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kc613
02-09-2012, 07:42 AM
66 yr old(electronically illiterate) with first post here, and my first time attempting anything like this, so here goes. trying to upgrade 4412 xovers, so, first question is about identifying the caps on the board. there are 4 caps on the board(excluding 4 bypass caps), 3 are round, aluminum exterior, blue colored caps, one for LF cicuit, two for MR circuit. in the HF circuit, there is a totally different cap, which is oblong and has a yellow plastic exterior. is this a different type of cap that effects the HF circuit in a different way (warmer, or?)? if so, should the replacement cap be of a different type also, to continue this line of thinking? i was planning on using Dayton 1% metallized polypropylene, unless there are better caps (at a reasonable price)that will give more improvement.

should i eliminate the bypass caps?

is there a benefit in using 2 caps in parallel instead of just the one cap? (reducing ESR) if so, can you piggyback the caps, with the top caps leads twisted around the bottom caps leads and the bottom caps leads soldered to the board, or must the leads from both caps be inserted and soldered to the board?

hope i'm on the right track in improving how these 4412's sound. thanks much, in advance.

p.s. i know these are silly questions for for a lot of people here on heritage, but i have to start somewhere.

grumpy
02-09-2012, 09:43 AM
mostly right, and good guesses (or reading) :)

blue/aluminum are almost surely bi-polar (this is important) electrolytic capacitors
and are often used as their size and cost vs. capacitance ratio is better, and they
can work well enough at lower frequencies. If something was going to go "bad"
from age, it would likely be these.

The yellow/plastic one is probably mylar. I'd leave (or replace) the bypass capacitor
(as opposed to leaving it out), unless you want to tackle "charge coupling" the crossover
... which might actually make "hot-rodding" your crossover with new parts worthwhile.

You'll want to look that last part up, about charge-coupling (or DC biasing the capacitors).
The cost will go up.

I'd suggest keeping the old crossovers intact, or regardless of how keen the new crossover
sounds, you will almost certainly reduce the value of your speakers.

Welcome aboard.

DavidF
02-09-2012, 11:49 AM
66 yr old(electronically illiterate) with first post here, and my first time attempting anything like this, so here goes. trying to upgrade 4412 xovers, so, first question is about identifying the caps on the board. there are 4 caps on the board(excluding 4 bypass caps), 3 are round, aluminum exterior, blue colored caps, one for LF cicuit, two for MR circuit. in the HF circuit, there is a totally different cap, which is oblong and has a yellow plastic exterior. is this a different type of cap that effects the HF circuit in a different way (warmer, or?)? if so, should the replacement cap be of a different type also, to continue this line of thinking? i was planning on using Dayton 1% metallized polypropylene, unless there are better caps (at a reasonable price)that will give more improvement.

should i eliminate the bypass caps?

is there a benefit in using 2 caps in parallel instead of just the one cap? (reducing ESR) if so, can you piggyback the caps, with the top caps leads twisted around the bottom caps leads and the bottom caps leads soldered to the board, or must the leads from both caps be inserted and soldered to the board?

hope i'm on the right track in improving how these 4412's sound. thanks much, in advance.

p.s. i know these are silly questions for for a lot of people here on heritage, but i have to start somewhere.

I am assuming you have the first generation 4412 network since it has four caps as you mentioned above.

http://www.jblproservice.com/navigation/Studio%20Monitor%20Series.html

I agree with Grumpy that the blue caps are targets for replacement due to possible out-of-spec condition from ageing. JBL generally used a better quality cap in the series with the driver so I am a bit surprised that the 17uF cap in line with the mid driver might be an electrolytic.

The “sound” of caps is a very subjective matter and I usually steer clear of making recommendations other than in a general way.

The 17uF cap should be replaced with a similar part due to age. If it is an electrolytic you may want to change this to a poly cap. Problem is that poly of that value will be larger.
The 3 uF part should be OK in terms of functionality. If you change up the 17uF cap in the mid you may as well use a similar make and grade cap for the tweeter.

The other electro caps are in parallel and not as high a quality for that reason. Replace them with similar type caps of the same value.

Retain the bypass on the electro caps. If you use good grade polys the need for bypass is not so important.

I use the Dayton 1% poly caps as a good poly substitute but mostly because of the tighter tolerances. I like to get a good match in component values between channels. Most caps you will find are 5% or more in tolerance. You may find better sound in higher priced caps but that sure is hard for someone like me to predict with certainty. I also use Clarity Caps but they are a jump in price.

I don’t think there is an advantage to using multiples of caps v. single other than trying to get at some value between standard cap values or trying to fit two smaller parts where the single just won’t fit.

On piggy back caps you can twist and solder the leads of the top cap to the leads of the bottom cap whose leads are in turn soldered into the circuit. Works fine as long as you have good conductor-to-conductor contact before soldering.