Hi-Z signals driving LED bar graph meter

[TijuanaKez]

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]

... 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]

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]

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]

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!

 

[TijuanaKez]

Ok thanks!

The LM3914 is rather old technology. You could roll your own meter using something like LM339 comparators with much higher input impedance.

What are the chances of being able to power such a circuit from +48V phantom do you think?

I know it has usually only around 10ma, but I've seen a few LEDs on phantom powered devices before. It wouldn't need to be bright.

Perhaps by charging some caps and PVMing the led graph at low duty cycle?

 

[JohnRoberts]

one could certainly design a discrete meter capable of running from 48V phantom but who wants to waste a mic input?

JR

 

[abbey road d enfer]

Problem to be solved is having a super quick way to check for the existence of signal directly before or after stage DI box.

Why do you think you need to check the signal before and after? Are you not confident enough in your DI?
Think of it: the proof of signal existence is in something thart can detect it. So you need a preamplifier to turn the signal into something capable of activating an indicator.
Whether you call that an instrumentation amplifier or a DI is irrelevant. In essence it's the same.
So you just need to control the signal at the output of the DI.
As JR suggested a LED ladder combined with low-power comparators could work within the limits of the 10 mA available from phantom power.

 

[JohnRoberts]

LEDs are commonly used built into cables to indicate presence of phantom power, actually kind of useful to troubleshoot bad cables or phantom power presence.

A simple phantom powered signal present circuit could be envisioned, but care must be taken to keep it common mode and slow moving to not introduce noise, or corrupt grounds. To detect mic level signals it would have to involve high gain and would surely overload in line level use.

JR

 

[TijuanaKez]

Ok thanks for the replies.

Why do you think you need to check the signal before and after? Are you not confident enough in your DI?

Not before and after, either would be fine. But the more I think on this, the ultimate would be to build it into a home-brew DI.
I play in a band and we need many DIs on stage. Frequently we arrive at a short changeover to our set at a festival, and there will be some kind of connectivity issue where the blame game gets thrown around. Is it the cable? Is it the patching wrong? Is it the source? or the DI itself. The stage techs are always quick to blame the source, where we, knowing our gear had no issues the previous gig, will blame the stage.

So 'what we need signal meter, or a DI with a meter in it' gets said a lot. That way the band, and the stage techs can know instantly who's end the problem lies.

Contrary to the title, and since I've reconsidered to best way to tackle this since posting, the common use would not be for Hi-Z, actually everything would be either line-level or instruments with onboard preamps.

Having a switch to switch the meter in and out is fine, if it would help protect the signal.
I would presumably need another switch to switch the range of the meter from line level, to a range more suitable to instruments.

As JR suggested a LED ladder combined with low-power comparators could work within the limits of the 10 mA available from phantom power.

In this case, do you think I would still need some kind amp to drive it?

I'm looking at simple design with maybe Cinemag 12:1 DI transformer.

Would it be better to meter before or after?
If after, I need to unbalance the balanced signal first though right? If I tap one leg, it could through out the CMRR correct?

 

[abbey road d enfer]

So 'what we need signal meter, or a DI with a meter in it' gets said a lot. That way the band, and the stage techs can know instantly who's end the problem lies.

Contrary to the title, and since I've reconsidered to best way to tackle this since posting, the common use would not be for Hi-Z, actually everything would be either line-level or instruments with onboard preamps.

Anyway, a proper meter needs some kind of buffer for sensing Hi-Z signals, so you'd better build a DI and graft a "meter" to it.

Having a switch to switch the meter in and out is fine
, if it would help protect the signal.

You wouldn't need a switch to disconnect the meter, since the buffer will isolate it.

I would presumably need another switch to switch the range of the meter from line level, to a range more suitable to instruments.

That is quite possible. It may also be possible to use an extended range meter, but it would be more complex.

In this case, do you think I would still need some kind amp to drive it?

Definitely. You need a rectifier to shape the signal into a usable form.

I'm looking at simple design with maybe Cinemag 12:1 DI transformer.

Since you'll have most of the ingredients of an active DI, I don't see the point in having a xfmr.

Would it be better to meter before or after?

Before or after is irrelevant. You nedd to see a sign that signal is happening.

If after, I need to unbalance the balanced signal first though right?

After a passive DI, yes, and that would be utterly crazy.

If I tap one leg, it could through out the CMRR correct?

It would, but also the meter indication would probably be seriously off.

 

[xander]

This, as an introduction, is a bit beyond the thread topic, but is just a take on my methodologies and development of metering in general. Personally I gave up on an assortment of displays. Including LED's beyond basic indicators lights and typical analog metering for a number of reasons; faceplate space for one. For the past 2 years or so I have favored a more comprehensive metering methodologies. Of those I tripped over the EVOR04, on to the EVOR04-slim.

A current project for a client is to interface a digital unit providing echo/doubler/signal delay/reverb/chorus/flanger/Trem/WahWah into a custom guitar amplifier I built for the client some years ago. The client request requires interfacing an accutronics BTSE-99FX DigitalSound Effector with an EVOR04-slim. I believe the AccuBell reverbs are part of the reverbs implemented in the BTSE-99FX (unconfirmed currently).

So as not to highjack this thread I will likely create a thread on this project as the project takes place. At this time parts are ordered and we shall see what the broken supply chain will do to the project timeline. Lately its been pretty bad.

The client's interests are, reducing the number of their floor pedals and to use the EVOR04-slim; for space taken and reduced noice (less cheap boxes, less additive noice). That, to present a controllable, programable effects listing table that selects any of the effects in the BTSE-99X; listed by name, not just a number display. Thus adding that functionality in place of the BTSE-99FX's suggested two digit display is a key task.

The outcome of other EVOR04-slim project have been very favorable. Data sheets, attached. The EVOR04 and EVOR04-slim do seem to be operationally equal.

In past projects and in my own equipment I have added compression level, input and output, and harmonics/saturation metering. The combined display functions do eliminate dozens of traditional analog meters and LED displays. Plus the EVOR04's has a small though effective Oscilloscope built in. Which has saved my clients and me on numerous live gigs when "things" went wrong.

 

[abbey road d enfer]

This, as an introduction, is a bit beyond the thread topic, but is just a take on my methodologies and development of metering in general. Personally I gave up on an assortment of displays. Including LED's beyond basic indicators lights and typical analog metering for a number of reasons; faceplate space for one. For the past 2 years or so I have favored a more comprehensive metering methodologies. Of those I tripped over the EVOR04, on to the EVOR04-slim.

That's an interesting product, as well as the others on their website. However I doubt this could be relevant to the subject of the thread, since the power requirements are about 4 times what's available with P48 powering.
As you indicated, I suggest you open a new thread in the Drawing board.

 

[xander]

Will do, thanks.