Re-chipping an old pro sound graphic EQ - diyAudio
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Old 29th November 2007, 08:06 PM   #1
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Default Re-chipping an old pro sound graphic EQ

I just obtained a pair of used Biamp EQ290 graphic equalizers from a sound contracting company. They date to the 1980s best I can tell, and the circuit boards are full of 4558s and 5532s - in fact, each band has its own dual op amp, and most of these are 4558s! So I'd like to re-chip them.

With so many 4558s to replace, I'm looking for something that's a significant performance improvement - mostly in terms of noise and gain-bandwidth product - but without a huge increase in supply current. Cost is also a factor. Seems like LM833s might be a good choice. I'd probably go with LT1124s if I had an unlimited budget.

The 5532s will probably be replaced by LM4562s, since they draw about the same current and are improved in almost every other respect.

Does anyone have other good suggestions for replacement chips?
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Old 30th November 2007, 03:16 AM   #2
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May be, u should think of changing the electrolytic caps also.

Gajanan Phadte
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Old 30th November 2007, 12:22 PM   #3
jaycee is offline jaycee  United Kingdom
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The 4558's probably serve as the gyrators, and as such are not directly in the audio path. Replacing them will improve nothing.

The 5532 is still a good chip. As gmphadte says, the biggest benefit would come from replacing the electrolytic capacitors first. Swapping out the opamps will make little, if any difference.
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Old 30th November 2007, 01:45 PM   #4
lohk is offline lohk  Europe
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I support that. It makes much sense also to check all other possibilities too, like cables and plugs.
You can possibly change the 5532s with 5532AN low noise types if they are still availlable. The 4558 are chosen because of their data, to replace them with something not fully similar will definitely bring no improvement.
I suppose there are lots of elcaps in the signal path - try to change them to Panasonic FC to start with.

(In the famous Trident 3000 - the british mixing desk in which so many good recordings were made - the the stereo remix line uses a 5534N for the zero-ohms summming amp, with 470yF/6V Roederstein elcaps in the signal path)
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Old 30th November 2007, 03:49 PM   #5
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Good point about the electrolytic caps, I'll definitely swap them out. I'm sure they've seen better days.

Can someone explain to me how the gyrators in a graphic EQ are "not in the signal path"??
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Old 1st December 2007, 09:04 AM   #6
Eva is offline Eva  Spain
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Given the age of the EQ, I would worry mostly about the faders...
I use to feel like the small child in The Emperor's New Clothes tale
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Old 1st December 2007, 09:30 AM   #7
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Given the age of the equipment, you are much better spending your time and money on parts that tend to deteriorate with age. Electrolytic capacitors, pots, faders and connectors would be where I would start. Changing all those opamps is more than likely going to make very little difference especially when compared with replacing the parts that are near the end of their life.
- Dan
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Old 1st December 2007, 11:17 AM   #8
jaycee is offline jaycee  United Kingdom
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The gyrators effectively simulate an inductor.. see this for an example:

Click the image to open in full size.

Only IC1a and IC2d are really in the signal path. You could replace the 4558's but there would be minimal, if any improvement to sound quality.

As others have stated, go for replacing parts that deteriorate with age first.
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Old 2nd December 2007, 10:13 AM   #9
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Default small story

once i had an eq more or less had the same problems so for starts dealed with precise caps arround the ic ( keep in mind that the circuit only uses 4558 ) then i meesed up a bit with power suply ...did some soldering but accidently small short damaged one of the 4558

in a very happy sunday there was no 4558 at my lab none at all so after looking at my stock oi looked at surplus devices existing in th shop

so i got my shelf one board from army cryptografy device ( something like a voice scrambler ) and there it was one of the boards was full with one gray coloured chip with gold tips only sauing 4558 on it -(no other data) made some time in the 70's and from the looks it looked like the old motorola construction

since i had somany and with military specs ha ha ha i thought ok let me change all 5 of them existing in the board .....

and then testing the one board compared to the other wearing normal st chips it was much more clean and delicate sound ....

i replced all 10 of them and i got a really working machine in my hands considering that its an eq really works clean and fine i still run it in small monitor or pa application and still dont have a clue about these chips but they play like hell
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Old 2nd December 2007, 01:58 PM   #10
jaycee is offline jaycee  United Kingdom
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Sakis, they were probably mil spec 4558's in extended temperature packing. The description sounds exactly like many mil spec IC's from that era. They were probably hand picked for lowest noise too.
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