Thursday, March 3, 2011
Transformers for the LA-2A
Thing continue to develop on the LA-2A component acquisition front. The LA-2A has three transformers.
The easy one was the power transformer. The one in the LA-2A is pretty much a bog standard PT - the schematic specifies a 250V-0-250V (500V CT) delivering 40mA, and 6.3V CT. The center tap on the 6.3V isn't even really necessary, since I plan on using DC elevation of the heaters to reduce hum, and will be implementing an artificial centre tap in any case. Frankly, there's not much mojo to be had in the PT - this is not like some guitar amps where the voltage sag of a poorly regulated transformer actually contributes to the sound. So there appear to be lots of good alternatives. Running down the list of usual suspects, it looked to me like the Hammond 269JX would do the job perfectly.
However, there are two audio transformers, and these almost certainly will make a noticeable contribution to the tone. I did a LOT of thinking about these choices, only a few of which I describe here.
The originals at different times in the production run had two different input transformers. The first was the UTC HA-100X, which has a fairly complex series of primary windings, which you can jumper in a variety of ways to get different impedances... 50 Ohm, 150 Ohm, 250 Ohm 333 Ohm and 500 (600) Ohm were all possible, although pretty much everyone, I assume, just wired it for a standard 600 Ohms. The secondary impedance is 60K. This transformer has a good reputation, but for all intents and purposes it's now unobtanium. The other input transformer in the originals, and which was evidently more widely used, was the UTC A-10. Overall, its specs look fairly close to the HA-100X, except that it had secondary wound for 50K ohms. Authentic copies of this transformer in good condition seem to be pretty damn hard to source as well.
So, the hunt was on for a suitable boutique-grade modern production alternative. The first option was to use the Jensen transformers JT-11-P-1. Although it is a 10K-10K bridging transformer, rather than the 600R - 50K of the original, many people have reported good results using it as an input transformer in LA-2As. Jensen also has a simple schematic showing how to adapt the LA-2A circuit for this transformer. On the one hand, it has a +20 dBu max input signal rating at 20Hz instead of the +15 or +16 dBu of the originals, so would have more headroom... probably a good thing now that signal levels tend to bigger these days than they used to be. On the other hand, of the options this is the one that seems most obviously different from the originals. For some applications, it should work fine, but I decided to give it a pass. For one thing, the 1:1 ratio would make it harder to use the LA-2A as a mic preamp with modest gain -- which you should be able to do just fine by just pulling out the T4B optical unit! Also, I've heard reports that these units can sound their best when being pushed fairly hard. And, er, I also gave them a pass because the folks at Jensen couldn't be arsed to respond to my emails, and also didn't pick up the phone.
Next up was the Cinemag CMQEE-3440A. While the impedances look right, this was rejected because it looks to be intended as a mic input device, and not for line levels... I'd worry that it wouldn't be able to handle the signal levels without pooching out. Someone suggested using the Cinemag CM-9589 10K:600 output transformer by wiring it in reverse, but that seems just too weird.
Another possible option was to have a company like Edcor wind a transformer to my specifications. On the plus side, they have good prices, and the guitar amp guys I know that have used them have been very pleased indeed. But on the downside, I'm less familiar with their reputation for producing the high-bandwidth pro-audio stuff. The real kicker though, was that because they custom wind every order, and usually have a backlog, it can sometimes take 6 weeks or more before your transformer is shipped.
Finally, I came across the Sowter transformers 1009. This one looks to be an accurate reproduction of the original UTC A-10, with a +15 dBu input signal. Moreover, there were really good reviews for LA-2As done with this transformer, and it was recommended in several places. The only drawback is that Sowter is in the UK, and that delivery to the US is naturally expensive and relatively slow. But at the end of the day, though, I decided to suck up the transatlantic shipping, and went with this. This was also influenced by the fact that Sowter also turns out to make the Sowter 1010, which appears to be an excellent quality 15K: 600 Ohm output transformer closely comparable to the UTC A-24. As you might have guessed, the A-24 was the output transformer found in the original. If anything, the 1010 has superior bandwidth. Anyway, I plopped down my money and placed the order with Sowter.
Of course, no sooner did I do this than an original A-24 came up on Ebay... It wasn't exactly cheap, but it wasn't outrageous either, so even though I'd just committed to buying the Sowter, I just couldn't pass up the opportunity to do a real-world A/B comparison with the original. In my experience, output transformers are where a lot of the magic happens. After dithering for a bit, I couldn't resist and ended up hitting "Buy Now" - an A-24 is presumably winging its way towards me now. The plan is to let the customer do their own listening comparison, and let them choose whichever of the two transformers sounds best to them. Maybe we could even do a blind comparison? Anyway, it should be an interesting experiment....