Using 2 transformers for 1 amp

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#21
Re: "Yes. I've seen that done" - Chance Gardener

mrfeedback said:
"it more or less wasts half of the transformer capacity" - Suck What ?.

Harry Gardeners advice is correct.
First thing is that a 225VA transformer is a 225 VA transformer, and using one or both secondaries will not change the total throughput capability of this transformer.

JD: Yes it will, because the windings are dimensioned to the avarage current. Now you only use half of the copper, this leads to greater losses and extra heating. There will be less throughput (although I admit it will not exactly half).

Because you can never be perfectly sure that each secondary has EXACTLY the same number of turns, connecting them in parallel is entirely unwise.
If one winding has at least one partial more turns than the other, you will have a shorted turns condition, which will thermally destroy a transformer.

JD: Agreed

The only way to ensure safety using four secondaries would be to use FOUR bridge rectifiers.

JD: Not at all. Check my earlier post, perfectly sensible engineering, only 2 bridges, uses xformers to the max, separate ground leads for star grounding, what else can you want?

As Harry said, the reason for totally seperate B+ and B- supplies is that the supply common point is at the supply stage center star earth point and not in the transformers.

JD: That what I said.

As he also said, use only one secondary per rail because adding an additional secondary in parallel introduces extra parasitic capacitances, and supply resonances.

JD: We already agreed not to parallel the two secondaries for even better reasons, no use to keep flogging a dead horse.

Any droop in supply voltage at full power because of winding resistance is insignificant.

JD: Possibly. Chance hinted at some unmentioned benefit, I thought he might refer to this.

Eric.
Click to expand...

I still think my approach is the better one based on the above arguments, considering the available stuff & its intended use.

Jan Didden
 
#22
djnigma;

You asked:

"So if you used only one secondary of a 225VA Xfmr, does this mean that you could draw 225VA from that one winding (5.6A at 40V)?"

As a rule of thumb, each of the secondary windings is sized to
carry half of the full load capacity of the transformer. If you try
to pull all the load from just one of the secondaries, you could
overheat that winding.
 
#24
Why not measure secondaries and when voltages are the same connect them in parallel. If it works fine without overheating than leave it like this, if not you can always disconnect one primary and use only the other.

What about doing it this way?
 

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#25
Electrics, Electrics Then The Electronics...

Parasitics, diodes, shottky, soft recovery, snubbers - (Useful DIY search terms) - go do the A/B thing and I well expect that you will go back to Harry's advice.
The secondaries will be large gauge wire so IR losses should not be too important.
More important are parasitic elements.
Also I regularly do run up testing on 500W/4ohm/ch and bigger amps via a 150VA isolation transformer and still get seriously loud on music program.
Those shelf systems with 2000 PMPO sticker ratings hang off a 150VA or so internal transformer so don't get too hung up about continuous ratings unless it is pure class A.
For class A/B concentrate more on correct earth arrangements and and supply non resonance.

JD: Possibly. Chance hinted at some unmentioned benefit, I thought he might refer to this.
Yup, he is talking about the above search terms and resonance issues.

Eric.
 
#26
sam9

Why not just make one transformer from two? By that I suggest looking at the hookup schematics on either the Plitron or Avel-Lindberg web sites and treat one transformer like the left (top?) half and the other as the lower. In effect that is literally what they are doing - winding two independant tranformers on a single core.


Alternatively, G.Randy Slone shows this in one of his books for a lab PS project using two fairly common transformer to essentialy create an equivalent unit of twice the value. The rest of the circuitry is then laid out no different from if you had a single tranformer.

There could be something wrong with this scheme and I'm sure I'll be informed otherwise should that be the case.
 
#27
Peter Daniel said:
Why not measure secondaries and when voltages are the same connect them in parallel. If it works fine without overheating than leave it like this, if not you can always disconnect one primary and use only the other.

What about doing it this way?
Click to expand...

Peter,

Why do you suggest a risky approach and at the same time give the schematic of the best way to do it? This is exactly what I suggested (using only half of the bridge, I mean if he has the bridge no sense throwing it away and buy two diodes).
Are you pulling my leg?🙄

Jan Didden
 
#28
Re: Electrics, Electrics Then The Electronics...

mrfeedback said:
Parasitics, diodes, shottky, soft recovery, snubbers - (Useful DIY search terms) - go do the A/B thing and I well expect that you will go back to Harry's advice.
The secondaries will be large gauge wire so IR losses should not be too important.

JD: The secondaries will be just large enough gauge that the manufacturer can get away with - ever heard of competition? Any use of only one secondary will worsen the heat condition, shortening the xformer life. If you think that's not too important, that's your privelege.

More important are parasitic elements.
Also I regularly do run up testing on 500W/4ohm/ch and bigger amps via a 150VA isolation transformer and still get seriously loud on music program.

JD: Your point?

Those shelf systems with 2000 PMPO sticker ratings hang off a 150VA or so internal transformer so don't get too hung up about continuous ratings unless it is pure class A.

JD: Unless I missed something big-time, the discussion was how to best use two transformers for a single amp, not the pro's and con's of amp spec and rating rules.

For class A/B concentrate more on correct earth arrangements and and supply non resonance.

JD: Yes, *after* having taken care of proper use of the transformers.

JD: Possibly. Chance hinted at some unmentioned benefit, I thought he might refer to this.
Yup, he is talking about the above search terms and resonance issues.

JD: He told you that? Or do you mean it is obvious?

Eric.
Click to expand...

Jan
 
#29
Feeding The Chooks...

Jan you are disapointing me here.
Of course it is obvious - Umm the title "Electrics, Electrics Then The Electronics..." is the first clue.
Then the search terms - go learn.
"JD: Your point?" - Umm for music and not sinewave and class A/B amp continuous rating is not as big a deal as you might think.
"For class A/B concentrate more on correct earth arrangements and and supply non resonance.
JD: Yes, *after* having taken care of proper use of the transformers."
Harry and I have already told you the proper usage.

Eric.
 
#31
Now now gentlemen

Uh gentlemen...

I did not intend to start a fight here. Just looking to save a few $$$ that's all. I reckon in the interests of all our sanity, i will just get a bleeding 500VA and be done with it. Point is, i saw the two 225VA's in the paper for $15 each so i just wanted a viable way of getting 450VA out of them both for a MONO amp.
 
#32
djnigma, Harry has an effect of confusing as you have noticed but I think you have had enough input to make your mind up?

My point of view is clear in this case. I would connect two secondary windings in parallel without hesitating but with two transformers in parallel I would check first for extra hum and heat. My experience with especially Toroid Transformers is very good.
http://www.toroid.se/

BTW: Check their special audio transformer. What do you think about it?
http://www.elfa.se/elfa/produkter/se/20/2020471.htm

Shielded with magnetic shield, 11 kg! Beautiful in real life.
 
#33
Re: Now now gentlemen

djnigma said:
Uh gentlemen...

I did not intend to start a fight here. Just looking to save a few $$$ that's all. I reckon in the interests of all our sanity, i will just get a bleeding 500VA and be done with it. Point is, i saw the two 225VA's in the paper for $15 each so i just wanted a viable way of getting 450VA out of them both for a MONO amp.
Click to expand...

The safest, least controversial way to use two power transformers to make one high power amp is to just make a two channel amp, powering each amp from one of the transformers and include a bridging circuit to permit you to use both amps to drive one speaker.

Rod Elliot describes how to do a bridging circuit in his Project 14. http://sound.westhost.com/project14.htm

This has the free side benefit that you end up with a medium power stereo amp as well for almost no cost increase. Most of the cost in a power amp is in the transformers, heat sinks and output devices which are not increased by doing this.

Only issue to doing this is that you will need to use lower voltage transformers since 1/2 power means lower rail voltages.

Phil
 
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