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Old 20th March 2006, 02:48 PM   #1
Minds is offline Minds  Indonesia
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Question Active x/o upgrade question

Hi all

i want to ask something about active crossover.

i had this NOS (new old stock) amp. just 2 channel with 2n3955 and it's 3055 pair. im using this to driving a 10" subwoofer.
but....the x/o was poor, the vocal and some instruments make their way to sub.

i have 2 options for this problem :
1. Put an external active crossover (i plan to stick these)
2. Improving my amp's active x/o.

of course the no.1 option would be the best, but somehow the option no.2 seems intriguing.

can we improving our amp's active x/o via maybe upgrading the opamps (the standard using SIP 4558 if i wasn't mistaken)?
i want to upgrading it to OPA 270 DIP (via socket) or double opamp LMs

sorry for my bad english i hope all the readers can understand this clearly and im waiting for the reply
thanks
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Old 1st April 2006, 01:22 PM   #2
TO-3 is offline TO-3  Peru
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What brand is your x-over? What are the cutoff slopes? The problem may not be completely related to the opamps, though I guess it is a good place to start.
I helped someone else here(moral support only) swap out his cheaps ops with some, uh, not so cheap ones. The Burr-Brown OPA213s are made for audio applications with very nice specs. The OPA275 is a nice opamp and, I think, has a fuller bass response. The AD823 is my favorite. It has really nice specs and very low noise but cost around $3.00US each. I have a Phase Audio LDX-23 and I replaced all 22 opamps in it for a very noticable difference.

First you should no that changing out the opamps isn't the only thing that needs to be done. Because the cheaper opamps don't require as much quiesent current, for bias, you will need to upgrade your power supply to accomodate the higer current.

You will also want to look for any electrolytic caps that are in the signal path. Get rid of them and use some polypropelyne or mylar caps in there place.

Good luck
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Old 1st April 2006, 10:07 PM   #3
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IMO...the best amps are the ones with little in the signal chain...get an external active xover and call it a day
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Old 2nd April 2006, 07:43 AM   #4
lndm is offline lndm  Australia
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I think the 4558 op amps are easy to improve on. I have replaced 4558's with 5532's (with no regard to quiescent current, but you'll check won't you ). Big difference. Less grain, more music. Also, electrolytic caps, should be changed for something better (IMO not mylar), start with the signal path. They will more likely be used for coupling than crossing over in which case the exact value would be negotiable allowing you to use practical values of a good capacitor type.
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Old 4th April 2006, 09:52 AM   #5
Minds is offline Minds  Indonesia
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i want to use JBL Active but later on i decide to swap my 2 channels with 4 channels from Soundcraftsmen (they do cars? oh yes they do )

yea i thought i want to use the ADs but ADs and BBs cost an arm and a leg herel. The AD 815s cost about 30USD in here if the stuff available (most cases not even the seller have any noticable opamps )

i also thought of get rid the elcos even in the supply line for better ones but it comes that upgrading to a better grade costs more

btw here's some internals
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Old 4th April 2006, 12:18 PM   #6
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Swapping op-amps will make absolutely no difference to the vocals etc coming out of the sub. The problem is that the filter slope is too shallow and/or the cut-off frequency is too high. Unless you can add another small board into your sub you need an separate external filter/crossover.
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Old 4th April 2006, 04:04 PM   #7
Minds is offline Minds  Indonesia
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so to improve these slopes what can we do?
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Old 4th April 2006, 04:50 PM   #8
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What amp is it?

If someone on the forum has a schematic, that would make it easier to find a solution. It could be something as simple as changing the value of a couple of resistors or capacitors to allow the crossover to go a little lower in frequency. If you REALLY want to modify the circuit and no one has a schematic, you can draw one out yourself. It may take an hour or so but it should be pretty easy on a relatively simple amp.

It's not easy to change the slope of a circuit without causing other problems. If you can change the slope, you'll very likely introduce ripple (uneven frequency response) in the passband. For bass it's not likely to be a problem (unless you're an audiophile with a high-end system).

Unless this is mainly a learning exercise, it's probably not worth modifying the circuit. Crossovers are VERY inexpensive. Even a cheap one with relatively shallow slopes will do the job if used in conjunction with the internal crossover.

As was mentioned before, changing the op-amps will do nothing to change the slope. The most you could hope for is a lower noise floor. When using hi-end op-amps, you must be very careful. Some are less stable than standard op-amps at unity gain. Instability can cause inaudible oscillation that can destroy the amp and/or your speakers. You would need an oscilloscope to check for oscillation.
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Old 4th April 2006, 04:55 PM   #9
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As I said if you can add a small board into your amp you can add another filter to increase slope. Otherwise it means adding an external filter/crossover.
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