JBL 3155 autoformers

[.:hb:.]

I am currently building my second version of 3155 crossovers for driving my 4355.
Version 1 has been the simplified circuit with the autoformers being replaced with a suitable network floating around on this board. Works so far.
Version 2 will be a full blown balanced DC biased one, and this time I'd really like to go with the autoformers.

My problem is, as usual in this case, finding suitable ones.
I do have autoformers with the required -4 dB tap, but there's one thing I am not sure of: How important is the inductance of those in this case? Is it an active part of the filter topology or is the autoformer just used for level (and impedance) transformation? If the latter is the case, using autoformers with a much larger inductance than the original ones (which are 1 mH and 0,16 mH, mine have about 5mH) might not be a problem.

Any ideas if this might work?

[martin2395]

Is there any special reason to go for the old, autoformer equipped 3155? :dont-know:

[.:hb:.]

Yes. It's the better solution. Having voltage dividers or just resistors before tweeters has proven to be a thing to aviod. I have learned that even tweeters benefit enormously (soundwise) from damping.
We have done the comparison with some project using a BMS coaxial driver (4590), and the differences beween an autoformer equipped filter and an otherwise identical using resistors were huge.

[ivica]

I am currently building my second version of 3155 crossovers for driving my 4355.
Version 1 has been the simplified circuit with the auto-transformer being replaced with a suitable network floating around on this board. Works so far.
Version 2 will be a full blown balanced DC biased one, and this time I'd really like to go with the auto-transformer.

My problem is, as usual in this case, finding suitable ones.
I do have auto-transformerwith the required -4 dB tap, but there's one thing I am not sure of: How important is the inductance of those in this case? Is it an active part of the filter topology or is the auto-transformerjust used for level (and impedance) transformation? If the latter is the case, using autoformers with a much larger inductance than the original ones (which are 1 mH and 0,16 mH, mine have about 5mH) might not be a problem.

Any ideas if this might work?

In the original 3155 design auto-transformer is a part of the network response, but it has HI-pass response influence on the lower frequency range, but ALSO LP-pass influence on higher frequency, so it is not only level 'shifter', impedance matching device, driver damping improvements, but influential network response device: in the lower section (primary inductance), and in higher section (secondary inductance)

regards
ivica

[.:hb:.]

Thanks Ivica. Even if this is not what I wanted to hear :crying:.

[4313B]

Tapped auto-transformers are about as popular with the designer of the 4345 and 4355 as mass rings in woofers are... ;)

Those tapped-autotransformers were kind of "inherited" from earlier JBL network designs (forties, fifties, sixties) when he first started working at JBL (seventies). He has stated that he was happy to stop using them, "they're awful". You sure won't find them in the Everest II for example. He has only gotten better at world class filter design.

Note that the M2 doesn't use them either, just plain old resistors, so someone at JBL Pro must feel the same way. Although, to be sure, I'd charge couple that protection cap on the D2430K if I had an M2... "If it's in the signal path then it's in the path of the signal."

[gibber]

Thanks Ivica. Even if this is not what I wanted to hear :crying:.

Hi Holger, if not successful with sourcing the right autoformers, bias the network you have and replace resistors with constantan wire. Will work wonders in the mid & treble sections, perhaps less so in bass and midbass. Should your 244x frams still be quite new, put a watt of pink noise in two opposite polarity drivers facing each other and leave hissing for a day. What you'd get when re-installing is the same as the change resistor to resistance wire. The wire comes in values like 10/33/100 ohms per meter, helps to avoid unwieldy loops. Perhaps also a nice substitute for the cabinet inside cabling when schematics demand a resistor interface a unit's terminal. Mind power rating, tho ;)

Ralph

[ivica]

Thanks Ivica. Even if this is not what I wanted to hear :crying:.
Hi,
Here on the AH FORUM you can find some 3155 transformer - less design that have almost the same F/R response as the original one, so may be can be easier to implement DC biasing.
I have not seen any comparative audio results between such realized network and the original one. Usually FR response has been done using "pure resistor loading", but it would more precise if the drivers response Have been done with drivers installed.

Regards
Ivica