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Does size really matter - SE Output

I've read a lot of posts about the importance of size in a single ended output transformer.
When I was choosing mine, after a lot of reading I decided to go w/ one of the smaller "build to order" (tho mine was a stock product) transformer companies, which is sometimes recommended here.
A friend of mine has the Hammond 16**sea series.
I paid about 1.75 times what he did, and my transformers are about 1/3 the size and weight.

My amp isn't finished yet, so we haven't been able to do any comparisons. His amp is also a completely different design (although both se 300b). I don't think a direct comparison will be fair when I do finish my amp.

It's only human nature to place importance on size, and I'm starting to feel a little inadequate here :).

So, is size really that important, or should I keep a positive outlook until the sound test.
 
I agree with kacernator that either power or bandwidth is reduced with the smaller transformer. Compared to a similar size P-P xfmr, the SE one only gets to use 1/2 the flux swing, this translates into 1/4 the power rating, unless the low frequency rating is compromised instead (which is the easy way out since the air gap reduces the inductance).
But there does seem to also be a limit to how big one can make a SE transformer before other factors kill the high frequency end. Hammond's big 75W one, 1642SE, is likely pushing the envelope for conventional audio SE xfmr design.

Hey, St Louis huh
I used to live in Brentwood, a long time ago.

Don
 

tubewade

Member
2007-06-03 8:54 pm
wicked1 said:
clip

... or should I keep a positive outlook until the sound test.

I would keep that positive outlook until you hear what they sound like. I, too, look for big transformers and often overdo a bit when I'm buying for a project, but several times in the past I've built amplifiers with what appeared smaller than optimum transformers (because I already had them on hand) and the resulting sound was far better than I expected.

I'm not trying to suggest that there is some way around the physics, just that the bandwidth may be just fine once you listen to them. If not, you know what you'll have to do.

Kind regards, Wade
 
There is another engineering compromise that can be made to keep bandwidth up and sorta keep power up with a smaller xfmr. That is to use smaller diameter wire. Then they can put sufficient turns on the smaller xfmr to get the inductance needed for the same low frequency response. SE xfmrs aren't limited by wire resistance heating like power xfmrs, so the high power spec can still be met with some increased coil heating.

But the increased primary resistance will cause the speaker damping factor to degrade some, since the wire resistance is in series with the load Z. Silver wire, with its lower resistance, could counter this effect some.

Also, a toroidal SE xfmr would come out a bit smaller than EI for the same power, but not much since the air gap is what sets the permeability of the core.

Don
 
A Hammond 1627 which may be appropriate for a 300B at certain operating points has a max current rating of 160mA. The size is required to avoid saturation.
A typical optimized 300b transformer would have a max current around 80mA.
The high Frequencies may be better in a smaller transformer... or not.
You have more versatility with a higher current rating - later you can satisfy your urge to try KT88's with 250V @ 140mA which I think sounds pretty good.
 
In my 300B SE Amp, I am currently using Tango XE-60-3.5-S, which is each rated for 30W, 180mA. They sound absolutely good, and they look beautiful too. I tend to agree that as long as the transformer has good bandwidth and has been built with good quality material, 30W is more than enough for an SE-amp which can only put out no more 9W per channel. I am using a pair of ProAC 2.5 clone (sensitivity = 85dB/W/m, I think) with this Amp and the bass output is comparable to an good qaulity 100-150W solid-state design, and even better than my friend's one which is also using the same transformer and connected to the same loudspeakers. My point is that output transformer is very important components in any Amp but it is not as important as the driver-stage.
 
According to Lundahl, (I shouldn't quote him without permission, but... I think he has some white paper on the subject on his website), the transformer should not be oversized. This has something to do with getting the flux just right, which will only happen with the correct size core and wire. So using a 30watt SE OT is not better than using a 10watt SE OT when the 300B provides about 10watts.
Regarding bandwidth it is easier to get good ditto when the tranny is small b/c its the leakage capacitance that dominates and that only gets worse as the size is increased. Triodes dont have very high rp, so most OTs have plenty inductance even when they are small.
 
Large cored output transformers generally require greater magnetizing currents and hence have greater core losses than their smaller counterparts. Many of these smaller, more expensive boutique wound transformers utilize better grade core materials (higher permeability) which allows them to use smaller cores as long as they don't exceed the maximum flux density for the material used..

Hammond makes affordable, decent transformers, not great ones. From all accounts the Onetics are better, but I have not heard any recently.

FWIW I generally tend to prefer transformers that some would consider undersized, and I like good bass too.
 
I would definately not use a SE xfmr of much larger wattage than the intended design Watts. All the xfmr design factors get tougher the higher the Watts rating. But I would also not use an undersized one either, and especially one that has had compromises made to bandwidth or winding resistance to meet the power spec. There is clearly an optimum size, although tilting toward a larger xfmr will reduce winding resistances for better damping factor.

The issue of magnetizing currents in large OTs (both SE and P-P) has some additional complications when max flux density is considered as selectable. It actually is easy to make a large transformer have less magnetizing current and losses than a small one by keeping the max flux level down (ie., a large core, but a low watt rating). Losses go up as the square of flux, so more material is a winning proposition here since max flux can be reduced. However, permeability peaks at some flux signal level in P-P designs, so there is an optimum size there. But for SE, with DC current, there is not much of any optimum for permeability. Also, an air gap in SE xfmrs essentially wipes out any advantages to higher permeability materials.

somewhat OT, but relevant to SE:

I might also note that the often quoted idea of DC biasing the core in SE xfmrs preventing some zero crossing effect in the flux is pure BS. The change in flux in a SE core still requires magnetic domains to flip. Its just that there are fewer of them available to flip when DC biased, so the permeability goes to h___. You will find that there is still the same hysteresis loop around the AC variation, but it looks smaller due to the low permeability. What does change is saturation effects and permeability variation become lopsided with signal, so now produce even harmonics rather than the odd harmonics seen in a P-P core.

Don
 
smoking-amp said:
<snip>

somewhat OT, but relevant to SE:

I might also note that the often quoted idea of DC biasing the core in SE xfmrs preventing some zero crossing effect in the flux is pure BS. The change in flux in a SE core still requires magnetic domains to flip. Its just that there are fewer of them available to flip when DC biased, so the permeability goes to h___. You will find that there is still the same hysteresis loop around the AC variation, but it looks smaller due to the low permeability. What does change is saturation effects and permeability variation become lopsided with signal, so now produce even harmonics rather than the odd harmonics seen in a P-P core.

Don

Interesting, I hadn't really thought much about this before, but having read your explanation it does make sense.
 
I agree with most of what has been said here so far. There is an optimum size range for a given power level. Too small and you lose bass and often reach core saturation on heavy bass notes. Saturation sounds particularly gross. On the other hand if a transformer is too big the bass can shake walls, but the highs may be rolled off and detail is lost. The Hammond 1628SEA falls into the too big category for some amps, which brings me to the point that nobody has made yet. Speakers.

If you have big speakers that can produce notes down to 40Hz and below (especially if the impedance doesn't rise in the low bass region) you may need a big transformer. Here the amp may be called upon to deliver substantial power into a load near 8 ohms at a low frequency. A small transformer will saturate, and the speaker can reproduce the ugly sound. I have a set of relatively efficient (96db) speakers with 15 inch woofers. Playing them loud with a $20 Edcor OPT is an ugly sounding event. The big Edcors (or the Hammond 1628SEA) in the same SE KT88 amp sounds great, and it gets very loud with wall shaking bass.

In my lab I have Yamaha studio monitors with 7 inch woofers. The response rolls off beginning at 80 Hz and is at least 15 db down at 40 Hz. The impedance rises to 20 ohms in the 80 Hz region. With these speakers you can crank the full power of a 300B into any OPT bigger than 1 pound (including the $20 Edcor) and not hear any saturation effects. If a bass note at 40 Hz were to get distorted, the speaker wouldn't reproduce it anyway, and the 10 to 20 ohm impedance doesn't tax the amp as hard in the bass region. The smaller transformers tend to outperform the really big ones on these speakers.

A friend has 106db horn speakers. Loud listening on these speakers requires 1/2 of a watt. Even the $20 Edcor can pass 1/2 of a watt at 20Hz and do a good job of it. I listened to the 1628SEA's on those speakers and all involved agreed that detail was lost, especially in the bass region! My theory is that too much of that 1/2 watt was lost in the 11 pounds of metal. When I measured transformers the Hammond was the most inneficient of the bunch.
 
thanks, Mr. Tubelab.
Helpful as always. It's your amp that i'm building, too.
I'll be sure to post results when I finally get the thing powered up.

I'm using little copper tubes to extend the 9 pin sockets and bring them up to the height of the other sockets right now (also your idea). One down. Taking a break to read some internet stuff, and I'll start cutting the little tubes for the next socket. After that I hook up the tx's and calibrate!
The task of making the chassis is next.

As far as speakers... I'm using the bk-16 speaker kit from Madisound. I forget the driver model right now, and don't feel like looking it up, but it's a fostex 6" full range driver w/ decent bass response. I do plan on supplementing w/ a powered subwoofer, tho, cause the bass isn't GREAT w/ these speakers.
It'll probably work out pretty well.