mcs

Using power transformers as output transformers
« Reply #20 on: June 15, 2004, 11:28:43 AM »
Quote from: "sismofyt"
I'd luv to see that mag! Brings back ol' memories  :grin:

Just send me your address for a copy
Quote
But if it was true dual singleended fashion, they'd have to be gapped, wouldn't they?

Correct - that's the problem :wink:
The transformers should be airgapped, but they were not... The fact that the amp was running with a high idle current makes it even worse.

Best regards,

Mikkel C. Simonsen


sismofyt

Using power transformers as output transformers
« Reply #21 on: June 15, 2004, 11:32:06 AM »
Hmm, me not understand  :?  :?: What exactly is the difference between using two trannies with a single primary compared to a single trannie with dual/split primaries? Why do they have to be airgapped?

I'll mail you my snailmail addy :grin:

mcs

Using power transformers as output transformers
« Reply #22 on: June 15, 2004, 11:53:52 AM »
With one transformer the DC current in the two parts of the primary will be in opposite directions, and will therefore cancel eachother out. That's why PP transformers don't need airgaps (or only small ones).

If you seperate the PP transformer in two you will have DC in both primaries with nothing to cancel it out, so you need two SE transformers.

Best regards,

Mikkel C. Simonsen

gyraf

Using power transformers as output transformers
« Reply #23 on: June 15, 2004, 11:56:46 AM »
I only got the mentioned construction half-way working - by splitting the two transformers primary windings to take opposite-phase DC. Never worked well..
..note to self: don't let Harman run your company..

sismofyt

Using power transformers as output transformers
« Reply #24 on: June 15, 2004, 11:57:06 AM »
Yes, Doh, they're not on the same core! What the hell was they, and I, thinking? I did follow that mag quite closely and if it didn't work, I'm sure they'd said so.. or maybe they liked that saturated sound?  :green:

SSLtech

Using power transformers as output transformers
« Reply #25 on: June 15, 2004, 12:27:28 PM »
Ah.. I got here late, but I also use the A2...

I like it's convenience, though it misses some of the 'communion' that you get from using analog analysers.

-W-a-y- faster, though!

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Peter Simonsen

Using power transformers as output transformers
« Reply #26 on: June 15, 2004, 12:37:16 PM »
Keith,

Do you know what the "normal" resale/used price of one of those A2´s would be..???

Thanks

Kind regards

Peter


sismofyt

Using power transformers as output transformers
« Reply #28 on: June 15, 2004, 08:00:46 PM »
Great PRR :thumb:

BradAvenson

Using power transformers as output transformers
« Reply #29 on: June 15, 2004, 09:49:34 PM »
Quote from: "Peter Simonsen"
Keith,

Do you know what the "normal" resale/used price of one of those A2´s would be..???


I think I paid between $1000-1500.  They don't seem to come up forsale very often.  

I know what you mean about it being too easy almost.  I sometimes forget to really interpret the results, or stop testing too soon.

There are some units by Audio Precision that have similar features.


jhaible

Using power transformers as output transformers
« Reply #30 on: July 14, 2004, 08:48:16 AM »
Quote from: "BradAvenson"
You might think about using a toroid.  In fact I think I'll see what the response would be of the talema I have right here.



Ok so this is a response curve for the talema 62032 12V toroid transformer.  I used a 10ohm load on the secondary and a low impedance drive on the primary.  Looks pretty good considering it only needs to work at one frequency.  



Just for fun ran a THD+N plot.  Seems like it could work as long as you got the loading correct.  

Now I can't say how much my test method really resembles the use as an output transformer, maybe CJ can comment on that.  I only put 21 dbu into the transformer, so not very much level for a power output.  I was dissipating .06W into 10 ohms.

Oh well, enough of a break for me, back to work.




I just found this thread - very interesting!
Are all Talema toroids that good?
I bought a tiny 1.3Watt 230V:22V+22V Talema today. Wanna see how it works as a 1:10 input transformer or 5:1 output transformer for a LA2A-like compressor. My expectations aren't too high (would be too good to be true, for 12 Euros !). But I have nothing to loose, I can still use it as for a small power supply. Hope I can hook it up for audio to make a rough test tonight.
JH.

dale116dot7

Using power transformers as output transformers
« Reply #31 on: July 14, 2004, 09:21:55 AM »
I tried them as line-level (push-pull plates to 600 ohm) transformers and they're kinda bad sounding in that application. They will probably work ok for speaker-level signals but there's a horrible amount of distortion when they're not loaded down a lot. I used the 62005 transformer for that test. I went back to a Hammond 850N really rapidly. The frequency response was surprisingly good but I felt the distortion sounded a lot like crossover distortion - and lots of it - even though the bias levels were set correctly.

gyraf

Using power transformers as output transformers
« Reply #32 on: July 14, 2004, 09:26:17 AM »
Toroid transformers are good at this because of their very efficient pri/sec coupling - that is, if you manage to balance out the DC currents. Toroids will saturate very easily with DC.

Jakob E.
..note to self: don't let Harman run your company..

jhaible

Using power transformers as output transformers
« Reply #33 on: July 14, 2004, 09:36:43 AM »
Quote from: "dale116dot7"
I tried them as line-level (push-pull plates to 600 ohm) transformers and they're kinda bad sounding in that application. They will probably work ok for speaker-level signals but there's a horrible amount of distortion when they're not loaded down a lot. I used the 62005 transformer for that test. I went back to a Hammond 850N really rapidly. The frequency response was surprisingly good but I felt the distortion sounded a lot like crossover distortion - and lots of it - even though the bias levels were set correctly.



So I suspect while the frequency response _would_ make them fit for audio applications, the hysteresis of the core material would cause too much distortion at low levels ... (Just drawing conclusions from your description "like crossover distortion"; no tests of my own done yet.)

JH.

BYacey

    Where dogs wear thermal underwear, Alberta, Canada
  • Posts: 769
Using power transformers as output transformers
« Reply #34 on: July 14, 2004, 11:21:37 AM »
You could use a 70V transformer for an audio output transformer, but the primary impedance needed to match a proper plate load for an output tube(s) will not yield much power. 4900 ohms tap on a 70V transformer equates to 1 watt at 8 ohms, and you will probably find the core size much to small.  This could  be OK for a little practice amp.
Bill Yacey
"Adjust R116 for least smoke"

jhaible

Using power transformers as output transformers
« Reply #35 on: July 14, 2004, 11:49:30 AM »
Quote from: "BYacey"
You could use a 70V transformer for an audio output transformer, but the primary impedance needed to match a proper plate load for an output tube(s) will not yield much power. 4900 ohms tap on a 70V transformer equates to 1 watt at 8 ohms, and you will probably find the core size much to small.  This could  be OK for a little practice amp.


I just want to drive 600 Ohms from a White Cathode Follower. (LA2A)
So we're speaking milliWatts, not Watts.
I guess if something will kill the application (and most probably it will), it's distortion at low levels, not power limitation. (These toroids were
dasigned for 1.3W @ 50Hz, so they should be able to operate a few
hundred milliwatts at 16Hz. But the thought of distortion makes me worry. The core material will certainly be optimized for low hysteresis, but it's one thing to have "low" hysteresis in terms of low power loss, and another thing to have low hysteresis in order to have low signal distortion at low levels ....
I'm not very optimistic here anymore.

JH.

BYacey

    Where dogs wear thermal underwear, Alberta, Canada
  • Posts: 769
Using power transformers as output transformers
« Reply #36 on: July 14, 2004, 12:57:07 PM »
Since your application is line level high quality audio, a 70V transformer is not the thing to use. Consider a good grade transformer from Lundahl, Jensen or the like.
Bill Yacey
"Adjust R116 for least smoke"

jhaible

Using power transformers as output transformers
« Reply #37 on: July 14, 2004, 02:41:58 PM »
Quote from: "BYacey"
Since your application is line level high quality audio, a 70V transformer is not the thing to use. Consider a good grade transformer from Lundahl, Jensen or the like.



I will. As I said, it would have been too good to be true.
But what's the reason? Am I on the right track with the hysteresis of the core material?

JH.

PRR

Using power transformers as output transformers
« Reply #38 on: July 14, 2004, 05:10:08 PM »
> use power transformers as output transformers to interface with say an 8ohm speaker. How do you figure out what the impendances of a power transformer would be?

Assume that the "120V" winding likes 1K-2K, "240V" likes 4K-8K. Working at lower impedance increases copper loss and leakage inductance treble-loss, but extends bass response. Working at higher impedances cuts bass response and may run into capacitiance losses.

Use the voltage ratio (squared) to estimate the secondary impedance.

For low distortion, keep the audio signal down to 1/3rd, preferably 1/10th, of the rated power voltage. That means a "240V" (336V peak) winding is only good for a B+ voltage like 100-150V, preferably more like 30-40V.

When measuring bass response, signal level matters. Bass response improves as you shift from very-very low level to medium level, then falls off at high level. This is true of all transformers, and is another view of bass distortion. So after you figure your maximum signal level, check bass response at -6dB, -20dB, and -40dB, to be sure it won't bass-cut weak signals too much.

> since they're designed for single-frequency use (60Hz), they're wound with no regard for winding and leakage capacitance.

That would be true if customers only used them for resistance loads. But most get used with rectifier-capacitor loads. Then leakage inductance can severely kill the DC output voltage. The spike waveform in the rectifier is equivalent to high harmonics, which if suppressed by leakage inductance will give disappointingly low voltage and rejected shipments.

Anyway, these things are all built as CHEAP as possible. With minimum material. Minimum copper leads to low leakage inductance and low capacitance. To keep it from burning up on power, they used fairly high-inductance iron, and it is the ratio of iron permeability to air permeability that sets the frequency range of a simple transformer. So for simple commercial reasons, they are not bad; it would cost more to make them NOT pass most of the audio band.

> They're also not designed to carry direct current,

No, but. There are two ways to deal with DC current. Use much more iron, or slip some air in the iron to keep flux density off the steep part of the curve. If all transformers were priced by the pound, obviously any tranny with DC should be air-gapped. But transformers today are not priced per pound: audio transformers are specialty items with specialty prices; power transformers are commodity items with incredibly competitive pricing (and vast quantity of over-stock on the surplus market). So the same job might be done with a 1VA special gapped core for $70, or with a 20VA commodity non-gapped core working at 1VA for $20.

Also the smallest trannies, around 10VA, are very often run with half-wave rectifiers that induce DC in the core. Since this is obviously the cheapest class of work, buyers don't expect precise operation, but the iron must not DC-saturate, suck huge AC current, and burn-up. So there must be some tolerance for DC flux at least in the smallest trannies.

As a very rough starting point, figure the AC current that the winding is rated for, and keep DC current to 1/10th of that (for E-I lamination, NOT for torroids). And if the next larger standard size is only a couple bucks more, get the bigger one. Too big won't do much harm, too-small sure will. The main limits on "too big" are really price and weight, and in the 10VA-50VA range the cost and weight are usually not problems.

Do NOT run DC through a torroid! You can get away with it in a stacked E-I core because the butt-lap joints between the iron sheets are in effect very small air gaps, delaying DC saturation. Torroids made by winding a single long strip of iron into a ring have no effective gap at all, and will saturate with DC current far-far-far smaller than their AC current rating. Since you might need a torroid 100 times bigger than a proper gapped core, or 10 times bigger than an "ungapped" stack-core, and torroids cost more per VA, they are poor ideas for any DC application.

jhaible

Using power transformers as output transformers
« Reply #39 on: July 14, 2004, 07:57:49 PM »
Quote from: "jhaible"
Quote from: "BYacey"
You could use a 70V transformer for an audio output transformer, but the primary impedance needed to match a proper plate load for an output tube(s) will not yield much power. 4900 ohms tap on a 70V transformer equates to 1 watt at 8 ohms, and you will probably find the core size much to small.  This could  be OK for a little practice amp.


I just want to drive 600 Ohms from a White Cathode Follower. (LA2A)
So we're speaking milliWatts, not Watts.
I guess if something will kill the application (and most probably it will), it's distortion at low levels, not power limitation. (These toroids were
dasigned for 1.3W @ 50Hz, so they should be able to operate a few
hundred milliwatts at 16Hz. But the thought of distortion makes me worry. The core material will certainly be optimized for low hysteresis, but it's one thing to have "low" hysteresis in terms of low power loss, and another thing to have low hysteresis in order to have low signal distortion at low levels ....
I'm not very optimistic here anymore.

JH.




Ok, I just come from my lab in the basement, where I have listend to some Gentle Giant, Neil Young, and classical solo piano music over that tiny power transformer at various levels.

I was surprised about the quality - better than I had expected!
Not really good, either. A bit thin and tinny. Certainly not the right choice for a LA2A output transformer. But it's remarkable how the whole audio range comes thru. This might have made a nice step-up transformer for a side chain / CV path of a LA3A, for instance.

JH.


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
1451 Views
Last post April 23, 2005, 03:41:06 PM
by gyraf
9 Replies
2398 Views
Last post April 13, 2008, 06:31:06 PM
by radiance
2 Replies
1144 Views
Last post August 11, 2008, 12:05:01 PM
by maxime
5 Replies
2444 Views
Last post June 09, 2011, 08:45:22 PM
by opacheco