Suitable transformer

[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

[wg_ski]

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.

[mantisory]

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

[AndrewT]

39Vac is near the limit for a gb150 feeding 8ohm speakers.
Go for it.

[syn08]

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.

[AndrewT]

significant magnetic imbalance in the core

How do you identify or measure that?

[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

[syn08]

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.

[syn08]

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