JLM AUDIO

ok. so i've got some interesting choices with the two BA's on the bench.

the opamps are choices between 1731, 2520 and the fabio bfb, so 18volt zener (but i haven't got a product ID yet)

for input transformers, i've got ones showing as 1.2ohm:63ohm, and others showing as 16ohm:1k6ohm

so what RL for vaiable impedence?
CL?
(RPAD and RG seem as though they'd stay at 120R and 68R respectively)

for output transformers, i've got a pair of 10:15 i'd like to save for a pair of 1290's, so that leaves a pair of green can Astors showing 400 : 400/800
you'd think the 400:400 would be good, but i tested passing signal through at mic level and they seemed pretty crunchy.

it'd be good to hear thoughts. i have yet to trawl through all options but i think i'm on target for the inputs, as i know the OEP 1:6.45 shows 10ohm:600ohm and a lundahl 1528 shows 42/42:420/420.

chris.mfdu

mmm that is a odd collection of transformers! any brand names, models or photos you can provide?

are those DC resistance measurements?

are the input transformers in some kind of shielding cans? if not you might have some problems with noise pickup and or oscillation i think...

i'd say try running NO output transformer at least to begin with. BAs i've made with just "impedance balanced" outputs sound great. then you can hear the sound of the input transformer and decide on your favorites and then try the output transformer options.

1290s need to have a gap in the laminations to prevent saturation from the power supply DC flowing through the primary winding... are the "10:15" transformers gapped? you can tell by looking at the E and I pieces - if all the "I"s are at one end rather than alternating you possibly have a gapped output transformer.

h

i imagine CL for an unknown transformer can only be determined by testing - there's some info over at prodigy-pro on a method of finding the flattest load for transformers.. it might be a jensen doc? or maybe something from cinemag.. i can't remember really. but if you look around there should be some info.

or you can just go with something average and then tweak if the top end (?) sounds off.

just try some possible permutations of transformers, opamps etc, record some samples while trying to keep as many factors equal as possible... and make up your own mind!

hi haima, and thanks for responding.

all trannies except the 10:15's are in cans. there are two harbuch's, two Astors, and four unknown.

i was certainly planning on putting signal through sans output trannie, but it is where i want the character of these to reside.

they'll be sharing a 3RU case (!!!) with a pair of 1290's, so it'll be my 'thick' box.

i'll grab some photos this eve and post them, for your delectation.

regards,
chris.mfdu

harbuch is still around - you could call them for the specs of those transformers.. they might be on their website even.

h

damn double post...

where's joe?

anyway, i'll check in with harbuch, but the othes will remain a scatch-and-sniff.

to get the 18v need for the opamps, the diode appars to be an 1N4746/BZX85C18 (Zener 1W 18V ) - correct?

will post photos of the collection later today.

c.

Chris

If you use the 160/1k6 for input and use CL/RL values for the LL1578 at HIGH setting from Joe's page, you'll be right.

Regards from a fellow turtlerocker.

thanks for that chrisP - i've gone ahead with your advice.

now - RZ and CZ - not fitted. i don't need to jumper them, do i? from the schematic it appears as though a jumper would be bad.

and the spot for a 10uf 63v electrolytic - i didn't get one of those. can't find one in the schematic.
is it required?

chris.mfdu

now - RZ and CZ - not fitted. i don't need to jumper them, do i? from the schematic it appears as though a jumper would be bad.

Correct - NO jumper, just leave them holes empty.

and the spot for a 10uf 63v electrolytic - i didn't get one of those. can't find one in the schematic.
is it required?

You got 6 x 470uf 50V electro's, yes? They are the only electrolytics so far as I am aware.

yeah i got the 470's. but thereis a spot beside #bd681 for a 10uf 63v cap.

i've emailed joe direct to ask . . .

c.mfdu

The latest BA PCB version has a extra 10uF 63v cap which is extra smoothing for the opamp reg when fitted. If the #BD681 is not fitted to lower the voltage rail for opamps than cannot run the full +/-24v then this cap is not needed but can be fitted anyway.