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In the Forum: Melquiades Amplifier
In the Thread: An ultimate transformer for narrow bandwidth?
Post Subject: Re: An ultimate transformer within a narrow bandwidth?Posted by cv on: 2/7/2005
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So, tantalisingly, I will suggest this; for the range 1k up, you would have no problem getting a small mumetal OPT made suitable for the 6C33/S2 combination. Actually, I have something similar but designed for VV52 (300B on anabolics), which I was able to directly compare to a respected full range OPT based on Silicon steel.
No contest; the mumetal device took the sound to a whole different level; it was so much more delicate, ethereal and composed (not unlike an S2 or my ribbon tweeters in their range) that I rapidly came to the conclusion that full range OPTs and non-exotic core materials are a waste of time if one is after the ultimate.
Sowter in the UK can make you a pair to try - I can help you spec if needed.
Other possibilities include amorphous which I have only heard so far driving ESLs... that was the usual, iron based Metglas alloy but the new kid on the block is an exceedingly costly Cobalt based material (amorphous and not to be confused with the Permendur type stuff offered by MQ from time to time). A friend has directly compared both amorphous types in a PP 2A3 amp and said the amorph cobalt is astonishingly clear sounding; space between notes/instruments, easier discrimination etc. This was the same winding geometry/bobbin, core size, just different core material.
Which brings us on to the cheapest and (possibly, for this application) best core material: air. For a 6c33 at 1kHz upwards, in purely engineering terms, it's quite doable. I have some bobbins that measure (-3db) 600Hz to 50 or 60k when driven from 600 ohms; the 6c33 will give you a lower LF cutoff. Also, these have an enormous ratio of almost 100:1, again, the 6c33 into Vitavox S2 is a less compromised design so you may get more top end as well.
Haven't listened to them yet - no time to build the blasted amp at the moment, but in terms of frequency response, they certainly "work". I expect the sonics should be something to behold.
I will dig up details of the bobbins and windings etc...
For LF, I would probably be inclined to go for the biggest core I could afford on the iron based Metglas, overspec'd to keep the flux low (less core distortion) and high primary inductance (less tube distortion). Hell, maybe even go to a *monster* core on silicon-steel and accept a low treble rolloff.
Sorry this is so rushed; I still hope to write something more structured covering sizing of the core, core selections etc...I will also leave you with this snippet: interwinding insulation material may be as important as the core/winding geometry in the final determination.Rerurn to Romy the Cat's Site