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Need to quiet down old mixer w/LM301s
Need to quiet down old mixer w/LM301s
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Old 11th June 2010, 04:49 PM   #11
Mooly is offline Mooly  United Kingdom
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Need to quiet down old mixer w/LM301s
It looks a bit crusty pleased you have sorted it.
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Old 11th June 2010, 04:54 PM   #12
Th3 uN1Qu3 is offline Th3 uN1Qu3  Romania
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It isn't crusty - it was wrapped in some sorta cardboard since the cap casing is bare metal and connected to the negative lead, and the case it was attached to is metal too, and connected to ground, so it wouldn't play nice if it were allowed to touch. You see a lot of cardboard in old Russian gear. As long as the glue holds it's a very effective insulator.

Next step, fitting a socket in one of the remaining busted channels, so i can swap out a few opamps and see how they perform. But i'm too lazy for that now. I'd try some different input+feedback resistors too, maybe the values can be lowered a little without bothering anything. AFAIK, lower value resistors make less noise.
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Old 11th June 2010, 05:11 PM   #13
Mooly is offline Mooly  United Kingdom
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Need to quiet down old mixer w/LM301s
While it's true thermal noise is reduced as impedances drop, I think you would be hard pushed to hear it on something like this.
It's worth looking at in conjunction with better opamps though.

Doug Self gave a rather illuminating example in his new book though... from memory something like,
you need an attenuator using two equal value resistors, which is quieter,
Two 50k resistor in series with the output taken from the junction, or use an NE5532 as a unity gain buffer, and take the output from two 1k resistors on the output.

The opamp wins out... but is it audible when you are talking in the -100db plus range.

I think you need to look elsewhere for your noise issues... the actual configuration and matching at the inputs.
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Old 11th June 2010, 05:11 PM   #14
jean-paul is offline jean-paul  Netherlands
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Did you use 4 x 4700 uf after the regs ?!
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Old 11th June 2010, 05:14 PM   #15
Th3 uN1Qu3 is offline Th3 uN1Qu3  Romania
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Quote:
Originally Posted by jean-paul View Post
Did you use 4 x 4700 uf after the regs ?!
Yes. It used to be 4x 1000. The regs aren't ICs, they're just plain one transistor regulators.
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Old 11th June 2010, 05:17 PM   #16
jean-paul is offline jean-paul  Netherlands
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Hi, and there are no resistors in series ? I think 4700 uF x 4 is a bit much after standard regs. I would use those before the regs to filter out ripple. After the regs such low values cause more problems than they solve as the regs may not like such a low ESR at their outputs. There should be no significant ripple after the regs so the large value caps serve no purpose. They do before the regs !

BTW how many opamps do you need and can you use dual versions ? You can design a convertor PCB for using dual opamps. Much more choice !

I would advise you to use good ones straight away as bad opamps won't give pleasure. You will be disappointed again and you will probably will want to change them again afterwards
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Last edited by jean-paul; 11th June 2010 at 05:24 PM.
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Old 11th June 2010, 05:23 PM   #17
Th3 uN1Qu3 is offline Th3 uN1Qu3  Romania
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Quote:
Originally Posted by jean-paul View Post
Hi, and there are no resistors in series ? I think 4700 uF x 4 is a bit much after standard regs.
Honestly i didn't bother looking. There are some 47 ohm resistors rated for a couple watts by their looks, but i can't say right now if they're in series with the caps or not. I'll check it tomorrow.

Quote:
Originally Posted by jean-paul View Post
BWt how many opamps do you need and can you use dual versions ? You can design a convertor PCB for using dual opamps. Much more choice !
There's a total of thirty-one dual opamps in this thing. Since i'm going to have to do point to point wiring anyway (non standard pinout of the original russian opamps), i'd rather use quads. One channel, one chip, one set of wires. For now i only want to fix the two channels that are still broken, then if i find good sounding opamps at a fair price i'll upgrade the rest as well.
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Old 11th June 2010, 05:25 PM   #18
jean-paul is offline jean-paul  Netherlands
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LME49860 is very good but they're dual.
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Old 11th June 2010, 05:30 PM   #19
Th3 uN1Qu3 is offline Th3 uN1Qu3  Romania
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The specs sure look nice, but the DIP is 6 euros a pop, since we're talking about 30 of 'em i'd be better off buying a new mixer...
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Old 11th June 2010, 05:33 PM   #20
jean-paul is offline jean-paul  Netherlands
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That is not entirely true as a new mixer is likely to have mediocre opamps too.

Just don't expect real good opamps in mixers.

BTW when you buy such numbers the price of the opamps will go down.
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