul 19, 2011
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Hey there knowledgable wizards. Still very much an inexperienced learner, but getting ready to tackle my first useful project.
Problem to be solved is having a super quick way to check for the existence of signal directly before or after stage DI box.

I'm imagining a 10 segment LED bar driven by an LM3914 or similar.
I thought I could use the input stage from this project as a base, but drive the LM3914 directly without Arduino's and such.

So before I waste too much time, my first important question is... is the 1M resistor in the linked input stage sufficient enough to protect the delicate signal of an acoustic guitar piezo pickup for example?
I need to be able to roughly meter signal but without altering it.
The quality of the amp is obviously unimportant, I only want to be able to get confirmation that a signal is present.

Ultimately, I will need to incorporate a switch to detect either Hi-Z signals or line level signals, depending on whats plugging into the DI.

thanks!
 

Bo Deadly

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Dec 22, 2015
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New Jersey, USA
... is the 1M resistor in the linked input stage sufficient enough to protect the delicate signal of an acoustic guitar piezo pickup for example?
I need to be able to roughly meter signal but without altering it.
...
Ultimately, I will need to incorporate a switch to detect either Hi-Z signals or line level signals, depending on whats plugging into the DI.

No. Piezos are basically a capacitance so it requires a charge amplifier or very high impedance amplifier like the bootstrapped high impedance amplifier (scan over that thread about piezos as well). Just about any current drawn from the piezo will affect it's output by at least loosing lows. You could have a momentary button to tap into it and just look and then release the button to disconnect the circuit but otherwise you will have to tap into after the DI. And presumably the DI is a special high impedance type for piezos. A conventional 1M will loose lows.
 

TijuanaKez

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Joined
Jul 19, 2011
Messages
45
No. Piezos are basically a capacitance so it requires a charge amplifier or very high impedance amplifier like the bootstrapped high impedance amplifier (scan over that thread about piezos as well). Just about any current drawn from the piezo will affect it's output by at least loosing lows. You could have a momentary button to tap into it and just look and then release the button to disconnect the circuit but otherwise you will have to tap into after the DI. And presumably the DI is a special high impedance type for piezos. A conventional 1M will loose lows.
Ok thanks for the reply!
Ok If we forget piezos actually for now, would it be ok for electric guitar pickups?
Thinking about things more, I don't think there would be a case where piezo wouldn't have some kinda of preamp before the DI.
 

Bo Deadly

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Dec 22, 2015
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Ok If we forget piezos actually for now, would it be ok for electric guitar pickups?

It would probably be ok but some folks around here would find fault with it. The problem is that if your DI is 1M and your circuit is 1M, then the load on the guitar will no longer be 1M but 1M||1M (the || means 'in parallel') or 500K. Personally I think 500K is just fine for just about any guitar pickup.

So again, if you really want to tap into a high impedance line, technically you need to do something like the bootstrapped input. Otherwise, take the signal from after the buffer (DI).

Or again, if you are just switching it in momentarily, then maybe it's ok.
 

JohnRoberts

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Hickory, MS
Hey there knowledgable wizards. Still very much an inexperienced learner, but getting ready to tackle my first useful project.
Problem to be solved is having a super quick way to check for the existence of signal directly before or after stage DI box.

I'm imagining a 10 segment LED bar driven by an LM3914 or similar.
I thought I could use the input stage from this project as a base, but drive the LM3914 directly without Arduino's and such.
The LM3914 is rather old technology. You could roll your own meter using something like LM339 comparators with much higher input impedance.

JR
So before I waste too much time, my first important question is... is the 1M resistor in the linked input stage sufficient enough to protect the delicate signal of an acoustic guitar piezo pickup for example?
I need to be able to roughly meter signal but without altering it.
The quality of the amp is obviously unimportant, I only want to be able to get confirmation that a signal is present.

Ultimately, I will need to incorporate a switch to detect either Hi-Z signals or line level signals, depending on whats plugging into the DI.

thanks!
 

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