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Old 7th February 2008, 12:54 PM   #1
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Default Suitable transformer

Hi all,
So, I was at my local surplus yesterday and saw a few toroids for sale, but the interesting ones were big dual secondary 39V, 10A toroids (390VA??) for 20$. I am thinking to use it to power a SKA amplifier (the GB150D)
Is there any reason to stay away from it - what should I look for?
I didn't notice the manufacturer name but I was thinking that a power transformer is nowhere near as important as an the various transformers (input, output, choke, etc) in a tube amp, say...is this assumption correct?
Thanks for your help,
Byron
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Old 7th February 2008, 02:53 PM   #2
wg_ski is offline wg_ski  United States
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If it has two 39 volt windings, then it's what you need for 100W/8R per channel. If it's 2x19.5, then you have two choices - 20-25 watts (with enough VA for class A) or stack two in series and get up to 200W/4R. Still only 40 bucks. And there really isn't any way it could be "bad" for DIY audio amps.

Deals like that are the only way you'll ever build an amp cheaper than buying it.
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Old 7th February 2008, 03:25 PM   #3
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yeah, i seem to remember that it was 39 Volt secondaries in parallel...thanks!
No, i was just wondering whether the is any type of 'quality' issues - everyone seems to use plitron, hammond, etc...all these name brands that seem to usually cost upwards of 75$...is it really necessary?
Thanks again,
Byron
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Old 7th February 2008, 03:57 PM   #4
AndrewT is offline AndrewT  Scotland
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39Vac is near the limit for a gb150 feeding 8ohm speakers.
Go for it.
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Old 7th February 2008, 04:09 PM   #5
syn08 is offline syn08  Canada
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Default Re: Suitable transformer

Quote:
Originally posted by mantisory
Hi all,
So, I was at my local surplus yesterday and saw a few toroids for sale, but the interesting ones were big dual secondary 39V, 10A toroids (390VA??) for 20$. I am thinking to use it to power a SKA amplifier (the GB150D)
Is there any reason to stay away from it - what should I look for?
I didn't notice the manufacturer name but I was thinking that a power transformer is nowhere near as important as an the various transformers (input, output, choke, etc) in a tube amp, say...is this assumption correct?
Thanks for your help,
Byron
I have a couple of those toroids, they are from Active Surplus, right? They are good, but not great. It's what I originally tried to use in the PGP amp and they have under full load a significant magnetic imbalance in the core (most likely due to asymetrical windings), leading to significant 60Hz even harmonics radiation. In a word: they are noisy. I gave up using them and got two toroids from Plitron, you can pick them up in Toronto, avoiding the shipping costs.

YMMV, but I wouldn't use them for any high end application.
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Old 7th February 2008, 04:14 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
significant magnetic imbalance in the core
How do you identify or measure that?
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Old 7th February 2008, 04:28 PM   #7
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thanks for the info - yeah, they were the active surplus ones...will steer clear then. Unfortunately the Plitrons are really expensive (+100$ or so) and the Hammonds from partsconnexion aren't much cheaper (and don't fill my needs)
B
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Old 7th February 2008, 04:53 PM   #8
syn08 is offline syn08  Canada
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Quote:
Originally posted by AndrewT
How do you identify or measure that?
Typo, that would be "odd" not "even".

It's an assumption. A magnetic core imbalance is known as creating mains frequency odd order harmonics. I'll try to find some references about and post here.

Experimentally I have found that these harmonics are extending well into the KHz range. Take a look at the spectra at http://synaesthesia.ca/distortion.html (the 1KHz IMD product spectra picture).

Once that I switched to Plitron toroids, these harmonics almost vanished in the noise floor.
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Old 7th February 2008, 04:56 PM   #9
syn08 is offline syn08  Canada
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Quote:
Originally posted by mantisory
thanks for the info - yeah, they were the active surplus ones...will steer clear then. Unfortunately the Plitrons are really expensive (+100$ or so) and the Hammonds from partsconnexion aren't much cheaper (and don't fill my needs)
B
Not all Plitron products are very expensive. Plitron has some surplus stock, selling for decent prices. Take a look here: http://www.plitron.com/shopping/shop...=Surplus+Stock
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Old 20th November 2012, 12:40 PM   #10
lineup is offline lineup  Sweden
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I want to ask if there is a standard for transformer voltages.
What I know there are:
- 15 VAC
- 18 VAC
- 22 VAC
- maybe 25 VAC
- maybe 30 VAC

In short what transformer voltages are out there?
What do the manufacturers offer?
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