LUN­DAHL MOV­ING-COIL TRANS­FORMER

Australian HIFI - - CONTENTS -

Swedish com­pany Lund­hal’s coils and trans­form­ers are found in many of the world’s best-per­form­ing high-end prod­ucts. Now the com­pany has started to sell them un­der its own name.

Lun­dahl isn’t ex­actly a house­hold name amongst au­dio­philes, yet you’ll find the com­pany’s trans­form­ers in many of the world’s best-per­form­ing high-end au­dio­phile prod­ucts. Yet de­spite hav­ing been in the trans­former and coil wind­ing busi­ness since 1958, when the com­pany was es­tab­lished in Swe­den by Lars Lun­dahl, the com­pany only started build­ing con­sumer prod­ucts in 2009. Un­til then, it had built prod­ucts specif­i­cally for other man­u­fac­tur­ers, which used its coils and trans­form­ers in in­dus­trial, elec­tron­ics, aerospace and pro­fes­sional au­dio ap­pli­ca­tions.

THE EQUIP­MENT

We ini­tially thought that the sam­ple we were provided for re­view was a pro­to­type, since all the cir­cuitry was housed in a small (120×80×100mm – HWD) wooden box with a hinged lid that’s kept in place when it’s closed by two mag­netic catches. But no, it turned out that we’d been loaned a fullfledged pro­duc­tion sam­ple—this is ex­actly what you’ll re­ceive when you buy one. Although it’s very prac­ti­cal (as you’ll dis­cover later), it’s rather left-field, so it’s likely to be some­thing you love or hate. Many peo­ple would pre­fer a clas­sic case, such as those used by Roth­well for its MCL step-up trans­former (which, in­ci­den­tally, uses Lun­dahl trans­form­ers!), which has a con­ven­tional chas­sis and face­plate, and the in­put, out­put and ground con­nec­tions on the rear panel.

Ob­vi­ously, it’s a step-up trans­former, which steps-up the volt­age from a mov­ing-coil car­tridge so it can drive a stan­dard mov­ing-mag­net phono stage, but it’s a stepup trans­former with a dif­fer­ence, be­cause buy­ers can cus­tomise ev­ery as­pect of it.

For ex­am­ple, our re­view sam­ple was sup­plied with a pair of Lun­dahl LL1931 trans­form­ers (one for each chan­nel). Th­ese use Lun­dahl’s un­cut amor­phous cobalt core and dual-coil struc­ture and the coils are wound with high-pu­rity cop­per wire made by Car­das. Ac­cord­ing to Lun­dahl, the dual-coil struc­ture im­proves a trans­former’s im­mu­nity to ex­ter­nal mag­netic fields, but to be on the safe side, the trans­form­ers are also en­cap­su­lated in a mu-metal shield/case.

How­ever you can spec­ify th­ese trans­form­ers to be wound with pure sil­ver wire in­stead, in which case the trans­form­ers in it will be LL1931Ag types. You can also spec­ify the trans­former core ma­te­rial, so if you’d pre­fer your step-up trans­former with a mu-metal lam­i­nated core, rather than a strip-wound amor­phous cobalt core, you can. You can also spec­ify whether you’d like the mu-metal core wound with cop­per or sil­ver wire. In this case, the trans­former des­ig­na­tions be­come LL1933 and LL1933Ag re­spec­tively.

All four step-up trans­form­ers are in­tended for medium-to-high out­put mov­ing coil car­tridges and so have se­lectable coil wind­ing ra­tios of 1:8 and 1:16. If you need even-higher turns ra­tios, Lun­dahl can oblige, with trans­form­ers wound with 1:32 and 1:64 ra­tios. Once again, you can spec­ify core and wire types, with the trans­form­ers be­ing types LL1941, LL1941Ag and LL1943 and LL1943Ag.

Once you have es­tab­lished which con­fig­u­ra­tion best suits your needs, you still have con­fig­urable op­tions, be­cause you can choose be­tween two gain set­tings (18dB or 24dB in the case of our re­view unit, but 24dB/30dB with dif­fer­ent trans­form­ers) via mi­cro-switches and, via links on the cir­cuit board, dif­fer­ent grounding ar­range­ments as well. You can also en­sure the Lun­dahl is pro­vid­ing the ex­act load for the phono stage you’re us­ing, with the par­tic­u­lar car­tridge you’re us­ing, be­cause there are tubu­lar posts on the PCB into which you can in­sert re­sis­tors (one for each chan­nel). You can use th­ese posts as ‘tem­po­rary’ con­nec­tions while you as­cer­tain the cor­rect value of re­sis­tance, then, once you’ve de­ter­mined that, you can solder the re­sis­tors per­ma­nently to the PCB us­ing the thru-holes along­side the posts. (Or, if you’d pre­fer not to do any sol­der­ing, you could just leave the re­sis­tors press-fit­ted into the posts.)

Our re­view trans­former had a gold-plated ground ter­mi­nal, but did not have gold-plated RCA ter­mi­nals for its in­puts and out­puts. We would have pre­ferred th­ese, but pre­sum­ably they’re avail­able as an op­tion, or you could eas­ily change them your­self.

Why Use A TrAns­former?

You’re prob­a­bly ask­ing your­self why you’d use a trans­former to step up the out­put from a mov­ing coil car­tridge when most peo­ple use ac­tive, pow­ered de­vices. It would be a good ques­tion, and it can be an­swered with just a sin­gle word: noise. Any pow­ered de­vice needs power (dah!) and that power usu­ally comes from the 240V mains—a volt­age source that is a per­fect con­duit for pick­ing up elec­tri­cal noise from other sources and chan­nelling it di­rectly to the power sup­ply of the de­vice that’s step­ping up the volt­age from the mov­ing-coil car­tridge.

The avail­able power so­lu­tions—bat­tery power, su­per-quiet fil­tered power sup­plies, and so on—are usu­ally ex­pen­sive, some­times ex­traor­di­nar­ily so. And if you go with one of the less ex­pen­sive bat­tery-pow­ered so­lu­tions, the lack of power (you’re usu­ally look­ing at around nine volts) and the re­quire­ment for min­i­mal cur­rent drain (to en­sure ad­e­quate bat­tery life) usu­ally ends up com­pro­mis­ing the qual­ity of the au­dio cir­cuitry, so you get limited band­width, low over­load mar­gins, re­duced chan­nel sep­a­ra­tion, higher dis­tor­tion (both THD and IMD) and in­creased noise.

A trans­former, on the other hand, has none of th­ese prob­lems, mostly be­cause it does not re­quire any power at all. The volt­age in one coil of wire (the pri­mary wind­ing) causes mag­netic flux that in­duces a volt­age in another coil of wire (the sec­ondary wind­ing). There is no elec­tri­cal con­nec­tion be­tween the two coils, and no other com­po­nents: just two coils of wire. If the coils each have an equal num­ber of turns, the volt­age in­duced in the sec­ondary coil will be equal to that of the volt­age in the pri­mary coil. If the sec­ondary has twice as many turns as the pri­mary it will dou­ble the volt­age, so for ex­am­ple if there’s one volt in­tro­duced to the in­put of the pri­mary wind­ing, there’d be two volts pro­duced at the out­put of the sec­ondary wind­ing. If the sec­ondary coil has half the num­ber of turns, the volt­age would be halved—so in our ex­am­ple, you’d end up with just 0.5 volts at the out­put of the sec­ondary coil. When the volt­age is in­creased, the trans­former is called a ‘step-up’ trans­former (which is the case with the trans­former in this re­view). Con­versely, when the volt­age is re­duced, the trans­former is called a ‘step-down’ trans­former.

Although step-up trans­form­ers are com­pletely noise­less and have an ex­tremely flat fre­quency re­sponse, they do have the prob­lem that the im­ped­ance of the pri­mary coil is much lower than the im­ped­ance of the sec­ondary coil, be­cause in or­der to have dou­ble the num­ber of turns on the sec­ondary side, you ob­vi­ously need twice the length of wire that was used on the pri­mary side. But this im­ped­ance change is not just dou­ble, as you’d ex­pect. In­creas­ing the volt­age in the sec­ondary by a fac­tor of 10 (an in­crease of 20dB) sees im­ped­ance in­crease by a fac­tor of 100. In other words, the in­crease in im­ped­ance is equal to the square of the in­crease in volt­age. You also get a re­duc­tion in cur­rent in the sec­ondary coil, com­pared to the pri­mary.

But there’s yet another ad­van­tage of us­ing a trans­former rather than an ac­tive step-up de­vice, which in­volves the type of dis­tor­tion pro­duced by both (and all step-up de­vices will in­tro­duce dis­tor­tion to the au­dio sig­nal. Whereas an ac­tive de­vice has a fairly con­stant re­sis­tive in­put im­ped­ance, the in­put im­ped­ance of a step-up trans­former is fre­quency de­pen­dent. This has the ef­fect that any har­monic dis­tor­tion pro­duced by a step-up trans­former is high­est at the low­est fre­quen­cies and drops as the fre­quency rises, whereas in most ac­tive step-up de­vices dis­tor­tion in­creases as the fre­quency rises. (There are, of course, ex­cep­tions… but they’re usu­ally very ex­pen­sive ex­cep­tions!)

TrAns­form­ers Ain’T TrAns­form­ers

Just as Sol here in Aus­tralia used to say ‘oils ain’t oils’ when pro­mot­ing a par­tic­u­lar brand of en­gine oil, Per Lun­dahl in Swe­den, if pre­sent­ing a sim­i­lar ad­ver­tise­ment, would prob­a­bly say ‘trans­form­ers ain’t trans­form­ers’. There are many cheap step-up trans­form­ers avail­able (most of them made in China) and they’re cheap not only be­cause the ma­te­ri­als used are not of a high stan­dard, but be­cause of the way they are wound. To see the dif­fer­ence you could use a piece of string and a piece of dowel. Rapidly wind the string around the dowel within two pen­cil marks 2cm apart. Have a look at the re­sult. That’s how cheap trans­form­ers are wound. Now un­wind the string, and wind again, slowly, mak­ing sure that each wind is tight up against the other, be­fore wind­ing a sec­ond layer.

You’re prob­a­bly ask­ing your­self why you’d use a trans­former to step up the out­put from a mov­ing coil car­tridge...

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