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Old 24th January 2013, 07:08 AM   #21
Mooly is offline Mooly  United Kingdom
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Common opamps are generally specified as short circuit proof on a continuos basis.
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Old 24th January 2013, 08:00 AM   #22
EUVL is offline EUVL  Europe
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Firstly you don't want to use normal opamps unless you are willing to accept the maximum voltage limitation.
Secondly there are well-proven, cleverer ways to parallel opamps.

Check out the datasheet of OPA551, Fig. 9 (and 10).
And yes, we have done this before.


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Old 24th January 2013, 03:47 PM   #23
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Quote:
Originally Posted by EUVL View Post
Firstly you don't want to use normal opamps unless you are willing to accept the maximum voltage limitation.
Secondly there are well-proven, cleverer ways to parallel opamps.

Check out the datasheet of OPA551, Fig. 9 (and 10).
And yes, we have done this before.


Patrick
Figure 9 illustrates a master/slave circuit, not a parallel circuit. Both op amps share the current load; but it is not true parallel. The second amp is a "slave" to the first amp. This avoids circulating currents entirely. It is indeed clever, although not unusual.

By the way, that op amp looks like it could be useful. I wonder how its audio performance is?
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Old 24th January 2013, 03:53 PM   #24
jcx is online now jcx  United States
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Vos still causes a current in the sharing R - the "slave" also must have input CM range up to the "master" output Vswing - not always the case with otherwise good op amps

plus "circulating current" isn't all bad - you can deliberately add Vos - to bias the outputs into Class A push-pull:

Click the image to open in full size.


there is a small stability hit too - the slave's phase shift adds a complex load to the master output - could be a probelm if very different GBW op amps are used together

Last edited by jcx; 24th January 2013 at 03:57 PM.
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Old 24th January 2013, 04:03 PM   #25
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Jan Meier did a parallel lm6172 amp but I can't seem to find it?!? Perhaps someone with better Google-fu...

LM6172: +- 18V, 60ma

You have to be careful with the faster opamps!
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Old 24th January 2013, 04:21 PM   #26
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Thanks for all the replies. The OPA551 seems a little too expensive and it only has one OpAmp per package, but the datasheet is handy.
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Old 24th January 2013, 04:21 PM   #27
jcx is online now jcx  United States
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Default it seems worth using better suited op amps

the TPA6120 are 100 MHz CFA - certainly need high frequency layout, 200 MHz 'scope highly recommended

but they are designed to put out 400 mA, swing to ~1.2 V of their +/-15 V rails

as dual op amps the paralleled output could be 800 mA - heatsinking is the next issue

they have the leadframe the die is attached to exposed on the undersurface where is is intended to be soldered to a PBC pwr plane for heat removal - rated for 2-3 W pwr diss

but I mounted them "belly up" and clamped to the CPU heat sink - no failures yet with >6W per chip Class A bias


may seem like some work - but then 1 TPA6120 replaces ~10 packages of LM6171 - and does it with much lower distortion, noise - awesome audio specs

and as I did you can fit 4-6 TPA under one CPU heatsink

Last edited by jcx; 24th January 2013 at 04:34 PM.
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Old 24th January 2013, 05:36 PM   #28
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You can get 50 NE5532s for U$9.99 (20 cents each) here:- 50 x NE5532 5532 IC Dual Low Noise Op Amp | eBay

They're through hole, not fussy about layout, don't need a heatsink. They're proven in a multi-opamp power amplifier, they will work. You can copy Doug Self's circuit. If you haven't got a copy you can download it from Elector if you join up. You can even buy PCBs.

There's an old principle, KISS. Keep It Simple, Stupid.
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Old 24th January 2013, 05:46 PM   #29
jcx is online now jcx  United States
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simple is buying a mass marketed amp for less than you can buy the power supply or case parts for in single quantity - you can consider the rest of the amplifier as thrown in for free by diy build cost standards


sometimes people seize on an idea - certainly go with it if it piques your interest - but there are technical and objective reasons it isn't done commercially, or even as a popular diy project approach

Last edited by jcx; 24th January 2013 at 05:49 PM.
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Old 24th January 2013, 05:51 PM   #30
paulb is offline paulb  Canada
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Quote:
Originally Posted by AudioLapDance View Post
Jan Meier did a parallel lm6172 amp but I can't seem to find it?!? Perhaps someone with better Google-fu...

LM6172: +- 18V, 60ma

You have to be careful with the faster opamps!
It used to be here
http://headwize.com/projects/meier3_prj.htm
but doesn't seem to be on the headwize site any more.
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