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How do you decide between output tranformer and OTL tube designs?

post #1 of 7
Thread Starter 
Just curious if confronted by both choices which you lean towards by default and why? An example would be when confronted by the WooAudio 3+ v. 6. There are obviously others. Any different traditional (even if lots of exceptions) characteristics in SQ between the two designs?
post #2 of 7
I am fortunate enough to have a choice at the flip of a switch - with the ASL MG Head OTL MK III. What I find to be the differences - note that a) many are not very prominent such that you often need to listen critically to hear them; and b) these can change with tube rolling:

Transformer:
- more authoritive and controlled bass
- cleaner, colder (more neutral) mids
- more extended highs (though very slightly) with more air/separation
- can be a bit sibilant depending on ancilary equipment and the recording
- more analytical in nature
- likes to rock, and swing, and do fast-paced acid jazz & electronica

OTL:
- bass can be a bit wooly/overhung; but also seems to go deeper
- warmer, more full-bodied (read colored) mids
- slightly rolled highs: or can sometimes sound that way as the soundstage tends to be smaller and more constrained)
- more forgiving in regards to other equipment used and recordings
- more musical in nature
- likes to sing, present vocals and instruments, and do calmer music (operatic, orchestral, classical)
post #3 of 7
The transformer choice is very important with a transformer coupled amp. Make sure a good one is used if you go that way.
post #4 of 7
Ditto on what your headphones are. Not just for attaining the required power to drive and control them, but also sonically. For example, I prefered the W5000 in transformer mode; I prefer my HD650s in OTL mode.
post #5 of 7
Tubes have inherently greater output impedance than transistors. This is a great oversimplification, but in general, output impedance should be lower than the impedance of the device being driven. Due to high output impedance, many tube designs have problems driving low impedance loads unless some form of impedance correction is provided. A transformer is one way to drop the output impedance of a tube amp, which makes it easier to drive low impedance loads. The lower the impedance of a headphone, the likelier that a transformer mode might be useful. That's why 300 ohm Sennheisers are likely to do better without a transformer than 40 ohm W5000's. The lower impedance of the W5000 will draw more current from the amplifier, and may be too difficult for the amplifier to handle.

For me the default is always OTL, since the best a transformer can do is be transparent, and precious few transformers actually accomplish this. I do not personally like transformer colorations in the sound. However, if OTL has difficulty driving a headphone, it can manifest as noise, poor low end, and possibly distortion. In that case, a transformer may be the only way to drive the headphone properly with that amp. Note that several manufacturers have designed tube amps that can drive any impedance headphone without a transformer.
post #6 of 7
For me the choice is almost always transformers, tubes are inherently high output impedance devices and though it is possible to get a decently low output Z with proper tube choice and design, I've found that my favourite tubes are compromised by this approach. Thus I prefer to match impedances with a transformer, it doesn't measure as well in terms of flatness and phase, the phase will do all sorts of "interesting" things once it gets to about 70-100kHz or so depending on the transformer, but the sound for whatever reason stays uncorrupted.
post #7 of 7
Transformers belong to the power supply, and nowhere else...

Kevin Gilmore

For high impedance OTL is mainly OK, but for low Z, even while OTL should be the default option as well, unfortunatelly there are just a very few tubes able to provide the current needed, at very low impedances (around 30-40 ohms) and also unfortunatelly non the ones we see commonly around in tube amps. So the most common, usual, but far from ideal solution is the transformer coupling to lower the output impedance as correctly Hirsch stated...Not my cup of tea at all...and as my main heapdhones are both low Z, SS for me...
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