BrianGT power transformer requirements

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Alcaid said:
Wouldn't it be advisable to multiply the output power by square root of 2 to cover the peaks?

Minimum VA rating = [(output power) x 1.4142] [VA]


In terms of voltage, I think that should already be covered by the rectification, but it's still not necessarily a bad idea. You also have to take into account the inneficiencies of the power supply and the amp. Even though the LM3875's seem to be pretty efficient, based on non-scientific temperature vs. output power testing (so non scientific I just touched the heatsink next to the chip, or the plastic casing to see if it was hot), I'd say it's close to 75% efficient, which is very good for an amp that isn't class D or G, but means that your power supply still needs to be able to supply about 35% more power than your chips will output. Compared to the amp, I suppose the power supply ineffiency is fairly negligible, so I'd just optionally add an extra 5-10VA just for kicks.

So based on the above assumptions, I'd say that the transformer should be rated for [(RMS output) * 1.35 + (anything you want to add for "headroom")] * [number of channels] VA. For a 50Wrms, 2-channel gain clone, that would be [(50 * 1.35 +10) * 2] = 155VA. So I guess for a "typical" stereo GC, I would recommend a minimum of a 150VA transformer. Feel free to correct me if I'm wrong, though.

EDIT: By minimum, I mean the bare minimum, unless you plan on listening at very low volume levels all the time. Otherwise, your results may be somewhat less than impressive.
 
If you choose a transformer which is a bit small you must also be aware of it and not use the amp for party and techno music. It's all about average power output from the transformer and the temperature inside. If the transformer is less than 105 deg C inside it's OK. 105 deg inside is 60-70(?) deg at the outside and the transformer is really hot!
 
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Joined 2002
Paid Member
Toroid Transformer headsup

I will be posting on Thursday or Friday on the Trading Post
Forum 3 new Toroids I have purchased.
I do have in stock NOW a :
Avel Lindberg Inc.
2 X 120V 50-60Hz Primary
2 X 25V 2 Amp Seconday
Measures 3 1/4" Dia X 1 1/2" Tall
Price $22.50ea

Others should be here by the end of the week:
Outputs; 2 X 21V and 2 X 28.4V

Steve @ Apex Jr.
818 248-0416 Voice
steve.apexjr@prodigy.net
 
diyAudio Editor
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People are using 300 VA transformers for 2 channels, which is
150 VA/ch
It's not as much availability, but if you are forced to buy them from a regular supplier rather than surplus, the difference between 200 and 300 VA is only about $7

Surplus providers are the way to go if they have an appropriate size of course!!
 
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The 2 X 25V are 100Va I'm not sure on the 2 X 21V
The 2 X 28.4 are 2.6 Amps. I'll wait until they arrive
before I post the exact Va or Amp rating since I don't
remember. They are already bought and the lader 2
will be in stock by the end of the week. Might be
good for a solid state amp or 1 for each channel.
I'll see what freight costs me and I'll adjust my price.
Steve @ Apex Jr.
818 248-0416
steve.apexjr@prodigy.net
 
Bringing this back from the dead...

I have been searching for a half hour now and I can't find out how to wire a toroid for a GC. I have a Avel one with 2x120V Pri, and 2x25V secondaries.

I recall Brian posting a picture of one in a thread and how to wire them. But can't seem to find that particular post. Shame on me for not bookmarking it.
 
chipco3434 said:
Avel transformer questions. Ask and ye shall receive.

http://www.diyaudio.com/forums/showthread.php?postid=359264#post359264

complete with pictures! No need to read!

I know, it helped me. Build on, brother.


Thats exactly what I was looking for. I swear it was further down that post and started around page 6 and higher. I had no clue it would be the 2nd page! :bawling:

But hey the colors do not match up! I can't even find the technical schematic for this particular toroid. AV-67530 on the avel website.

I've got a pair of purple, and greys, then a brown, black, peach, blue, redd wire.
 
chipco3434 said:
no schema on the toroid?


No. I purchased this from Steve at Apex jr. Perhaps this is an older model, or perhaps some sort of OEM product?

The writing on this is exactly as follows
Code:
Avel Linberg Inc.                  E115159
P/N : AV-67530
Pri : 2x120V, 50/60Hz
Sec : 2x25V/2A
                                   SL 0305

I've emailed Steve for some input. I've also googled for the part number with no luck.
 
mitch808 said:


But hey the colors do not match up! I can't even find the technical schematic for this particular toroid. AV-67530 on the avel website.

I've got a pair of purple, and greys, then a brown, black, peach, blue, redd wire.

You can get a good idea of what wires are what using an ordinary multimeter set to measure resistance (probably best in the 200 ohm range).

Hook one of your meter's probe leads to one of your transformer windings, and then move the other probe lead across the other transformer leads until you get a reading.

Your primaries are likely to be the thinner of the wires, and you meter will measure something in the region of 50 ohms ish. If you are on 120V supply, you'll want to wire these in parallel, or series if you are on 230v supply.

Your secondaries should be much thicker wires, and will measure a very low resistance with your multimeter. Probably less than 1 ohm.
If you are rectifying each winding separately (eg if you're using Brian's rectifier board), then as long as you have the secondary windings "paired up" it shouldn't matter which way round each of the wires in each pair goes when you connect it to your rectifier.

Hope this helps.

Steve
 
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