Hi Guys,
I've been working on adding a pair of 11 LED VU meters to my DIY Boombox. I built up the kits and tested them on an old National Boombox external speaker outputs and both kits work fine. I powered the led kits by the DIY box SLA battery.
The problem I have is that soon as I connect the kits to the Lepai speaker output terminals, the Lepai cuts out like it's sensing a short.
I've triple checked my wiring and all looks ok. I Incidently tried a spare speaker on the wiring that goes to the led kit inputs and the speaker works fine.
Any ideas?
I'd appreciate any help.
Thanks,
James.... 🙂
I've been working on adding a pair of 11 LED VU meters to my DIY Boombox. I built up the kits and tested them on an old National Boombox external speaker outputs and both kits work fine. I powered the led kits by the DIY box SLA battery.
The problem I have is that soon as I connect the kits to the Lepai speaker output terminals, the Lepai cuts out like it's sensing a short.
I've triple checked my wiring and all looks ok. I Incidently tried a spare speaker on the wiring that goes to the led kit inputs and the speaker works fine.
Any ideas?
I'd appreciate any help.
Thanks,
James.... 🙂
Quick guess.
The amp uses a bridged output stage and you are attempting to connect the meter 'across' the speaker which then shorts the one side of the amp out via the common ground connections.
You need to connect the meter input to one side of the speaker only... either side it doesn't matter. Do not connect any other wire/s to the speaker.
The amp uses a bridged output stage and you are attempting to connect the meter 'across' the speaker which then shorts the one side of the amp out via the common ground connections.
You need to connect the meter input to one side of the speaker only... either side it doesn't matter. Do not connect any other wire/s to the speaker.
Thanks for the quick reply. I've worked all day on this!' So at the moment I have a positive and negative wire piggy backing the speaker positive and negative wires at the amp outputs.
Each LED board (one for each channel) has a positive and negative input terminal. So for a noob like me, exactly how should I wire it? Are you suggesting I just connect one wire to the led board?
I hoping this will save the day.
Thanks in anticipation.
James.... 🙂
Each LED board (one for each channel) has a positive and negative input terminal. So for a noob like me, exactly how should I wire it? Are you suggesting I just connect one wire to the led board?
I hoping this will save the day.
Thanks in anticipation.
James.... 🙂
I think you will find that the power supply negative (or ground as you might call it) for the LED meter and the negative or ground meter input are one and the same point.
So when you wire it all up you are placing a dead short on one of the amplifier outputs.
What you need to do is wire the LED meter power supply as normal and then connect just the positive meter input connection to either speaker terminal. Doesn't matter which.
That will prevent the short circuit. Assuming the meter input is AC coupled (with a capacitor) it should all work. If it doesn't then we need to see the circuit details. You might have to add a cap to block the DC voltage that is on each speaker terminal.
So when you wire it all up you are placing a dead short on one of the amplifier outputs.
What you need to do is wire the LED meter power supply as normal and then connect just the positive meter input connection to either speaker terminal. Doesn't matter which.
That will prevent the short circuit. Assuming the meter input is AC coupled (with a capacitor) it should all work. If it doesn't then we need to see the circuit details. You might have to add a cap to block the DC voltage that is on each speaker terminal.
I've tried having the positive and negative power connected to the LED board and either just the positive or negative speaker input cable connected. The led board does not function properly with only one connection. I did notice however that if you just connect the negative input to the LED board, it maxes out with all LEDs lit even with the volume turned right down.
I'm not sure how to post pics on this forum but the power terminals and the input terminals of the LED board don't look to be in the same circuit.
I'm not sure how to post pics on this forum but the power terminals and the input terminals of the LED board don't look to be in the same circuit.
Scratch that, you are right, the negative of the power input and the negative signal input of the LED board are indeed in circuit together via a small cap.
Ok, after further testing, I've found that if I connect both of the speaker inputs and just the positive power power to the LED board, I can get it to sort of work. Only half of the 11 LEDs light up with high volume - even with the trimmer pot turned up to max. If I try any another connection combo either nothing happens of the Lepai cuts out momentarily.
This is the kit I'm talking about:
http://pages.ebay.com/link/?nav=item.view&alt=web&id=371216743342
http://pages.ebay.com/link/?nav=item.view&alt=web&id=371216743342
Your amp has two power amps per channel. That is 1 and 2 here (the speaker IS NOT ground referenced). When you connect your meters input ground to the speaker that is why it shorts it out. So leave the input ground floating.
It all runs off the 'red' supply (I assume), that is to say amp and meter share the same supply.
Blue is ground. Your meter connects via one wire, the green.
If it doesn't work as it should then we would need to see diagrams for it. It should work.
If the input sensitivity to he meter is wrong (all or not enough LED's lighting) then you would have to look at altering the gain of the meter circuit.
It all runs off the 'red' supply (I assume), that is to say amp and meter share the same supply.
Blue is ground. Your meter connects via one wire, the green.
If it doesn't work as it should then we would need to see diagrams for it. It should work.
If the input sensitivity to he meter is wrong (all or not enough LED's lighting) then you would have to look at altering the gain of the meter circuit.
Attachments
Thank you very much for your assistance.
I've tried connecting the positive and negative power supply wires plus just the positive wire for the LED board input but I can get only 4 of the 11 leds to light up. If I connect the negative wire from the speaker to the negative of the led board inputs I get all leds lit?!?
Details of the 2 x LED boards (one per channel).
I've tried connecting the positive and negative power supply wires plus just the positive wire for the LED board input but I can get only 4 of the 11 leds to light up. If I connect the negative wire from the speaker to the negative of the led board inputs I get all leds lit?!?
Details of the 2 x LED boards (one per channel).
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That looks a pretty basic circuit and not really suited to what you want. The diode D1 is a simple rectifier. That means the circuit needs at least 0.4 volts to overcome the volt drop of the diode and so light the first LED. Both speaker terminals will have around 7 volts DC on them. That will cause the LED's to light.
You need to AC couple the input with a series capacitor. Try a 47uf 25 volt cap with the positive end to the speaker.
So its wired like this.
1/ The 12 volt supply connected to the two points marked -12 and +12.
2/ A single wire from the input at D1 wired to the negative end of a 47uf cap. The positive end of the cap goes to the speaker terminal.
So just three wires in all. How well it might work is open to question. Its not ideal as an audio power meter.
You need to AC couple the input with a series capacitor. Try a 47uf 25 volt cap with the positive end to the speaker.
So its wired like this.
1/ The 12 volt supply connected to the two points marked -12 and +12.
2/ A single wire from the input at D1 wired to the negative end of a 47uf cap. The positive end of the cap goes to the speaker terminal.
So just three wires in all. How well it might work is open to question. Its not ideal as an audio power meter.
Thanks again. I'll source some caps today and give that a try. I've tested these meters on old Boombox speaker outs and they work well. I guess my limited understanding of electronics didn't allow me to realise just because it works on an old school amp doesn't mean it will work on the Lepai.
If your suggestion doesn't pan out, is there a LED VU meter you would suggest? Would using a meter that taps from the amp input side be a better solution or will that affect the strength of input signal the amp receives from an MP3 player for example?
You've been a great help here, I do appreciate it.
James..... 🙂
If your suggestion doesn't pan out, is there a LED VU meter you would suggest? Would using a meter that taps from the amp input side be a better solution or will that affect the strength of input signal the amp receives from an MP3 player for example?
You've been a great help here, I do appreciate it.
James..... 🙂
Ok, just tested with the cap as instructed and it's now working! The only issue is that with the trimmer pot at max, the LEDs don't dance until the volume is above 11 o'clock. If you turn it up to the 2 o'clock position, I can get all LEDs to light with a bass hit.
Is there a way to increase the sensitivity slightly?
Is there a way to increase the sensitivity slightly?
Sorry for all the noob questions.
Well after a couple of hours of use, it appears to be loosing sensitivity - I now have to crank up the volume higher to get the LEDs to light up.' Any ideas?
Well after a couple of hours of use, it appears to be loosing sensitivity - I now have to crank up the volume higher to get the LEDs to light up.' Any ideas?
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The only way to make it more sensitive would be to add a gain stage to the front end of the meter. You would also need to know some idea of the meters basic sensitivity to get an idea of what was needed. You could do that by trying this.
1/ Starting with the trimmer on its least sensitive position.
2/ Connect the meter input to the 12 volt supply.
3/ Turn the trimmer up and the LED's should all light in sequence.
4/ When the top LED just lights, measure the DC voltage on the trimmer wiper (middle leg).
That will tell you how much voltage you need to light them all. Knowing that will tell you if its achievable to get them all to light with this amp.
The circuit really needs a proper front end and precision rectifier. Look at post #62 here which is a similar kind of issue.
http://www.diyaudio.com/forums/everything-else/267760-lm-3915-vu-meter-problem.html#post4226036
1/ Starting with the trimmer on its least sensitive position.
2/ Connect the meter input to the 12 volt supply.
3/ Turn the trimmer up and the LED's should all light in sequence.
4/ When the top LED just lights, measure the DC voltage on the trimmer wiper (middle leg).
That will tell you how much voltage you need to light them all. Knowing that will tell you if its achievable to get them all to light with this amp.
The circuit really needs a proper front end and precision rectifier. Look at post #62 here which is a similar kind of issue.
http://www.diyaudio.com/forums/everything-else/267760-lm-3915-vu-meter-problem.html#post4226036
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