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Old 27th April 2004, 01:30 PM   #1
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Default BrianGT power transformer requirements

I've seen many different types of power transfomers with varying topologies and current capabilities used with BrianGT's boards. So, my question is, "what is the minimum current requirement?" That way I know anything beyond is nice, but not a must have.
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Old 27th April 2004, 01:37 PM   #2
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If you have the same VA rating as the output power, that's a good start for home use.
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Old 27th April 2004, 01:42 PM   #3
Alcaid is offline Alcaid  Norway
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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]
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Old 27th April 2004, 01:44 PM   #4
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I would say at least 80 - 100VA per channel, for any reasonanble performance.
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Old 27th April 2004, 01:58 PM   #5
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Thanks guys, just what I neded. I was wondering why people were using 300VA transformers. I guess it was more an issue of availability.
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Old 27th April 2004, 02:10 PM   #6
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Quote:
Originally posted by Alcaid
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.
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Old 27th April 2004, 02:14 PM   #7
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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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Old 27th April 2004, 02:17 PM   #8
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Quote:
Originally posted by Peter Daniel
I would say at least 80 - 100VA per channel, for any reasonanble performance.
Seems pretty OK to me.
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Old 27th April 2004, 04:22 PM   #9
Apex Jr is offline Apex Jr  United States
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Default 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

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Old 27th April 2004, 04:52 PM   #10
Variac is offline Variac  United States
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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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