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Old 1st June 2008, 08:31 AM   #1
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Join Date: May 2008
Default Technics pre amp op-amp upgrade?

Hi guys, first post over here

I restore audio gear as a hobby, usually older vintage amps, cassette decks and turntables. No, I know the old gear isn't perfect, don't get me wrong, but I enjoy my music however it comes. Anyway, on to my post

I hope this is the right section for this, in late 2000 I bought a Technics SU-A900D Mk2 pre/power combo. Cut a long story short I was never really thrilled with it and it ended up in my audio storeroom. The other week I pulled it out and powered it up, it's developed a gain control on the L/ch in the preamp. After some probing with the scope I've singled it out to the tone control op amp. Original intention was to repair it and sell it, I'm now wondering that if I've got to replace some parts anyway I can upgrade it and see what I think of it afterwards.

My 'faulty' IC is an NJM4580DD, I've heard before there's better possible parts out there, one thread I found on here mentioned the OPA2228 which sounds a good replacement.

The one that's got me blank though is the phono eq amp which is an AN6558. Can't even find a datasheet for this. It feeds from the same +-15v rail as the others and pinout seems the same on the preamp schematic but is this a decent quality part in the grand scheme of things or is there a better one? I use a turntable and this amplifier was always dull through the phono section, the amp section did sound much better with another pre rigged up to the amp inputs. I also managed to break one of the flimsy input sockets so making an aluminium chassis for some gold plated ones is also on my list, also toying with replacing the (few) electrolytics with Black Gate caps. The circuitry seems quite direct from source to amp output, it's just these couple of ICs that I think are killing the dynamics of the sound.

I can do the schematics and understands things like slew and gain but substituting chips of any description isn't something I touch often, any advice on IC quality or replacement would be appreciated

Regards, Bruce.
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Old 1st June 2008, 05:36 PM   #2
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
Hi, The AN6558 is a Dual opamp...Here is the data sheet:

http://www.datasheetcatalog.org/data...nic/AN6557.pdf

I suppose there are many opamp that would be good replacements for both of those Chips...like..

OPA2134
OPA2137
LM4562
OPA2107
AD823
OPA2604

Pluss there are many others that might be suitable....

Maybe buy a few and put in sockets and try a few different ones out and see what sounds best to you...

Cheers
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Old 1st June 2008, 08:55 PM   #3
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Join Date: May 2008
Hi Minion, thanks for the input. Not sure what was wrong with me last night, couldn't find the 6558 anywhere, must've been tired and put the prefix in Google too. The datasheet helps a lot though, seems the better chips have much higher specs. Most of them seem to have the same pinout, so it's mainly checking pinout, case style, voltage and then seeing if there's any stability problems then?

With the phono amp I was thinking about the extra gain it needs. I'm not really happy with the characteristics, in the handbook it gives 30hz LF cut-off, presumably because there's no subsonic switch. I think it could be better even on the current circuit in there.

If just upgrading a few parts improves it, I'm happy. Otherwise I'll let it go and build myself a pair of monoblock amps

Getting the upgrade bug badly now, got an old Pioneer CD player, bought it ages ago and changed the pickup, I like it over my other CD players but it always sounded a bit woody, just looked inside and it's got a pair of 4558 as outputs. Upgrade looms

Installing some sockets seems a good idea, I'll do what you say and buy a few different types and then solder in the one I like best.

Thanks again, any more suggestions on good upgrades would be great.

Regards, Bruce
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