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Old 25th November 2014, 05:11 PM   #1
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Default Basic Transformer Question

I am new building amplifiers, beyond building complete kits anyway, and I am going to build the FirstWatt Pass De-Lite amp just for the novelty of it. Nelson first has a tx with secondaries listed at 50v, then a future version uses a 60v tx (with no other PS changes), and finally a third version uses a 75v tx, again with the same PS parts. First, I can't find a tx that has secondary windings that can be combined into 75volts, only 70 or 80. So far research says that either one is close enough and will work fine, but the 80 is better....is that correct, given the schematic shown on the link below?

Article here for convenience: De-Lite PDF

Is there anyway I can use the 80v tranny on the earlier versions of the amp safely? Otherwise, I have to buy two transformers as I follow the upgrade path....
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Old 26th November 2014, 12:23 PM   #2
AndrewT is offline AndrewT  Scotland
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Note that Vac is different from Vdc. Don't get them confused.

Transformers give Vac.

The pdf shows 50Vac and 60Vac and 75Vac transformers.

The higher the voltage, the more risk of blowing up the amplifier and the more risk of injuring yourself.

I suggest you start low.

The 75Vac transformer is shown as getting around 100Vdc on the operating amplifier. When the PSU is built you are likely to find the capacitor voltage is very much higher. DO NOT use 100Vdc capacitors just because the schematic states 100Vdc.

You will need >125Vdc smoothing capacitors. The available voltage exceeding that minimum is 160Vdc.
These will cost two to three times what a 63Vdc capacitor will cost. Even an 80Vdc capacitor is much cheaper than a 160Vdc

A 60Vac transformer may just keep you under 100Vdc when on no load. You might get away with using 100Vdc with a 60Vac transformer, but you really do need to check your actual operating voltages and calculate your worst case PSU voltage.
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Last edited by AndrewT; 26th November 2014 at 12:29 PM.
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Old 26th November 2014, 12:31 PM   #3
AndrewT is offline AndrewT  Scotland
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Since you are new, I suggest a much lower voltage amplifier for your first build.

Read Decibel Dungeon and build a chip amp (without regulated supply) as an easy project from which you can learn many of the basics and on which you can make some informed decisions on what comes after that success.
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Old 26th November 2014, 07:43 PM   #4
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Thx for the info Andrew, I did find some 150V caps that are within my price range from United Chemi-Con. I was looking at 100v caps at the beginning version of the amp, but I didn't adjust that for the later versions with higher supply voltages.

I do have a friend with a lot more electrical experience than myself to help me during the build, but neither of us have a lot of exp with audio eq specifically.

Also, thanks for the tip about DecDun, there is a lot of info there to read, keep me busy this holiday weekend! I have built a chip amp with a regulated supply and a tube amp, but again, both kits with nice instructions.

I think I would feel safe enough to proceed though once I hook up something to drain the caps as I upgrade.
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Old 27th November 2014, 08:46 AM   #5
AndrewT is offline AndrewT  Scotland
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Use a Mains Bulb Tester to power up every new, or (even slightly) modified project.

Especially important for first power up of a mains transformer. The bulb allows you to wire up the transformer and guarantees that you will not damage the transformer, no matter how wrong you get the wiring.
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Old 29th November 2014, 05:40 AM   #6
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OK, sounds like something I should have Thanks again for your advice in this post and my others!
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