For my parafeed headphone amp, I first used a pair of Amveco 10VA toroidal transformers with dual primaries and dual 12V secondaries. Both primaries and secondaries were wired in series, for a turns ratio of 9.6:1. With closed loop, the response is pretty much flat to 20kHz, but the roll-off near the gain crossover region was not optimal, and the phase margin was insufficient as shown in the gain-phase plots I posted. I'm thinking of retrying these transformers with a zobel network to see if some damping will improve things. It certainly helped with the Transcendar transformers I tried later on. To see the gain-phase plots, go here:
SE VT Headphone Amplifier
SE VT Headphone Amplifier
The only problem I can see with the parafeed design is the requirement for a large high quality coupling cap, if you want to handle a wide range of headphone impedance. If you accept a requirement for headphones 32 ohms and above, you can get by with 10uF.
I made a 6p15p headamp for a friend of mine using Antek 115+115:5+5 transformers with primaries and secondaries each in series. The reflected load was about 30k so a much smaller coupling capacitor could be used (.47uF). However this only worked because of sufficient mu, otherwise a driver would have been in order. The center tap on the power transformer should be grounded, obviously
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Looks to me like this isn't going to be a great option for this amp.
Also, when I asked about what VA ratings people had luck with, it wasn't concerning saturation but rather the overall behavior of the transformer.
I did some quick back-of-the-envelope calculations and I'm fairly certain that core saturation will be an unavoidable problem with these transformers.
I'm not real satisfied with the commercial offerings in terms of HF performance without spending $200 per transformer.
At this point I think I'm going to wind my own, and I'm thinking of using C-cores. Does anyone know of US companies that will sell these at reasonable prices in small quantities, or am I out of luck?
Also, when I asked about what VA ratings people had luck with, it wasn't concerning saturation but rather the overall behavior of the transformer.
I did some quick back-of-the-envelope calculations and I'm fairly certain that core saturation will be an unavoidable problem with these transformers.
I'm not real satisfied with the commercial offerings in terms of HF performance without spending $200 per transformer.
At this point I think I'm going to wind my own, and I'm thinking of using C-cores. Does anyone know of US companies that will sell these at reasonable prices in small quantities, or am I out of luck?
I don’t mean to dissuade you from winding your own OTs, but you might want to check out the Toroidy line of toroidal output transformers. Some people over in the Baby Huey threads used their 50 watt output transformers with 6.6k primaries with good results. TME sells these for under $70 each and shipping from Poland to Midwest USA is $9.90 per transformer. I have bought from TME several pairs of OTs and they arrived in 3-4 days each time. You may have to contact Toroidy directly to glean the parameters you were looking for. (I have no connection with Toroidy or TME)
See:
TTG-EL34PP TOROIDY - Transformer: speaker | 50VA; O115x65mm; 0.008/54kHz; 200mA | TME - Electronic components
Alternatively, I understand that Toroidy is willing to wind custom transformers upon request for a modest additional fee. I have not done this so I don’t know the cost or wait involved.
See:
TTG-EL34PP TOROIDY - Transformer: speaker | 50VA; O115x65mm; 0.008/54kHz; 200mA | TME - Electronic components
Alternatively, I understand that Toroidy is willing to wind custom transformers upon request for a modest additional fee. I have not done this so I don’t know the cost or wait involved.
If you plan to use a power transformer for a push-pull application, your options are mainly dictated by volt-seconds. For a power transformer with 115/230 V capability, what you get is 115 VRMS (160V peak) of capability per side at 50 or 60 Hz. Given the economics, it's probably safe to assume that the transformer flux density will be pushed close to the limit to save on copper and iron, so, 12-15 kG. If you want 20Hz capability, the voltage that the windings will support will be lower by a factor of 2/5 or 1/3 . The Antek toroids have an advantage that they're designed for 50 Hz operation, so you get that extra edge in voltage capability (they brag about it). If you are willing to restrict bandwidth at the low end, the voltage capability will go up accordingly.
You may be able to play some games by assuming that the designers sized the turns and the core area to handle the upper line range (132/264 VAC), but that may be a dangerous assumption. You can probably get a handle on this by feeding the unloaded transformer with a variac having 130V boost capability and monitoring the excitation current. Once you get close to saturation, the excitation current will shoot up like a rocket, as silicon steel has a very square hysteresis loop
The people getting away with using power toroids in their push pull designs may be getting away with it because their program material does not call for full power at 20 Hz. Putting a current probe on one side of the primary and feeding the amp with a 20Hz sine would probably be distressingly revealing.
Using a power transformer in a parafeed application is a safer option, as then you have the whole primary to play with if necessary, so your voltage capability goes up by a factor of two.
You may be able to play some games by assuming that the designers sized the turns and the core area to handle the upper line range (132/264 VAC), but that may be a dangerous assumption. You can probably get a handle on this by feeding the unloaded transformer with a variac having 130V boost capability and monitoring the excitation current. Once you get close to saturation, the excitation current will shoot up like a rocket, as silicon steel has a very square hysteresis loop
The people getting away with using power toroids in their push pull designs may be getting away with it because their program material does not call for full power at 20 Hz. Putting a current probe on one side of the primary and feeding the amp with a 20Hz sine would probably be distressingly revealing.
Using a power transformer in a parafeed application is a safer option, as then you have the whole primary to play with if necessary, so your voltage capability goes up by a factor of two.
New scope should be home waiting for me, so I will try to get some hard data this evening. I also ordered a pair of 10VA Anteks that I intend to wire in series and take some measurements to see if that is a viable option. For $10 a piece, it won't hurt to look. I am curious about the parasitic capacitance when 2 transformers are hooked up this way. Series connection suggests that it should be halved, but I have a feeling that they very well could double. I intend to look at that as well, but if anyone has any input, it would be appreciated.
I do two in series (interleaved). I get full power at 30Hz... I get about 60% maximum at 20Hz.
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I am a single ended kinda guy. But two in series could be used in parallel feed push pull. And slightly off topic... my scope didn't arrive today, so my testing will have to wait another day or two.
I couldn't tell you exactly... I can tell you the secondary load was 6R, and the primary was about 1k Ra-a. 320V B+... Triode connection.
I'd love to try them as outputs in a circlotron topology- with some sweep tubes as outputs I bet it would put out a good bit of low distortion power, and with both windings in series it should handle a good bit of swing.
All I'm going to say is that the Anteks will saturate far too soon. Like I said, I should have done my back-of-the-envelope calculations before I posted this thread, since I'm going to have a bit more than 230V of swing on the primary. This wasn't supposed to be a small amp.
All hope might not be lost for the Anteks though, as one interesting possibility is this:
AS-1T275 - 100VA 275V Transformer - AnTek Products Corp
Saturation would likely be less of an issue using the 275V windings as the primary. I could then either use the 6.3V windings (which would give me something like a 15k primary impedance with an 8 ohm load, so I'd realistically be limited to 2 ohm and 4 ohm speakers) or wind a new secondary onto it. I have my doubts that the performance could be made acceptable, but at $35 I may give it a shot for the sake of science.
The Toroidy parts are very interesting, and the prices are quite impressive. They may be my best option.
I still may try to design my own. It may take a few tries to get it right, but I think it's doable. Calculations aren't too bad, though trying to get the leakage inductance down will be a challenge.
All hope might not be lost for the Anteks though, as one interesting possibility is this:
AS-1T275 - 100VA 275V Transformer - AnTek Products Corp
Saturation would likely be less of an issue using the 275V windings as the primary. I could then either use the 6.3V windings (which would give me something like a 15k primary impedance with an 8 ohm load, so I'd realistically be limited to 2 ohm and 4 ohm speakers) or wind a new secondary onto it. I have my doubts that the performance could be made acceptable, but at $35 I may give it a shot for the sake of science.
The Toroidy parts are very interesting, and the prices are quite impressive. They may be my best option.
I still may try to design my own. It may take a few tries to get it right, but I think it's doable. Calculations aren't too bad, though trying to get the leakage inductance down will be a challenge.
You might be better off getting your toroidal output transformers from someone like Amplimo out of Holland, much better characterized transformers.
Toroidy out of Poland seems to have a better line of pre-designed transformers for P-P applications. Having said that, I have a pair of 3k p-p toroidal transformers from Amplimo that I got through Ebay, so they can do it as well.
This may be a silly question, but dont the Anteks being spoken about here, come in a 380/400/415/440 V primary version.
That would seem to be the easiest way to avoid saturation.
FWIW I have been experimenting with toroids as OPT on a very small HA, in parafeed, with limited success.
In my case I have been using an old RS branded 30VA, 115+115: 15+15 windings for around 8:1 voltage ratio.
F-3dB ends up about 4Hz and 25kHz with no resonance obvious to me.
This worked far better for me than the Block PCB mount 6VA I tried beforehand (not a toroid as far as I can tell)
That would seem to be the easiest way to avoid saturation.
FWIW I have been experimenting with toroids as OPT on a very small HA, in parafeed, with limited success.
In my case I have been using an old RS branded 30VA, 115+115: 15+15 windings for around 8:1 voltage ratio.
F-3dB ends up about 4Hz and 25kHz with no resonance obvious to me.
This worked far better for me than the Block PCB mount 6VA I tried beforehand (not a toroid as far as I can tell)