Author Topic: How to "copy" signal for digital VU meters?  (Read 2691 times)

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Offline algorhythmTopic starter

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How to "copy" signal for digital VU meters?
« on: April 24, 2016, 01:46:16 am »
Hello EEVBlog.

I'm putting together a relatively simple audio system for my workshop:

raspberry pi + hifidac -> lm2b unbalanced-balanced conv. -> dbx231s eq -> dbx 233xs xover -> art sla-4 amp

It sounds great so far.  I bought a couple of the cheapo Banggood AS30 VU Meter kits to fill the extra rack space with blinky goodness (see pics).

When I split signal from the raspberry pi into the meters, they severely distort the signal.  When playing a 440Hz sine wave through the system and looking at it on the scope, I notice that approximately the top 1/3 of the waveform is "rounded" flat (i.e. soft-clipped).  It's really strange.  I like these meters since they're simple looking and fit well in the extra rack space, but they murder the signal.  It looks like there's no input stage.  The audio signal goes straight into the uc.

Is there a good way to "copy" a signal for the meters without affecting the signal going into the audio system?  What's the best way of doing it?

I've tried feeding the meters from the balanced outputs of the eq, using various combinations of +/-/g, but the problem persists.  I've got an audio isolation transformer coming in the mail for a different project, and will probably try it here to see if anything changes, but I have no idea if it will make a difference.

Can anyone point me in the right direction with this?

 

Offline algorhythmTopic starter

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Re: How to "copy" signal for digital VU meters?
« Reply #1 on: April 24, 2016, 03:04:39 am »
Isolation transformers just arrived.

Each meter is being used as a 'mono' meter since I have two, so I'm just putting left channel into one meter and right into the other.

Attached is a schematic of the left channel and what I tried with a transformer, and it actually seems to work, with only minor attenuation of the signal and no distortion.  Please see the picture.

What are the problems with this?
 

Offline algorhythmTopic starter

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Re: How to "copy" signal for digital VU meters?
« Reply #2 on: April 24, 2016, 04:04:23 am »
Ok great. The solution is easier than I had hoped. $2.52 is no problem.

Can I get away with a circuit as simple as the one in the picture here?  Are caps a good idea, or unnecessary?

 

Offline algorhythmTopic starter

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Re: How to "copy" signal for digital VU meters?
« Reply #3 on: April 24, 2016, 04:16:00 am »
Ok excellent.

Something like 1k resistor between buffer output and vu input?

I actually have a TL072 in my drawer and am going to mess around with it right now.  5v is the minimum supply voltage, so hopefully it works, otherwise I'll order a couple OPA2365s.
 


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