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Old 22nd April 2013, 03:05 PM   #1
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Default A DIY amp project to match my transformer?

I've acquired a transformer I'm hoping I can use in a DIY amplifier project. I'm having trouble locating something this transformer would be useful for. I was wondering if anyone here has any ideas?

The transformer is 300VA toroidal. Secondaries are 36v-0-36v and 20.8v-0-20.8v. There is no serial number on the transformer but I have tested and the output is as reported.

I've noticed that a lot of the projects seem to use something with much lower voltage on the secondaries. Am I going to get much use out of this thing at all?

I appreciate any input you might be able to give.
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Old 22nd April 2013, 03:11 PM   #2
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Do the wires from the secondaries look about the same gauge or are they different, if so, which is heavier?
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Old 22nd April 2013, 03:17 PM   #3
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16AWG on the 36v and 22AWG on the 20.8v.

It seems like the smaller of the two is meant to be for a preamp.
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Old 22nd April 2013, 03:47 PM   #4
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Well, there you have it. An integrated pre/power amplifier.

300VA, taking it as 60V average, about 5A. The split is ~ 1A on the 20V and ~ 4A on the 36V. Work that backwards, 1*40 + 4*72 = 328 - not that far out. 4A isn't a lot of current @ +/-50V rectified, although you probably won't swing quite that much after rectification, but you can still build something useful. 80W into 16 ohms is just over 2A.
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Old 22nd April 2013, 03:53 PM   #5
AndrewT is offline AndrewT  Scotland
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300VA, the majority of which is in the 36+36Vac secondary, can support upto 300W of total maximum output power.

Expect it to work well with a two channel 100W + 100W into 8ohms amplifier.
It would also work well with a single channel 200W into 4ohms amplifier.

But, is it really a 36+36Vac transformer?
How did you measure the input and output voltages?
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Old 22nd April 2013, 04:43 PM   #6
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Good question, I am new to this so any correction is welcome.

I identified the primaries. There is a common (brown) a 115V lead (orange) and a 230V lead (red). Applying US line voltage (which I measure to be 115V) to the proper primary leads, I just used a multimeter to measure the output voltage on the secondaries. I think this is what you call "no load".

I asked some questions and gave some info about the transformer in another thread.
Testing a transformer without a data sheet
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Old 22nd April 2013, 06:08 PM   #7
JMFahey is online now JMFahey  Argentina
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This is something you can build successfully.
Don't despise it because of the "beginner" label.
No 550W capable amp is for beginners by the way
QUASI Amplifier for Beginners
The basic circuit is very versatile, can be adapted to lots of different PSU rails (including the roughly +/-45V you'll get) and lots of different transistors.
A cool, flexible design, for which a PCB design is offered, a big plus.

You might build an around 80/90W into 8 ohms per channel stereo amplifier or a 180/200W into 4 ohms mono power amp, for PA or DJ work.
Read that thread end to end and ask your doubts.
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Old 22nd April 2013, 06:52 PM   #8
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there are three wires for the AC primary?
Or 4 wires?
Three wires is an unusual connection scheme for a 120/240 volt primary.

Another thing you can try is to use the entire AC primary and see what your secondary voltages end up being...

Best idea is to use a small AC transformer of 5-12vac for the primary, measure the actual voltage and then the secondary voltage and multiply the measured secondary voltage by the ratio that would give you 120vac on the primary... that will tell you what it ought to produce for voltage.

You ought to get 1/2 the voltage using the full primary on 120vac... which will actually make it a bit easier to find a good project to start with...

Here's one to consider: Class A, 4 Watt, No Feedback, Simple Circuit, Great Sound

Simple, easy to build, and all you really need is a big heatsink...
something to consider to get ur feet wet.

Maybe post a jpeg of ur iron showing the leads clearly and maybe with a ruler in the picture or something else to show scale?

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Last edited by bear; 22nd April 2013 at 06:56 PM.
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Old 22nd April 2013, 08:20 PM   #9
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@JMFahey, thanks. That is exactly what I'm talking about. I'm sure that will be a great learning experience.

@bear, that idea about using the primary that is intended for 230V is interesting. I have to wonder if there are any other trade-offs to doing that. I'll try to get a better picture of it soon. My camera died so I'm just using my phone camera.
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