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Old 26th September 2012, 07:44 PM   #1
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Default multiple output transformers per channel

I have many questions spinning in my head about advantages and problems related to "size" of output transformers, without a rule of thumb. Having wound a few, there seem to be considerable complications in making a really really good large push/pull output transformer. Later outer windings have more resistance per winding turn. High resistance of long wires for large windings requires heavier-guage wire.

I've used transformer outputs in series for voltage, but never in parallel... For one channel, can I just use 4 identical smaller lower-wattage output transformers, with inputs and outputs in series then those series-pairs in parallel? Hopefully each tranny would handle half the voltage and half the current. Are there resonance problems with audio-frequency transformer inputs or outputs in parallel? If it works, I can see some advantages, like PERFECTLY matched push and pull sides without complicated winding patterns, compact low-resistance coils around half-size cores. I imagine there might be a little more total iron weight with 4? But all coils would be smaller, around half-size cores, push and pull sides could match PERFECTLY. Small-signal performance might improve (or not). I just don't know the dangers and downsides of parallel... If there's some imbalance and cross-coil flow in paralleled inputs, don't they disappear in the paralleled outputs? Or is there a better multi-tranny scheme, with primarys in series and outputs in parallel, to reduce the turns ratio?

Not discussing cost here, just performance...just need to get a weird idea clarified and out of my head.

Thanks.
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Old 26th September 2012, 08:20 PM   #2
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Are you talking about paralleling both primary and secondary, or only primary or secondaries? No sense in doing the first think.
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Old 27th September 2012, 08:09 PM   #3
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I'm considering schemes where the output secondaries from multiple transformers end up in parallel. But is that ill-advised?

Last edited by cyclecamper; 27th September 2012 at 08:13 PM.
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Old 27th September 2012, 09:09 PM   #4
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Something like a PSE with separate transformers ??
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Old 28th September 2012, 07:05 PM   #5
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As one case. But many others. For instanace, very large mono bass amps use enormous output transformers. Several smaller output transformers might work much better (and be easier to package, and have more surface area for cooling, and smaller windings), IF their outputs (secondary windings) can be paralleled.
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Old 28th September 2012, 08:18 PM   #6
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I hesitated to write about such an approach being afraid that prices on flathead signal transformers may skyrocket. I used them with primaries in series, secondaries in parallel. They have quite good insulation between windings, so it is safe to stack them for P-P guitar amps.
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Old 26th December 2014, 05:36 PM   #7
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idea of multiplying small vintage output transformers is on my mind for some years.. and i recall i've read something like someone on Triode festival having an OT, which was in fact multiple small vintage transformers inside the can. but i can't find any info about this practice (no wonder)
my personal aim is to try effect of old iron/wire/winding to mids and highs, while gaining some H for lows. Is there one rational way to do it ? Can i treat primaries and secondaries just as a separate coils, ignoring the fact they are on different cores?
thanks for opinions
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Old 26th December 2014, 05:59 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by cyclecamper View Post
I'm considering schemes where the output secondaries from multiple transformers end up in parallel. But is that ill-advised?
With a stereo tube amp that has a ground referenced output, you can parallel the two channels (with the same input signal to both channels).
For a 4 Ohm speaker (on one of the 4 Ohm taps), jumper the 8 Ohm taps.
For an 8 Ohm speaker (on one of the 8 Ohm taps), jumper the 16 Ohm taps.

With two identical mono tube amps (with the same input signal to both), jumper the taps as above, and also the 0 Ohm (ground) taps.

Last edited by rayma; 26th December 2014 at 06:03 PM.
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Old 26th December 2014, 06:17 PM   #9
rayma is offline rayma  United States
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Quote:
Originally Posted by cyclecamper View Post

I've used transformer outputs in series for voltage, but never in parallel... For one channel, can I just use 4 identical smaller lower-wattage output transformers, with inputs and outputs in series then those series-pairs in parallel?
Remember that the net DC current in a transformer primary must be zero to avoid saturation in a core without a gap.
This condition is satisfied in a standard push-pull circuit, since the two plate currents flow in opposite directions in their winding halves.
This would not happen in the connection that you suggest.
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Old 26th December 2014, 09:18 PM   #10
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If I understand correctly, OP want to parallel two series pair of OPT. That is first taking two OPT and series both the primary and secondary. Wire up another pair exactly like the first pair. Then parallel both series pairs. Just like taking 4 8 ohm speaker, put two in series, then parallel the two pair and get the final impedance of 8ohm.

First pass, the impedance part seems to work out, you created a composite OPT.

BUT that does not work at all. The reason is when you connect two primary in series, the point where the two primary connect together becomes the center tap. The DC current through the primary for push pull will not be cancelled because the two opposite currents are on different core. So you don't have cancellation of the magnetic flux. You need to have gaped core to handle the net DC current. That complicates the design again.
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