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Old 28th February 2009, 02:52 PM   #1
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Default op amp recommendations

I've got a circuit of an active crossover from the 1980's which used LF351's as sallen & key filters and response shapers.

Not up to date on latest chips so am looking for modern amps unconditionally stable for unity gain and with low noise.

There are 9 amps per channel, one input buffer and 3, 3, 2 used for a 3 way split so cost is somewhat of a consideration, 5 each for 18 amps would start to add up

Any kits for regulated +12 -12V too? I've got some transformers left over from my building days.

thanks.
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Old 1st March 2009, 02:30 AM   #2
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the LF351 is an OK op amp, but you could use a TL071, or the newer TLE2071. the newer op amp has lower noise and higher bandwidth than the older BIFET op amps like the LF351 and TL071. if you don't mind using a higher component density on the board, you could go with 9 TLE2072's or 5 TLE2074's (with 2 unused op amps available for something else, like output buffers or whatever).

for +/-12V a power supply would be real simple, a transformer, bridge rectifier and a 7812/7912 pair of regulators and a few electrolytic caps. i used to build split 12 and 15V supplies in my sleep for various projects. the supply current for each op amp in the TLE07x series is about 2ma each amplifier, so for an 074 this would be 8ma (the data sheet actually lists this as 6.5ma) per chip. you could probably use 7812 and 7912 regulators without heatsinks for supplying these currents, although if i was building it i'd use a heatsink anyway in case i wanted to add other goodies to the circuit later.
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Old 1st March 2009, 02:40 AM   #3
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You might find that helping out the power supply would bring an even greater sonic improvement

fwiw -- Nat Semi has several really great opamps optimized for audio -- the LM452 and its cousins.
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Old 1st March 2009, 05:07 PM   #4
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Hi , thanks for the suggestions. The 2071 looks good. Any links for suppliers, I don't have access to any trade accounts now.

I've read about super regulators and wondered if 78 and 79 types are ok or if something a bit more specialised would be worthwhile without having to spend a fortune
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Old 3rd March 2009, 10:05 AM   #5
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thanks to all above for your useful suggestions. OPA134's at Farnell are only 1.20 so those they will be. The last time I bought some they were much more expensive!

My last query is on grounding. The circuit is effectively 3 parallel circuits from phono in gnd to 3 phono out gnds per channel.

I envisaged star earthing each channel 3 signal gnds at the channel input gnd.

But where do I connect the supply centre gnd?

Should I have a star at the circuit output connected to supply centre gnd and connect both channel three gnds there and run all phono gnds from there?

Or am I just thinking too much about power amp grounding and it does not matter so much in a circuit where a bunch of op-amps are just pulling some tens of mAs?
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