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Old 12th December 2008, 07:46 AM   #1
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Default Toroid power transformers - are they a good idea?

Toroid power trannies are used in many SS amps these days but have yet to gain wide acceptance for tube amps, it seems.

IMHO, the advantages of a toroid (compact, lightweight, cool running, low level of EMI) make it worth the effort to overcome its main disadvantage (high transfer of mains-borne noise to the secondary, due to its high band-width).

However, I'd be interested to hear other people's views.
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Old 12th December 2008, 08:01 AM   #2
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What I have done occasionally in preamps is use a 230:12 into a 12:230 to get the HT, so using two transformers instead of one (and getting a free -15DC feed for a CCS).

I guess I should put some smoothing between the two transformers - not sure if this double arrangement has any benefits on smoothing out the crud.

andy
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Old 12th December 2008, 04:29 PM   #3
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On paper toroids may offer the ideal solution but in practice the windings are never quite symmetrical resulting in flux spew and this can be quite large. If you are designing a preamp with one, keep long leads and able to rotate the doughnut for min hum. Bear in mind safety issues.
Another point, Bmax (flux excitation) is higher than for EI types and can result in high inrush currents. typically an extra long duration/ antisurge fuse is used but if power types (+150W) are used then some active inrush limiting i.e relay and out resistor is required. Reason; a 10A anti surge fuse is hopeless for a circuit consuming 3A. It won't protect before the switchboard chickens out.

Toroids for power amps have always been a cosmetic issue and no manufacture likes reducing Bmax for the sake of more winding and less space. Tube amps have always had lower Bmax for the sake of flux and magnetic fields. From day one the upright mains tranny has always been on the chassis.
The temperature rise for toroids isn't as good as for EI types.
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Old 13th December 2008, 12:16 AM   #4
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Quote:
Originally posted by richwalters
On paper toroids may offer the ideal solution but in practice the windings are never quite symmetrical resulting in flux spew and this can be quite large.
That's interesting. Do you mean large relative to ideal or compared an equivalent quality EI? Another negative of course is sensitivity to DC on the primary. I used to consider the extra bandwidth of toroids a problem until doing a spectral analysis of my house utility. The standards have fallen so low, with so many high level harmonics close to the fundamental, that easily filtered cruft above that is the least of my worries. On the plus sides most toroids in my experience drive less mechanical energy into the chassis (potentially the primary energizer of microphonics) and the secondaries almost always have significantly lower impedance.
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Old 13th December 2008, 01:32 AM   #5
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I've been using them for years, Ray, and I do so every time I can find the right voltage and rating. They're smaller, lighter, more efficient, and generate less heat and strayfield than an EI.  I saw the argument about problematic winding geometry and thus strayfield, but I don't think this is significant, given the efficiency of operation they present.

Only problem I have is that they're ugly as sin, so they have to be covered in cans for commercial use, which adds to the expense, but this is true of about half the EI iron out there as well.

The Antek guys have really expanded the range of available units; I have several here I'm building around.

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Old 13th December 2008, 02:44 AM   #6
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Quote:
The Antek guys have really expanded the range of available units; I have several here I'm building around.
I have also used several Antek toroids with good results. Despite my reputation, I have not seriously abused them much. I have a 4T360 running a Simple SE and it runs stone cold, but it is barely taxed with this load.

I bought the first Maryland Toroid ever built for tube amps (a few years ago). It was in a 60 WPC P-P amp for a couple of years. It has been powering two Simple SE's simultaneously running EH KT88's at 100 mA each for a few months. It runs cold to the touch. These toroids are designed especially for tube amps:

http://www.toroid.com/standard_trans...rmers_tube.htm

A conventional EI transformer can be momentarilly overloaded considerably without incident, some can be continuously operated well above the published ratings without any problems. I expected the little Allied 5K56VG in my Tubelab SE to smoke when I started running 300B's in an amp designed for 45's, but that was over 2 years ago. It runs too hot to touch, but hasn't fried yet.

One thing that I have noticed with some toroids is that you will reach a point where the core saturates upon overload causing a serious increase in primary current. I have seen this in some filament transformers when I stuffed a 6336 in a 6AS7 socket. The primary fuse blew instantly even if I put a 5 amp fuse in place of the 2 amp. I have seen the little Antek 100VA toroid eat some big fuses when overloaded. The exact overload was not known, but I am guessing about 200VA. Beware of this issue in class AB or class B amps.
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Old 13th December 2008, 03:21 AM   #7
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On the subject of Antek, anyone notice they're marketing a line of toroid OPTs now?
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Old 14th December 2008, 10:27 PM   #8
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Quote:
Originally posted by rdf
On the subject of Antek, anyone notice they're marketing a line of toroid OPTs now?
I use Antek toroids in my dac and in some of the filament supplies in my 300B SE amplifier, they are generally extremely quiet and cool running until significantly overloaded and then they get hot and noisy.. I'm quite pleased with them overall, and they are one of the few vendors with anything approaching reasonable prices.

The key to the use of any toroid is to size them appropriately for their intended use, for AC filament heating you can get pretty close to or even slightly beyond the sinusoidal current rating without a problem, but when using them for dc heating duties I would derate to 60% or less of their secondary current rating(s) in order to avoid overheating, noise, and problems induced by momentary (on peaks) or continuous core saturation.

You can greatly improve the situation in terms of external emi/line noise transmission by putting a common mode/normal mode choke in front of the primaries.. Coiltronics and Coilcraft both make suitable chokes for this purpose - a few milli-henries is more than enough to do the job.

I also noticed they are selling a limited line of PP OPTs when I was doing research for an order I just placed with them. I don't know how good they actually are but would also be interested in hearing more..
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Old 15th December 2008, 01:43 AM   #9
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Heres the link to their output transformers.

http://www.antekinc.com/output.html

After things settle down here I plan on building some kind of solid state amp to compare against my current tube setup and Antek looks like a good deal for an 800VA unit.
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Old 15th December 2008, 02:25 AM   #10
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"anyone notice they're marketing a line of toroid OPTs now?"

The frequency response data they give for them does not specify the driving and load impedances, so one can not tell the quality. They also don't specify the primary inductances. I would expect to see much better curves for progressive wound xfmrs intended for audio. My guess is those curves are for a 50 Ohm signal generator source. So I am assuming they are just the usual random wound cores, like for 60 Hz power xfmrs, but with the correct turns ratios. Until I see some real data, I'm building my own toroid winder.

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