Re: Poor student DIY toroid output xfos
- From: "Doug Bannard" <ve3spf@xxxxxxxxxx>
- Date: Thu, 29 Nov 2007 19:57:59 -0500
"GLM" <labbe-morissette.guillaume@xxxxxxxxxxxxxxxx> wrote in message
news:08F3j.13029$qP4.6959@xxxxxxxxxxxxxxxxxxxxxx
Hi,
I'm a 3rd year uni student and I've completed my first complete tube amp
design up to the output stages. I've chosen my output tube (EL34 as SE
pentode), and most of the parts and got a nice preamp/tone control module
up and working. Now I'm at the point where I need to consider output
transformers...I've designed most of the amp according to the parts and
tubes I could easily get my hands on (budget constraints), summing at
about 35$ per channel [God bless surplus Sovtek tubes] (doing it mono 1st,
once proven it'll go stereo :P) NOT INCLUDING the xfo. Looking at the
price for 40W transfos, my jawbone is dropping at the prices (70$USD
including shipping is WAY out of question).
I'm not unexperienced to winding transformers with critical impedences and
turn ratios [did a few RF baluns and xfo secondary-voltage-divider-based
PSUs], and I still have my material at home and I have access to cheap
enameled copper wire, so the idea of winding them myself obviously came
right to me. Reading across previous posts on this forum, I came across
the toroids OPT thread. Winding toroids is a daisy so this sounds like
the most interesting option. However, being of very experience in the
tube field, I'm a bit sceptical about the overall size of a decent toroid
able to drive 40W in 8 ohm speakers (although the self-shielding property
does take a bit off the xfo casing's size and complexity).
What topology would you recommend to use? Seperate windings on opposite
core sides or 2-layer winding around the core? I'm also not used to
computing core saturation issues with AF and such high inductances (RF
obviously requires so much less).
<snip>
The toroid is not really a good DIY fit for this project. If you were to
use a toroid it would need to have an air gap sawn through its cross section
so that it would be able to support the DC ampere turns from your SE output
stage without saturating. As far as the winding configuration is concerned,
if you were to use a toroid you would wind both primary and secondary
completely around the core (avoiding the immediate area of the sawn air
gap). Winding around the entire circumference is done to reduce leakage
inductance and is particularly necessary if your feedback loop (if any)
includes the output transformer.
That having been said, I would suggest a combination of the ideas put forth
by Peter and Mick. Get hold of an old push-pull output transformer (perhaps
take Peter up on his kind offer?). Something rated for about 5000 ohms
plate to plate with taps on the secondary for 4, 8, and 16 ohms if possible.
Carefully disassemble the laminations being careful not to bend them. Now
re-assemble, with, as Mick suggested, all the "E"s in the same direction,
butting up against the stack of "I" pieces. In other words, no more
interleaving. Try as a start an 0.005" thick paper spacer at the interface
between the "E"s and the "I"s.
The larger the gap spacer, the more DC amp-turns that can be absorbed
without core saturation...but the price that you pay is reduced primary
inductance. You will know if the primary inductance is too low if the
amplifier has inadequate low frequency response. If the gap is too small on
the other hand you will be rewarded with large amounts of 2nd harmonic
distortion.
So use the entire primary for your plate load. You are probably looking for
a load impedance of ~2000 ohms for a single pentode connected EL34. So if
you use a 5000 ohm plate to plate primary with taps at 4, 8 and 16 ohms and
you have for example an 8 ohm speaker, you will be able to get ~2500 ohms
across the entire primary by connecting the 8 ohm speaker to the 16 ohm
secondary tap.
If you really want to design and wind your own from scratch, here are a few
references that you can study, but I'll caution you in advance that these
are not "cookbook" type references which enable a quick design. A cookbook
design does not an optimum transformer make:
1) Electronic Transformers and Circuits......Author is Reuben Lee. There a
three editions, any of which is fine.
2) Magnetic Circuits and Transformers........MIT press (the best and
arguably only good treatment of DC bias calculation in cores). Book is long
out of print but should be available in your EE library.
3) For the closest to a cookbook approach, there is the Radiotron Designer's
Handbook, 4th edition. The information is good although very dated. If you
do a design based on RDH4, it will work, but you won't gain the same insight
into what is actually going on that you will from the first two references.
4) There are also supposed to be good texts written by Robert Wolpert, but I
cannot comment on these as they aren't in my collection.
5) Get your lamination data from Tempel Steel or Magnetic Metals (both
should be on the web).
Hope that this was of some help.
Doug R. Bannard, P.Eng.
.
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