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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Nov 2020, 9:12 am   #21
GrimJosef
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Default Re: Circuits for toroidal push-pull output transformer

It seems then that it is possible to make toroids with relatively gentle saturation. Without more detail, particularly about the core materials and perhaps about the measurement technique, it's hard to reach any definitive 'all other things being equal' conclusion. Another couple of bits of isolated evidence come from posts no 39 and 40 in this thread https://www.vintage-radio.net/forum/...=133238&page=2 where two forum members report their first-hand experience of toroids with very sharp saturation.

Cheers,

GJ
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Old 25th Nov 2020, 11:15 am   #22
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Default Re: Circuits for toroidal push-pull output transformer

I did a bit of reading because I noted the Single Ended Toroidal Output transformers are available. and of course these will have a large standing DC current.

It seems Toroidal transformers are available Gapped in the same way as E, I and for the same reason.

To facilitate winding the gap has to be filled making the process more expensive.

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Old 25th Nov 2020, 11:25 am   #23
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Default Re: Circuits for toroidal push-pull output transformer

In the greater scheme of things, whether a transformer is on a toroidal core, C-core, E and I core or an R-core shouldn't matter, but the devil is in the details.

As Ed has said the business of splitting windings into sections and interleaving them has been done many times on traditional E&I transformers, but would be a major difficulty on a toroid. But the toroid is a more efficient thing in the first place in terms of keeping leakage flux down. Then again, good broadband toroids need the windings distributing evenly round the full circumference of the donut.

However, one issue with toroids has as much to do with human psychology as physics. Because the cores are tape-wound, non-isotropic magnetic steels can be used with all of the flux going in the favoured direction. At this point many designers were like kids getting let out of school early, and with much whooping and cheering set about seeing how few turns per volt they could get away with, and thus cutting manufacturing costs.

For mains transformers this has resulted in acoustically noisy transformers which sit there making Frankenstein film sound effects. For output transformers, being parsimonious with magnetising inductance spoils the low frequency cutoff.

So for output transformers, there is not necessarily much wrong with toroids, provided you can get them designed and made by someone who will do a proper job and not cut corners.

There shouldn't be any need for different circuitry just because of the shape of the output transformer core.

David
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Old 25th Nov 2020, 11:37 am   #24
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Default Re: Circuits for toroidal push-pull output transformer

Crossed with Mike (I either need to think faster or to type faster...)

Cutting a toroid and glueing it back together with a gap (or two gaps) will give a volume of intense fringing flux around the gap. This can induce transverse-plane circulating currents in thick wire windings and spoil behaviour at high frequencies. You get a similar problem with gapped pot cores.

With tape wound cores you can get the effect of some gapping by winding a non-magnetic spacer in between turns of the GOSS strip. This detracts from the effective iron cross section area so the whole thing has to be a lot bigger.

For single-ended amplifiers, the good old E&I core is hard to beat. I wouldn't consider toroids for anything other than push-pull.

If toroidal cores are being considered as a way of going beyond what E&I transformers can do, then why stop there? Go transformerless. But the OP said he already has toroidal transformers. If his primaries are wound for push-pull, then that's what it has to be. If their primary inductance is low, then lower impedance speakers and lower voltage/higher current valves will win back some of the lost bass.

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Old 25th Nov 2020, 11:54 am   #25
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Default Re: Circuits for toroidal push-pull output transformer

Thanks for the additional info David very useful.

Most of those telling you about gapping also sell Toroidals so want to push you down that path I feel.

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Old 25th Nov 2020, 8:43 pm   #26
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Default Re: Circuits for toroidal push-pull output transformer

Well said David, it sounds a bit as though the manufacturer has invested in an expensive toroid winding machine ans is now trying to get its utilisation up with throughput of a higher selling price product.
If you really want this type of thing I would put my money on C cores. They look better too!

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Old 25th Nov 2020, 9:23 pm   #27
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Default Re: Circuits for toroidal push-pull output transformer

Once again Ed has the idea!! All other comments are pertinent too. I use toroids for power supplies only. But I always run lower flux densities rather than higher. I design toroids with a BIG core and lots of turns so I get flux densities down to about 1 Tesla or in my language 64500 lines per square inch. Toroids above about 500 watts can be run at 2.25 Teslas, or close to 150,000 lines per inch. Apart from the noise factor mentioned, the heat becomes a BIG problem even with modern insulation ratings in the class D or even E range. That means the transformer gets as hot as the output valves.

Leakage with toroids is really the only benefit ( apart from size) as it is inherently very low, but to keep it that way the windings, as mentioned must cover the whole toroid evenly. Van der Veens partial cure for this is that he winds five or six secondaries in parallel to cover more of the toroid. Thinner wire can be more expensive too so really there is little to gain. To stop almost all flux leakage a copper strap can be over wound on the outside of the toroid that "catches" any flux that does get away, also adding to the keeping the heat inside the toroid. I suppose they have their place, but after buying the book I was a little disappointed with its contents, and this audiophool decided there were bigger phools than himself.

Joe
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Old 26th Nov 2020, 10:26 am   #28
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Default Re: Circuits for toroidal push-pull output transformer

Thanks again gents, for this very informative discussion. I hope that it's not to frustrating to you that I'm not able to get into my garage and get on with this project yet. On the other hand, time spent planning is never wasted.

It began just as a way of building a stereo power amp using EL34s, as a way of learning a bit more about the Leak stereo amps. I got the power transformer a year ago, but a matching single Leak OPT turned out to be faulty. Meanwhile, at the back of my mind, I've always been wondering about the mixed press toroidal output transformers get, so when I got the chance to buy a pair with the right primary impedance, UL taps produced by a reputable company at a price I could afford, I thought I'd give it a go. I have no way of knowing about the quality of the transformers, apart from the manufacturer, size and the fact that they work when plugged into a 5-20 circuit.

Joebog1, thanks for posting a circuit and for your interesting comments about the van der Veen book.
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Old 26th Nov 2020, 10:16 pm   #29
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Default Re: Circuits for toroidal push-pull output transformer

qualityten, if you want more VDV information you are welcome to PM me

Joe
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Old 27th Nov 2020, 10:11 pm   #30
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Default Re: Circuits for toroidal push-pull output transformer

Thanks joebog1!
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