Posted by Tom Ronan on November 11, 2004 at 19:45:00

There are more in the Lundahl data sheets than meets the eye. :-)

In trying to find a good match for an output transformer for my proposed WE 205d and/or 10/vt-62 amplifier, I've run into a snag, and have a question. The main issue is that I have 4 ohm speakers (Altec 755As in Lowther horns these days) and there aren't a lot of 10K 1 watt SE output transformers out there, let alone ones with 4 ohm taps.

At first glance, the data sheets indicate that an LL9202, used with secondary config B will give a nice 11.5K primary - perfect for the 10 or 205d. However, this nice transformer comes at a price 20mH - almost double the primary leakage inductance of the LL1620. (See below.)

Secondary connection Primary leakage inductance
for 4/8/16 W (all in series)

-/B/C B/C/D C/D/E
LL1620 11.5 k 6.0 k 3.3 k 11 mH
LL9202 23 k 11 k 6.5 k 20 mH

There seems to be no 4 ohm option for the 11.5k ohm primary on the LL1620. However, if you look at the connection option that would seem to go in that suspicious "-" blank there, option A would seem to suggest itself. Interestingly enough, option A is listed farther down in the data sheet, and is simply all secondaries in parallel, giving the highest ratio for the transformer.

Plus, who would ever need a 23k transformer, and even if they did, what madness is that, anyway :-) Single ended 26 POWER amp? Something on the order of 0.1 watt? Ok, I'm nuts, but not _that_ nuts.

Though this amp may be full range - full range isn't _full_ range really for me. 40Hz is plenty low enough. Actually, it may end up doing duty as a tweeter (Jensen RP-302) only amp, and in that case the tweeters are 16 ohms anyway. My concern there is that I want as much high-end bandwith as possible for natural highs, so lower leakage inductance is important.

So here's my (rather long winded) question - can I use the LL1620 in connection A for a 11.5K to 4 ohm OPT. And while I'm asking, do they come in 25mA or 30mA versions somewhere in the secret labs?