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Old 1st January 2002, 10:11 PM   #1
JoeBob is offline JoeBob  Canada
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Default need simple preamp

I'm working on my LM3886 based amp and was thinking that after I should make a preamp for it. I'm almost done my amp modules and have to wait for my transformers so maybe now would be a good time to work on a preamp, the thing is what design to use. I'd like to keep it simple and use a single IC based idea (as is my amp) whlie not trading sound quality. Any ideas would be apreciated. Since my amp boards are almost done I'd like to start the preamp in a week or two...
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Old 1st January 2002, 10:25 PM   #2
paulb is offline paulb  Canada
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I'd recommend something very simple with an audio grade op-amp like the TI OPA2134. Selector switch, volume control, op-amp with gain of 5 or so. Do you want/need tone controls? Balance control? Remote control (!)?
You can use a stepped attenuator instead of a potentiometer for volume if you wish.
There are many DIYers at headwize.com that have done evaluations of different op-amps, you may want to browse around there for a while. Use an IC socket and you can plug in different types to see if you can hear the difference.
You may be able to get away with no opamp at all if your source is, say, a CD player.
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Old 1st January 2002, 10:34 PM   #3
JoeBob is offline JoeBob  Canada
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No, I don't need tone controls, or balance controls, just selection and volume. I'll probably use mainly a CD or PC soundcard, so I know I could probably not even use a preamp (which is what I was originally planning), but I'd like to build one and use an opamp so that if ever I wanted to expand and use different devices I could.

I was looking at the LM358, would that workout fine?
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Old 1st January 2002, 11:13 PM   #4
paulb is offline paulb  Canada
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LM358 is an older design that is probably very poor for audio, at least by today's standards. LM833 would be better.
But I think both are duals. Build it with a 358, get it working, then plug in an 833 (or OPA2134) and see if you can tell the difference.
Pick an op-amp that is unity gain stable; while you're not using it at unity gain it will give you more stability at small gains as well.
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Old 1st January 2002, 11:15 PM   #5
JoeBob is offline JoeBob  Canada
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Ok, I'll try the 833, I like nationals service, took only a day to get my 3886's, and I like to be consistent. But if TI's opamp is of noticable better quality, I'll go for it...
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Old 1st January 2002, 11:29 PM   #6
JoeBob is offline JoeBob  Canada
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Well, after looking them both over, the OPA2134 looks better. Especially since an older post said that the LM833 caused softer bid-bass (although that poster said replacing with an LF353 solved his problems).
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Old 2nd January 2002, 01:41 AM   #7
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JoeBob,

The LM3886 based amps usually need around 750mV RMS to acheive rated output .... most equipment should be able to supply this so you probably wont need any gain in the preamp.
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Old 2nd January 2002, 01:43 AM   #8
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JoeBob,

How did you get your 3886's from National in a day?
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Old 2nd January 2002, 02:12 AM   #9
JoeBob is offline JoeBob  Canada
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24hr overnight ups shipping, from noth dakota to here in quebec canada.

Since I based my amp on Rods LM3875 amp, could I use the OPA2134 in Rods "Simple High Quality Hi-Fi Preamp" project 02 I do believe? Instead of the NE5532s?

And sure I porbably won't NEED gain, but it's fun, sho why not?
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Old 2nd January 2002, 02:17 AM   #10
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which supplier did you purchase the parts from?
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