Does anyone know why this voltmeter causes squealing?

[Esppse]

Hello,

I installed a voltmeter like this on a pedal. When the meter is disconnected, the pedal sounds like it should. When I introduced this meter into the unit, there's a high pitch squeal. Is there anything I can do to remove the squealing?

https://www.ebay.com/itm/New-Auto-Car-Mini-Red-Voltmeter-Tester-3-Digital-Voltage-Test-Battery-DC-0-30V/183598466535?hash=item2abf5245e7:g:2zcAAOSwP4FaSvWH

3 wires:
Black is gnd
Red powers the meter, white reads the power, so both together to 9V.

Thanks

[Phoenix]

What is the pedal (schematic?), and how is the DC voltmeter connected? Also, you say it doesn't squeal when disconnected, do you mean when power is disconnected from the meter, or when the white lead is disconnected from the circuit? Photos of the guts would be good too.

[amptramp]

There is usually an internal clock to drive the A/D converter and the LED displays.  In most cases, the varying current drain is if no consequence.  In audio, the clock is either within the audio range (which is possible because anything at 9000 Hz or above requires FCC testing for interference).

[Rob Strand]

It would be good to get an answer to Greg's questions.  Especially if the white wire is connected to an audio line and if the squeal goes away when you disconnect the white wire.    I've seen microprocessors with unbuffered A/D inputs which can inject pulses on high-impedance lines.  It's like there is capacitor based sampler, or chopper, periodically shorting a cap across the input then charging up.  That effect will produce an audible effect for sure.

[Esppse]

It's just a simple gain boost Zvex Super hard type circuit actually. Both red and white are connected where 9V in would be.

I have tested 3 of the same red voltmeters and they all squeal. I have the same in blue, that squeals too. However, I tried a green one, that was dead quiet and worked perfectly. Very strange.

When the led display drops from 8.57 to 8.56 etc... the pitch of the squeal changes. As if the led is bleeding through somewhere.

It may be a faulty design, it's a cheap meter, I just wanted one to monitor battery levels.

[Rob Strand]

It may be a faulty design, it's a cheap meter, I just wanted one to monitor battery levels.

I couldn't say it squeals because it is cheap.

My guess is it has something to do with how the display is driven.  The display could be multiplexed and that naturally has a update frequency.    It's hard to know what is going on without probing the waveforms.  A tthe end of the day it does what it does and you have to work around it.

A simple test would be to put 100uF across the power supply rails. 

A better filter might be:

9V--> 100uF to gnd ---> Display +ve Power ---> 100 ohms --->  100uF to gnd -> Display input ---> Pedal

All that is probably overkill but if it fixes the problem you know you have crap on the supply.
If you can be bothered, after that you could try pull stuff out and try using smaller caps  to come-up with a minimalist solution.

BTW things like transistor and mosfet boosters are susceptible to crap on the power rails.   That why you get hum problems with wall-warts.

[Phoenix]

In addition to Rob's excellent suggestions re: RC power filter, I would also make sure to connect the voltmeter directly to the DC jack/battery snap pads to minimize the loop area of the digital meters current pulses. I would also make sure the meters power leads do not go near any audio signal wires, or if they must cross, they do so at right angles to reduce any coupling.

[Esppse]

Thank you very much for the help, I'll attempt these workarounds!