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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: Double C-core output in CAT JL1 posted by Ben H on April 30, 1999 at 07:37:06:
***The CAT JL1 (standard and new Signature version) uses C core output transformers.***They say that this is the most expensive type of output transformer, but that it is the most linear BECAUSE of the air gap and the ease of controlling the amount of airgap in a C-core. They claim that the air gap, which is used by ALL single ended amps (to avoid DC saturation of the core) makes the magnetic response of the transformer more linear and that THIS is the reason that single ended amps have a livelier, direct sound, and that this is why the JL1 SOUNDS like an SE amp in the midrange (but of course has the power and bandwidth of a push-pull amp).
I suspect that Ken was actually saying something a bit different, because the way it is stated above is generally speaking incorrect.
One of the main distortion mechanisms in the output transformer can be considered similar to the crossover distortion that result in a typical transistor output stage with insufficient bias. You have high distortion spike when you swith the magnetization direction. Since that usually happens around zero in a typical p-p output tranny, the small signal gets distorted most. You don't switch in the single-ended amps, they run in the linear area.
The single-ended amps have gaps for very different reason - to avoid the DC saturation. That gap is usually detrimental to the overall transformer performance and is a painful solution they have to endure, because they really have no alternative (*practically* speaking).
Just like in the case of the SS amps, one good solution to reducing distortion is actually in creating some DC bias in the output transformer - shifting the operating point for small signals into a linear region, where no switching takes place. You can take a typical p-p output transformer with no gap (none is normally required because of zero-net DC current in it) and inject a controlled amount of DC current into its primary, or into a separate winding dedicated to this job. The amount of this current is determined based on many considerations (just like the bias in a SS amp is), but the result can be very impressive.
Needless to say, the output transformer used that way must be designed to be able to take this additional bias with no ill results - one can't just DC bias the first available part. The part will be bigger and more expensive than a typical zero-DC output transformer. Some good parts come with specification on how much DC they can take and one can try those with some DC bias. One can, for example, use two slightly different output tubes (say, two mismatched 6550's), making sure the disbalance is not too high (withit the spec) and actually get much better sound. Nothing is free and the penalty will be probably lower maximum output power and maybe a loss in low frequensy response. But the overall sound might get more enjoyable. Please check with the amplifier manufacturer before trying this on any product. If you can't get the Max. DC spec, be very careful.
Best of luck,
Victor.
Output transformer design is another example of multi-dimensional optimization. There are several conflicting objectives there and good design requires high level of skills. There are some true experts in this area out there.
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Follow Ups
- Output Transformer Bias, Gaps, etc. - Victor Khomenko 08:23:23 04/30/99 (0)
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