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Old 21st August 2004, 10:48 AM   #1
hacknet is offline hacknet  Singapore
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Default Toroidals as OPTs?

i`ve seen a posting on using toroidals as OPTs. i would like to know more and how it sounds as i`m interested in a budget tube amp.

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Old 21st August 2004, 03:06 PM   #2
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The toroid core can be used to make an exceptionally good output transformer but those on the marketplace often cost more than standard E-I core type OPT's. Plitron is one company I know of that sells allegedly (I have no experience with them) good toroidal output xfmers.
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Old 21st August 2004, 11:41 PM   #3
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As it happens, I tried a 2 x 120V to 2 x 9V 50VA transformer as an audio transformer just the other day. If you do the sums, you find that the voltage ratio is appropriate (primaries in series, secondaries in parallel) for treating the transformer as a 5ka-a to 7R load. Note that SE operation is not permissible (unless you use an anode loading choke and capacitor couple to the output transformer). In a push-pull circuit, there is theoretically no DC magnetising current, and an ungapped core (toroid) is feasible.

I drove the transformer from the output of an oscillator from a source resistance of 2k4 (representative of typical triode conditions) and loaded the secondary with a 6R8 resistor. I applied a 10kHz square wave and achieved a surprisingly good result - significantly better than that achieved by a vintage EI Woden 20W output transformer under the same conditions. Even more surprisingly, the low frequency performance was good too, although I would expect the core to be unable to sustain 50Hz at >20W without significant distortion.

Before you get too exited, bear in mind that it will be very difficult to to persuade a practical push-pull output stage to balance the output stage currents to the degree needed by an ungapped core. This is the weakness. You save money on the output transformer, but pay for it in the bias arrangements....
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Old 22nd August 2004, 01:06 AM   #4
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Current balance is the one thing I've always wondered about, too. For commercial toroids purpose-built as OPTs (e.g., the Plitrons), how much current unbalance can be tolerated at idle and under signal?
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Old 22nd August 2004, 02:17 AM   #5
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Default commercial power toroids

One caution I have found about using commercial power toroid transformers. The 130 VA Avel-Lindberg Ltd xfmr I found surplus has the two 120 V primaries wound as random bifilar, ie, both windings were wound at the same time to save fixture setup times. This causes the two primary windings to have 21000pF capacitance between them. Also, there is little effective isolation from primary to secondary, with 850pf common mode. It measures impressive bandwidth from a 50 Ohm source, but not so hot with a realistic source impedance. So, I would check for this problem before using one in an amplifier.

One way to avoid this capacitance problem might be to use a 120V to 120V (or 240V,240V if you can find one) toroidal isolation xfmr, which would presumably have some insulation between the windings. Then wind your own 8 Ohm secondary on top. Unfortunately, most isolation transformers do not have the same number of turns on primary and secondary in order to compensate for voltage drop from winding resistance. So, may be hard to find a good xfmr, or will have to add some turns to one of the primaries to equalize.

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Old 22nd August 2004, 03:12 AM   #6
hacknet is offline hacknet  Singapore
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thanks for the response!

from this i gather that the amp has to be push pull and it can`t take extreme biasing.

i`m thinking of a pair of nuvotem talema transformers to play with.

for 7k PP, what transformer ratings should i look out for?

i`m thinking of a 6L6 or a EL84.
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Old 22nd August 2004, 09:18 AM   #7
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Default Re: commercial power toroids

Quote:
Originally posted by smoking-amp
The 130 VA Avel-Lindberg Ltd xfmr I found surplus has the two 120 V primaries wound as random bifilar, ie, both windings were wound at the same time to save fixture setup times.
That's most surprising. I'd expect safety problems if that transformer was configured for 240V mains.


Quote:
Originally posted by SY
Current balance is the one thing I've always wondered about, too. For commercial toroids purpose-built as OPTs (e.g., the Plitrons), how much current unbalance can be tolerated at idle and under signal?
Yes, I've wondered about that, too. Although the Plitron output transformer specifications note that they were achieved with balanced DC currents, there is no mention of what imbalance (if any) might be tolerated.

Hacknet: For 7ka-a to 6R (a good choice for nominal 8R loudspeakers), that's an impedance ratio of 1167:1. The voltage ratio is the square root of the impedance ratio, so it's 34.1:1. If you have a 2 x 120V primary, that's 240V total, to you need 240V/34.1 = 7V. You won't find that, so I'd try a 6V transformer. If you can find a 2 x 110V with 2 x 6V 50VA transformer, then that should do nicely. Parallel the two 6V windings, but connect the 110V windings in series.
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Old 22nd August 2004, 02:09 PM   #8
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Default Re: Re: commercial power toroids

Quote:
Originally posted by EC8010



Hacknet: For 7ka-a to 6R (a good choice for nominal 8R loudspeakers), that's an impedance ratio of 1167:1. The voltage ratio is the square root of the impedance ratio, so it's 34.1:1. If you have a 2 x 120V primary, that's 240V total, to you need 240V/34.1 = 7V. You won't find that, so I'd try a 6V transformer. If you can find a 2 x 110V with 2 x 6V 50VA transformer, then that should do nicely. Parallel the two 6V windings, but connect the 110V windings in series.
Thanks For the informative post!

technicalities aside, would a toroidal sound better than an equivilantly priced hammond?
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Old 22nd August 2004, 05:18 PM   #9
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Quote:
Originally posted by EC8010
Before you get too exited, bear in mind that it will be very difficult to to persuade a practical push-pull output stage to balance the output stage currents to the degree needed by an ungapped core. This is the weakness. You save money on the output transformer, but pay for it in the bias arrangements....
got a hack saw? gap the core ! as I pointed out on the National Semi website, Sanjay Maniktala has an excellent outline of the equations needed to work with gap sizes:

http://www.national.com/onlinesemina...ics_033104.pdf
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Old 22nd August 2004, 05:22 PM   #10
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Default Re: Re: Re: commercial power toroids

Quote:
Originally posted by hacknet
Technicalities aside, would a toroidal sound better than an equivalently priced Hammond?
Who knows? A toroidal main transformer will certainly be cheaper than an output transformer designed for the job. I suspect in the end that it will be down to whether you can keep those anode currents in balance. If you can, then the HF performance will probably be better.
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