Stereo VU meter kit BUILD THREAD

[Cfeedback]

Hi,
I have installed the VU buffer in my diy power amplifier, and I get a mains hum as soon as I plug in the plugs of the unbalanced inputs.
When I disconnect the inputs from the VU buffer again, the output stage is absolutely quiet.
I wonder if it is due to the hidden pin4 because I didn't solder a wire there.The humming only occurs as soon as IN + is plugged in.
I've already tried three different wiring options, but that didn't work.

[Joe Malone]

Hi,
I have installed the VU buffer in my diy power amplifier, and I get a mains hum as soon as I plug in the plugs of the unbalanced inputs.
When I disconnect the inputs from the VU buffer again, the output stage is absolutely quiet.
I wonder if it is due to the hidden pin4 because I didn't solder a wire there. The humming only occurs as soon as IN+ is plugged in.
I've already tried three different wiring options, but that didn't work.

What power supply are you using to run for the VU buffer ? Meter lamps?
Is that what the extra small transformer and reg pcb on the right is for?
Is its reg output referenced (linked) to the 0v of the audio amps or not?

I can see no connection to the buffer pcb to 0v which maybe correct depending on 0v reference

Looking at your amp no power supply should be needed as looks like a LM3886 amp ic which should have +/-35v max so the vu buffer can be run directly from the +35v rail with just a 1k series resistor to smooth the unreg rails more for the buffer. Also the VU meter lamps could be run in series with a series resistor like 3k from the -35v.

[Cfeedback]

Yes, that's right, I am currently running the VU buffer with a 12 V regulator in the top right corner of the picture.
The regulator is fed by the print transformer.
The output from the reg goes exclusively to the + and - connection of the VU BUffer.

[Joe Malone]

Yes, that's right, I am currently running the VU buffer with a 12 V regulator in the top right corner of the picture.
The regulator is fed by the print transformer.
The output from the reg goes exclusively to the + and - connection of the VU BUffer.

Since that supply is floating you will need to link the 0v from 12v supply to the 0v of the amp supply. You can do this just by linking the -in to the 0v on the VU2 inputs. As I presume the -in is the shield of the unbalanced signal coming from the amp 0v already?

But as explained above no extra power supply is really needed.

Let me know if you are still having problems

[Cfeedback]

Since that supply is floating you will need to link the 0v from 12v supply to the 0v of the amp supply. You can do this just by linking the -in to the 0v on the VU2 inputs.

With the VU 2 inputs you mean the +/- connection for the power supply, correct?

As I presume the -in is the shield of the unbalanced signal coming from the amp 0v already?

Yes that's right.
I only used a balanced cable between the RCA and the volume pot.
An inner conductor lies together with the screen on the RCA input on GND and the screen on the potentiometer is open, there is only one inner conductor on the pot GND and the other on pot IN.
From the potentiometer it goes once to the amplifier pcb and once to the VU2 buffer. And you're right, the two lines are coaxial with only one inner conductor

But as explained above no extra power supply is really needed.

Yes, great, I really like the idea of using just one power supply.
There are +/- 29 volts after the rectification, I just assumed that a regulated power supply, a regulated voltage for the VU2 must be used.

[Joe Malone]

Since that supply is floating you will need to link the 0v from 12v supply to the 0v of the amp supply. You can do this just by linking the -in to the 0v on the VU2 inputs.

With the VU 2 inputs you mean the +/- connection for the power supply, correct?

As I presume the -in is the shield of the unbalanced signal coming from the amp 0v already?

Yes that's right.
I only used a balanced cable between the RCA and the volume pot.
An inner conductor lies together with the screen on the RCA input on GND and the screen on the potentiometer is open, there is only one inner conductor on the pot GND and the other on pot IN.
From the potentiometer it goes once to the amplifier pcb and once to the VU2 buffer. And you're right, the two lines are coaxial with only one inner conductor

But as explained above no extra power supply is really needed.

Yes, great, I really like the idea of using just one power supply.
There are +/- 29 volts after the rectification, I just assumed that a regulated power supply, a regulated voltage for the VU2 must be used.

The -DC of the VU2 needs a connection to the 0v centre ground of the amp supply to make both referenced to the same 0v.

This can be done by a wire directly between -DC and amp 0v

or if Amplifier 0v is at left and right -in on VU2 just solder link 0v (used 3rd pin on input) to -in on VU2 inputs.

[Cfeedback]

The -DC of the VU2 needs a connection to the 0v centre ground of the amp supply to make both referenced to the same 0v.

This can be done by a wire directly between -DC and amp 0v

I'm going to put -DC from VU2 to PGND from amplifier.
PGND from the left and right output stages are connected to each other, so I can see them as one.

or if Amplifier 0v is at left and right -in on VU2 just solder link 0v (used 3rd pin on input) to -in on VU2 inputs.

You mean with amplifier 0V left and right as with dual mono, where both amplifier channels are completely separated from each other?

[Joe Malone]

The -DC of the VU2 needs a connection to the 0v centre ground of the amp supply to make both referenced to the same 0v.

This can be done by a wire directly between -DC and amp 0v

I'm going to put -DC from VU2 to PGND from amplifier.
PGND from the left and right output stages are connected to each other, so I can see them as one.

or if Amplifier 0v is at left and right -in on VU2 just solder link 0v (used 3rd pin on input) to -in on VU2 inputs.

You mean with amplifier 0V left and right as with dual mono, where both amplifier channels are completely separated from each other?

I don't know how to explain it any clear sorry. Just solder the 0v solder pad next to the -in on the 2 x Vu2 inputs which will link -DC to Amp PGND

[Cfeedback]

Okay, we got along right.
Because that was now my train of thought.

First of all, thank you very much for the great help


greetings

[Cfeedback]

Great, that was the solution.

I connected -DC of VU2 with PGND of the output stages and now everything is wonderful.

Thanks again for your help...

[Joe Malone]

Great, that was the solution.

I connected -DC of VU2 with PGND of the output stages and now everything is wonderful.

Thanks again for your help...

Cool great to hear that fixed it. When you have 2 power supplies like this you have to reference them to each other.

[verticalmammal]

Greetings!

Just completed my VU meter kit and I must have done something wrong. The left channel works perfectly, but the meter on the right panel latches high immediately upon power-up. It doesn't seem to respond to adjusting the potentiometer in the back. When I remove power, the meter slowly falls back down. If I switch which meter is plugged into which channel the issue stays with the board, the meter that was on the right previously works great when wired to the left channel of the board, so it's probably not an issue with the actual VU meter component. The behavior makes me suspect that a capacitor somewhere on the right channel circuit is somehow involved in the issue, but I can't see anything obvious when I look it over. Here is an album with pictures of my build. What do you think, anything jumping out?

Thanks!

[Joe Malone]

Greetings!

Just completed my VU meter kit and I must have done something wrong. The left channel works perfectly, but the meter on the right panel latches high immediately upon power-up. It doesn't seem to respond to adjusting the potentiometer in the back. When I remove power, the meter slowly falls back down. If I switch which meter is plugged into which channel the issue stays with the board, the meter that was on the right previously works great when wired to the left channel of the board, so it's probably not an issue with the actual VU meter component. The behavior makes me suspect that a capacitor somewhere on the right channel circuit is somehow involved in the issue, but I can't see anything obvious when I look it over. Here is an album with pictures of my build. What do you think, anything jumping out?

Thanks!

OK nothing stand out except under the pcb is very hard to see problems as the pcb needs a good clean with an old toothbrush and isopropyl alcohol. Clean under the pcb and attach here a large clear well lit in focus photo of the pcb bottom. And also give a list of all voltages measured to 0v on the 8 opamp pins. Make sure your power supply is 12v or more.

Post up here the photo and voltages

[verticalmammal]

OK nothing stand out except under the pcb is very hard to see problems as the pcb needs a good clean with an old toothbrush and isopropyl alcohol. Clean under the pcb and attach here a large clear well lit in focus photo of the pcb bottom. And also give a list of all voltages measured to 0v on the 8 opamp pins. Make sure your power supply is 12v or more.

Post up here the photo and voltages

Awesome, thanks! I'm using a 12v ifi power supply. Voltages as follows, using this numbering:

1. 6.30
2. 6.30
3. 6.21
4. 0.00
5. 6.20
6. 6.22
7. 6.22
8. 11.90

Here are some more PCB pictures that will hopefully be more helpful. It looked fine after one round, but then I took that first picture and wow, the camera showed me whole worlds of gunk I couldn't see with my naked eye, so I cleaned it some more and took another picture.





Let me know if you'd like another round of cleaning and some more pics, or different angles or voltages or anything. Hugely appreciate your help, thanks again!!

[Joe Malone]

OK nothing stand out except under the pcb is very hard to see problems as the pcb needs a good clean with an old toothbrush and isopropyl alcohol. Clean under the pcb and attach here a large clear well lit in focus photo of the pcb bottom. And also give a list of all voltages measured to 0v on the 8 opamp pins. Make sure your power supply is 12v or more.

Post up here the photo and voltages

Awesome, thanks! I'm using a 12v ifi power supply. Voltages as follows, using this numbering:

1. 6.30
2. 6.30
3. 6.21
4. 0.00
5. 6.20
6. 6.22
7. 6.22
8. 11.90

Here are some more PCB pictures that will hopefully be more helpful. It looked fine after one round, but then I took that first picture and wow, the camera showed me whole worlds of gunk I couldn't see with my naked eye, so I cleaned it some more and took another picture.


Let me know if you'd like another round of cleaning and some more pics, or different angles or voltages or anything. Hugely appreciate your help, thanks again!!

Voltages are correct. Have you retried the pcb since cleaning it? Also first turn the trimpot back 25 turns encase you have one on full gain. With no meters connected measure volts across +M- on both sides and let me know what volts you read?