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Old 31st January 2004, 02:58 AM   #1
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Default Kill the Crummy Opamp!!

Have here a Harmon-Kardon Citation 16 amp that I'd like to bring a little up to date. I know it's never going to be a world-beater, but I believe it can be made a whole lot better just by gettng rid of the cheezy LM1303 (other channel has µA739, same thing)opamp.

The feedback loop for the opamp is global...call me a nOOb but I've not seen this done with an opamp. If something modern with a nice, normal high input impedence can be substituted for the 1303, then I should also be able to get rid of the huge 150µf electrolytic cap and replace with a 2.2µf film cap of some sort.

What am I likely to run into in trying to replace this dinosaur of an opamp with, say, an OPA604? Stability?
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Old 31st January 2004, 03:03 AM   #2
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Rip out its guts and replace it all with something up to date. Be it a Leach or Aleph or something else but get rid of that mess!

Mark
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Old 31st January 2004, 03:22 AM   #3
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Quote:
but get rid of that mess
....agreed. But it's my buddy's amp, and he believes the old HK solid-state amps are God's Gift to audio. I thought perhaps I might be able to make it a little less horrid by updating a few components.

So...anyone else...on perhaps a bit more constructive note?
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Old 31st January 2004, 05:29 AM   #4
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Before you do anything, does the amp work properly and how does it sound like? The amp is antique I suppose and I think it's a shame to ruin it with new parts if the amp is working.
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Old 31st January 2004, 05:48 AM   #5
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Yes, it does work. Distortion measures pretty low (about 0.06%) at 130W, and 10KHz square wave looks decent.

The sound? Doesn't exactly blow me away. Sounds OK. I've certainly heard lots better, but nothing stands out in its performance as being horrid.

The most annoying thing is the turn-on and turn-off transients. Not real pleasant. If ever an amp needed an output relay, this would be it.
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Old 31st January 2004, 08:19 AM   #6
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Actually, as much as I hate to see an opportunity to junk an old opamp get away, I believe I am going to simply renew a few of the old electrolytics and call it good. I did temporarily swap the old opamp for a OPA604, but it honestly made little difference in either the sound or the measurements at the output (although it was perfectly stable, which shocked me...)

Still, classic or not, I'm going to recommend that a output relay be added. Those thumps might be fine in a PA system, but are pretty unacceptable in a home environment.
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Old 31st January 2004, 09:05 AM   #7
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That is a good opamp topology, one of the best. An OPA604 would be a good choice for a better opamp to drive it. The only thing to watch out for would be all the small compensation caps around the opamp. With an FET opamp the input cap could be as small as you like, 0.1µF even. Look at a schematic for a Carver M1.0T, same front end with a better opamp.

The ouputs are quasi-comp, probably to hard to change over to full comp. You might be able to add a Baxendall diode to the bottom half, but then you would need to fiddle with the bias too.

Just changing out the caps will make a big difference.

If driving low impedance loads you may want to increase C18 and C19 to a much larger value.
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Old 31st January 2004, 10:11 AM   #8
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Without a proper adapter of some sort, the 604 has to be mounted on a daughter card, and connected to the driver board with butt-ugly spaghetti wiring. Too asthetically unpleasing for me. I suppose I'm likely to leave the opamp alone simply because of that. I already know it will work, but if it looks hacked then I'll probably maintain the original.
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