Just did a little searching for ferrite toroids on Ebay, and they have gotten a lot more expensive. Used to buy a whole box of them to experiment with. Now it's $25 for a couple of moderate size toroids. Not going to solve the OT crisis easily with ferrites. I think we are paying way too much for sending somebody's blue rockets up.
Ferrites pretty much look alike - you may think you are getting a decent power-type ferrite toroid, but instead are getting stuck with a dissipative Ni/Zn ferrite with relatively low permeability, good only for RF suppression duty. I don't buy unknown ferrites, period.
Ferrites are way behind metals in terms of saturation flux density, so you won't be solving the OT crisis with them anyway. I have sorta looked at using them for tweeter driver duty, say 5kHz and above, with a decent multipole high pass filter ahead of the amplifier. That's the only place I'd use them, as ferrites are severely limited in terms of flux swing.
Ferrites are way behind metals in terms of saturation flux density, so you won't be solving the OT crisis with them anyway. I have sorta looked at using them for tweeter driver duty, say 5kHz and above, with a decent multipole high pass filter ahead of the amplifier. That's the only place I'd use them, as ferrites are severely limited in terms of flux swing.
I measure the dimensions and inductance to figure the permeability. If it's 2000 or above it's likely the MnZn type. The saturation flux density can also be checked with a winding and Variac. A Current probe to the scope. Volts per turn at Bmax. Most of the large cores are likely to be for switching power designs. Have to avoid powdered iron ones though. I found one set of double cores, with windings for a saturable reactor. Square hysteresis curves.
I got some sample 6 inch NiZn cores from Fair-Rite. Was going to order a few more to make a direct audio OT until I saw the price for low quantities.
I did find some very interesting "cores" at a tech. junk yard. They were made for RF ionizing of gas for semiconductor processing. They were approx. 1/8" thick slices, ground flat, like laminations, with mylar insulation between the slices. 4" by 3" U's with matching 1" by 4" I's. Got enough to make a couple of small audio OT's. Haven't tried them yet, way too many turns to wind.
I got some sample 6 inch NiZn cores from Fair-Rite. Was going to order a few more to make a direct audio OT until I saw the price for low quantities.
I did find some very interesting "cores" at a tech. junk yard. They were made for RF ionizing of gas for semiconductor processing. They were approx. 1/8" thick slices, ground flat, like laminations, with mylar insulation between the slices. 4" by 3" U's with matching 1" by 4" I's. Got enough to make a couple of small audio OT's. Haven't tried them yet, way too many turns to wind.
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WOOPs, should be 6 inch MnZn cores from Fair-Rite above. Posting too late last night. And I guess the "Blue Rockets" are actually Amazon related.
The ferrite OTs I was mentioning earlier are HF switch mode, similar to the D. Berning impedance converters. There are several HF OT topologies. 200KHz makes up for the low Bsat. The Mosfet switches operate in self demodulating configuration. Audio gets modulated on the HF carrier to go thru the ferrite "OT". I think Berning charges $30,000+ for his. But the ferrite OT's are actually quite suited to low cost if done properly. Ferrite cores can be quite cheap in quantity. But $25 ones off Ebay won't be much help for DIY.
The ferrite OTs I was mentioning earlier are HF switch mode, similar to the D. Berning impedance converters. There are several HF OT topologies. 200KHz makes up for the low Bsat. The Mosfet switches operate in self demodulating configuration. Audio gets modulated on the HF carrier to go thru the ferrite "OT". I think Berning charges $30,000+ for his. But the ferrite OT's are actually quite suited to low cost if done properly. Ferrite cores can be quite cheap in quantity. But $25 ones off Ebay won't be much help for DIY.
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It takes two ferrite cores to do both polarities, for P-P or SE. Then you have the inverter PC board and parts. Plus the DIYer winding two moderate turns windings on both toroids. (each has to cover the full core ) Then you need a HF filter and definitely need N Fdbk around the whole thing, since the output tends to be high Z due to leakage L suffered at the HF in the cores. Will it have stability or HF noise issues? If this all gets up above 50% of the cost of a standard OT, it may not be so attractive.
Another idea might be to use long E laminations. These were commonly used for constant voltage xfmrs. 20 years ago a lamination manufacturer sent me 50 lbs of them for free. "Can't hardly sell them any more." May be scarce now? I think Temple steel had then. Then stack short bobbins of well insulated small wire, bifilar wound on them.
With a stack of those on the lamination core, you series connect one bifilar set for the primary side(s) and parallel the other bifilar sides for the secondary. Pre-wound bobbins would be real nice.
Another idea might be to use long E laminations. These were commonly used for constant voltage xfmrs. 20 years ago a lamination manufacturer sent me 50 lbs of them for free. "Can't hardly sell them any more." May be scarce now? I think Temple steel had then. Then stack short bobbins of well insulated small wire, bifilar wound on them.
With a stack of those on the lamination core, you series connect one bifilar set for the primary side(s) and parallel the other bifilar sides for the secondary. Pre-wound bobbins would be real nice.
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The Berning approach tosses a huge boatload of parts into the mix to get around the audio transformer size issue. Debugging that amp will be very much like dealing with the bastard child of a tube amp and a SMPS. Closing the feedback loop will be interesting, to say the least.
The Long E laminations scheme continued:
Polyimid HV insulation on the wires, or maybe Teflon on the secondary to lower distributed capacitance. These DIY OTs will be rather a bit more bulky than a standard OT, but the performance achievable may be outstanding.
Switchmode OT continued:
Yeah, the approach used for HV Switching inverters usually splits up the HV winding into several smaller, lower V, windings. Each has its own rectifier bridge and HF ripple filter, then the DC voltages get combined in series to make the HV for the tube to shunt.
The Berning topology also does not allow for CFB readily, but there is an alternative converter arrangement that does. Already on DIYaudio. Same number of parts.
This complexity with N Fdbk/stability issues and HF noise could easily overwhelm the typical DIYer who just wants an OT to wire in. It would have to be pre-engineered to be cook-book straightforward. Just a PC board you populate the parts, then wind your cores and connect them too. Probably some current limiting safety circuitry for incorrect core connections. Debugging a non working one could be night-marish with a scope mandatory.
Polyimid HV insulation on the wires, or maybe Teflon on the secondary to lower distributed capacitance. These DIY OTs will be rather a bit more bulky than a standard OT, but the performance achievable may be outstanding.
Switchmode OT continued:
Yeah, the approach used for HV Switching inverters usually splits up the HV winding into several smaller, lower V, windings. Each has its own rectifier bridge and HF ripple filter, then the DC voltages get combined in series to make the HV for the tube to shunt.
The Berning topology also does not allow for CFB readily, but there is an alternative converter arrangement that does. Already on DIYaudio. Same number of parts.
This complexity with N Fdbk/stability issues and HF noise could easily overwhelm the typical DIYer who just wants an OT to wire in. It would have to be pre-engineered to be cook-book straightforward. Just a PC board you populate the parts, then wind your cores and connect them too. Probably some current limiting safety circuitry for incorrect core connections. Debugging a non working one could be night-marish with a scope mandatory.
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How thick are the long E lams? 6 mil is the standard for audio XFMRs vs line frequency power. Last time I looked, Edcor was offering lam stacks and bobbins for sale- maybe their prices have ballooned on those just like what happened with their end product.
I looked at the Edcor laminations here: (they say from Tempel Steel Corp. )
https://edcorusa.com/collections/laminations# ( EI-125 for Edcor 50 or 60 Watt OTs )
$243.69 for a 9.24" stack, 23.24 Lbs, M6 steel, 14 mil or 0.014" thk
Edcor 50 or 60 Watt OTs use a 2" stack of these, so 9.24" would do 4.62 OTs.
50/60 Watt Edcor Ots are $133/$141 each now.
and I looked at the Tempel Steel laminations here: (page 18 for the EI-125 lams)
https://22383675.fs1.hubspotusercon.../TempelTransformerIndexCatalog_01.17.2022.pdf
Both are 0.014" thk, 14 mil = 29 gauge, M6 steel, these EI-125 lams have a 1.25" wide center tongue by 1-7/8" long tongue (the thicker 16 mil lams are more typical for cheap power xfmrs)
Looks like I will have to call them for prices
Now the Long E lams came from Tempel Steel too. (they were known for also doing smaller quantity sales, 20 years ago at least)
part # FR-1375 14 mil 0.014" thk too, M6 1-3/8" wide tongue, by 4" long tongue, so two standard bobbins fit easily
I don't see the FR-1375 part # listed in their present catalog, would have to call them about availability and price still. I do see what looks like an identical part listed on page 46, FR-5731
Long E laminations are known for much lower leakage L than the usual scrapless E-I laminations. One famous old OT maker used them for high BW OTs. Maybe I should order a stack of smaller long Es anyway.
And there are many smaller and bigger long E laminations listed in the FR catalog section too. Typically twice as long a tongue as the standard (scrapless) EI type laminations.
20 years ago they sent me 50 Lbs of the FR-1375 lams (Es and Is ) for free, Was still in their catalog then, maybe was going obsolete. Still have the old physical catalog here.
https://edcorusa.com/collections/laminations# ( EI-125 for Edcor 50 or 60 Watt OTs )
$243.69 for a 9.24" stack, 23.24 Lbs, M6 steel, 14 mil or 0.014" thk
Edcor 50 or 60 Watt OTs use a 2" stack of these, so 9.24" would do 4.62 OTs.
50/60 Watt Edcor Ots are $133/$141 each now.
and I looked at the Tempel Steel laminations here: (page 18 for the EI-125 lams)
https://22383675.fs1.hubspotusercon.../TempelTransformerIndexCatalog_01.17.2022.pdf
Both are 0.014" thk, 14 mil = 29 gauge, M6 steel, these EI-125 lams have a 1.25" wide center tongue by 1-7/8" long tongue (the thicker 16 mil lams are more typical for cheap power xfmrs)
Looks like I will have to call them for prices
Now the Long E lams came from Tempel Steel too. (they were known for also doing smaller quantity sales, 20 years ago at least)
part # FR-1375 14 mil 0.014" thk too, M6 1-3/8" wide tongue, by 4" long tongue, so two standard bobbins fit easily
I don't see the FR-1375 part # listed in their present catalog, would have to call them about availability and price still. I do see what looks like an identical part listed on page 46, FR-5731
Long E laminations are known for much lower leakage L than the usual scrapless E-I laminations. One famous old OT maker used them for high BW OTs. Maybe I should order a stack of smaller long Es anyway.
And there are many smaller and bigger long E laminations listed in the FR catalog section too. Typically twice as long a tongue as the standard (scrapless) EI type laminations.
20 years ago they sent me 50 Lbs of the FR-1375 lams (Es and Is ) for free, Was still in their catalog then, maybe was going obsolete. Still have the old physical catalog here.
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You can also do balanced P-P (symmetric wound) OTs using the long E laminations. Two standard bobbins fit easily.
Sections of each P-P side occur one each bobbin, and the two bobbin windups are identical. You get identical electrical characteristics for each P-P side. (unlike virtually all of the commercial OTs using scrapless laminations)
Hmm, FR-1375 then reverse the digits you get FR-5731 must be identical, maybe some tweak to bolt hole size or something. Metric?
Sections of each P-P side occur one each bobbin, and the two bobbin windups are identical. You get identical electrical characteristics for each P-P side. (unlike virtually all of the commercial OTs using scrapless laminations)
Hmm, FR-1375 then reverse the digits you get FR-5731 must be identical, maybe some tweak to bolt hole size or something. Metric?
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