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Old 22nd April 2003, 01:27 AM   #1
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Default op amp recommendations

I have built a few amps using discrete devices in the past. Now, I am thinking about trying out an op amp based mosfet. I have a few devices in my mind (from the good old days), like tl082 or ad835. The first is a little too slow (in terms of slew rate) and the 2nd might be too fast (50v/us) so it could be a source of parasitic oscilation.

The amp will be an op amp driving a complementary-feedback based output stage (so I don't have to have a voltage amplification stage).

I like to get about 100w out of this amp so I am looking for an op amp with slew rate roughtly in the 35-40v/us range.

Any suggestions? thanks in advance.
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Old 22nd April 2003, 09:21 AM   #2
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Slew rate is certainly a consideration, but to judge your choice specifically on slew is silly.

You will get a lot of opinions here...my own being one based on 'bang-for-the-buck'. In this catagory, the OPA2604 rules (25V/µS). I also really like the OPA627, but it is hardly the bargan that the 2604 is.

I realize I need to give some of the Analog Devices opamps a try, but all the ones I want to check out are SMD. Too much trouble and money for so little gain.
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Old 22nd April 2003, 01:47 PM   #3
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Yes, the OPA2604 is good, the OPA627 is the best, but expensive, and you could try also the LM6171/LM6172.
Very good and cheap.
3000V/us!
You'll have to bypass them properly, but they sound very good.
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Old 22nd April 2003, 04:14 PM   #4
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thanks, everyone. The opa2604 seems to be a very good choice. has anyone tried it vs. the LM3171/3172?

What happened to the TL082 crowd? They were hot some 10-yrs ago when I was really into DIY and now you don't seem them get even mentioned.
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Old 23rd April 2003, 05:08 AM   #5
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I've built an amp with the NTE equilivant of the TL072 and FETs for current drive. It sounds very good, clean and VERY little noise. I'm sure that there are better OP amps and I plan on trying some out. But one thing to consider is how much voltage will the OP amp swing (before clipping). Take the OPA227 as an example. Operation is +- 18, working V at +- 15. Even if this is what you do swing, this is your limiting factor in output power. SO, I=V/R=15/8=1.875 and Wattage = V*I= 28.125 peak power assuming 100% effiency. Realistcly this would be 18-20 Watts. I guess that my point is that voltage of the OP amp should also be taken into consideration if you are trying to get 100 Watts. Check the TI site for Power Operational Amps for additional options. But, I do have to say that I run my OPA based amp in my studio with a pair of '70s Advents and a seperate Sub and it is quite loud enough for the front rooms of my house, even during parties.

-Dozuki
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Old 23rd April 2003, 07:08 AM   #6
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The AD8610 is FANTASTIC....if you have max +-13 V! OK, not very low noise but otherwise
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Old 23rd April 2003, 08:49 AM   #7
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Quote:
Originally posted by Dozuki
I've built an amp with the NTE equilivant of the TL072 and FETs for current drive. It sounds very good, clean and VERY little noise. I'm sure that there are better OP amps and I plan on trying some out. But one thing to consider is how much voltage will the OP amp swing (before clipping). Take the OPA227 as an example. Operation is +- 18, working V at +- 15. Even if this is what you do swing, this is your limiting factor in output power. SO, I=V/R=15/8=1.875 and Wattage = V*I= 28.125 peak power assuming 100% effiency. Realistcly this would be 18-20 Watts. I guess that my point is that voltage of the OP amp should also be taken into consideration if you are trying to get 100 Watts. Check the TI site for Power Operational Amps for additional options. But, I do have to say that I run my OPA based amp in my studio with a pair of '70s Advents and a seperate Sub and it is quite loud enough for the front rooms of my house, even during parties.

-Dozuki
If you look in the thread "need advice on this amp..." you will see an example of an amp that has some gain in the output stage, so it runs with the input opamp at +/- 15V and the output stage much higher.

Jan Didden
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Old 23rd April 2003, 09:35 AM   #8
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Are we mixing power amps with pre-amps?
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Old 23rd April 2003, 10:38 AM   #9
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Quote:
Originally posted by Dozuki
But one thing to consider is how much voltage will the OP amp swing (before clipping).

-Dozuki
Dozuki: a nice reminder but it wouldn't be a factor since the op amp will be driving a cf ops with about 10x voltage gains.

have you compared the tl072 vs. tl082?
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Old 24th April 2003, 02:27 AM   #10
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I did try the amp with a TL082. I don't remember the reason, but I felt that the TL072 worked better with the circuit I had put together. Either it didn't sound right to me, or I couldn't get it to swing enough voltage.

Sorry, I forgot that a CF OPS has voltage gain. This will definately help. And using BJTs as opposed to FETs in the OPS will bump the power up a bit too. I should actuall try using a CF OPS as I've recently come into a bunch of different OP amps to play with this same type of amplifier. When I was trying different OP amps I bread boarded the whole circuit on a small BB. It was easy to tweak and a fun little amp to play with. I could post my schematic if there is interest and I can get my scanner working. I probably made some mistakes, but I've been using it for 6 months or so now and I'm quite pleased with the sound.

-Dozuki
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