Circuit modification: powering a VU meter board

[blackeyeliner]

That's a decent sign that you can add the doubler to the Crack and not move the grounding.  I would recommend wiring up the doubler to give +/-6V, which would mean that the grounded terminal on the 6.3V winding should go to the junction of the two caps.

Cap sizes are determined by how much ripple you can tolerate, I'd say start with 4700uf and see how that works.

-PB

So it's exactly like it's done on the scheme I proposed (this one - http://www.daenotes.com/images/full-wave-voltage-doubler.png), right? I am ready to go in this case and will do it by weekend.

[Paul Birkeland]

Yeah, I would put some resistors across each of those caps, 1K ought to be enough, wattage rating isn't particularly important with only 6V across them, 1/8W is enough.

[blackeyeliner]

Yeah, I would put some resistors across each of those caps, 1K ought to be enough, wattage rating isn't particularly important with only 6V across them, 1/8W is enough.

Across - you mean, in parallel? Will do. Just out of curiosity, why is that desirable to do?

[Paul Birkeland]

It's generally good practice.  IMO, it matters more for high voltage circuits, where you really want to know that each cap is seeing an equal amount of voltage.

[blackeyeliner]

I made it and - it worked! However, I messed up with polarity while plugging it in and one of the capacitors is destroyed, so I will have to replace it. However, the doubler gave me 13V DC which was pefrectly fine for powering the board.

Interestingly enough, while looking at the board I understood it could be powered with AC, as it has diode bridge on it (and AC IN marking). However, directly hooking up 6.3V AC from Crack to the board did not work - it barely produced enough voltage to light up the backlights, and meters did not work. With doubler however, it works just fine.

[blackeyeliner]

Here's how it looks:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FME0WGlv.jpg&hash=556d32b56a97e6ac99f0cdaa3bb5194f7b0ccfc3)

[Paul Birkeland]

A terminal strip or a piece of proto board can make that more durable/safe.  A 6V winding into a diode bridge will give 4-7V, depending on load and diodes.

-PB

[blackeyeliner]

A terminal strip or a piece of proto board can make that more durable/safe.  A 6V winding into a diode bridge will give 4-7V, depending on load and diodes.

-PB

So I rebuilt the doubler with a new capacitor. It is weird but it's working perfectly when the sound inputs are not connected to the board, the lights go on, doubler produces it's 13.5V DC, but if I connect the sound input, lights on the board (meters illumination) go off and it starts humming into the sound circuit.

I obviously measured the current flowing through the doubler and with the sound input connected, it's 2+ amperes, while without sound inputs connected, it's normal 0.08A. Is there something I am terribly missing?

[Paul Birkeland]

Is there something I am terribly missing?

Yes, undoubtedly something is very wrong there, but it's hard to know without knowing a lot about the VU board setup.

-PB

[blackeyeliner]

Yes, undoubtedly something is very wrong there, but it's hard to know without knowing a lot about the VU board setup.

-PB

So you think there's nothing I can do on my own at this point? I guess the board design will be hard to get somewhere. It's pretty simple, but I don't understand how connecting audio can make it draw 2+ amperes, really. At the same time, any external power source makes it run perfectly well.

[blackeyeliner]

I just thought that probably an easier way would be to use the fact that the board can be powered with AC (it has a diode bridge on it and a stabilizer afterwards) and just find a very small low power transformer, install it inside the case and power in parallel from the switch...

[Paul Birkeland]

That's probably a better idea.  12V transformers are very easy to find.

-PB

[blackeyeliner]

That's probably a better idea.  12V transformers are very easy to find.

-PB

So it appeared that 12V is not enough for this board to function. It was technically working, but it has a 12V regulator after the diode bridge, that needs 18-19 volts DC to output 12V stabilized. I found a small 24V transformer, put it on a small board, drilled a couple of holes in the casing and here it is, working like a charm. The only problem is that the transformer gets pretty hot.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F0sbsyDd.jpg&hash=7fc31c605c0d24d155d3984612d0492ba6d8f0a9)

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FFQmxFyV.jpg&hash=6f502655312521dc1925c1e5f91d1ac080914b3c)

[blackeyeliner]

So to close this up, I ended up installing a radiator on top of the transformer, replaced the transformer with a lower voltage one so it generates less heat. It's still hot, but the rest of the amplifier is the same temperature.

So I guess this is it, it seems to work, spend the day on, like 8 or 10 hours already.

[mcandmar]

What is the VU board you are using?  Most voltage regulators need higher voltage then their output, for example for 7800 series or LM317's they drop out when the source supply is less than 2v above its output voltage so +4v is usually a good target to aim for.  Going too high, like feeding a 12v regulator 24v will cause it to run very hot as it has to dissipate the extra voltage.

A hot transformer could indicate the load is higher than the transformers current rating.  Generally those small PCB transformers do run hot so its usually a good idea to fit one rated for more current than it needs to supply, assuming you have the physical space.