Got a 6-0-6 v ca 72 VA transformer, what should I do with it? - diyAudio
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Old 23rd March 2015, 09:37 PM   #1
OskarD is offline OskarD  Sweden
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Default Got a 6-0-6 v ca 72 VA transformer, what should I do with it?

Hi guys,

Been hooked surfing Diyaudio for a while but never posted anything yet.
Just built a chipamp (small Gainclone-LM1875) using an expensive toroid transformer.

Want (need!) to build more stuff and as I just got two throw-away SLA chargers from the local dump that I hope I can use for something fun. One is a single 12v secondary, while the other is dual 6v with ca 72 VA of power.

Any suggestions of what I can use them for? Need to be something that uses the low voltage yet some of the Amps else smaller transformers would be more suitable.
Hope to use the 6-0-6 transformer as the casing is really nice and retro (1982 orange and white plastic)

Or should I just toss them again?

Cheers,
Oskar
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Old 24th March 2015, 03:32 AM   #2
paulb is offline paulb  Canada
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Maybe look for surplus 5 V - 10V input switcher modules. You might be able to get, say, a +/-15V output that could power a preamp.
Don't throw it away! !
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Old 24th March 2015, 06:52 AM   #3
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12 V ct might be useful for supplying power to the filaments of the 12 XXX family tubes.
For amplifiers, very low power can be achieved. I use a TDA 7264 ( took it from a TV ) with a 24 V ct and it outputs around 10 W p/ch.
MAybe a little class A amp...2 or 3 W. Rail to rail is 12 V, minus the losses in the junctions of the semiconductors
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Old 24th March 2015, 08:22 AM   #4
wwenze is offline wwenze  Singapore
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Power a tripath with 12V, voltage double it to 24V and power a tpa3116 or +-12V for a dac or headphone amp. 72VA is too much to waste on a not-amp.
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Old 24th March 2015, 10:33 AM   #5
AndrewT is online now AndrewT  Scotland
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Create a +-18Vdc supply from the TWO transformers.

The total 144VA will power amplifiers from 70W to 140W reasonably well.
A 5.1 amplifier with 6 channels of 10W to 20W might be a good project.
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Old 24th March 2015, 12:37 PM   #6
OskarD is offline OskarD  Sweden
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Cool ideas, thanks!

Maybe a Class D will fit into the case, but will then be right next to the EI-transformer, will that be an issue with hum as with the gainclones? With the TPA3116 or a Tripath there would be no issue with fitting a large heatsink so this would be possible if I just get it PCB-mounted. The rectifier or voltage doubler circuits will fit on the other side of the transformer away from the space where a amplifier could be placed.

AndrewT, the two transformers are slightly different, one is 6A and the other 6,5A, the voltage is also slightly different (at 12V). Won't that be an issue with the ground not being at 0V? But I guess there is sollutions to nearly everything... I might build a smaller amp of the cool orange 6-0-6, and try to get my hands on another single 12V transformer on my next visit to the dump :-)

Can put up a picture later...

/Oskar
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Old 25th March 2015, 07:57 AM   #7
AndrewT is online now AndrewT  Scotland
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join the two supplies together.
The output will be:
0Vdc, ~+17.5Vdc, ~+36Vdc
or
~-17.5Vdc, 0Vdc, ~+18.5Vdc
or
~-36Vdc, ~-18.5Vdc, 0Vdc

Notice, in all three cases the difference between the tappings is always the same.
YOU decide which one of those tappings becomes your Zero Volts. Choose that arrangement and the other two tappings give your output voltages relative to your Zero Volts.

Most dual polarity supplies have a tiny difference in the +ve and -ve voltages.
Most dual polarity amplifiers do not care on that tiny difference and will tolerate quite enormous differences in the +ve and -ve supplies.
The only time it tends to affect performance is when you push an amplifier to clipping. The lower voltage half waveform will clip a bit earlier.
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Last edited by AndrewT; 25th March 2015 at 07:59 AM.
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Old 31st March 2015, 11:32 AM   #8
OskarD is offline OskarD  Sweden
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Soldered in a GBU804 rectifier bridge chip and two large 10000uF caps and next to the trafo, now connected as a single 12V supply, getting 18,6V after the bridge.

As there is only a tight space left over for an amplifier and no room for large heatsink I've just ordered a D-amp based on the TPA3116 from China that accept 18-24V. Hope the trafo won't go lower than this under load, or that the amplifier will accept it anyway.

Am a bit concerned of the box all made of plastic and no easy way to place safety ground. Most my wall power outlets ar not grounded anyway so maybe not an issue I could deal with anyway. Added a picture of this if anyone have any extra warnings or suggestions concerning the box. I think it will look great in my garage or work room!

Maybe a bit early to start fiddling with this as I haven't got the amplifier yet, but my multimeter showing 0,2 Vac after the bridge, should I add some voltage controll to get rid of this? Only measured without any load, so the ripple shouldn't be any cause? The rectifier bridge is for 8A and if I remember correctly 200 v.

And the amperemeter, would it be possible to connect that in line with the power after the capacitors, or how would that affect the amplifier? Haven't seen any amplifier with this, usually only voltmeters connected to the loudspeaker outputs I assume.

Thankfull for all input on my loose thoughts .

Oskar
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Old 31st March 2015, 12:21 PM   #9
AndrewT is online now AndrewT  Scotland
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some DMM set to Vac cannot measure this accurately when there is a high DC voltage.

I am lucky all my 4 DMM give good readings that agree with each other.

You can use a capacitor to block the DC and measure the AC voltage across a 100k load resistor. See if that reads similar.
This is a high pass filter. Connect a 1uF 100V plastic film in series with a 100k resistor. Measure the AC voltage across the resistor. The DMM is isolated from the DC by the capacitor. The high pass filter has an F-3dB of 1.6Hz

BTW, 0.2Vac in what scale?
a 199.9Vac scale cannot measure low mVac ripple.
even 19.99Vac will struggle to give a good reading of ripple.
I use 1.999Vac if ripple is very high or 199.9mVac if ripple is low.
The scope shows ripple as peak to peak.
The DMM reading is scaled to roughly read Vac as if it were a sinewave.
Very roughly, 0.2Vac from the DMM would show as 600mVpp on a scope (6 divs when set to 100mV/div)
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Last edited by AndrewT; 31st March 2015 at 12:26 PM.
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Old 31st March 2015, 01:24 PM   #10
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Good paperweight...

keep for use when in several years you find that you need such a thing... I expect it would be best for running stuff like relays or lighting, etc...
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